AMER. ZOOL., 24:333-343 (1984)
By Jove!! Why Do Alternative Mating Tactics
Assume So Many Different Forms?1
EDWARD C. WALTZ AND LARRY L. WOLF
Department of Biology, Lyman Hall, Syracuse University, Syracuse, New York 13210
SYNOPSIS. Tremendous diversity exists in the form of alternative mating tactics (AMTs)
employed by males of many species. We develop a general framework in which to view
alternatives that vary in (i) their frequency in the population, (ii) their fitness value with
respect to the primary tactics, (iii) the extent to which the alternative tactic is site-fixed
and (iv) the intrinsic ability of males to change tactics.
The frequency and fitness value of alternatives should be influenced by Resource Holding Potential (RHP), which generally varies with age, size, and perhaps energy reserves.
AMTs should be unequal in fitness value when RHP increases at an accelerating rate with
age. "Subordinate AMTs" can result when various factors favor males attempting to
reproduce before reaching the age with maximum RHP. An asymptotic relationship
between age and RHP should result in most males in a population having essentially equal
RHP. Several ways exist for males to partition the set of mating opportunities between
two or more "equal AMTs."
Transient tactics may occur if (i) resources for females and territorial males differ and
do not covary positively in their distribution, or (ii) local areas are so attractive to females
that males effectively cannot defend them. We suggest Levins' (1968) "fitness set" analysis
as a useful model predicting whether a male should specialize on a single tactic, or partition
its effort between the two tactics.
INTRODUCTION
snow-white bull, a mortal man, and a
shower of gold (Norton and Rushton,
1969). Though less spectacular, perhaps,
than Jove's many roles, the alternative mating tactics of many species also assume
diverse forms. Some alternatives differ
genetically (e.g., Cade, 1981), others relate
to ontogenetic changes in body size (e.g.,
Howard, 1978), and still others appear relatively free of phenotypic constraints (Rubenstein, 1984; Waltz and Wolf, in preparation). Males using alternative tactics may
be "parasites" of individual territorial
males (e.g., Campanella and Wolf, 1974);
independent, but non-territorial, residents
(e.g., Rubenstein, 1984; Waltz and Wolf, in
preparation); or transients, wandering
among various areas, including other males'
territories (e.g., Alcock et al., 1977).
This paper addresses the selective reasons for the observed diversity among different forms of AMTs. We assume that the
potential for AMTs exists, and focus on
variations in their expression. Investigators
usually have studied AMTs as a question
in behavior. We consider them as an integral component in the evolution of life his* From the Symposium on Alternative Reproductive tories, and try to develop a more general
Tactics presented at the Annual Meeting of the Amer- perspective on their diversity. Although
ican Society of Zoologists, 27-30 December 1982, at
females also may have alternative tactics
Louisville, Kentucky.
Behavioral ecologists traditionally have
viewed species or populations as having single, specific tactics for obtaining mates (e.g.,
Wilson, 1975). Variation in mating tactics
within populations recently has received
increased attention (see reviews by Alcock,
1979; Cade, 1980; Rubenstein, 1980;
Waltz, 1982; Dunbar, 1982; Gross, 1983;
Austad, 1984; Dominey, 1984). Variation
within populations typically is expressed as
males being either aggressive or nonaggressive in their attempts to obtain matings (e.g., "territorial" and "satellite" tactics in the Ruff, see Hogan-Warburg [ 1966];
scientific names in Table 1). Observations
of alternative mating tactics (AMTs) now
are so widespread, we may assume reasonably that most males at least have the
potential for AMTs (see also MeVey, 1981).
In classical mythology, the god Jove
(a.k.a. Zeus or Jupiter) employed the first
recognized alternative mating tactics when
he seduced mortal females. At various
times, he disguised himself as an eagle, a
333
334
E. C. WALTZ AND L. L. WOLF
for mate selection (see Noonan, 1981;
Thornhill, 1984), the dearth of information on female tactics requires that we
restrict our discussion to males.
