Blackwell Science, LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066The Linnean Society of London, 2005? 2005
862
253264
Original Article
VOCALIZATION BARRIER TO QUAIL HYBRIDIZATION
J. M. GEE
Biological Journal of the Linnean Society, 2005, 86, 253–264. With 5 figures
No species barrier by call in an avian hybrid zone between
California and Gambel’s quail (Callipepla californica and
C. gambelii)
JENNIFER M. GEE*
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544–1003, USA
Received 1 September 2004; accepted for publication 7 December 2004
Acoustic signals sometimes act as premating isolating barriers between animal species, but we know little about the
circumstances that dictate the presence and strength of these barriers. Among insects, barriers to backcrossing are
strengthened by acoustic signals that are under genetic control. Hybrid signals tend to be intermediate to parental
signals, and signals are recognized only by like-types, which results in reinforced species boundaries. This is not typically the case in avian taxa. Instead, acoustic signal transmission is controlled by some combination of genes and
learning, and perhaps as a consequence of this variation, vocalizations play a diversity of roles in avian hybrid zones.
I used California and Gambel’s quail (Callipepla californica and C. gambelii), hybridizing birds that do not learn to
vocalize, to explore whether genetically determined vocalizations function as a species barrier. Using spectral analysis, I measured temporal features of calls of uniquely colour-banded quail that were recorded across one area of the
California and Gambel’s quail hybrid zone. Species discrimination is known to occur under captive conditions,
though its basis is unexplored. Here I show that differences in the calls of parental species are likely great enough
to permit species discrimination. Hybrid call components were intermediate to those of the parental species and
covaried with genetic traits, as assessed with seven highly polymorphic microsatellite loci. Contrary to expectation,
males as frequently called in response to unlike- as like-type females who had initiated antiphonal calling, which is
a courtship call between a female and a male. Furthermore, paired males and females did not share like-type assembly calls, nor was there a correlation between the female’s genetic or plumage traits and her mate’s advertisement
call. Based on these results, I conclude that California and Gambel’s quail recognize each other and hybrids as potential mates and backcrossing occurs frequently. Thus, compatible mating signals could contribute to increased mixing
of gene pools and slow the rate of speciation. I suggest that selection to respond to wide signal variation within species and imprinting on calls of mixed-species coveys may cause mating signal compatibility between classes within
the area of hybridization. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005,
86, 253–264.
ADDITIONAL KEYWORDS: bird song - sexual imprinting – speciation - species recognition – vocalization.
INTRODUCTION
When different animal populations meet, they may
mix and mate in a zone of hybridization, despite differences in the way they look, sound, and behave.
Within a hybrid zone, individuals may pair with like
or unlike mates, with respect to those differences. The
cause of this variation rests in part on the type of
mechanism that is used to locate and identify appropriate mates, which is often facilitated by the
exchange of species-specific mating signals. For some
*E-mail: [email protected]
organisms, mating signals are genetically determined.
In others, genes determine the basic signals, but the
latter are subject to modification through learning. It
remains an open area of research to explore how evolution proceeds between pairs of hybridizing species
that are at different points along the continuum of
mating signal determination, from signals that are
fixed to signals that are highly modifiable through
learning.
Acoustic signals are under strict genetic determination among several insect species, e.g. the brown
planthopper (Nilaparvata lugens; Butlin, 1996), bushcricket (Ephippiger ephippiger; Ritchie, 1992), and
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
253
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J. M. GEE
green lacewing (Neuroptera: Chrysopidae; Wells &
Henry, 1998). Genetically determined signal production and preference are consistently transmitted
across generations and are invariant throughout the
lifetime of an individual. These are the conditions that
drive population divergence, and under which speciation is essentially complete (Lande, 1981, 1982; Kirkpatrick, 1982). Individuals that formerly would have
bred together cease to identify each other as possible
mates, and mate with like-calling individuals. Intermixing may still be possible only if certain factors disrupt normal signal transmission or reception, such as
certain ecological conditions.
Bird species differ widely in the amount that they
can modify their songs and song preferences through
learning, which makes them an excellent group for
investigating how learned species recognition affects
reproductive barriers. The majority of avian hybrid
zone studies have focused on a speciose subset of birds,
called songbirds. Songbirds have complex vocal organs
(syringes), and vary in several different aspects of
song behaviour, including the timing of vocal learning,
sex patterns of song production, and the number of
songs that are learned (i.e. repertoire size). To a
greater or lesser extent, songbirds modify certain song
components by copying the vocalizations of other birds
(reviewed in Baptista, 1996), though in all avian species, vocalizations are constrained by an innate auditory template (e.g. Marler & Pickert, 1984; Kroodsma
& Canady, 1985; Baptista, 1996).
