Behavioral Ecology Vol. 14 No. 6: 823–840
DOI: 10.1093/beheco/arg072
Ecological and behavioral correlates of
coloration in artiodactyls: systematic analyses
of conventional hypotheses
C. J. Stoner, T. M. Caro, and C. M. Graham
Department of Wildlife, Fish, and Conservation Biology, University of California, Davis,
CA 95616, USA
To test the generality of adaptive explanations for coat coloration in even-toed ungulates, we examined the literature for
hypotheses that have been proposed for color patterns exhibited by this taxon, and we derived a series of predictions from each
hypothesis. Next, we collected information on the color, behavioral, and ecological characteristics of 200 species of even-toed
ungulates and coded this in binary format. We then applied chi-square or Fisher’s Exact probability tests that pitted presence of
a color trait against presence of an ecological or behavioral variable for cervids, bovids, and all artiodactyls. Finally, we reanalyzed
the data by using concentrated-changes tests and a composite molecular and taxonomic phylogeny. Hinging our findings on
whether associations persisted after controlling for shared ancestry, we found strong support for hypotheses suggesting even-toed
ungulates turn lighter in winter to aid in concealment or perhaps thermoregulation, striped coats in adults and spotted pelage in
young act as camouflage, side bands and dark faces assist in communication, and dark pelage coloration is most common in
species living in the tropics (Gloger’s rule). Whereas white faces, dark legs, white legs, dark tails, and white tails did not appear to
assist in communication alone, legs and tails that were either dark or white (i.e., conspicuous) did seem to be linked with
communication. There was moderate support for hypotheses that countershading aids concealment, that white faces are
a thermoregulatory device, and that white rumps are used in intraspecific communication. There was weak support for spots in
adults and stripes in young providing camouflage and for dark leg markings being a form of disruptive coloration. We found little
or no evidence that overall coat color serves as background matching, that side bands are disruptive coloration devices, or that
white rumps help in thermoregulation. Concealment appears the principal force driving the evolution of coloration in ungulates
with communication, and then thermoregulation, playing less of a role. Key words: bovids, cervids, communication, concealment,
countershading, disruptive coloration, pattern blending, phylogeny, thermoregulation, ungulates. [Behav Ecol 14:823–840
(2003)]
he adaptive significance of coloration in ungulates has
never been addressed systematically. Indeed, for mammals, many of the current hypotheses about coloration are
still the same ones formulated more than 100 years ago
(Beddard, 1892; Buxton 1923; Mottram, 1915, 1916; Poulton,
1890; Roosevelt, 1911; Thayer, 1909; Wallace, 1889) and have
received remarkably little attention ever since (but see
Ortolani, 1999). Hypotheses to explain the function of
coloration in mammals can be grouped into three broad
categories: concealment, communication, and regulation of
physiological processes (Cott, 1940).
Animals can remain concealed when their overall color
resembles, or matches, the natural background in which they
live (Endler, 1978). This phenomenon of background
matching, also known as general color resemblance, includes
crypsis in which overall body color resembles the general
color of the habitat, or pattern blending, in which color
patterns on the body match color patterns of light and dark in
the environment (Cott, 1940; Poulton, 1890). Background
matching may be subtle in that an individual may change its
pelage or skin coloration annually if backgrounds change
between seasons (also termed variable background matching), or may vary with age if the background environments of
immatures and adults differ in predictable ways. Alternatively,
T
Address correspondence to C.J. Stoner. E-mail: cjstoner@ucdavis.
edu.
Received 17 August 2001; revised 16 January 2002; accepted 21
January 2003.
2003 International Society for Behavioral Ecology
animals may achieve concealment through disruptive coloration by sporting contrasting colors or irregular marks that
break up the body’s outline (Cott, 1940). Lastly, animals
may attain concealment by having a pelage that lightens
toward the ventral surface. This counteracts the sun’s effect of
lightening the dorsum and shading the ventrum when it
shines from above (Kiltie, 1988, 1989; Thayer, 1896, 1909).
All of these types of coloration are found in even-toed
ungulates.
When coloration is conspicuous, as in the black and
white faces of oryx species, intraspecific communication
(Wickler, 1968) or communication between prey and
predators is thought to be involved (Rowland, 1979). For
example, patches of color may be used to signal danger to
conspecifics (Alvarez et al., 1976) or to signal young to follow
their mothers (Leyhausen, 1979). Alternatively, they may be
used to signal to predators, the classic example being
aposematic coloration in species that have noxious defenses,
such as skunks (Spilogale putorius) (Guilford, 1990; Johnson,
1921). Many ungulates exhibit conspicuously colored
heads, legs, and anal regions (Guthrie, 1971; Meyer and
Kranzle, 1999).
Finally, coloration may be related to thermoregulation or
protection against ultraviolet rays (Hamilton, 1973). Thus,
dark-colored animals may be expected to absorb solar
radiation better than light-colored animals do and might
therefore be found in colder climates (Burtt, 1979). Even-toed
ungulates exhibit a range of hues from black through gray
and red to pale. In contrast to birds, none of the major
hypotheses for coloration in mammals pertains to mate
choice (see Badyaev and Hill, 2000).
Behavioral Ecology Vol. 14 No. 6
824
Table 1
Summary of suggested hypotheses for the adaptive significance of coloration in even-toed ungulates and
examples
Hypothesis,
Example
Source
Overall body coloration
Uniform coloration and background matching
Reddish coat matches rocky habitats in markhor
Uniform coloration and thermoregulation
Pale coat minimizes heat exposure in hartebeest in desert
Lighter coats in winter and background matching
Seasonal color changes reduces conspicuousness in red deer in snowy arctic
Spots on adults and pattern blending
Dappled coat provides camouflage for giraffe in dappled light of woodland
Spots on adults and reduced aggression
Dappled coat assists in lowering aggression in axis deer living in groups
Stripes on adults and pattern blending
Striped coat in greater kudu blends in with riverine habitat
Stripes on adults and reduced aggression
Striped markings in group-living eland lower aggressive interactions
Spots on young and pattern blending
Dappled coat in young mule deer provides camouflage in dappled light of woodland
Stripes on young and pattern blending
Striped coat in young red river hog aids concealment in grassy vegetation
Side bands and disruptive coloration
Dark stripe breaks up springbok’s form in open environment
Side bands and maintaining gregariousness
Side bands assist Thomson’s gazelle in maintaining group cohesion
Countershading and concealment
Light ventrum in the diurnal dibitag counteracts shading from the sun
Body parts
Dark face markings and communication
Black face assists in signaling to group members in oryx
White face markings and communication
White face assists in signaling to group members in vicuna
White face markings and thermoregulation
White face helps pronghorn reflect heat in desert
Conspicuous face and communication
Black and white face in impala assists in signaling to conspecifics
Dark leg markings and disruptive coloration
Dark leg markings in lechwe break up form in open environments
Dark leg markings and communication
Black knees assist in signaling to group members in addax
White leg markings and communication
White legs in bontebok act as visual signals in daylight
Conspicuous leg markings and communication
White legs in okapi signal to stalking predators
Dark tail and communication
Black tail in Grant’s gazelle signals to coursing predators
White tail and communication
White tail in blue duiker is prominent at night
Conspicuous tail and communication
White tail in white-tailed deer signals to other group members
White rump and communication
White rump in Soemmering’s gazelle signals to other group members
White rump and thermoregulation
White rump in bighorn sheep acts to deflect heat in desert
1, 2
2
3
3
4
5, 6
7
5, 8
1
6
4
3, 9
2, 10
11
12
13
14
2, 6, 12
12, 15, 16
4
1
1, 6, 17
18
1, 6, 19
2, 6
Sources: 1 Kingdon, 1982; 2 Spinage, 1986; 3 Cott, 1940; 4 J. Wolff, personal communication; 5 Thayer, 1909;
6
Estes, 1991; 7 Lobao Tello and van Gelder, 1975; 8 Linnell and Andersen, 1998; 9 Fogden and Fogden,
1974; 10 Hasson, 1986; 11 Poulton, 1890; 12 Geist, 1987; 13 Guthrie and Petocz, 1970; 14 Roosevelt and Heller,
1914; 15 Kingdon, 1997; 16 Danilkin, 1996; 17 Bildstein, 1983; 18 Geist, 1978; 19 Ward, 1979.