Much of our appreciation for the diversity of AMTs has developed from our work
on the mating behavior of dragonflies. In
this taxon, alternative tactics often take the
form of site-fixed satellites, forced into a
secondary role because they are unable to
hold territories (Campanella and Wolf,
1974; McVey, 1981). We have found alternatives that differ from this pattern, and
from AMTs described in other taxa. We
briefly introduce these alternatives here to
emphasize the diversity among AMTs.
against conspecific males (Wolf and Waltz,
1984). A territorial male's activities are
restricted essentially to the defended area,
in which he has fairly exclusive access to
females.
We recognize at least two non-aggressive
alternatives to this "typical" mating tactic.
Transient males perch in the vegetation a
meter or more back from the shore and fly
over the pond in search of females. They
occasionally perch briefly on the pond, but
never defend an area. Temporary swarms
of two to ten transients often pursue a single female. Resident males are much less
common. They remain perched in the same
area of the pond for an hour or more, but
do not defend the site. Residents defend only
T H E LEUCORRHINIA INTACTA
a "personal space" a few centimeters in
MATING SYSTEM
radius, and clearly differ from territorial
Leucorrhinia intacta is a small dragonfly males. The resident tactic probably is the
similar in general biology and behavior to least expensive energetically, requiring
other members of family Libellulidae (see neither defense nor patrolling. We will limit
Walker and Corbet, 1975). We have stud- subsequent discussion to the two common
ied the mating behavior of male L. intacta tactics, territorial and transient.
in a population near Syracuse, NY for sevUnlike most species, male L. intacta
eral years. We summarize our salient find- appear to be nearly free of phenotypic conings here; a more detailed description will straints on their tactical choice. We have
appear elsewhere (Waltz and Wolf, in prep- been unable to discern any influence of
aration).
relative age or size on the choice. Males
Mature L. intacta males spend much of frequently change tactics from day to day,
the day perched around the margin of a and some change even within a day. Tranpond, where essentially all mating and ovi- sition matrix analysis of several pairs of
position, but no feeding, occurs. Females consecutive days suggests that the tactical
visit only long enough to copulate and ovi- choice is independent from day to day.
posit. When a female appears, she imme- Similarly, males that change tactics within
diately is chased, often by several males, a day do not appear to follow any typical
and is clasped by one of them. She almost "trajectory" of tactic (cf., Campanella and
always copulates with the male. Females Wolf, 1974).
oviposit by hovering just above the water
Our tentative interpretation of this syssurface and dipping the abdomen into the tem is that males arrive at the pond and
water, releasing small groups of eggs on select a tactic based primarily on the qualeach dip. The mate usually hovers nearby ity of available sites and the frequencies of
during oviposition, and chases any males the two tactics. We observe nearly equal
attempting to mate with her. At most pop- daily mating success of territorials and
ulation densities, more than 80 percent of transients, consistent with an hypothesis of
oviposition bouts end with the female being negative frequency-dependence. Territomated by another male. Most females mate rial males settle preferentially in areas with
with three or more different males during large numbers of perches and with suba visit to the pond.
merged vegetation suitable for oviposition.
The
existence of the alternatives, however,
Males have several tactics for obtaining
mates. Many are territorial while at the does not result simply from territorials fillpond. They defend small (2 m or less ing in good sites, and the remaining males
radius), perch-centered "dominions" acting as transients. The frequencies of the
DIVERSITY OF ALTERNATIVE MATING TACTICS
two tactics remain nearly constant at about
60% territorials over a range of from 3 of
4 to over 80 males on the pond. Even when
only 4 or 5 males are present, 2 or 3 clearly
act as transients. The frequencies of the
tactics appear to be the predominant influence on a male's choice of tactics.
AMTs in our population of Leucorrhinia
intacta appear to differ from those in other
species in several important respects. The
alternatives in L. intacta are present in
higher and more stable frequencies, especially at low densities {e.g., McVey, 1981;
but see Howard, 1981). Males apparently
can choose between tactics of nearly equal
value; they are not simply "making the best
of a bad lot" (see Maynard Smith, 1979;
Cade, 1980). The predominant alternative
to being territorial in our population is to
be a transient, whereas most dragonflies
and many vertebrates have site-fixed satellites. The transient tactic is similar to
AMTs observed in other insects, including
another dragonfly species (review in
Alcock, 1979; Ueda, 1979), and in some
fishes (Warner and Hoffman, 1980).