Depending upon the conditions, highly plastic,
learned song may either rapidly erode or fortify barriers to gene flow between species (Achlan & Servedio,
2004). Newly arisen songs have the potential to disrupt signal recognition among populations. Namely,
song changes that affect species recognition may be
able to spread through a single population, isolate it
and drive its divergence from parent populations
(Baptista & Trail, 1992; Slabbekoorn & Smith, 2002).
On the other hand, learned vocalizations that
increase compatibility between populations may
cause reproductive barriers to deteriorate. For
instance, in hybrid zones, learning through song copying often causes new vocalizations to arise that combine elements from the two parental species (e.g.
Haavie et al., 2004). As a result, individuals may
attract heterospecific mates or develop a preference
for them. In species that produce and recognize vocalizations through mechanisms that are mostly under
genetic control, variations in vocalizations would be
less likely to arise and would be transmitted more
slowly. Consequently, signal compatibility would be
more readily maintained across populations, and species boundaries would be less subject to sudden
changes in permeability, assuming that vocalizations
were central to their maintenance.
Many galliforms produce simple vocalizations that
are transmitted through strict genetic inheritance
(Konishi, 1963; Lynch, 1996). Though genetically
determined acoustic signals may promote speciation
among insects, we have only begun to study their
effect in birds (e.g. De Kort et al., 2002). In this study,
I report how acoustic signals contribute to mating patterns in a hybrid zone between California quail and
Gambel’s quail, avian species that are thought to
inherit, not learn, vocalizations. The hybrid zone
between the two species is at least 100 years old (Henshaw, 1885). The area of species overlap is narrow, corresponding to a steep transition in habitat types. This
suggests that the hybrid zone is restricted by ecological conditions (Gee, 2004), consistent with a bounded
model of hybridization (Moore, 1977; Moore & Price,
1993). An important factor that allows for hybridization is the formation of mixed species flocks, or coveys,
and pairing that occurs within the covey. Pairing
within the covey leads to earlier breeding and greater
reproductive success (Gee, 2003). However, still open
for question is how individual pairs are formed within
the mixed species coveys, and what premating reproductive barriers permit or prevent interbreeding.
Ideally, to investigate whether acoustic signals are
recognized across classes, one would quantify the
behavioural response of individuals to whom recorded
vocalizations are played. However, California and
Gambel’s quail are not territorial and respond weakly
to playbacks of vocalizations (J. M. Gee, unpubl. data).
As an alternative, I took advantage of natural situations of counter-calling between males and females.
Antiphonal calls functioned similarly to a playback of
female vocalizations in that females initiate the
antiphonal call to which males respond. I also
recorded the long distance contact call (assembly call),
which is often used between paired birds. If acoustic
signals are genetically determined and are incompatible among classes, this would suggest the following:
(1) signals differ across classes and correspond with
genetic, phenotypic and morphometric traits used to
determine classes; (2) males respond only to the courtship calls (antiphonal calls) initiated by females of
their type, and (3) paired males and females have
similar calls.
METHODS
STUDY
SITE AND SAMPLING
Vocalizations were recorded between 1997 and 2001
near Deep Canyon Desert Research Station (33∞75¢ N,
116∞50¢ W), Palm Desert, California, USA. The area of
hybridization studied is about 20 km wide (Fig. 1). A
Sony TCM-5000EV recorder and Sennheiser ME66
directional microphone were used to record vocalizations. In order to be able to identify individuals at a
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
VOCALIZATION BARRIER TO QUAIL HYBRIDIZATION
distance, I trapped and marked each individual with a
numbered metal band and colour bands. Approximately 500 individuals were trapped in sympatry,
about 60% of which were hybrids. Methods have been
described in detail elsewhere (Gee, 2004). Briefly,
before pairs formed, individuals were trapped in sympatry with seed-baited walk-in funnel traps (Smith,
Stormor & Godfrey, 1981).
I observed natural behaviour from the beginning of
covey break-up (late January-February) to at least the
end of the breeding season (late June-August) for a
minimum of 8 hours daily, between dawn and c. 12:00,
and between 15:00 and sunset. Quail that were
present after the covey split into pairs tended to
remain in the area and were considered the breeding
population. Adults were aged as either yearlings or
older than yearlings based on the plumage of upper
primary wing coverts (Leopold, 1939, 1977). From
each individual captured, I collected a small blood
sample (<50 mL) by brachial venipuncture, and then
blotted these samples onto individual filter papers saturated with 0.5 M EDTA (Petren, Grant & Grant,
1999). The samples were allowed to dry and stored
desiccated at ambient temperature for several months
before laboratory analysis. Of 500 individuals trapped
in the areas of sympatry, 280 were genotyped using
microsatellite markers developed for these species
(Petren et al., 1999; Gee, 2003).
