Download Female-specific color is a signal of quality in the striped plateau

Behavioral Ecology
doi:10.1093/beheco/arl001
Advance Access publication 5 June 2006
Female-specific color is a signal of quality in the
striped plateau lizard (Sceloporus virgatus)
Stacey L. Weiss
Department of Zoology, Duke University, Durham, NC 27708-0325, USA
Recent theoretical and empirical studies confirm that male mate choice and/or female–female mate competition can be expressed
in the absence of sex-role reversal. Such reproductive patterns may select for the evolution of female traits that indicate female
phenotypic or genotypic quality among non–role-reversed species. Although attention to the evolution and function of female
ornaments is increasing, additional focus is needed on female-specific ornaments (those not expressed in conspecific males) and
on nonavian systems in order to gain a broad understanding of how selection acts directly on ornamentation of female animals. In
the striped plateau lizard, Sceloporus virgatus, only females develop orange throat patches during the reproductive season. The color
peaks in expression near the time of ovulation and appears to stimulate male courtship. Here, I examine whether this femalespecific ornament can be used by males to reliably assess female phenotypic quality. Using multivariate regression analyses, I show
that the area of the orange color patch predicts body condition and mite load, the chroma (i.e., saturation) of the color patch
predicts body size, and both patch area and chroma reliably predict average egg mass. Thus, female reproductive color may
function as a condition-dependent signal, indicating phenotypic quality to potential mates. Key words: condition-dependent
ornament, female ornament, female reproductive signals, parasites, sexual selection. [Behav Ecol 17:726–732 (2006)]
emale choice, male–male mate competition, and the elaborate male traits that result from these forces of sexual
selection are a primary focus of studies in evolutionary biology
and behavioral ecology. Analogous studies concerning the evolution of female behavior and ornamental traits are far less
common, though they have been increasing in recent years
(Amundsen 2000; Houde 2001). Historically, female ornaments
have been considered nonfunctional byproducts of genetic correlation to males that are sexually selected to express ornaments
(Darwin 1871; Lande 1980; Muma and Weatherhead 1989). As
such, few studies addressed whether elaborate female traits are
the product of direct selection on females. Challenges to this
view emerged in the 1990s by phylogenetic studies that demonstrated that female showiness is evolutionarily labile, with more
evolutionary increases than decreases (Irwin 1994; Omland
1997; Burns 1998), and by studies that directly tested female
ornaments for potential function (e.g., Rowland et al. 1991;
Potti and Merino 1996; Amundsen et al. 1997; Jones and
Hunter 1999; Amundsen and Forsgren 2001). It is now recognized that female ornamentation may evolve via natural
selection or sexual selection acting directly on females (e.g.,
West-Eberhard 1983; Amundsen 2000).
To date, most empirical work on the function of female
ornaments has focused on instances where females express
a reduced form of a male sexually selected trait. Additionally,
the majority of this work has been conducted on birds
(Amundsen 2000). Work with other taxa is required to establish a general understanding of the evolution of female ornamentation. Here, I address the function of a female-specific
ornament—expressed by females but absent or much reduced
in males—in a lizard species with conventional sex roles. Such
cases are not readily explained by traditional sexual selection
theory or by genetic correlation and thus provide clear evi-
F
Address correspondence to S.L. Weiss, who is now at the Department of Biology, University of Puget Sound, 1500 North Warner Street
#1088, Tacoma, WA 98416-1088, USA. E-mail: [email protected].
Received 24 August 2005; revised 20 April 2006; accepted 27 April
2006.
The Author 2006. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: [email protected]
dence for direct selection on female ornamentation (e.g.,
Amundsen and Forsgren 2001; Roulin, Dijkstra, et al. 2001;
Hanssen et al. 2006).