Within these three broad categories, a number of more
specific hypotheses have been advanced for overall body
coloration and specific parts of the body in artiodactyls
(Table 1). Most of these functional hypotheses for coloration
were formulated by using one or a handful of ungulate species,
and are based on qualitative arguments. Although they may well
apply to the species in question, the extent to which they are
generally applicable to other species in the order Artiodactyla
is completely unknown. The purpose of this study was to test
these conventional hypotheses systematically in order to
determine their generality within the order. Our main focus
was on overall body coloration and color of specific parts of
the body rather than a constellation of body parts such as
‘‘contrasting markings’’ (see Geist, 1978) because hypotheses
Stoner et al.
•
Color of artiodactyls
have, in general, been formulated for specific areas of the
body in this order and our data set allowed us to test such
hypotheses at this relatively fine-grained level. The value of
the comparative approach used here is that it allows us to
investigate the adaptive significance of morphological traits by
using a very large number of species, and make conclusions
that have wide generality. Similar comparative analyses on
coloration in birds have been successful in opening up debate
on coloration signaling unprofitability to predators (Baker
and Parker, 1979), on linking coloration to lekking (Hoglund,
1989), and in explaining patches of white plumage in waders
(Brooke, 1998).
METHODS
Species
Recent discoveries of new species in the order Artiodactyla
(see Groves and Schaller, 2000), a growing number of
phylogenetic studies focusing on this order, and controversies
regarding the distinctness of numerous species and subspecies have led to continual changes in artiodactyl taxonomies. We started analyses based on the species listed in Nowak
(1999) and refined this list to exclude (1) purely domesticated species; (2) species that are, or were at one time, extinct in
the wild; and (3) species for which very little morphological or
behavioral data could be found. Purely domestic species, such
as domestic cattle (Bos taurus), domestic goats (Capra hircus),
and domestic sheep (Ovis aries), were excluded because their
pelage coloration may reflect artificial selection rather than
adaptations to natural environments. The llama (Lama glama)
and alpaca (Lama pacos) were similarly excluded as Nowak
(1999) and Franklin (1982) note that these are generally
thought to be domestic descendants of the wild guanaco
(Lama guanicoe). Dromedary camels (Camelus dromedarius),
were excluded because some taxonomists (see Corbet and
Hill, 1991, 1992) believe they never existed in the wild (see
Nowak, 1999). Although we included some species that are
found both in the wild and in domestic or feral populations
(e.g., wild boars [Sus scrofa], Bactrian camels [Camelus
bactrianus], banteng and domestic Bali cattle [Bos javanicus],
guar [Bos guarus], and yaks [Bos grunniens]), we focused on
the coloration or ecological characteristics of wild populations
for our analyses. One such species, the water buffalo (Bubalus
bubalus), was excluded because some taxonomists feel this
scientific name refers only to domestic populations of the
species (see Corbet and Hill, 1991, 1992 and discussion in
Nowak, 1999).
To be conservative, we excluded species that are currently,
or were at one time, extinct in the wild: the cave goat,
Myotragus balearicus; three gazelle species, Gazella arabica,
Gazella rufina, and Gazella saudiya; the blue buck, Hippotragus
leucophaeus; and Schomburgki’s deer, Cervus schomburgki. We
similarly excluded Pere David’s deer, Elaphurus davidianus,
European bison, Bison bonasus, and the Arabian oryx, Oryx
leucoryx, because at one time they existed only in captive
environments (see Nowak, 1999). Thus, the species retained
in our analyses are currently, and have presumably always
been, extant in the wild.
Finally, we excluded species for which little information on
pelage coloration or behavior was available. For example,
information regarding Pseudonovibos spiralis is based primarily
on horns and accounts from local hunters. Gazella bilkis and
Naemorhedus baileyi were also excluded, as their descriptions
are based on rare specimens collected in the past (see
Grzimeck, 1990; Nowak, 1999). As a result of this selection
process, our analyses were based on a total of 200 artiodactyl
species (see Supplementary Material online).
825
Data collection
Information on overall body coloration, seasonal coloration
change, and coloration of underparts was obtained from
descriptions in the literature; face, leg, rump, and tail
coloration was determined from photographs (Table 2 and
appendix). Information on ecological characteristics was
taken from descriptions in the literature and was placed into
categories; latitude characteristics were determined from
range maps (Table 3). With a few exceptions, we principally
looked for associations between coloration and ecological
rather than behavioral variables because behavioral hypotheses for a color marking, such as ‘‘follow-me’’ signals, are
difficult to test. Such signals might apply between mother and
young (Schaller, 1967), between members of small groups on
the move (Hickman, 1979), or between members of large
social groups; they are more amenable to tests through direct
observation.
For all variables, a value of one was assigned to species
demonstrating a given trait, zero to species that did not
display the trait, and a ‘‘?’’ for species for which little or no
information on the variable was available (supplementary
appendix online). Variables were scored present or absent as
variation in pelage characteristics and ecological and behavioral characters was unknown for most species. Categories
were not mutually exclusive. For example, a species demonstrating light red coloration would be assigned a value of one
in both the ‘‘pale’’ and ‘‘red’’ color categories.
Analyses
Nonparametric tests
To identify general trends between coloration traits and
ecological variables, we first applied chi-squared tests separately to the two largest families, cervids (N ¼ 39 species) and
bovids (N ¼ 125 species), to identify associations that were
present within particular families. When sample sizes were
inadequate to perform chi-squared statistics, we instead used
Fisher’s Exact probability tests. We then performed these
analyses for all the artiodactyls (N ¼ 200 species) in order to
include important families such as the Suidae and Giraffidae
and to determine whether associations held across the whole
order. For each test, we included only those species for which
information was available for the coloration and ecological
variables of interest. Only significant results are reported in
the text (p , .1 because we were searching for associations
using very coarse behavioral and ecological measures). We felt
it important to present cross-species comparisons in light of
the current controversy over the applicability of phylogenetically controlled methods (Irschick et al., 1997; Price, 1997),
although in our discussion we attach far more weight to
results that controlled for shared ancestry (see below).
Phylogenetic comparisons
Cross-species comparisons fail to account for the fact that
species values are nonindependent: shared character states
may reflect common ancestry rather than independent
adaptations (Harvey and Pagel, 1991). To control for the
potential effects of shared ancestry, we analyzed the same
hypotheses as in the nonparametric tests by using a composite
phylogenetic tree and Maddison’s concentrated-changes test
(Maddison, 1990) as implemented in MacClade (Maddison
and Maddison, 1992). Concentrated-changes tests have been
reported to detect such correlated changes statistically based
on fewer character state changes compared with that of other
methods (Cooper, 2002; Cooper et al., 2002; Ridley, 1983;
Sillen-Tullberg, 1993). Although concentrated-changes tests
rely on the topology of a given phylogeny rather than on
Behavioral Ecology Vol. 14 No. 6
826
Table 2
Descriptions of coloration variables (see Appendix for sources)
Variables
Overall body colorationa
Pale
Red
Gray
Dark
Seasonal coloration change
Lighter in winter
Underparts
Countershading
a
Descriptions
White, sandy, light brown, pale gray, light gray, buffy gray, light buff, light red, fawn,
tawny, or yellowish-brown coloration
Red, rusty, chestnut, tawny, rufous, ochraceous, cinnamon, or auburn tones
Gray or grayish coloration
Black, blackish, or dark brown coloration
Pelage becomes white or lighter seasonally
Ventral coloration described as paler than upper parts, white, dingy-white, whitish,
or tinged with white
Color patterns
Presence of spots in young
Presence of spots on adults
Presence of stripes (vertical or horizontal) on young
Presence of stripes (vertical or horizontal) on adult
Presence of side band
Black or white spots on dorsal surface, rear or legs
Black or white spots on dorsal surface, rear or legs
Black or white stripes on dorsal surface, rear or legs
Black or white stripes on dorsal surface, rear or legs
Prominent line along side of body
Tail
Light
Dark
Markings on tail white or lighter than coloration of rump
Markings on black or darker than coloration of rump
Rump
White markings
Presence of rump patch or white markings on rump
Face
Conspicuous light markings
Conspicuous dark markings
White or light markings on face that contrast with the overall body coloration
Dark or black markings on face that contrast with the overall body coloration
Legs
Conspicuous light markings
Conspicuous dark markings
White or light markings on legs that contrast with the overall body coloration
Dark or black markings on legs that contrast with the overall body coloration
Sexually dichromatic species were scored as both colors.