This research was performed under conditions
approved by the Princeton University Institutional
Animal Care and Use Committee and the California
State Department of Fish and Game. All birds were
released where they were trapped.
SPECIES
ASSIGNMENT THROUGH GENETIC ANALYSIS,
SCORED PLUMAGE AND MORPHOMETRICS
In the great majority of cases, individuals were
assigned to a species based on genotype. When genotyping was not possible, for instance, in cases where I
borrowed recordings from collections, I classified individuals based on the location where the recordings
were made. In all of these instances, recordings were
made in allopatry. I relied on plumage and morphometric measures to assign species to sympatric individuals that could not be genotyped (as described
below).
I determined the genotype of each individual at
seven microsatellite loci, all developed for use in Callipepla species (Gee, Calkins & Petren, 2003). I used a
guanidine-based method (Ausubel et al., 1988) to
extract genomic DNA from blood samples and estimated DNA concentration by agarose gel electrophoresis (Petren et al., 1999). I assigned each
individual multilocus genotype to a genetic cluster
that was built using a Bayesian model-based method
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(STRUCTURE: Pritchard, Stephens & Donnelly, 2000;
Gee, 2004). I designated individuals as Gambel’s quail
if they had cluster assignment probabilities of > 90%.
Following this assignment scheme, individuals
between 0 and 10% were identified as California quail.
Individuals with a 10–90% assignment probability
were classified as hybrids and categorized by individual probabilities of species assignment.
The following set of measurements was taken with
dial calipers (tarsus and wing) and a ruler (head
plume and throat) to the nearest 0.1 cm: lengths of
wing (unflattened), tarsus, head plume (flattened),
throat patch (from base of beak to trailing edge of
patch) and belly patch (distance along midpoint from
the leading edge to the bottom edge). Weight was measured to nearest 0.1 g with a 250-g pesola spring balance and preweighed holding bag. For males the
following plumage traits were scored from 0 to 5
(C. californica/C. gambelii): colours of crown (chestnut/rusty), forehead (grey/black), belly patch (scaled
chestnut/black) and flanks (chestnut/rusty). Individuals were often trapped multiple times within a season
and across years, and all measurements were averaged to increase accuracy of measurements. I
employed principal components analysis (PCA) first to
identify traits that are correlated with the major axes
of variation in the dataset. For males, these traits
were flank, belly colour, forehead, and cap, and in
females, topknot, flank and tarsus (Gee, 2004). I then
reran the PCA with only these variables. From this
second analysis, I used individual PC1 loadings as a
means to measure overall phenotype, which I could
then use in subsequent analyses.
ANALYSIS
OF VOCALIZATIONS
California quail and Gambel’s quail produce structurally and contextually similar calls (Ellis & Stokes,
1966; Williams, 1969). I examined three different
calls: the assembly, antiphonal, and male-advertisement calls. Quail most often give the assembly call
when they become separated from their coveys or their
mates. The assembly call is also the female component
of male-female antiphonal calls. During the breeding
season, a female initiates antiphonal calling with the
assembly call, during which unmated males or her
mate respond with the ‘sneeze’ call (California quail)
or ‘meah’ call (Gambel’s quail) (Stokes & Williams,
1968). Males also give the sneeze or meah call during
aggressive encounters (Stokes & Williams, 1968; Williams, 1969). Like the antiphonal call, the male advertisement call is used strictly during the breeding
season (Williams, 1969), and is thought to announce
the male’s unmated status (Stokes & Williams, 1968),
though mated males also routinely give this call (J.M.
Gee, pers. observ.).
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
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J. M. GEE
To identify homologous features in calls, I followed
the same criteria used for identifying homology in
morphological traits (Brooks & Mclennan, 1991).