Finally, unlike in most other species, L.
intacta males may choose a tactic with few
or no intrinsic constraints, and they can
change tactics between and within days.
They appear relatively unconstrained by
age or fighting ability (cf., Alcock et ai,
1977; Howard, 1978; McVey, 1981); or by
genetic or long-term developmental differences between tactics (cf., Cade, 1981;
Dominey, 1980; Gross and Charnov, 1980).
335
3, appear closely tied to the life history
pattern of the organism. In this section we
examine selective forces shaping life histories, to suggest general principles governing the evolution of alternative mating
behaviors.
Factors influencing the frequency of
alternative tactics in populations
Influences on AMT frequencies within
populations should be the same factors that
favor the existence of alternatives. Critical
ecological variables regulating satellite, or
parasitic, tactics include environmental
heterogeneity, operational sex ratio and
synchrony in female arrival rates, tacticrelated differences in survival, and time
costs of "handling" females (see Cade,
1980; Rubenstein, 1980; Waltz, 1982). We
focus on proximate ecological and ultimate
demographic forces that favor a variety of
alternatives. We consider two classes of tactics in which (i) younger or smaller males
are forced into "subordinate AMTs"; and
(ii) two groups of equally competitive males
adopt different, but "equal AMTs." In
proposing some selective reasons for equal
AMTs, we introduce some advantages of
transient vs. resident tactics; and we will
return to this topic later.
The existence of either equal or subordinate AMTs should depend on the effects
of age (or size) on the ability of individual
males to acquire and defend either females
or the resources necessary to attract them
(i.e., approximately, Resource Holding
Potential [RHP]; Parker, [1974]; Emlen
SELECTION FOR VARIOUS FORMS OF
and Oring [1977]). We presume that a
ALTERNATIVE MATING TACTICS
male's fitness, measured as reproductive
Three critical attributes to understand success (RS), generally is constrained by his
RHP, although this need not be so. Two
the diversity of different AMTs are:
general relationships between age and RHP
1) the frequency with which alternatives or RS can be imagined (Fig. 1). First, if
occur in the population,
RHP increases throughout a male's life2) the mobility of the alternative tactic, time, older males should have the greatest
i.e., whether the tactics are roving tran- RS. If resources or females limit males' RS,
sients vs. site-fixed residents or satel- younger males will be forced into suborlites,
dinate roles (see Howard, 1978, 1981).
3) the flexibility of the male's decision, i.e.,
The accelerating effect of age on RHP
the extent to which choice of tactics is shown in Figure 1 should be most common
constrained by the non-behavioral phe- when age gives a multiple advantage in
notype.
obtaining mates. For example, when the
These attributes, especially numbers 1 and entire body or at least the weapons used in
336
E. C. WALTZ AND L. L. WOLF
intrasexual combat grow indeterminately
(e.g., red deer, bullfrogs), older males are
both larger and more experienced than
young males (Gadgil, 1972). Female preference for older males (Cox and LeBouef,
1977; Yasukawa, 1981) also would enhance
the effects of size or experience on RS. The
ability of females to select older males is
facilitated by obvious differences in males'
size or adornments. Subordinate AMTs
need not be age-related. Even males in
prime age classes may adopt subordinate
AMTs if their RHP is low because of small
size or infirmity (McVey, 1981; CluttonBrock et al., 1982; Wirtz, 1982).
Second, RHP might increase rapidly in
young age classes, and then remain essentially constant throughout the rest of a
male's lifetime (Fig. 1). This asymptotic
pattern should be more likely in organisms
with determinant growth of the body and
weapons, such as most insects, birds, and
some mammals. RHP and RS might be
nearly independent of age when females
are not defendable, because they are highly
clumped or widely scattered in space and
time, or because competitor densities are
high. In general, conditions favoring
"scramble" competition for mates might
produce an asymptotic relationship
between RHP and age.