incorporating branch lengths estimates like more rigorous
methods (see Nee et al., 1996; Pagel, 1994; Read and Nee,
1995), we deemed these tests appropriate for our analyses
owing to the scarcity of branch length and divergence time
estimates for ungulate taxa (Pérez-Barberı́a and Gordon 1999,
2000, 2001; Pérez- Barberı́a et al. 2002).
Figure 1 demonstrates the hypothesized phylogenetic tree
on which analyses were based. Owing to the lack of consensus
over the placements of Artiodactyla taxa in the literature, we
based this composite tree primarily on the findings of recent
molecular studies and supplemented these with taxonomies
and studies based on morphological traits. The general
positions of the Cervidae, Bovidae, Giraffidae, Antilocapridae,
Tragulidae, Hippopotamidae, Suidae, and Camelidae were
based on cladistic analyses by Gatesy et al. (1999). Additional
sources were used to resolve the genus Moschus (Su et al.,
1999), the Tragulidae (based on taxonomy in Grubb, 1993),
Suidae (Groves, 1981; Randi et al., 1996; Thenius, 1970),
Peccaridae (Theimer and Keim, 1998), and Camelidae (based
on Gatesy et al., 1999: Figures 11 and 12).
Cervidae branch. The general positions of the CervinaeMuntiacus, Hydropotes-Alces-Capreolus, and Rangifer-New World
odocoileinae clades were inferred from Cronin et al. (1996),
Douzery and Randi (1997), Gatesy et al. (1999), and Randi
et al. (1998). Resolution within Cervinae-Axis-Dama was based
on Emerson and Tate (1993) and Randi et al. (2001), the
Elaphodus-Muntiacus clade was based on Amato et al. (2000),
and the placements of Hydropotes, Alces and Capreolus were
based on Randi et al. (1998). The placements of Hippocamelus,
Pudu, Blastoceros, and Ozotoceros (in relation to Rangifer,
Odocoileus, and Mazama) were based on Webb (2000).
Bovidae branch. All information gained from the simultaneous analysis of data sets in Gatesy and Arctander (2000) was
retained in the Bovidae tree. Gatesy and Arctander (2000) was
used to infer the basic branches within Bovidae and
supplemented with information from the following sources.
Resolution within the bovini and tragelaphini tribes was based
on Schreiber et al. (1999) (Bulbus, Bos), Gatesy and Arctander
(2000) (Syncerus, Psuedorx, Boselaphus), Hassanin and Douzery
(1999a,b) (Tetracerus quadricornis in clade with Boselaphus
tragocamelus), and Mathee and Robinson (1999) (Tragelaphus,
Taurotragus). Resolution within the Antilopini clade was based
on Gatesy et al. (1999), Mathee and Robinson (1999),
Rebholz and Harley (1999) (general positions of Rachicerus
and Madoqua clades), and Brashares et al. (2000) (resolution
within the Rachicerus and Madoqua clades). Resolution within
the Gazella-Antilope clade was based on Rebholz and Harley’s (1999) parsimony cladogram. Placements of Litocranius
walleri, Ammodorcas, Antidorcus, Dorcatragus were based on
Brashares et al. (2000). Within the Alcelaphini, the position of
the Neotragus/Aepyceros clade (Hassanin and Douzery, 1999b;
Mathee and Robinson, 1999) was based on Gatesy and
Arctander (2000). Resolution within Neotragus was taken from
Brashares et al. (2000). Positions of Connochaetes, Damaliscus,
Acelaphus, and Sigmoceros were taken from Gatesy and
Arctander (2000), whereas Brashares et al. (2000) provided
Stoner et al.
•
Color of artiodactyls
827
Table 3
Descriptions of ecological and behavioral variables (see Appendix for sources)
Variables
Descriptions
Latitude categories
Tropics
Subtropics
Transitional
Subarctic
Arctic
Part
Part
Part
Part
Part
Environmental categories
Open
Closed
Grasslands, deserts, or tundra habitats
Swamp/riverine, or dense forest habitats
Habitat categories
Grassland
Bushland
Woodland
Light forest/woodland
Dense forest
Desert
Rocky
Tundra
Swamp/riverine
Occupies prairie, savannah, meadows, or steppe grasses habitats
Occupies scrub, bushland, riparian, thicket, or shrub vegetation habitats
Occupies woods, woody areas, or woodlands
All species included in the woodland category, plus those occupying open, sparse, or light forests
Occupies alpine, tropical, boreal, deciduous, mixed, timberland, or dense forests
Found in deserts or semideserts
Occupies rocky areas such as talus, boulders, rocky outcrops, crevices, or cliffs
Found in tundra
Occupies swamp, marsh, bogland, moorland, reedbeds or riverine habitats
Activity pattern categories
Diurnal
Crepuscular/nocturnal
Primarily active during the day
Primarily active during the night, or in the morning or late/afternoon evening
Group size categories
Solitary
Intermediate groups
Large groups
Primarily found alone or in pairs
Primarily found in groups of 250 individuals
Forms aggregations of more than 50 individuals
Hider/follower behavior
Hider
Follower
Young lie concealed for more than 1 week after birth
Young follow mothers within 1 week of birth
Hunting style of principal predators
Stalker
Courser
Predator usually observes/follows/stalks prey before attacking
Predator usually runs down/exhausts prey
of
of
of
of
of
range
range
range
range
range
falls
falls
falls
falls
falls
in
in
in
in
in
tropic
tropic
tropic
tropic
tropic
the topography within Damaliscus. Within the Caprini-Rupicaprini-Ovibovini clade, the general placement of Ovis,
Oreamnos, Capra, Hemitragus (species assumed to be monophyletic), Pseudois (assumed to be monophyletic), Pantholops,
Ovibos, Rupicapra, Nemorhaedus (assumed to be monophyletic),
and Capricornis was inferred from Gatesy and Arctander
(2000). Resolution within Ovis was based on Hassanin et al.
(1998), Ludwig and Knoll (1998), Ludwig and Fisher (1998),
and Hiendleder et al. (1998). The placement of Budorcas
taxicolor is based on Hassanin and Douzery (1999a,b). The
topography of Capra species is based on Hassanin et al. (1998)
and Manceau et al. (1999). The placement of Ammotragus with
Rupicapra was based on Hassanin and Douzery (1999b),
whereas the topography of Capricornis species was based on
Hassanin et al. (1998). In the tribe Hippotragini, the
placement of Oryx, Addax, and Hippotragus was based on
Gatesy and Arctander (2000). The general placements of
Redunca, Kobus, Pelea, and Cephalophus branches in the
Redunci-Cephalophini tribes were based on Gatesy and
Arctander (2000). Relationships within these genera were
derived from additional sources: Brasheres et al. (2000)
(Redunca, Cephalophus), Gatesy and Arctander (2000) (Kobus),
Hassanin and Douzery (1999b), and Mathee and Robinson
(1999) (placement of Oreotragus in clade with Cephalophus).