Namely, I studied the following: similarity of relative
position, special similarities in fine structure, and continuity through intermediate forms (Brooks & Mclennan, 1991; Price & Lanyon, 2002). I used the sound
analysis program Canary 1.2.4 (Cornell Laboratory of
Ornithology, Ithaca, NY) on a Power Macintosh G4
computer to digitize recordings at 16 bits, and
44.1 kHz resolution. Measurements were made by
visual inspection on spectrograms produced with a filter bandwidth of 175 Hz, sampling frequency of
21.53 Hz, FFT size of 2048 points, an overlap of 75%,
and a frame length of 1024 points. I periodically
checked spectrograms against waveforms for consistency and accuracy. This method allowed for fine
temporal analysis of sound (Rebbeck, Corrick & Eaglestone Stainton, 2001). In preliminary analyses, I
incorporated several measures of frequency, including
frequency range of each element. However, acoustic
separation between the parental species was sufficient
on the basis of temporal features alone (see below),
and temporal measures were easy to assess even in
poorer quality recordings. For each analysis, a particular call of an individual was used only once, except in
comparisons between paired birds, in which I included
only unique pairs. During the span of the study, several individuals paired more than once, usually after
they had lost their mates.
the two species and hybrids with the coefficient of
variation, followed by 2-way F-tests (Zar, 1999). Additional recordings from areas of allopatry were provided by the Borror Laboratory of Bioacoustics,
Department of Evolution, Ecology, and Organismal
Biology, Ohio State University, Columbus, OH, and
the Grace Bell Collection, Royal British Columbia
Museum, Victoria, BC (N = 5 for California quail;
N = 8 for Gambel’s quail).
I compared male advertisement calls across classes
by measuring the call duration and time to highest
frequency. The assembly call components were compared between species with a MANOVA. I used the
first principal component factor (Table 1) in an
ANOVA to assess differences in assembly call within
species, between the sexes.
SPECIES
RECOGNITION IN ANTIPHONAL CALLING
Antiphonal calling is composed of sex-specific calls.
California and Gambel’s quail both give antiphonal
calls during the breeding season. If California and
Gambel’s quail have incompatible acoustic signals,
response to female calls should be from males of the
same class, as assessed by genetic and plumage and
morphometric traits.
MATING
SIGNAL COMPATIBILITY AS EVIDENCED BY
PAIRS: ASSEMBLY CALLS, MALE ADVERTISEMENT CALLS,
PLUMAGE AND GENETICS
SPECIES
DIFFERENCES: ASSEMBLY AND MALE
ADVERTISEMENT CALLS
Assembly calls were measured for syllable and intersyllable duration (six elements). The assembly call of
Gambel’s quail sometimes ends with one (‘chicago’) or,
more often, two short syllables (‘chicago-go’), whereas
that of California quail always ends with a single syllable (‘chicago’). I took this variation into account by
measuring the entire duration of the call cycle, though
the duration of the fourth syllable itself was not analysed. To stabilize the variance in assembly call measures between California quail, Gambel’s quail, and
hybrids, I log-transformed measures before employing
ANOVA. Univariate analyses (ANOVA) were
employed to test for variation in individual acoustic
parameters described above (dependent variables)
among the parental types and hybrids (independent
variables). I considered the assembly call as a whole
by comparing two different parameters across groups:
the entire call cycle length and PC1. I used MANOVA
tests on log-transformed measures to reveal features
of the assembly call (dependent variables) that differed between the sexes within a species (independent
variables). I compared variation within and between
If species-specific calls are important to pair formation, one would expect a positive relationship between
the call elements of males and females in mated pairs.
However, this relationship would only hold true if
individuals have adequate opportunities to pair with
mates of their call type within winter flocks. Using relative proportions of each sex and species of breeding
individuals, a previous study (Gee, 2003) showed that
pairing was not assortative by species. Though species
designations were assigned by genotype and phenotype, here I show that call type, genotype and phenotype are correlated. Thus, availability of like-calling
mates likely did not constrain pairing.
To investigate whether pairs formed between likecalling individuals in the area of sympatry, I
regressed the first principal component (PC1) of
assembly call (extracted from six elements) against
PC1 of the mate’s assembly call. Males and females
may not necessarily use the same kind of traits to
identify conspecific mates. To explore whether
females mate according to male advertisement call
type, PC1 of plumage and morphometric traits was
regressed against the advertisement call of the male.
I assessed whether genetically similar individuals
mate by regressing the genetic species assignment of
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
VOCALIZATION BARRIER TO QUAIL HYBRIDIZATION
a bird against that of its mate. I assessed the relationship between genetic species assignment and
assembly call types by regressing PC1 of hybrid
assembly calls (both male and female) against their
genetic species assignment scores. Hybrids were initially classified both genetically and by appearance of
plumage (Gee, 2004). Plumage, vocalizations and
genetics between mates were simultaneously examined to further capture any patterns of trait similarity in relation to natural pairing. In order to avoid
generating artefactual correlations, pairs involving
pure species (at the extreme ends of the genetic species assignment axis) were not included in these linear regressions.
Pairing status was based on the criteria that two
individuals were either seen together at least twice at
the beginning of the laying season, over at least a 1month period, or seen once together accompanying
young chicks.