When most males in a population are
equal in RHP, AMTs of equal fitness value
may result from males dividing mating
opportunities, especially if females do not
select mates based on their tactic. Partitioning of mating opportunities may derive
from variation in the spatial distribution of
females, perhaps in relation to the
availability of critical resources. Receptive females may be available in local
sites (=microhabitats) lacking essential
resources, such as perching sites, for territorial males. Non-territorial males, especially transients, should use those areas. In
our population of L. intacta, many areas
suitable for oviposition lack perches for
territorial males. Transient males obtain
matings, and their mates oviposit in these
areas. Territorial males mate in areas containing both suitable oviposition habitat and
adequate perches. The extent to which
resources needed bv females and territo-
O
ex
6
LU
O
AGE
Fie. 1. Two possible relationships between Resource
Holding Potential (RHP) and a male's age. Line I is
referred to as an "asymptotic" effect; line II as an
"accelerating" effect of age. Dashed line indicates
that, for both effects, very old males may become
senescent and decline in RHP.
rial males vary independently in their distribution should regulate the frequencies
of AMTs in a population.
Males in other species may mate with
females at two or more different sites. Some
males of the anthophorid bee, Centris pallida, dig up newly emerging females, while
other males mate with older females in
feeding areas (Alcock et al., 1977). (These
males are constrained by size in their success as "diggers," but they illustrate the
basic point.) If females in both areas are
economically defensible by males, then two
groups of territorial males might occur; if
not, then the potential for non-aggressive
AMTs exists.
Mating opportunities may occur in all
areas, but males still may adopt distinct
AMTs. We have modified the cost-benefit
model of territoriality developed by Gill
and Wolf (1975) (see Fig. 2) to illustrate
this point. Assume that potential territorial
sites vary in their attractiveness to females,
and that the expected gross benefits
(=number of matings) rises linearly with
site quality. Because of increased attractiveness to competitor males, the costs of
defending a site probably rise at an accelerating rate with site quality (see Gill and
DIVERSITY OF ALTERNATIVE MATING TACTICS
NBw/
+
•
—
NBt
J_
LLJ
^
.
NBr.
Z
LLJ
CO
'""i
l-Y/ / SITE
M
QUALITY
/
FIG. 2. Hypothetical relationships between the quality of a local site and the net benefits obtained there
by being territorial (NB,), wandering (NBW), and nonterritorial resident (NBr). Regions I, II, and III indicate ranges of site quality in which males should be
resident, territorial, and wandering, respectively.
Wolf, 1975; Pyke, 1979), so the net benefits of territoriality increase at a diminishing rate (Fig. 2).
Two alternatives to territoriality, transient and resident tactics, require no defense
costs and their net benefits should rise linearly with site quality. For all tactics, a "site"
is approximately the zone around the male
within which he can detect and respond to
females. The "site," therefore moves with
the transient, and may be taken as the average of all areas visited by the individual.
Neither resident nor transient males have
exclusive access to females in the sites they
occupy, so their benefits might rise at a lower
rate than territorials' (i.e., slope < 1; see
residents in Fig. 2). Transients, however,
move through a variety of areas. Thus, they
can preferentially visit areas containing
females at the time, and can exploit mating
opportunities in "interstices" between territories. These advantages should help
counteract the lack of exclusive access to
females, so the net benefits for transients
may increase faster than residents' (Fig. 2).
Since transients have higher costs of transport, the y-intercept of their net benefits
line is below that of residents. These costbenefit considerations suggest that residents, territorials, and transients may assort
337
over a gradient of female arrival rates (Fig.
2). Territorial males should occupy sites
with intermediate arrival rates, while residents and wanderers utilize the low and
high extremes, respectively.