Female-specific ornaments have been documented in more
than 30 species of lizard (Cooper and Greenberg 1992) but
have been empirically examined in only a few. These studies
have focused on the relationship between female color and
reproductive state and perhaps male response to such color
(Ferguson 1976; Cooper et al. 1983; Watkins 1997; Cuadrado
2000; LeBas and Marshall 2000; Hager 2001; Weiss 2002a;
Baird 2004). The presence or absence of the color advertises
female reproductive status, and males respond by increasing
courtship to receptive females and/or reducing courtship to
nonreceptive females. The importance of the among-female
variation in ornament expression has previously been considered in only one lizard species, Ctenophorus ornatus (LeBas and
Marshall 2000), where no relationship between color expression and female reproductive quality was found. In fact, very
little work in any taxon has assessed the possibility that femalespecific color (often termed nuptial color) varies in expression and that this variation may have some signal value
(Amundsen and Forsgren 2001). It remains possible that lizards vary in their expression of female-specific color in a way
that reflects phenotypic or genotypic quality. If they do, males
may be selected to discriminate among females not solely
based on the presence or absence of the ornament but on
the degree to which it is expressed.
The female-specific ornamental color of striped plateau lizards, Sceloporus virgatus, changes seasonally with female reproductive state (Figure 1a,b); both color patch size and intensity
peak near the time of ovulation (Weiss 2002a). Within the
seasonal pattern of color expression, there is a great deal of
variation among females in the amount of color expressed at
peak color development (Figure 1b–d). This variation does
not appear to influence female–female social interactions
(Weiss 2005). However, during the courtship season, males
respond to among-female variation in color expression by
maintaining closer proximity to and tending to interact more
intensely with females expressing dark orange color relative to
females expressing pale or no orange color (Weiss 2002a).
Weiss
•
Female-specific color indicates phenotypic quality
Figure 1
Variation in female reproductive color. (a) Female Sceloporus virgatus
prior to expression of orange reproductive color. (b) The same individual as in (a) at peak expression of reproductive color. (c and d)
Two other individuals, each at peak color expression.
Here, I examine whether the expression of ornamental
color by female S. virgatus can reliably predict female phenotypic quality. Characteristics of interest include body size, body
condition, mite load, clutch size, and egg mass. These traits
could be important to female fitness. Body size of reptiles may
indicate juvenile growth rate (Halliday and Verrell 1988) or
survival ability, though the relationship between size and survival ability may be complex and vary among years (Forsman
1993; Abell 1998; Willemsen and Hailey 2001). Body condition is expected to reflect the female fat reserves, which may
relate to general health, as well as the amount of energy available for egg production. Relatively low mite loads indicate
a reduced susceptibility to mite infestations and increased
health (Hamilton and Zuk 1982; Roulin, Riols, et al. 2001),
as well as reduced costs of infection. Trombiculid mites have
been found to reduce weight gain in a closely related congener (Sceloporus undulatus hyacinthinus), suggesting that the infestations are costly (Klukowski and Nelson 2001), perhaps
especially so to females trying to maximize the energy allocated to clutch production. Mite loads are also known to influence behavior and offspring life-history characteristics in
lizards (Sorci and Clobert 1995; Main and Bull 2000). Clutch
size is a direct measure of annual reproductive output. Finally,
egg mass likely reflects the resources provided to offspring by
the female and positively influences both hatchling size and
survivorship (Vleck 1988).
METHODS
Study species
During the reproductive season, female S. virgatus develop
orange color that surrounds and often covers small, sexually
monomorphic blue throat patches (Figure 1a,b). Outside of
the reproductive season, male and female S. virgatus are
monomorphic, with the exception of body size (females:
61.5 6 0.3 mm, males: 55.4 6 0.3 mm, N ¼ 273; SL Weiss,
727
unpublished data). Thus, relative to most congeners, visual
signaling of S. virgatus appears to be reduced in males (i.e.,
reduction of blue throat color and absence of blue belly
patches common within the genus; Stebbins 1985; Wiens
1999) and increased in females (i.e., occurrence of bright
orange reproductive color absent among sister species;
Cooper and Greenberg 1992).
The reproductive cycles of females and males are relatively
synchronous, both within and between the sexes. Females produce one clutch each year that weighs nearly 30% of female
body mass (Vinegar 1975) and that is carried in the oviducts
for approximately 1 month before being oviposited. The male
testicular cycle peaks once a year, just before female ovulation
(Ballinger and Ketels 1983).