Some species were excluded from the composite phylogeny
because relatively little information could be found regarding their phylogenetic position (e.g., Bos sauveli, Tragelaphus
latitudes
latitudes
latitudes
latitudes
latitudes
(010 degrees from equator)
(1130 degrees)
(3150 degrees)
(5160 degrees)
(more than 60 degrees)
buxtoni, Cephalophus rufilatus, Cephalophus natalensis, Cephalophus nigrifrons, Modoqua piacentii), or great disagreements exist
regarding their placements (e.g., Saiga saiga, Oribi oribi, and
Procapra species). To resolve polytomies in the composite tree
(as MacClade requires a fully dichotomized tree for concentrated-changes tests), we used an online library database
(BIOSIS) to count the number of citations retrieved for each
species in the polytomy (Nunn C, personal communication).
Species with the least number of citations (indicating those
for which less information is known) were excluded from the
polytomies until a fully dichotomized tree remained. These
species included Ovis vignei, Ovis nivicola, Hemitragus jayakari,
Capra wallie, Capra falconeri, Capra pyrenaica, and Oryx dammah.
To prepare for phylogenetic analyses, we first used
MacClade to map the coloration (dependent) and ecological
(independent) variables onto the phylogenetic tree using the
matrix depicting each species’ character states (one, zero, or
‘‘?’’; supplementary appendix online). From the distribution
of character states across all species, MacClade reconstructs
the evolutionary history of a given trait throughout the tree by
using parsimony. This makes it possible to count the number
of evolutionary gains (change in a character state from
a zero to a one) and losses (change from a one to a zero) in
either the coloration or ecological variables (Maddison and
Maddison, 1992). In those instances in which character reconstruction was ambiguous (i.e., the tree contained areas in
which both zero and one were equally parsimonious), within
828
Behavioral Ecology Vol. 14 No. 6
Figure 1
The hypothesized phylogeny artiodactyl for species included in concentrated-changes tests was based on previously published phylogenetic
studies. Upper star denotes origin of the Cervidae; lower star, origin of the Bovidae.
Stoner et al.
•
Color of artiodactyls
MacClade we used ‘‘most parsimonious reconstruction
mode,’’ which includes all reconstructions produced by the
deltran and acctran options, to generate all possible reconstructions of the character. Because multiple ambiguities for
the same character can lead to many possible reconstructions,
we selected the first and the last reconstructions for the first
analysis (following the method of Ortolani 1999; Ortolani and
Caro 1996). We examined the first and last reconstructions as
these are the reconstructions with the maximum number of
gains and fewest losses, and minimum number of gains and
most losses, respectively.
Next, we used concentrated-changes tests to test the
probability that gains in the coloration variable were associated with the ecological variable more than could be expected by chance, and losses in the coloration variable
occurred in the presence of the ecological trait less than expected by chance. Thus, we examined whether the presence
of the ecological trait facilitated the maintenance of the color
trait over evolutionary time. The null hypothesis, tested
against a distribution derived through simulation and
10,000 replicates, was that gains and losses in a given coloration variable were randomly distributed on the tree with
respect to the ecological variable. When ambiguity in character reconstruction existed, the multiple reconstructions
resulted in two or four probability values for each test,
depending on whether one or both of the dependent and
independent variables displayed ambiguity. Because, in
concentrated-changes tests (but not the nonparametric tests),
a dependent variable could be matched to an independent
variable more than once because of multiple phylogenetic
reconstructions, we applied a standard Bonferroni correction.
This is a more conservative way to apply this test than by using
a sequential Bonferroni (Rice, 1989). Thus, if four tests had
been conducted between a coloration and ecological variable,
we multiplied each p value by four and reported it only if the
product was less than 0.1. Following the method of Rice
(1989), we limited application of the Bonferroni correction to
multiple tests of the same pair of dependent and independent
variables rather than across one dependent and several
independent variables because each independent variable exhibited a different phylogenetic reconstruction and was used
to test a different hypothesis. If several concentrated-changes
tests were conducted for a given prediction, we report only the
most significant p value corrected using the standard
Bonferroni technique.
For the sake of convenience, even-toed ungulates is
a general term we use loosely to refer to all species that we
analyzed, cervids refers to Cervidae, bovids to the Bovidae,
and artiodactyls to all species in the supplementary appendix
online. As a short hand, ‘‘cross-species comparisons’’ refer to
species counts analyzed by using chi-square tests or Fisher’s
exact probability tests that do not control for phylogeny,
whereas ‘‘phylogenetically controlled tests’’ refer to the results
of concentrated changes tests using the reduced species list,
termed CCT. For Fisher’s and chi-square tests, true p values
are given, but for CCT, p values are given after Bonferroni
corrections. We consider an association worth discussing if any
of the phylogenetic reconstructions was significant after
a Bonferroni correction.
RESULTS
Coloration for concealment
Uniform coloration
To discover whether ungulate coloration aids in concealment,
we first tested whether uniform pelage color typically matches
habitat background. Cross-species comparisons showed that
829
pale bovids and artiodactyls were likely to be found in open
environments (N ¼ 125 species, v2 ¼ 9.28, p , .01; N ¼ 194,
v2 ¼ 13.26, p , .01, respectively), but results failed to reach
significance when confounding effects of shared ancestry
were taken into account. Pale bovids and artiodactyls were
also associated with one habitat category, deserts, but only in
cross-species comparisons (N ¼ 125, v2 ¼ 6.91, p ¼ .03; N ¼
194, v2 ¼ 10.19, p , .01, respectively). Next, we tested gray
and red pelage coloration against rocky habitats and found
gray bovids (N ¼ 125, v2 ¼ 8.20, p ¼ .02) and artiodactyls (N ¼
194, v2 ¼ 8.80, p ¼ .01) were significantly more likely to live in
this habitat, but again, these results disappeared using
phylogenetic tests. Surprisingly, red pelage was associated
with living in habitats that were not rocky (bovids: N ¼ 125,
v2 ¼ 9.80, p , .01). Dark bovids and artiodactyls were found
significantly more often in closed environments (N ¼ 125,
v2 ¼ 4.37, p , .0001; N ¼ 195, v2 ¼ 33.2, p , .0001,
respectively), and dark cervids, bovids, and artiodactyls were
found in dense forest habitats (Fisher’s test: N ¼ 38, p ¼ .02;
N ¼ 125, v2 ¼ 16.16, p , .01; N ¼ 195, v2 ¼ 28.23, p , .0001,
respectively), although not using CCTs. An association
between dark bovids and swamp habitats approached significance (N ¼ 125, v2 ¼ 5.30, p ¼ .08).
Lighter coats in winter
If seasonal pelage coloration contributes to concealment,
species displaying a lighter winter coat should occupy habitats
which become lighter seasonally. Cervids, bovids, and artiodactyls were likely to turn lighter in winter if they lived in
arctic latitudes (Fisher’s test: N ¼ 39, p ¼ .07; N ¼ 125, v2 ¼
2.72, p , .01; N ¼ 197, v2 ¼ 24.79, p , .0001, respectively;
CCTs for both bovids and artiodactyls: p , .001). Bovids and
artiodactyls with lighter winter coats were also associated with
tundra habitats (N ¼ 125, v2 ¼ 12.72, p , .01; N ¼ 197, v2 ¼
24.37, p , .0001, respectively), and this association held true
for all three taxonomic groups using CCTs (p ¼ .07, p , .01,
p , .001, respectively). In short, there was strong backing for
ungulates taking on lighter coats in arctic and tundra regions.