The net benefits postulated in this simplified model suggest that males would
"prefer" to be transients and then territorials, and accept resident status only if
excluded from these two tactics. But if the
success of the tactics vary inversely with
the number of males adopting them, the
benefits of the alternatives may equilibrate
(see Fretwell, 1972; Maynard Smith and
Price, 1973). The three tactics might attain
equal reproductive success, depending on
the ability of territorials to exclude prospective settlers in their areas (by definition, transients do not exclude territorial
males), the availability of sites of different
quality for attracting females, and the
number of males in the population.
In our L. intacta population, the benefits
of territorial and transient tactics appear
to vary negatively with their respective frequencies (Waltz and Wolf, in preparation).
Therefore, the tactics are predicted to have
(on average) equal reproductive success,
and .the benefits for all territorial males
should be equal, independent of the intrinsic quality of their site. But when territorial
males can be "despotic" (see Fretwell,
1972), they should have higher RS than
residents. Thus, co-occurrence of territorials and residents in despotic situations
{e.g., bullfrogs) almost presupposes that
residency is a subordinate AMT, especially
since despotism should result from large
differences in RHP.
When little variation in RHP exists, males
have less opportunity to be despotic; they
may settle disputes by conventions, such as
"resident always wins" {e.g., Davies, 1978).
They should adjust their tactical choice in
response to the varying frequencies of the
alternative tactics in the population, and
achieve (on average) equal fitness. Unlike
our results for L. intacta, most studies of
AMTs indicate that reproductive success
differs markedly between the tactics (see
review in Cade, 1980; Austad, 1984), so
most systems probably represent subordinate AMTs.
338
E. C. W A L T Z AND L. L. W O L F
Why don't males defer reproduction
until their prime?
In situations with subordinate AMTs
(and possibly also where equal AMTs exist),
males attempt to reproduce before reaching peak RHP, apparently because factors
other than RHP favor early reproduction.
We need to examine these other influences
to understand why subordinate AMTs
occur. We introduce here some factors
thought to favor decreasing the age of first
reproduction (see Stearns [1976], Wittenberger [1979], and Bell [1980] for more
comprehensive reviews), and discuss their
significance for subordinate AMTs.
Potential advantages to early age of first
reproduction exist in the contribution of
offspring to population growth (Cole, 1954;
Lewontin, 1965). Even a slightly earlier age
of first reproduction can outweigh advantages in fecundity for competitors that
begin breeding later. Early reproduction
should be favored as a "bet-hedging" strategy in increasing populations when adult
mortality is high and unpredictable, or in
more stable environments with variable
juvenile mortality (Schaffer, 1974). Even
in stable or declining populations, random
adult mortality should favor early reproduction (Charlesworth, 1970), particularly
if (i) mortality is so high that males are
unlikely to reach the age of prime RHP,
and (ii) there is little loss of residual reproductive value from early reproductive effort
(see Pianka and Parker, 1975).
In seasonal breeders, reproducing early
in a season affords the offspring an opportunity to grow for a longer time before the
next season (Perrins, 1965). This could
allow for greater survivorship (e.g., Livdahl,
1982); it also could increase the RHP of
male offspring and the egg-laying potential
of female offspring during the following
season. For organisms, such as dragonflies,
that breed iteroparously within a single
season (see Fritz et al., 1982), these within
season effects are analogous to the effects
that apply between seasons to organisms living more than one year.
These potential advantages to early
reproduction merit practical consideration
by field workers attempting to estimate fitness, or RS. Most investigators estimate RS
strictly as some measure of the numbers of
offspring produced (see Howard, 1979).
But these advantages to early reproduction
suggest differential value of offspring
derived from different contributions to
population growth or from differences in
RS, based on the timing of birth. Thus,
realized fitness is a function of both offspring numbers and the time of their production (see Williams, 1966; Bell, 1980).
Ultimately, the optimal age of first
reproduction will depend on interactions
among these factors and the RHP-age
function (see Bell, 1980; Wittenberger,
1979). Their combined effect often is that
some males attempt to reproduce before
they can compete effectively with other
males. Older males then force them to
adopt tactics that, in the short term, are
inherently less profitable (see also Howard,
1978). If early reproduction is favored
when equal AMTs already exist, a mixed
assemblage of several AMTs may result.