During the courtship period, males are more active than
females (Rose 1981; Merker and Nagy 1984). Rose (1981)
suggests that female inactivity is selectively advantageous as
it allows females to allocate more energy to the production
of eggs and less to locomotion and social interactions, including mate searching. Sedentary females may be visited by 3–6
courting males in a single day (Smith 1985), and aggressive
male–male interactions nearly always occur in close proximity
(1–2 m) to a female (Smith 1985). In some cases, close spatial
relationships temporarily form between given male–female
pairs. The associated male is the most likely sire of that
female’s clutch, although forming an association is no guarantee of paternity; approximately 30% of hatchlings are sired
by males other than the associated male (Abell 1997). Thus,
male S. virgatus may exhibit mate choice by differentially allocating their investment in time, courtship intensity, and male–
male competition as they travel from female to female.
Data sets and animal maintenance
Data were collected May–July 1996 and 1997 within 3.2 km of
the American Museum of Natural History’s Southwestern Research Station (SWRS), Cochise County, Arizona. Lizards used
in this study were captured by noose and permanently marked
by toe clipping. The same data set was used for analyses of
body size (snout-to-vent length [SVL]) and body condition.
Females (N ¼ 210) were measured and either brought to
SWRS for use in other experiments (N ¼ 86) or released at
the site of capture after being marked with unique combinations of paint spots on the dorsum for rapid field identification (N ¼ 124). Some released animals were recaptured and
remeasured throughout the reproductive season (see Weiss
2002a). To ensure statistical independence, I randomly selected one measurement of each female to use in analyses.
A different, but related, data set was used for the analysis of
mite load. The females (N ¼ 123) were from the same captureand-release population as above, and 97 females are included
in both the body size/condition data set and the mite load
data set. However, for females that were measured multiply, I
randomly selected one measurement per female separately
for the 2 data sets. The data sets overlap for 71 females.
A third data set was used for analyses of clutch size and egg
mass (N ¼ 26). These animals were housed in outdoor enclosures at SWRS in visual contact with a conspecific male and
were allowed 20-min periods of physical contact (including
copulation) once or twice each week. The color of these females was measured immediately after each interaction period (for details, see Weiss 2002a). Using this housing
regimen, females express normal reproductive behavior and
physiology, with the exception of oviposition (Weiss 2002b).
To obtain measures of reproductive output, I induced
oviposition by intraperitoneal injection of 0.1 cc. oxytocin
approximately 1 week after most free-ranging females had
oviposited.
Behavioral Ecology
728
Measures of female color
I defined female reproductive color by 3 variables: area, value,
and chroma of the orange coloration of the throat patch
(hereafter, orange area, value, and chroma). To calculate orange area, I photographed the right throat patch of each
female along side a ruler (for purposes of calibration),
scanned photographs, selected orange pixels using the automated ‘‘color range’’ command of Adobe Photoshop 4.0, and
determined the area of the selection using National Institutes
of Health Image 1.60. I determined the value (relative lightness or darkness) and chroma (degree of saturation) by
matching the orange color of females to Munsell color chips.
Each Munsell color chip is classified by separate measures of
hue, value, and chroma. All females were matched to Munsell
hue 10R but varied in both value (range: 3.0–8.5) and chroma
(range: 3.5–14.0). Higher numbers for value and for chroma
indicate darker and more saturated color, respectively.
I personally matched the color patches of all females to the
Munsell chips on the live lizard under a standardized light
source (Pelican Super SabreLite) in order to remove the effect of variation in normal human color vision and in light
conditions on the accuracy of color matching (see Endler
1990). The bias of the human visual system, however, cannot
be eliminated by this method. Recent work with spectrometry
shows that the orange color of female S. virgatus is nonreflective in the ultraviolet wavelengths and has a typical ‘‘orange’’
spectrum (SL Weiss, K Foerster, and K Delhey, unpublished
data), similar in shape to that of the matching Munsell
color chip.
Measures of female phenotype
I measured body size using a transparent ruler and used the
residual of a regression of body mass (measured with a Pesola
spring scale) on SVL3 as my measure of body condition (N ¼
210 free-ranging females). I counted the total number of
trombiculid and pterygosomatid mites on 123 free-ranging
females. These mites are external and easy to count and identify to the family. For these 3 estimates of female phenotypic
quality, all measurements on a given individual were made
by me within a 5-min period. I sampled females at various
dates across the reproductive season and therefore included
date as an independent variable in the regression models
(described below).