Spotted coats
If spotted coats facilitate crypsis through pattern blending,
having the appearance of dappled light would be favorable for
diurnal, solitary, forest-dwelling species and those that hide
their young. Cervids with spotted adult coats were diurnal
(CCT: p ¼ .06) but were less likely to be solitary (Fisher’s test:
N ¼ 37, p ¼ .03). Conversely, bovids with spotted young and
spotted adult coats were likely to be solitary (N ¼ 125, v2 ¼
.79, p ¼ .03, N ¼ 125, v2 ¼ 5.17, p ¼ .08, respectively), as were
species with spotted young across all artiodactyls (N ¼ 193,
v2 ¼ 13.90, p , .01; CCT: p ¼ .02). Spotted adult bovids were
associated with light forests (N ¼ 125, v2 ¼ 9.97, p , .01; CCT:
p ¼ .08). Cervids with spotted young were strongly tied to
grassland/bushland habitats (CCT: p , .0001), bovid spotted
young to light forests habitats (N ¼ 125, v2 ¼ 11.66, p , .01;
CCT: p ¼ .06), and spotted young across all artiodactyls to
dense forests (N ¼ 196, v2 ¼ 13.62, p , .01). Species with
spotted young were significantly more likely to hide after birth
in all taxonomic groups (cervids, Fisher’s test: N ¼ 38, p ¼ .09;
bovids: N ¼ 123, v2 ¼ 5.60, p ¼ .06; artiodactyls: N ¼ 194, v2 ¼
23.40, p , .0001, CCT: p ¼ .05) (Figure 2). Indeed, spotted
cervid, bovid, and artiodactyl young were significantly less
likely to follow their mothers (Fisher’s: N ¼ 38, p ¼ .09; N ¼
123, v2 ¼ 5.90, p ¼ .05; N ¼ 194, v2 ¼ 24.61, p , .0001,
respectively). In short, there was patchy support for adult
spotted coats helping with pattern blending, whereas in
young, spotted coats were found in forest bovids, grassland
cervids, and hiding and solitary artiodactyls, strongly supporting a concealment hypothesis.
830
Behavioral Ecology Vol. 14 No. 6
Figure 2
Association between spotted coats in young artiodactyls and hider species. The first reconstruction was used for both phylogenies, and the
associated p value is .05 after Bonferroni correction. Black lines indicate species with spotted coats; white lines indicate nonspotted species.
For the boxes on the right, black boxes indicate species that are hiders; white boxes indicate species that are not hiders.
Stoner et al.
•
Color of artiodactyls
Striped coats
Similar analyses of striped patterns revealed that adults
displaying stripes were significantly associated only with light
forests in bovid and artiodactyl species (N ¼ 125, v2 ¼ 15.15,
p , .001, CCT: p , .1; N ¼ 197, v2 ¼ 11.32, p , .01, CCT:
p ¼ .09, respectively) (Figure 3). We found marginal
associations between bovids with striped young and light
forests (N ¼ 125, v2 ¼ 15.15, p , .001, CCT: p , .1) and
artiodacyls with striped young and dense forests (N ¼ 197, v2
¼ 4.88, p ¼ .09). There was a tendency for artiodactyls with
striped young to be solitary (N ¼ 194, v2 ¼ 5.33, p ¼ .07), but
this did not hold after controlling for phylogeny. Bovids and
artiodactyls with striped young were chiefly those that
exhibited a hiding strategy (N ¼ 123, v2 ¼ 5.87, p ¼ .05; N
¼ 195, v2 ¼ 11.60, p , .01, respectively), and they did not
exhibit following behavior (N ¼ 123, v2 ¼ 6.34, p ¼ .04; N ¼
195, v2 ¼ 12.21, p , .01, respectively); however, these results
did not hold up after controlling for phylogeny. In short,
striped coats in adults were found in light forests, and striped
young were associated with hiding species.
Dark side bands and dark leg markings
If side bands act as disruptive coloration by acting to break up
the body shape, they may aid in concealment for species that
are diurnal or are found in open and sunny environments in
which other forms of crypsis are limited. Artiodactyls with
prominent side bands were principally diurnal (N ¼ 187, v2 ¼
6.41, p ¼ .04) and found in open environments (N ¼ 194,
v2 ¼ 14.73, p , .001), as were bovids (N ¼ 124, v2 ¼ 11.84,
p , .01). There were strong associations between side bands
and desert-dwelling for both bovids (N ¼ 124, v2 ¼ 10.00,
p , .01) and artiodactyls (N ¼ 194, v2 ¼ 14.71, p ¼ .0006).
Reindeer (Rangifer tarandus), the only cervid with side bands,
is found in tundra (Fisher’s: N ¼ 38, p ¼ .03). None of the
analyses of side bands remained significant after controlling
for phylogeny.
Dark leg markings, which may serve as another form of
disruptive coloration, should similarly be associated with open
environments. We found that artiodactyls with dark legs were
likely to live in deserts (N ¼ 196, v2 ¼ 8.36, p ¼ .0.02; CCT:
p ¼ .07), although there was no association with other open
habitats such as tundra.
Countershading
As predicted if countershading acts to conceal shadows cast
on the lower body, countershaded bovids were diurnal (CCT:
p ¼ .06) and strongly associated with desert habitats (CCT: p ¼
.01). Comparisons across all countershaded artiodactyls
similarly revealed an association with desert-dwelling (CCTs:
p ¼ .03). Conversely, there were fewer countershaded species
living in tundra than expected by chance (bovids: N ¼ 125,
v2 ¼ 12.11, p , .01; artiodactyls: N ¼ 195, v2 ¼ 10.03, p , .01).
Coloration for communication
Markings may be most effective as visual signals if they are
associated with conditions in which they are most visible (e.g.,
diurnal activity patterns and open habitats), associated with
gregarious species (for intraspecific communication), or
associated with particular predator types (for interspecific
communication).
Spotted and striped coats in adults
Spotted adult pelages might provide a mechanism to reduce
intraspecific aggression because spotted coats resemble those
of young in many species and might therefore act as a signal
of subordination. Spotted cervids were more likely to be
831
found in intermediate-sized groups (two to 50 individuals;
Fisher’s test: N ¼ 37, p , .01), but there were no associations
between spotted coats in adults and living in intermediatesized or large (more than 50) groups after controlling for
phylogeny. There were no significant associations between
striped coats in adults and intermediate or large group sizes.
Side bands
If side bands are used in communication between conspecifics, perhaps to maintain gregariousness, they should be
found in group-living species. Artiodactyls with side bands
lived in intermediate-sized (N ¼ 192, v2 ¼ 8.56, p , .01) and
large groups (N ¼ 192, v2 ¼ 15.91, p , .0001; CCT: p ¼ .01),
providing support for this hypothesis.
Face markings
Dark visual signals for intra or interspecific communication
might be most noticeable in diurnal species occupying open
habitats. Cross-species comparisons revealed that cervids with
dark faces tended to be diurnal (N ¼ 36, v2 ¼ 6.77, p ¼ .03),
but there was no evidence that dark-faced species in any of the
three clades lived in open environments. Dark-faced bovids
and artiodactyls lived in intermediate (CCTs: p , .01; p , .01,
respectively) and large-sized social groups (CCTs: p ¼ .02; p ¼
.02, respectively) (Figure 4), as is expected if dark faces are
used to communicate with conspecifics. Although dark-faced
artiodactyls were apparently subject to predation by stalking
predators (N ¼ 188, v2 ¼ 8.79, p ¼ .01), there were also
indications that bovids and artiodactyls with dark faces were
pursued by coursers after controlling for shared ancestry
(CCTs: p , .01; p , .01, respectively).
If white face markings are used in communication, they
might be prominent signals for either diurnal or nocturnal
species. Artiodactyls with white faces were likely to be diurnal
(CCT: p ¼ .01) but not nocturnal, and to live in intermediatesized social groups (CCT: p ¼ .06). Bovids with white faces
were likely to be pursued by coursers (N ¼ 119, v2 ¼ 5.92,
p ¼ .06), although this result was not upheld using phylogenetic tests.