Some males may practice "subordinate"
tactics, while others adopt tactics with success equal to that of territorial or haremholding males. Mixtures of multiple AMTs
that may fit this description are known in
several populations (L. intacta [this study]
and water striders [Rubenstein, 1984]), but
why they exist remains unclear.
Transient vs. site-fixed tactics
We have described two possible advantages to transient tactics. First, our
"resource uncoupling hypothesis" (see
p. 336) may apply when territorial males
and receptive females require different
resources. If the two kinds of resources are
distributed independently, transients may
mate in areas suitable for females (e.g., good
oviposition habitat), but unavailable to territorial males (e.g., no perches). Second, the
"local female arrival rate" hypothesis suggests that transients may exploit mating
opportunities in sites where the female
arrival rate is so high that territorial defense
is infeasible (Fig. 2). Increased activity in
these "hot spots" would increase harassment of mating pairs, so even non-territorial residents may not be able to reside
there. These two explanations for transient tactics as an equal AMT are not
DIVERSITY OF ALTERNATIVE MATING TACTICS
mutually exclusive. We treat them separately to focus attention on proximate
determinants of the transient tactic.
The frequency of transients may vary,
depending on their proximate determinants. For example, the arrival rate of
females in a local area probably varies considerably through time. Under the female
arrival rate hypothesis, transients are
expected in the population only when local
female arrival rates are above some threshold (Fig. 2), so the presence of transients
should vary with female arrival rates (see
also Campanella and Wolf, 1974). Under
the resource uncoupling hypothesis, however, transient mating opportunities should
exist in some areas so long as any females
are available.
The female arrival rate hypothesis
assumes differences in mean arrival rates
in different areas. Transient mating opportunities also may arise when female arrival
rates vary unpredictably in time or space.
Among different dragonfly species, for
instance, predictable arrival rates apparently favor territorial behavior in some
populations (Campanella and Wolf, 1974),
whereas in otherwise similar species,
unpredictability in female arrivals appears
to favor non-territorial wandering as the
predominant mating tactic (Campanella,
1975). This is an example of different mating tactics between, rather than within,
populations; but the decision rules about
being territorial should be similar on the
two scales, and similar even for different
resources that are defended (e.g., Stiles,
1973).
Younger or smaller males also may adopt
transient tactics as a subordinate AMT (e.g.,
Alcock et al., 1977; Fairchild, 1984). In
those cases, they may obtain matings for
the reasons outlined above, but it is more
likely that they rely on sneaking into territories or exploiting the few mating
opportunities between territories. The high
proximate cost of being transient generally
should favor subordinate males adopting
low cost, resident tactics whenever feasible. Residency also may reduce the risk of
young subordinate males not reaching
prime breeding age.
Variations in the costs and speeds of dif-
339
ferent modes of locomotion (e.g., Tucker,
1975) should influence the likelihood of
transient tactics, as either an equal or a
subordinate AMT. Roving transients usually are found in swimming or flying organisms (insects [review in Alcock, 1979; Ueda,
1979; this study], fishes [Warner and Hoffman, 1980; Gross, 1982], and birds [review
of similar tactics in Rohwer, 1982]), but
apparently not in "ambulatory" species
such as red deer or elephant seals. However, site-fixed residents often are found
in the same taxon as transients (e.g., L.
intacta), so the mode of transport does not
absolutely determine the form of the AMT.
The mobility of the AMT may influence
their frequency in the population. Because
they move freely and in swarms among territories, transients may exert a greater fitness detriment to territorial males than
would individual satellites. Transients can
intercept females in interstices, and preclude them from moving into a territory.
Transients may have a higher probability
of finding mates since they search over
much wider areas than residents or satellites. This suggests that systems with transient AMTs may be predisposed to higher
frequencies of alternatives than systems
with site-fixed residents. This would be true
especially when transient fitness is positively
frequency-dependent over some range of
frequencies, e.g., because groups of wanderers can invade a territory more successfully than can single individuals (Stiles,
1973; Robertson et al, 1976).