RESULTS
DISTINCT
SPECIES CALL TYPES
Based on analysis of 38 California quail, 29 Gambel’s
quail, and 49 hybrids, I found that California quail
and Gambel’s quail have distinctly different assembly
and male advertisement calls (Figs 1, 2; Tables 1, 2).
PC1 and PC2 explained 81% of variation between
assembly calls of the parental species (individual contributions given in Table 1). Durations of the call cycle
and of the last syllable (‘go’) were most associated with
257
high PC1 and PC2 values. A MANOVA indicated that
call elements differed between species and produced
results consistent with PCA (see Table 2, 3rd syllable
duration: F2,104 = 11.69; P < 0.001; cycle duration;
F2,105 = 14.09; P < 0.001). There was no sex difference
within species (MANOVA; F6,81 = 1.62; P = 0.151). The
duration of male advertisement calls was greater in
Gambel’s quail than California quail (F2,32 = 7.18;
P = 0.003) while time to the highest frequency did not
show a clear difference between species (F2,32 = 3.04;
P = 0.062). Hybrid assembly calls and male advertisement calls are intermediate to the parental species
(Fig. 1). Assembly calls differed for several call components between parental and hybrid individuals
(Table 2).
VARIATION
OF ASSEMBLY CALLS WITHIN
AND ACROSS GROUPS
Assembly calls of hybrid individuals showed more
variation than those of California quail, as measured
by PC1 (F44,33 = 3.31, P < 0.001), cycle length
(F44,33 = 2.28, P < 0.05), and the third call element
(F44,33 = 1.54, P < 0.05) (Fig. 2). I could not reject the
null hypothesis of equal individual variation in assembly calls between hybrid and Gambel’s quail (PC1
F44,27 = 0.91; cycle length F44,28 = 1.15), or between California and Gambel’s quail (PC1 F33,27 = 0.27; cycle
length F33,28 = 0.48; third element F33,27 = 1.17). The
third call element between hybrid and Gambel’s quail
did differ significantly (F44,27 = 1.81).
Figure 1. Sample locations from distributions of California quail (Callipepla californica) (filled circles) and Gambel’s quail
(C. gambelii) (open circles). Abbreviations: ASC, assembly calls; ADC, advertisement call. Samples in C. californica allopatry were from: British Columbia (ASC = 2; ADC = 1), Oregon (ASC = 2; ADC = 1), and California (ASC = 1; ADC = 1).
Samples from C. gambelii allopatry were from California (ASC = 1; ADC = 1), Arizona (ASC = 5; ADC = 2), and New Mexico
(ASC = 1; ADC = 1). The half-filled circle shows the sympatric study site, where recordings were taken from just outside
the area of species overlap (C. californica: ASC = 22, ADC = 1; C. gambelii: ASC = 25, ADCs = 3), and in several populations
across the hybrid zone (ASC = 47; ADCs = 21). Spectrograms of each parental species and hybrids where the top panel
shows the ASC given by males and females and lower panel depicts the male ADC used only by males.
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
258
J. M. GEE
Figure 2. Examples of assembly calls. Only males are shown and are grouped in columns by vocal type: California quail
(leftmost), hybrid (centre), Gambel’s quail (rightmost).
Table 1. Principal component loadings for assembly call,
using both allopatric and sympatric individuals. Factor 1
explained 64.2% of the variation in assembly calls, and
factor 2 explained an additional 16.4% of variation
Syllable 1
Pause 1
Syllable 2
Pause 2
Syllable 3
Cycle duration
Factor 1 (64.2%)
Factor 2 (80.6%)
0.002
0.259
0.518
0.085
-0.015
0.796
0.193
-0.448
-0.091
0.094
0.790
0.347
INTERSPECIFIC
DUETTING
Of all antiphonal calls observed involving two individuals of known species, calling occurred between individuals of different types in 52% of cases (N = 31)
(Fig. 3).
VOCAL
TYPES CORRESPOND TO GENETIC ASSIGNMENT
For male and female hybrids, the third syllable of the
assembly call was significantly associated with genetic
species assignment (R2 = 0.13; F1,34 = 6.21; P = 0.018)
(Fig. 4). Of all call elements, only the third syllable of
the assembly call was considered because it was most
strongly associated with both first and second PCs (see
Table 1).
CALL
TYPES DO NOT CORRESPOND BETWEEN MATES
Results did not support the hypothesis that mating
occurs between individuals of the same vocal type
(Fig. 5). Multivariate pair-wise correlations revealed
no significant relationships between the vocalizations
(PC1 of assembly call), morphology (PC1 of morphological measures), and genetic species assignments of
mates (all pair-wise correlations: N > 24, P > 0.05).