Because any conspicuous markings might provide an
effective signal, rather than dark or white facial markings
per se, we repeated the analyses for species with either dark or
white markings, or a combination of the two. Cervids and
artiodactyls with conspicuous faces inhabited grassland/
bushland habitats (CCTs: p ¼ .03; p ¼ .03, respectively), but
no other associations were significant. Bovids with conspicuous faces were less likely to be found in large groups than
expected by chance (N ¼ 125, v2 ¼ 6.05, p ¼ .01). In short,
there was rather little support for conspicuous faces having an
obvious communicatory role.
Leg markings
Artiodactyls with dark leg markings were associated with
deserts (N ¼ 196, v2 ¼ 8.36, p ¼ .02; CCT: p ¼ .07), where
this color pattern is presumably prominent. Artiodacyls
with dark legs are found in large social groups (CCT:
p , .1), as expected if such markings facilitate intraspecific
communication; there was no association with stalking or
coursing predators for any taxonomic group.
White leg markings were associated with diurnality in
artiodactyls (CCT: p ¼ .04), but we found no associations
between these markings, open habitats, gregariousness, or
predator types.
We repeated leg marking analyses combining species with
either dark or white leg coloration and found that both bovids
and artiodactyls with conspicuous legs were diurnal (CCTs:
p ¼ .02; p ¼ .08, respectively) and principally lived in deserts
832
Behavioral Ecology Vol. 14 No. 6
Figure 3
Association between striped coats in adult artiodactyls ungulates and living in light forests. The first reconstruction was used for both phylogenies,
and the associated p value is .09 after Bonferroni correction. Black lines indicate species with striped coats; white lines indicate nonstriped species.
For the boxes on the right, black boxes indicate species living in light forests; white boxes indicate species not living in light forests.
Stoner et al.
•
Color of artiodactyls
833
Figure 4
Association between artiodactyls with dark faces and living in large groups. The first reconstruction was used for both phylogenies, and the
associated p value is .02 after Bonferroni correction. Black lines indicate species with dark faces; white lines indicate species lacking dark faces.
For the boxes on the right, black boxes indicate species living in large groups; white boxes indicate species that do not live in large groups.
Behavioral Ecology Vol. 14 No. 6
834
(CCTs: p ¼ .04; p ¼ .08, respectively). Artiodactyls with
conspicuous legs were associated with also grassland/bushland habitats (CCT: p ¼ .05). These findings suggest a role in
communication, but because no significant associations were
uncovered with group sizes or predator hunting style, it is
unclear whether intra or interspecific communication is
involved.
Tail markings
Chi-square analyses gave some backing for dark tail markings
acting as visual signals. Dark tails were found among diurnal
bovids (N ¼ 121, v2 ¼ 9.00, p ¼ .01) and artiodactyls (N ¼ 188,
v2 ¼ 7.17, p ¼ .03). Bovids with dark tails tended to form large
groups (N ¼ 125, v2 ¼ 8.76, p ¼ .01), whereas artiodactyls with
dark tails were associated with intermediate (N ¼ 192, v2 ¼
8.33, p ¼ .02) and large groups (N ¼ 192, v2 ¼ 8.69, p ¼ .01).
Dark-tailed bovids and artiodactyls were associated with
stalking predators (N ¼ 119, v2 ¼ 9.47, p , .01; N ¼ 187,
v2 ¼ 6.16, p ¼ .05, respectively), and there were no significant
associations with coursing predators. None of these results
were upheld in phylogenetic tests, giving little support to
conventional hypotheses for dark tail coloration.
White tails were associated with diurnality in bovids (N ¼
121, v2 ¼ 7.01, p ¼ .03) but not with being diurnal or living in
open environments in other clades. There were few associations between white tails and gregariousness, except whitetailed artiodactyls were weakly associated with large-sized
groups (CCT: p ¼.1). There was an association between whitetailed bovids and stalking predators, but only in cross species
comparisons (N ¼ 119, v2 ¼ 10.82, p , .01). Thus, there was
little support for white tails having signal value.
In examining species with either black or white tails, we
found that bovids and artiodactyls with any conspicuous markings were diurnal (N ¼ 121, v2 ¼ 9.34, p , .01, CCT: p , .01;
N ¼ 188, v2 ¼ 7.40, p , .01, CCT: p ¼ .06, respectively).
Artiodactyls with tail markings were associated with intermediate-sized (N ¼ 193, v2 ¼ 3.09, p ¼ .08, CCT: p ¼ .07) and
large (N ¼ 193, v2 ¼ 4.47, p ¼ .03, CCT: p ¼ .09) groups, as
were bovids (N ¼ 125, v2 ¼ 5.88, p ¼ .02). Conspicuous tails
could be involved in interspecific communication, as bovids
with conspicuous tails were attacked by stalking predators
(N ¼ 119, v2 ¼ 12.48, p , .001), although cervids were
actually less likely to be attacked by stalkers (Fisher’s test:
N ¼ 36, p , .01); significance for these analyses were lost
after controlling for phylogeny. In short, there was reasonably strong support for conspicuous tails being associated with
intraspecific communication.
White rumps
In examining whether white rumps serve a communicatory
role, we found white rumps were associated with diurnality in
artiodactyls (N ¼ 188, v2 ¼ 5.15, p ¼ .07; CCT: p ¼ .09) and
open habitats in bovids (N ¼ 125, v2 ¼ 21.14, p , .0001; CCT:
p ¼ .05) and artiodactyls (N ¼ 195, v2 ¼ 33.29, p , .0001;
CCT: p , .0001). There were strong associations using crossspecies comparisons between white rumps and living in
intermediate-sized groups for cervids (Fisher’s test: N ¼ 36,
p , .0001; CCT: p , .01), bovids (N ¼ 125, v2 ¼ 16.48, p ,
.001), and artiodactyls (N ¼ 193, v2 ¼ 32.32, p , .0001) and
living in large groups (cervids: N ¼ 36, v2 ¼ 7.29, p ¼ .04;
bovids: N ¼ 125, v2 ¼ 23.64, p , .0001; artiodactyls: N ¼ 193,
v2 ¼ 33.37, p , .0001). Bovids and artiodactyls with white
rumps were likely to be pursued by coursers (N ¼ 119, v2 ¼
13.88, p , .1; N ¼ 187, v2 ¼ 8.48, p ¼ .01, respectively), but
absence of significance in phylogenetically controlled tests
casts doubt on their role in signaling to predators. No
associations were found between species sporting white rumps
and those preyed on by stalking predators.
Coloration for thermoregulation
Uniform coloration
Bovids and artiodactyls with dark pelages were found
principally in the tropics (bovid CCT: p , .0001; artiodactyl
CCT: p ¼ .01), as expected from Gloger’s rule. Pale bovids and
artiodactyls were associated with deserts in chi-square tests (N
¼ 125, v2 ¼ 6.91, p ¼ .03; N ¼ 194, v2 ¼ 10.19, p , .01,
respectively) but not after controlling for phylogeny.
Face markings
To test whether white face markings serve to reduce heat
stress, we checked whether they are found in species living in
open and warm environments. White-faced cervids were more
likely to live in open environments (Fisher’s test: N ¼ 39, p ¼
.06; CCT: p ¼ .06). White faces were associated with living in
grassland/bushland in bovids (N ¼ 125, v2 ¼ 6.19, p ¼ .05)
and in artiodactyls (N ¼ 196, v2 ¼ 4.82, p ¼ .09; CCT: p , .01).
White faces were also found in desert bovids (N ¼ 125,
v2¼ 6.19, p ¼ .05). In sum, white faces were associated
with diurnality, open environments and grassland/bushland
habitats. These findings provide evidence for a thermoregulatory role.
White rumps
Turning to the idea that rumps may also deflect heat in eventoed ungulates, we found significant relationships between
species having white rumps and those occupying deserts
(cervids, Fisher’s test: N ¼ 38, p ¼ .08; bovids, N ¼ 125, v2 ¼
24.37, p , .0001; artiodactyls, N ¼ 195, v2 ¼ 33.29, p , .0001)
and grassland/bushland (bovids, N ¼ 119, v2 ¼ 7.92, p ¼ .05;
artiodactyls, N ¼ 187, v2 ¼ 8.39, p ¼ .02). These associations
were not upheld when shared ancestry was taken into account,
however.