Selection for flexibility in the
male's tactical options
In some populations, male tactics are
fixed, by either genetic or ontogenetic differences between the tactics; but in others,
males can change tactics, sometimes even
within a day (Table 1). Several authors have
observed qualitatively that fixed tactics may
be favored when different phenotypes are
required for alternative roles (Rubenstein,
1980; Waltz, 1982), but the question of
why various levels of flexibility exist seems
to have received little attention (but see
West-Eberhard, 1979). We suggest that
"fitness set" models of specialist vs. generalist strategies (Levins, 1968) provide a
340
E. C. WALTZ AND L. L. WOLF
TABLE 1. Two time scales of tactical plasticity: Some selected examples.
Time scale
Lifetime
Species
Field cricket
(Gryllus integer)
Hymenoptera
(various spp.)
Scorpionflies
(Panorpa spp.)
Dragonflies
Plathemis lydia
Erythemis simplicuollis
Leucorrhinia mtacta
Bluegill sunfish
(Lepomis macrochirus)
Three-spined stickleback
(Casterosteus aculeatus)
Frogs
Green tree frog
(Hyla anerea)
Bullfrog
(Rana catesbiana)
Ruff
(Philomachus pugnax)
Elephant seal
(Mirounga angustirostris)
Red deer
(Cervus elaphus)
Fixed
"Seasonal"
Plastic
Fixed
Plastic
References*
X
X
Cade, 1981
X
X
Alcock, 1979
X
X
Thornhill, 1981
X
X
Campanella and
Wolf, 1974
McVey, 1981
This study
Gross, 1982
X
X
X
X
X
X
X
X
van den Assem,
1967
X
X
Perrill el al.,
1978
Howard, 1978
X
X
X... .
X
X
van Rhijn, 1966
X
X
LeBouef, 1974
X
X
Clutton-Brock
etal, 1982
* Most recent reviews cited where appropriate.
useful framework within which to consider
the evolution of fixed vs. flexibly-determined AMTs for individuals.
The models' general predictions can be
summarized as follows (see Levins, 1968
and Fretwell, 1972 for more detailed expositions). When very different phenotypes
are required for success in two habitat types
(i.e., mating tactics in the present context),
the fitness set is said to be "concave,"
because fitness is highest for the two
extreme phenotypes. When a single phenotype has the highest in both habitats (tactics), the fitness set is "convex." The former should favor specialization, and the
latter, generalization, so long as the environment is "fine-grained" (see Levins,
1968). In a sufficiently coarse-grained or
patchy environment, generalization always
is favored because the costs to search for
and move to a patch of preferred habitat
are sufficiently high. In general, mating
opportunities for two AMTs are not sep-
arated in time or space, so the predictions
for fine-grained habitats usually should
apply. When the mating areas for two tactics are separated, such as for Centrispallida
(Alcock et al, 1977), the predictions for
coarse-grained environments may be more
appropriate.
Applying fitness set models to this question provides two new insights into the
determination of AMTs. First, although the
general predictions regarding the shape of
the fitness sets in fine-grained environments have been suggested previously
(Rubenstein, 1980; Waltz, 1982), the possibility of generalization being favored in
coarse-grained environments appears not
to have been recognized. Second, by using
the fitness set framework, we may be able
to predict quantitatively how different two
optimal phenotypes need to be before specialization is favored. This approach may
suggest why, for example, bluegill sunfish
and three-spined sticklebacks have similar
DIVERSITY OF ALTERNATIVE MATING TACTICS
mating systems and alternative male
behaviors, but apparently very different
determination of the two alternatives (contrast van den Assem [1967] with Gross and
Charnov [1980]).
Several reasons exist for fitness sets of
mating tactic morphs to be concave, rather
than convex. First, pronounced sexual
dimorphism should facilitate recognition
by territorial males of interlopers on their
territories. Thus, "sneaker" or "satellite"
males are favored to have an inconspicuous, or even pseudofemale phenotype.
Second, the alternative tactics may interact
quite differently with females. If, for example, the time available for satellites to mate
with females is very limited, they might be
selected to release as large a quantity of
sperm as possible, in order to maximize
their fertilization success (e.g., Gross, 1982).