Linear regressions revealed no significant relationship between the assembly call (PC1) of mates (R2 =
-0.032; F1,21 = 0.3188; P = 0.578), the genetic species
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
VOCALIZATION BARRIER TO QUAIL HYBRIDIZATION
259
Table 2. Mean duration (in seconds) of assembly call elements, assembly call PC1 score, and male advertisement. Latency
to squill is the mean amount of time that males took to interrupt the assembly call of a female during antiphonal calling.
Hybrids are defined by a 10–90% chance of genotypic assignment to either parental class (see Methods). The coefficient
of variation (for call element length) or standard deviation (SD for assembly call PC1) follows in parentheses. Traits marked
with an asterisk (*) are significantly different from the other species (Tukey test, P < 0.05)
Species
Callipepla californica
Hybrid
C. gambelii
N
1st syllable
1st pause
2nd syllable
2nd pause
3rd syllable
Cycle length
Assembly call PC1
Male advertisement
Latency to squill
0.10 (29.3)
0.10 (36.7)
0.18 (32.2)
0.11 (33.0)
0.20* (32.3)
0.71* (14.7)
-0.04* (SD 0.03)
0.38* (26.1)
0.59 (28.4)
0.09 (19.7)
0.17 (37.5)
0.25 (37.9)
0.11 (31.7)
0.16 (40.1)
0.82 (22.7)
0.01* (SD 0.05)
0.45 (21.1)
0.49 (40.5)
0.10 (23.6)
0.17 (36.3)
0.30* (27.3)
0.10 (42.0)
0.14 (29.8)
0.94* (21.2)
0.04* (SD 0.05)
0.04 (11.9)
0.63 (45.0)
34, 45, 29
34, 45, 29
34, 46, 29
34, 45, 28
34, 45, 28
34, 45, 28
34, 44, 28
8, 16, 11
8, 22, 6
assignment of the female and either her mate’s phenotype (morphological PC1) (R2 = -0.014; F1,31 = 0.572;
P = 0.455), or his vocal type (PC1 of assembly call)
(R2 = 0.033; F1,27 = 0.1.954; P = 0.173) (Fig. 4). Plumage and morphological traits of a female (PC1) were
not associated with the duration of her mate’s advertisement call (R2 = -0.12; F1,8 = 0.01; P = 0.927).
DISCUSSION
Mating signals have a direct influence on the incidence of heterospecific pairing within hybrid zones
(Gill & Murray, 1972; Emlen et al., 1975; Baker &
Baker, 1990), but whether the mating signals decrease
or increase, the rate of divergence may depend in part
on the extent to which acoustic signals are learned.
Hybrids of oscine passerine species can learn pure
parental songs, which may lead to backcrossing and
introgression. In contrast, in species that give genetically determined acoustic signals, isolation may be
promoted by preference for mating between individuals of like, intermediate-sounding signals. This study
focused on California and Gambel’s quail, which like
many galliforms have predominantly genetically
determined vocalizations. I posed the question concerning whether intermediate acoustic signals act as a
reproductive barrier by generating incompatibility
between acoustic signal classes.
Unlike the genetically heritable signals of hybridizing insects, results suggest that calls do not function
as a reproductive barrier between California and
Gambel’s quail in an area of species overlap, likely
because there is no discrimination between vocal
types. Vocalizations appear to permit backcrossing
and could lead to formation of a hybrid swarm if
hybrids are as fit or fitter than parentals. As opposed
to restricted pairing between like-calling individuals,
mating occurred across vocal classes. Several alternative explanations may account for the mating pattern.
Namely, signal differences could be misrecognized,
imperceptible, or ignored by one or both sexes. In contrast to song production, song recognition could be
learned, or if genetically determined, unlinked to song
production.
Signal differences could be imperceptible relative to
signal noise. Vocalizations of non-oscine birds, though
for the most part genetically heritable, may be highly
complex (Zuk et al., 1990; Beani & Dessi-Fulgheri,
1995; Guyomarc’h et al., 1998). If individual differences in calls are great within species, as they are in
California and Gambel’s quail, preserving the ability
to recognize conspecifics in spite of great individual
variation may interfere with the ability to detect
differences between species (Ryan & Rand, 1993).