DISCUSSION
After taking phylogeny into account, we found that many
aspects of coloration in ungulates are related to concealment
and are less involved in communication. Thermoregulation
apparently played only a minor role in the evolution of
coloration in this taxon. Although our results are preliminary,
they point to predation pressure being a key selective force
acting on coloration in this order of mammals. The
importance of mate choice was not assessed but is likely to
be of minor import, judging from the small number of
ungulate species that are sexually dichromatic. If correct, this
contrasts sharply with birds, although a phylogenetically
controlled analysis of sexual dichromatism and mating system
in ungulates would still be useful.
Before discussing our findings in more detail, we must
allude to five points that could have influenced our results.
First, we know that our behavioral and especially our ecological categories are broad. Thus, dense forest constituted
alpine, tropical, boreal, deciduous, mixed dense forests, and
timberland, where lighting conditions are likely to be
extremely variable (Endler, 1993) and would influence the
extent to which prey might remain cryptic. Indeed, variation
in lighting within subcategories of dense forest might even be
greater than between dense forest and light forest/woodland,
but it is impossible to measure this across the range of
environments in which species live. More refined categories
might increase our ability to detect associations.
Second, the large scope of the study meant that we lacked
detailed data on most species and were forced to use a coarse
categories that could be ascribed to most species, such as
habitat or group size. If analyses were restricted to a smaller
sample of well-known species, one might be able to use more
Stoner et al.
•
Color of artiodactyls
sensitive measures, such as ‘‘is’’ or ‘‘is not’’ preyed up by
cheetahs (Acinonyx jubatus) and thereby be able to make far
more specific predictions about prey-predator signaling, for
example (see Caro, 1994).
Third, our attempts to tease out predictions that would lend
support or cast doubt on conventional hypotheses may have
been misplaced. Thus, it may be inappropriate to suggest that
if a color patch is used in communication, we would expect it
to be found in diurnal or open country species; perhaps they
would be found in crepuscular species or forest-dwelling
species too.
Fourth, our phylogenetic reconstruction was of low
resolution, as a complete molecular phylogeny is not available. In the absence of such data, we constructed a composite
tree based on the most recent molecular studies and
supplemented this with taxonomies based on morphological
traits. Certain species were additionally excluded because of
lack of information or strong disagreements over their
phylogenetic position, and we resolved polytomies by using
the arbitrary procedure of excluding species with fewest
citations. Nevertheless, this is the most comprehensive tree
based primarily on molecular studies that is currently available
for Artiodactyla. Although a more resolved tree might in the
future change our results because concentrated changes tests
are notoriously sensitive to inclusion or exclusion of species
(Sillen-Tullberg, 1993), we think that this would not produce
more significant associations. Nonetheless, future reanalysis
of data in the supplementary appendix online by using
a thorough molecular phylogeny will definitely be worthwhile.
Fifth, we used stringent criteria to accept that an association
between a coloration pattern and an ecological or behavioral
variable was indeed significant by focusing attention on
phylogenetic reconstructions that were significant only after
a standard and therefore conservative Bonferroni correction.
With these points in mind, we now discuss specific associations summarized in Table 4.
Overall body coloration
If analyses had been restricted to simple cross-species
comparisons, there would have been strong support for
overall coat coloration being an adaptation for background
matching or thermoregulation (Kingdon, 1982; Spinage,
1986). Pale coats were found in open environments and
desert habitats, and dark coats were found in closed environments and dense forests. Gray coats were found in rocky
habitats, which would support only a background-matching
role. Nonetheless, none of these significant associations were
maintained once shared ancestry was taken into account.
Moreover, on some of these tests, species counts revealed that
there were actually more species not associated with the
variable of interest (pale bovids not found in deserts; gray
artiodactyls not found on rocks), despite there being more
pale species in deserts than non-pale species in deserts. In
contrast, parallel comparative analyses of overall body
coloration in carnivores (Ortolani and Caro, 1996) and
lagomorphs (Stoner et al., 2003) revealed that pale coloration
was associated with desert habitats, making the even-toed
ungulate results even more surprising. In conclusion, despite
several extraordinarily experienced naturalists having linked
overall coat color to habitat on the basis of observations
made on single species or guilds of ungulates, there is
no comparative systematic support for background matching
across the even-toed ungulates. Although a more resolved
phylogeny might, in the future, be more supportive of
background matching, we speculate that overall coat coloration, such as being pale, serves different functions in different
species, and that ascribing the function of all pale coats either
835
to background matching or to reflecting heat in hot climates
is too simplistic.
Cross taxonomic support for Gloger’s rule is equivocal. This
study showed that dark even-toed ungulates live in the tropics;
similarly, in canids, ursids, and herpestids, dark species are
found in tropical forests (Ortolani and Caro, 1996). Assuming
many tropical areas are humid, this result provides indirect
support for Gloger’s rule, although the causal mechanisms
maintaining the association remain opaque. In contrast, dark
lagomorphs were not associated with tropical latitudes after
controlling for phylogeny (Stoner et al., 2003), so the
generality of this rule in mammals is still open to question.
Ungulates that assume lighter coats in winter are associated
with arctic latitudes and tundra habitats. This might be an
adaptation to match their snowy background (Cott, 1940) or
a thermoregulatory device, as white coats are translucent and
let heat and sunlight penetrate, which are then caught by
darker inner hairs. Dark outer hairs would only absorb heat
remaining on the surface that is quickly carried away by wind
(Wolff J, personal communication). Similar matches between
white coats and living in the arctic have been found in
phylogenetic comparisons in carnivores (Ortolani and Caro,
1996) but not in lagomorphs (Stoner et al., 2003).
It seems obvious that deer are spotted to blend in with
dappled light (Beddard, 1892; Schaller, 1967; Thayer, 1909),
so we were surprised to find few significant associations
between species in which adults are spotted and living in
forests or grassland. This was undoubtedly owing to shared
ancestry because spotted species are closely related, particularly in the cervids. Nonetheless, spotted cervids were diurnal,
and spotted bovids were found in light forests (although there
are only five species that are spotted). In sum, spotted coats
may be an adaptation for concealment based on these two
tests, but other predictions concerning habitats and being
solitary were not supported. Among carnivores, dark spots
were associated with living in closed habitats and having
ungulates as the main prey item, suggesting that, in that order,
their function is aggressive camouflage (Ortolani, 1999).
There was definite support for striped coats aiding in
concealment (Cott, 1940; Estes, 1991) given that both bovids
and artiodactyls with such coats were more likely to inhabit
light forests after controlling for ancestry. Nonetheless, there
was no association with being diurnal, living in grassland, or
being solitary and possessing a striped coat. In carnivores,
however, vertically striped coats were associated with living in
grassland and with being terrestrial, bolstering the background-matching hypothesis (Ortolani, 1999). In even-toed
ungulates, there was no evidence for spotted or striped coats
being involved in intraspecific communication because they
were not associated with gregariousness.
Many of the predictions regarding spotted coats in young as
helping in concealment were well supported (Cott, 1940;
Thayer, 1909). Each of the three taxonomic groups was found
in forests or grasslands, and solitary artiodactyls were
particularly likely to have spotted young (see Danilkin,
1996). Extraordinarily, every one of the 30 species with
spotted young exhibited a hiding strategy, demonstrating
a tight link between this morphological character and this
type of antipredator behavior. Mothers of species that have
spotted young all leave their neonates alone when they forage.
Support for species with spotted young using the follower
strategy (Macdonald, 1984) was lacking.