Territorial males may maximize their success by spending longer with the female.
Their energy would be allocated more profitably to growth, allowing better territorial
defense. Third, behavior patterns required
for alternative roles may require different
morphological adaptations. Transients
need very efficient locomotor structures,
whereas territorial males need structures
to aid in territorial defense (e.g., Stiles,
1973). If these optimal phenotypes differ,
individuals may have to allocate energy to
one or the other, resulting in a concave
fitness set.
Fitness set predictions may be applied to
two different time scales of mating tactic
determination, across and within an individual's lifetime (Table 1). The degree of
similarity of the two optimal phenotypes
apparently may or may not permit organisms to vary their tactics across their lifetime. Many organisms appear not to change
their tactics even within a "season" (Table
1). We use the term "seasonal" variation
for convenience, although a "season" is on
the scale of a day or two for "annual"
insects, but on the order of months for
perennially breeding species. Fitness set
models suggest that this within-season specialization results from intrinsic phenotypic constraints in a fine-grained environment (but see later).
Mating tactics for other organisms are
341
more flexible, even on this seasonal scale.
In these cases, the optimal phenotypes for
the two tactics probably differ very little.
In our dragonfly population, for example,
males employing the two tactics fly for
nearly equal durations over the course of
an hour; they differ principally in the numbers and durations of flights. Both territorial and transient males are involved in
fights, but territorial males initiate the contests.
Fitness set models suggest when intrinsic
flexibility, or fixity, should be favored.
However, having the flexibility to select
tactics does not presuppose that males have
free choice of tactics. The tactical choice
still may be limited by ecological constraints. Although scorpionflies, for example, can switch tactics within a day, their
choice is constrained in part by their finding or not finding a prey item to offer a
female (Thornhill, 1981). In addition, the
frequencies of intrinsically flexible tactics
may be limited by other males in a multimale game (Maynard Smith and Price,
1973). The problem of frequency-dependent payoffs probably is the major limitation in applying fitness set models to AMTs.
Further theoretical development is needed
to deal with this problem.
The interaction of intrinsic and ecological constraints has two important ramifications. First, even if males have flexible
tactical choice, the AMTs still may not have
equal (average) RS, because of ecological
constraints. Second, many organisms whose
tactics appear fixed (Table 1), in fact, probably have the ability to be flexible, but are
limited by other males. Young males
adopting subordinate AMTs often appear
not to develop conspicuous "badges" of
subordinate status, as do males in other
systems (see Rohwer, 1982). By not signalling their secondary status, they retain
the ability to adopt territorial tactics if the
opportunity occurs (e.g., Howard, 1981).
Only experimental manipulations can
resolve whether males adopt an alternative
"volitionally," or if they are forced to do
so (see Gross, 1982). Unfortunately, the
answers to such experiments are not always
clearcut (e.g., Perrill et al., 1978).
One of our primary purposes in sug-
342
E. C. WALTZ AND L. L. WOLF
gesting the application of fitness set models
to AMTs is to emphasize the fundamental
decision that a male must make—should it
allocate all of its investment to a single
option, or partition its investment between
two, competing alternatives? Phrasing the
decision as an allocation problem suggests
the underlying commonality with questions of specialist vs. generalist habitat usage
(Levins, 1968), hermaphroditism vs. dioecy
(review in Charnov, 1982) and semelparous vs. iteroparous reproduction (Schaffer
and Schaffer, 1979). Perhaps by recognizing the common aspects of these seemingly
different questions, we can understand better the general rules all organisms use in
making decisions regarding the allocation
of their limited investments.
ACKNOWLEDGMENTS
Our work has been supported by grants
from the National Science Foundation,
from the Syracuse University Senate
Research Fund, and by BRSG Grant 507
RR077068-18 awarded by the Biomedical
Research Support Program at the National
Institutes of Health. We thank Janet Wolf
for sharing her knowledge of classical
mythology, and S. Austad and P. Waser
for valuable suggestions on the manuscript.
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