To examine whether female color can predict clutch size or
average egg mass, I induced 26 gravid females to oviposit by
oxytocin injection and then counted and weighed all eggs. For
these measures of reproductive phenotype, I controlled for
date by measuring female color at the time when each female
was first determined by palpation to have oviductal eggs. At
this time, females are at or near peak color expression (Weiss
2002a). Thus, date was not included in the regression models
for these variables. For the analysis of clutch size, I included
SVL and postoviposition body condition in the initial regression model to control for the effect of female size on reproductive output. Due to missing data, analyses that include
postoviposition body condition have a sample size of 18 individuals. One female died from unknown causes before oviposition was induced. She is only included in the analysis
of clutch size, which was determined for this individual by
dissection.
Statistics
Multivariate regression was used to examine the specific question: does female reproductive color predict female phenotypic quality? I separately tested each phenotype measure
using a general linear regression model. In every case, the 3
color variables were independent variables, and the given phenotype measure was the dependent variable. Regression models may also include additional independent variables; these
‘‘control variables’’ examined the effect of date, body size, or
body condition in cases when that variable 1) was suspected to
influence the relationship of interest, 2) was significantly correlated to the phenotype variable, and 3) had a significant
effect on the regression model. Body size never contributed
significantly to a model and thus was not included in any final
model. See Table 1 for the specifics of each final model.
I examined the collinearity diagnostic ‘‘condition index’’ of
each regression model. All models had condition indices
greater than 30, indicating a potential problem with collinearity. An assessment of the variance proportions indicated that
the problem was likely caused by a high linear dependence
between 2 of the color variables: value and chroma. Because
value did not significantly contribute to any of the regression
models, I repeated all analyses excluding that variable. The
removal of this factor reduced all condition indices to less
than 20 (size: 7.2, condition: 7.2, mite load: 7.4, clutch size:
18.8, egg mass: 18.4) and did not influence the interpretation
of the analyses. Because collinearity issues did not affect the
interpretation of the models, I chose to present the analyses
using the 3 separate color variables.
When control variables were included in the regression
model, the resulting F statistic and R2 were relative to the
combined effects of color and the control variables, rather
Table 1
Multivariate regression assessing the relationship between color expression and aspects of phenotype in female Sceloporus virgatus
Color variablesa
a
b
Control variablesa
Phenotype measure
Full model statisticsb
Orange area
Value
Body size
Body condition
Mite load
Clutch size
Average egg mass
9.34; 4,205
19.23; 4,205
26.76; 5,117
0.60; 3,22
11.32; 3,21
0.07
0.50
0.29
0.19
0.40
0.26
0.10
0.16
0.08
0.02
(,0.001)
(,0.001)
(,0.001)
(NS)
(,0.001)
(NS)
(,0.001)
(0.001)
(NS)
(0.01)
(NS)
(NS)
(NS)
(NS)
(NS)
Chroma
Date
Condition
0.51
0.20
0.31
0.21
0.55
0.14 (0.04)
0.16 (0.01)
0.77 (,0.001)
—
—
—
—
0.41 (,0.001)
—
—
(0.001)
(NS)
(0.06)
(NS)
(0.01)
Female color was defined by 3 variables: orange area, value (relative lightness or darkness), and chroma (degree of saturation). Control variables
were included when a given variable was correlated to the phenotype variable and had a significant impact on the regression model. Date refers to
the date on which a female was measured. Condition refers to female body condition, calculated as the residual of a regression of body mass on
SVL3.
Cells show standard coefficients and P values (in parentheses). A dash indicates that the variable was excluded from the model. NS: P . 0.05.
Cells show the F statistic, df, and P values (in parentheses).
Weiss
•
Female-specific color indicates phenotypic quality
729
than being specific to the effects of color alone. To determine
the significance of a subset of predictors (the 3 color variables) to the regression model, I calculated the partial F statistic (Fp) and the coefficient of partial determination (Rp2 ). The
Fp statistic examined the null hypothesis that, given the inclusion of the control variable, the 3 color variables combined
did not significantly improve the regression model. The null
hypothesis was rejected when the 3 color variables together
(hereafter, female color) explained a significant amount of
variation in the phenotype measure. The Rp2 statistic indicates
the proportion of variation in the phenotype measure that was
explained by female color while controlling for (i.e., holding
constant) date or body condition.