The amount of interindividual variation is similar in
California quail, Gambel’s quail, and hybrid quail,
even though recombination of parental traits in
hybrids might be expected to produce greater call variation than in parental species. This may be explained
by the occasional repetition of the third call element in
Gambel’s quail and hybrids, but not in California
quail. The repetition causes variation to be greater in
cycle length in Gambel’s quail and hybrids, but the
duration of the third call element in hybrids can
appear like that in California quail. Species-specific
signals presumably evolved amidst wide signal variation in allopatry. Individuals in sympatric populations
may overcompensate for great interindividual calling
variation and respond to heterospecific calls as if they
were conspecific calls.
Signal recognition could be mislearned. The evolutionary consequences of mislearned song may be far
reaching. For example, song convergence in sympatry
may lead to the movement of hybrid zones (Secondi,
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
260
J. M. GEE
A
A
5.0
4.0
3.0
2.0
1.0
kHz
0.0
Figure 4. Comparison between vocal traits (PC1 of assembly call) and probability of genetic assignment to Gambel’s
quail based on microsatellite genotypes (N = 36). The figure
shows parental types for comparison, but the analysis
included only hybrids.
0.5
S
B
B
5.0
4.0
3.0
2.0
1.0
kHz
0.0
S
C
C
0.5
1.0
5.0
4.0
3.0
2.0
1.0
kHz
S
0.0
0.5
Figure 3. Antiphonal call between opposite-sex individuals of (A) the same species (Callipepla californica), (B) a
female C. californica with a male C. gambelii and (C) the
same species (C. gambelii).
DeBakker & ten Cate, 2003). Across many taxa
(Hinde, 1962; Immelmann, 1972; Kendrick et al.,
1998), learning forms the basis for developing mate
preferences and species mate recognition (Lorenz,
1937; Bateson, 1966; Clayton, 1990; Grant & Grant,
1996). Learned recognition of heterospecific calls and
individuals and avoidance of siblings may result in
interbreeding (ten Cate, Vos & Mann, 1993). Quail live
in a sedentary covey, a flock of coalesced family groups
formed after the breeding season (Johnsgard, 1973).
Learned individual recognition, known as filial
imprinting, allows offspring to recognize and follow
parents (Hinde, 1962; Bateson, 1966). One component
of learned recognition could be the learned calls of parents and in addition to recognition of parents, a more
general process of learning to recognize and follow
other individuals of the covey, which in sympatry is
composed of both species and their hybrids (Gee,
2003). Because pairs are formed within the covey
(Leopold, 1977; Brown et al., 1998; Calkins, Hagelin &
Lott, 1999), the processes of filial imprinting and sexual imprinting, that is generalizing parental traits to
identify potential conspecific mates, are intrinsically
interwoven. Imprinting on calls of siblings through
individual recognition may result in avoidance of matings between close relatives, while imprinting on calls
of covey mates may both allow for protection of young
by the covey and later acceptance of covey members as
mates. Inbreeding avoidance through non-random
mating occurs within the mixed species breeding pool
of sympatric coveys (Gee, 2003), and evidence exists in
other systems that individual recognition may be
established in flocks (e.g. Whitfield, 1987; Schimmel &
Wasserman, 1991).
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
Male assembly call (PC1)
VOCALIZATION BARRIER TO QUAIL HYBRIDIZATION
Male genotype
1.0
0.8
0.6
0.4
0.2
0.0
0.0 0.2 0.4 0.6 0.8 1.0
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.12 -0.08 -0.04 0.00 0.04 0.08
Female genotype
Female assembly call (PC1)
2.0
0.15
Male plumage (PC1)
Male assembly call (PC1)
261
0.10
0.05
0.00
-0.05
-0.10
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Female genotype
1.5
1.0
0.5
-0.0
-0.5
-1.0
-1.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Female genotype
Male plumage (PC1)
2.0
1.5
1.0
0.5
-0.0
-0.5
-1.0
-1.5
-0.12 -0.08 -0.04 0.00 0.04 0.08
Female assembly call (PC1)
Figure 5. Comparison of mates by genetic species assignment (N = 68), vocal types (PC1 of assembly call; N = 23), female
genetic species assignment against her mate’s vocal type (N = 29), female genetic species assignment against her mate’s
phenotype (PC1 of morphology) (N = 68), and female vocal type against her mate’s phenotype (N = 33). All pairs were
observed during 1999–2000 in one field site (Royal Carrizo) where California quail, Gambel’s quail and their hybrids
coexist. Only hybrid-hybrid pairs were included in regression analyses.
© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 253–264
262
J. M. GEE
Signal differences could be ignored by at least one
sex. Males and females may be differently attuned to
perceiving or responding to species-specific signals,
and males are more likely to ignore species-specific
differences. In antiphonal calling, females initiate
exchange of calls, and males may not be very selective
in their response to female vocal type, whereas
females may have greater sensitivity to call differences. Playback of vocalization to both sexes would
make this distinction possible. Male birds of many
hybridizing species, including red-legged and rock
partridges (Ceugniet & Aubin, 2001) and indigo and
lazuli buntings (Emlen et al., 1975) respond aggressively to recordings of both heterospecific and conspecific vocalizations. However, females may be more
selective. In collared doves, which, like partridges and
quail, maintain a system where song learning does not
occur, females and males respond differently to
recordings of conspecific, altered conspecific and heterospecific song; females respond only to unaltered
conspecific song, whereas males respond to artificially
altered song (Secondi et al., 2002).
Acoustic signals may be ignored or be of minor
importance to pairing. It is unlikely that backcrosses
and parentals cannot discriminate between their
respective signals. In captivity, parental-type individuals from sympatry can discriminate between species
(Gee, 2003); however, rather than acoustic cues, discrimination may be made on the basis of plumage or of
a composite of traits (Calkins & Burley, 2003), as is the
case in zebra finches (Brazas & Shimizu, 2002). Social
dominance, which is more important in pair formation
than species-specific traits in hybridizing blackcapped and Carolina chickadees (Poecile atricapillas
and P. carolinensis) (Grubb et al., 2003), has not been
shown to significantly influence mate choice in Gambel’s quail. However, behaviour in general is central to
mate choice in other galliforms (e.g. Brodsky, 1988;
Zuk et al., 1990; Mateos & Carranz, 1995).
Species-specific acoustic signals may form a solid
behavioural barrier to hybridization that facilitates
speciation (Hoy, Hahn & Paul, 1977; Payne, 1986;
Irwin & Price, 1999). Species will form only if male
acoustic signal traits and female preferences for those
traits are coordinated (e.g. Lande, 1981; Wu, 1985;
Liou & Price, 1994; Kondrashov & Shpak, 1998). In
birds that learn song, misimprinting that occurs early
in life may cause females to mate with heterospecifics,
which results in leaks in reproductive barriers (Grant
& Grant, 1997; Baker & Boylan, 1999). In California
and Gambel’s quail, acoustic signals are fixed, but signal recognition may be flexible. Flexibility in signal
recognition may break down species barriers and may
slow divergence of California and Gambel’s quail. The
results cannot unambiguously distinguish between
the possible reasons for acceptance of heterospecific
signals. Quail either overcompensate for individual
variants, thereby mistakenly recognizing heterospecific calls, learn to prefer vocal types other than their
own type, or entirely disregard signal differences. In
order to more fully understand the role of genetically
determined call as a reproductive barrier, further
studies are necessary to assess how call production
and preference are transmitted and whether acoustic
signals are important in mating. Behavioural assays
on imprinted males and females could determine
unequivocally whether response to the call type of
each species is stable or whether it becomes modified
according to early social experience (e.g. Gelter, 1987;
Clayton, 1990; Slagsvold et al., 2002). If recent contact
has occurred in certain areas of the hybrid zone, one
would predict that the populations would not interbreed if imprinting were instrumental in maintaining
the hybrid zone. Similarly, allopatric populations from
opposite sides of the hybrid zone would have
incompatible communication signals if compatibility
depends on learning and not simply the inability or
insignificance of call differences in species discrimination and pairing.
ACKNOWLEDGEMENTS
I thank B.R. Grant, P. R. Grant, M. Hau, K. Petren, M.
Wikelski for suggestions that contributed to this
manuscript. I thank C. M. Stillwell for helping with
figures. W. Turner recorded many of the calls. I am
grateful to the staff at the University of California,
Natural Reserve System, Philip L. Boyd Deep Canyon
Desert Research Center for logistical assistance. F.
Barbeau and M.A. Barbeau, G. Morin and S. Morin, S.
Adams and D.K. Adams, and S. Arrowsmith graciously
permitted access to their land. Recordings made outside of the study area were provided by the Borror
Laboratory of Bioacoustics, Department of Evolution,
Ecology, and Organismal Biology, Ohio State University, Columbus, OH, and the Grace Bell Collection,
Royal
British
Columbia
Museum,
Victoria,
BC. Financial support was provided by a National Science Foundation Predoctoral Fellowship, a National
Science Foundation Doctoral Dissertation Improvement Grant DEB-0073271, an Environmental Protection Agency Science-to-Achieve-Results Graduate
Fellowship U-91572901–0, Sigma Xi Grants-in-Aid of
Research, the American Ornithologists’ Union Betty
Carnes Memorial Award, and The Reserve Community Association of The Reserve, Palm Desert, CA,
USA.
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