Striped coats in young also aid in concealment. Bovids with
striped young were found in lightly forested habitats. Species
whose young had striped coats were significantly more likely
to be hiders not followers, but based only on crossspecies comparisons. Thus, the relationship between pattern
blending in spotted young and the hiding strategy was
Behavioral Ecology Vol. 14 No. 6
836
Table 4
Degree of support for associations tested in the present study
Concealment
Coloration
Overall body coloration
Uniform color
Lighter in winter
Spotted adults
Striped adults
Spotted young
Striped young
Side bands
Countershading
Body parts
Dark face
White face
Conspicuous face
Dark legs
White legs
Conspicuous legs
Dark tail
White tail
Conspicuous tail
White rump
a
BM
Communication
PB
DC
CS
Intra
Physiological
Inter
T
Yesa
None
Strong
Weak
Strong
Strong
Weak
None
None
None
Strong
Moderate
Weak
Strong
Weak
None
Weak
Strong
None
None
None
Moderate
Weak
Strong
None
Weak
None
Moderate
None
None
None
None
None
BM indicates background matching; PB, pattern blending; DC, disruptive coloration; CS countershading; Intra, intraspecific communication;
Inter, interspecific communication; and T, themoregulation.
Refers to Gloger’s rule.
replicated in striped young although without the same degree
of certainty.
It is difficult to interpret results regarding predictions about
side bands acting as a form of disruptive coloration (Estes,
1991). Although artiodactyls with side bands were diurnal and
lived in open environments and in both desert and tundra
habitats, and although some of these findings were also found
separately in cervids and bovids, none of these results held up
after controlling for phylogeny. The clearest result from crossspecies analyses was that artiodactyls with side bands lived in
open environments (24 out of 24 species), but this is not
a clear-cut expectation from the disruptive coloration
hypothesis (e.g., disruptively colored woodcock, Scolopax
minor, chicks are found in leaf litter); thus, we cannot be
certain that side bands serve a disruptive function. In contrast,
artiodactyls with side bands were found in large groups,
suggesting that they might be used in intraspecific communication, perhaps allowing individuals to keep in sight of each
other.
There was a strong association between bovids and
artiodactlys that were countershaded and inhabiting deserts.
Because countershading is thought to aid in concealment by
reducing shadow in well-lit environments (Cott, 1940; Fogden
and Fogden, 1974), the hypothesis is partially supported
despite no significant associations with other open environments. In contrast to even-toed ungulates, lagomorphs that
were countershaded were not more likely to be found in open
habitats that included deserts (Stoner et al., 2003), so the
widespread importance of countershading in helping to
conceal mammals is still questionable.
Body parts
Focusing on phylogenetically controlled comparisons, darkfaced bovids and artiodactyls were found in intermediate- and
large-sized social groups, which implies an intraspecific
communicatory function (see Kingdon, 1982; Spinage,
1986). The prediction that dark faces might be used to signal
to stalking predators that they have been noticed (see Caro,
1995) was supported only weakly (in a chi-square test on
artiodactyls), but in contrast, there were highly significant
associations between dark faces and being pursued by
coursers, the biological significance of which is mysterious.
Because even-toed ungulates are likely to flee from coursers,
predators would be unlikely to see their faces. Perhaps the
extent of blackness on the face signals ability to outdistance
a predator, but this is pure speculation. In carnivores, dark
patches around the eyes were associated with crepuscular
activity, perhaps serving as a way to reduce glare (Ortolani and
Caro, 1996).
White faces were associated with being diurnal (all
artiodactyls) and living in open environments (cervids and
artiodactyls). Species with white faces were found in grassland/bushland habitats in all three clades. If, as we imagine,
grassland habitats are sunny environments with relatively little
shade, this would lend (albeit weak) support to a thermoregulatory role in which white faces are used to lower
temperature, as suggested by Geist (1987). There was weak
support for white faces being used in intraspecific communication based on patterns of grouping but no support for
signaling to predators (see Poulton, 1890). Similarly, there
was very little widespread support for conspicuous faces being
used in communication.
We found weak support for dark legs acting as disruptive
coloration (see Roosevelt and Heller, 1914) because artiodactyls with dark legs were found in deserts. Phylogenetically
controlled comparisons gave some support to a role for dark
legs being used in intraspecific signaling, but only for
artiodactyls and living in large groups. Hypotheses suggesting
white legs might be an aid in communication (Danilkin, 1996;
Geist, 1987; Kingdon 1982) received weak support. Conspicuous leg markings were more obviously associated with
Stoner et al.
•
Color of artiodactyls
diurnal communication in open habitats, but the type of
communication, to predators or conspecifics, was unknown.
Conventional hypotheses that dark tails are involved in
intraspecific communication (Kingdon, 1982) or signal to
predators were supported only in nonparametric tests.
Backing for white tails acting in communication (Bildstein,
1983; Kingdon, 1982) was similarly weak, and cross taxonomic
analyses did do not support the suggestion, based on single
species studies, that white tails are used to signal to predators
(for a review, see Caro et al., 1995). However, our examination
of species with conspicuous tail markings (be they dark or
white) revealed that these species are more likely to be diurnal
and gregarious than those without distinct tail markings.
Thus, conspicuous tails seem to be involved with intraspecific
communication in even-toed ungulates. In lagomorphs,
contrasting tail tips were similarly associated with sociality
(Stoner et al., 2003) although not in carnivores (Ortolani and
Caro, 1996), making it difficult to form general conclusions
about mammals.
White rumps might either act as signals to conspecifics
(Guthrie, 1971; Ward, 1979) or to predators (Caro, 1986;
Fitzgibbon and Fanshawe, 1988), or be used in thermoregulation (Spinage, 1986; Estes, 1991). White-rumped species in
all three ungulate clades were found in species inhabiting
open environments and, after controlling for phylogeny, in
intermediate-sized social groups, lending weight to intraspecific communication. There was some evidence of signaling to
predators because white-rumped species were pursued by
coursers, although not in concentrated changes tests. That
white rumps were found in the open, in deserts, and in
grassland/bushland habitats lends credence to the thermoregulation hypothesis, but we did not replicate all these results
by using phylogenetic controls. The function of white rumps
remains an enigma, although intraspecific communication
and thermoregulation both remain promising candidates.
837
adaptive significance of coloration in animals and that they
elevate the study of coloration from fascinating anecdotes to
testable hypotheses about a wide array of species.
APPENDIX
Sources used to obtain morphological and ecological
information for each species are as follows: Bauer, 1995,
Chadwick, 1983; Chapman and Chapman, 1997; Clark, 1994;
Cloudsley-Thompson, 1980; Dagg and Bristol-Foster, 1976;
Danilkin, 1996; Estes, 1991, 1993, Grzimek, 1990; Heptner
et al., 1989; Hoefs, 1985; Holmes, 1974; Huntingford and
Turner, 1987; Johnson and Lockard, 1983; Jones et al., 1985;
Kingdon, 1982; Lindsey et al., 1999; Macdonald, 1984;
Mungall and Sheffield, 1994; Nabhan, 1993; Nowak, 1999;
Miura et al., 1993; Oliver, 1993; Payne and Francis, 1985;
Putman, 1988; Prior, 1987; Schaller, 1977, 1998; Shackleton,
1999; Shrestha, 1997; Soma, 1987; Sowls, 1984, 1997; Spinage,
1968, 1982; Spitz, 1991; Strahan, 1983; Stuart and Stuart,
1993, 1997; Valdez, 1982; Valdez and Krausman, 1999; Van
Wormer, 1969; Walther, 1984; Walther et al., 1983; Wemmer,
1987; Whitehead, 1972, 1993; Wilson, 2000.
We thank the University of California for supporting C.J.S. and the
McNair Scholars Program for supporting C.M.G., Kate Trimlett and
Janelle Vargas for help with coding variables, Marisa Flores for
running supplementary analyses, John Eadie and Charlie Nunn for
discussion, John Byers for drawing our attention to the possibility of
white rumps being used in thermoregulation, Jerry Wolff for
suggesting we examine conspicuous coloration patterns and more
behavior, and Dave Westneat, Jerry Wolff, and an anonymous reviewer
for very helpful comments.
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