RESULTS
Body size and condition
Body size and condition were independent of each other (r ¼
0.03, P . 0.60, N ¼ 210) and thus could be analyzed separately. Female color explained a significant amount of variation in female body size (Rp2 ¼ 0.10, Fp ¼ 7.47, degrees of
freedom [df] ¼ 3,205, and P , 0.001) and body condition
(Rp2 ¼ 0.27, Fp ¼ 25.62, df ¼ 3,205, and P , 0.001). Individuals
with more female color were larger and had higher condition
indices than those with less female color.
The relationship between female color and body size was not
simply a result of larger animals having larger throats to contain larger patches. Chroma explained a larger amount of variance in body size than did orange area (Table 1, Figure 2a).
Conversely, orange area explained a larger amount of variance
in body condition than did chroma (Table 1, Figure 2b).
Mite load
Mite load significantly increased throughout the reproductive
season of S. virgatus (bdate ¼ 0.61, P , 0.001), and date alone
explained 37% of the variation in mite load. Additionally, mite
load was correlated to body size (r ¼ 0.20, P ¼ 0.02, N ¼ 123)
and condition (r ¼ 0.19, P ¼ 0.04, N ¼ 123). However, body
size was nonsignificant in the initial regression model (bsize ¼
0.05, P . 0.50) and thus was removed from the model.
While controlling for date and body condition, female color
reliably predicted mite load (Fp ¼ 8.49, df ¼ 3,117, and P ,
0.001) and explained 18% of the variation in mite load. Individuals with more female color were less susceptible to mite
infestations and therefore were healthier than individuals with
less female color. There was a significant negative relationship
between mite load and orange area (Table 1, Figure 2c) and
a nearly significant negative relationship between mite load
and chroma (Table 1).
Reproductive output
Clutch size was significantly correlated to both body size (r ¼
0.47, P ¼ 0.02, N ¼ 26) and postoviposition body condition (r ¼
0.48, P ¼ 0.04, N ¼ 18). However, neither variable contributed significantly to the initial model (bsize ¼ 0.51, P . 0.10;
bcondition ¼ 0.31, P . 0.30), and each was therefore removed. The final regression model could not reliably predict
female color (F ¼ 0.60, df ¼ 3,22, and P . 0.60; Table 1).
Including either or both of the control variables did improve
the predictive value of the model, but in no case was the
model significant (P values of these alternative models ranged
from 0.14 to 0.20).
Egg mass was not significantly correlated to either body size
or body condition (P . 0.50); thus, no control variables were
included in the regression model (Table 1). Female color
Figure 2
Partial regression plots showing the significant relationships between color and phenotype in female Sceloporus virgatus. (a) From
the regression model examining female body size, residuals of
chroma are positively related to residuals of body size (N ¼ 210).
(b) From the regression model examining body condition, residuals
of orange area are positively related to residuals of body condition
(N ¼ 210). (c) From the regression model examining mite load,
residuals of orange area are negatively related to residuals of mite
load (N ¼ 123). See Table 1 for statistics.
explained a significant amount of variation in egg mass (R2 ¼
0.62, F ¼ 11.32, df ¼ 3,21, and P , 0.001). Females with
heavier eggs had larger orange area (Figure 3a) and higher
chroma (Table 1, Figure 3b) than females with lighter eggs.
730
Figure 3
Partial regression plots showing the significant relationships between color and average egg mass for female Sceloporus virgatus.
From the regression model examining average egg mass, residuals of
(a) orange area and (b) chroma are positively related to residuals
of egg mass (N ¼ 25 for each). See Table 1 for statistics.
DISCUSSION
Female ornamentation of S. virgatus can reliably indicate multiple aspects of female phenotypic quality. The information
content of the color signal is complex, with size and saturation
of color patches indicating different aspects of the female
phenotype. Females with large color patches are in better
body condition, with fewer mites and larger eggs, than females
with small color patches. Females with more saturated color
patches are bigger, with larger eggs and a tendency to have
fewer mites, than females with less saturated color. These relationships make it theoretically possible for males to assess
female phenotypic quality on the basis of their reproductive
color expression. Previous work shows that male behavior is
influenced by female color in the direction predicted by these
relationships. Male S. virgatus associate more closely to moreorange females than to less-orange females during the courtship season (Weiss 2002a). The results herein suggest that by
doing so, males are preferentially associating with high-quality
females.
Positive relationships between female ornamentation and
aspects of phenotypic quality similar to those presented
herein have been found in a handful of studies in other species. For example, display coloration correlates positively with
body size and/or condition in female bluethroats (Amundsen
et al. 1997), red-winged blackbirds ( Johnsen et al. 1996), and
northern cardinals ( Jawor et al. 2004). More ornamented females are known to carry reduced parasite loads in pied fly-
Behavioral Ecology
catchers (Potti and Merino 1996), barn owls (Roulin, Riols,
et al. 2001), and a calopterygid damselfly (Cordoba-Aguilar
et al. 2003). Additionally, female ornamentation has been
found to correlate positively with measures of reproductive
output in birds (Jawor et al. 2004; Griggio et al. 2005) and
baboons (Domb and Pagel 2001), as well as the ability to
tolerate reproductive costs (Hanssen et al. 2006). Only a small
number of these studies focus on female-specific ornamentation (Domb and Pagel 2001; Hanssen et al. 2006) or on nonavian systems (Domb and Pagel 2001; Cordoba-Aguilar et al.
2003).
Aspects of lizard mating systems allow the taxon to serve as an
important contrast to avian species. Whereas in birds female
mate choice for male indicator traits is common (Andersson
1994), in lizards there is remarkably little evidence for
female mate choice or for the presence of male signals that
could be used by females to assess male phenotypic quality
(Olsson and Madsen 1995; Tokarz 1995; but see Cooper and
Vitt 1993). For instance, in S. virgatus, the display color of
males has been reduced over evolutionary history (Wiens
1999), and recent studies suggest that it has no effect on
female mate choice and only a weak effect on male–male
interactions (Abell 1999; Quinn and Hews 2000). The relationship between the lack of reliable male signals on which
to base direct female choice and the occurrence of conditiondependent female-specific signals may be an interesting line
of future investigations.
Given that females of many lizard species, including
S. virgatus, show no evidence of direct mate choice for males
expressing specific phenotypes (Olsson and Madsen 1995;
Tokarz 1995; Abell 1999), what strategies might they have to
maximize offspring quality? And might these strategies select
for female condition-dependent signals? Female lizards may
benefit from minimizing mate searching and maintaining
a sedentary lifestyle during vitellogenesis in order to maximize
energy allocation to egg production (Rose 1981; Olsson and
Madsen 1995) and thereby improve offspring viability (Vleck
1988). Female lizards may also benefit from mating with multiple males, by promoting sperm competition and reducing
the risk of genetic incompatibility (Madsen et al. 1992). Sedentary females that benefit from mating with multiple males
are likely to also benefit by attracting males into their home
range (e.g., Cox and Le Boeuf 1977), which may result in
direct selection on females to signal their quality to potential
male mates.
Such selective pressure is expected to be particularly strong
in species like S. virgatus with short, synchronous reproductive
cycles (Weiss 2002a). Such a breeding system limits an individual’s ability to obtain multiple mates due to time limitations (Emlen and Oring 1977). From the perspective of
sedentary, multiply mating females, such a time limitation is
expected to increase the benefits of mate attraction. From the
perspective of males, time limitations tend to lower the degree
of polygamy (Emlen and Oring 1977), thereby increasing the
benefits of male discrimination among potential female mates
(e.g., Dewsbury 1982; Langmore et al. 1996). Based on the
results presented herein, S. virgatus males that appropriately
respond to female color by allocating more reproductive effort to more-orange females are likely to benefit by siring offspring with larger, healthier mothers that have larger eggs.
The costs of producing the female color signal and how
the signal remains an honest indicator of female phenotype
are unknown. Pteridines, not carotenoids, appear to be responsible for the production of red and orange social signals
among lizards (Ortiz and Williams-Ashman 1963; Cooper
and Greenberg 1992), and preliminary work has detected
pteridines (and no carotenoids) in female S. virgatus color
patches (J Hudon and SL Weiss, unpublished data). Although
Weiss
•
Female-specific color indicates phenotypic quality
little attention has been paid to the potential costs of pteridinebased pigmentation, pteridines do have important biological
functions including antioxidant functions (see reviews by
Ortiz and Williams-Ashman 1963; McGraw 2005). As such,
the use of pteridines in morphological social signals may limit
their availability for other physiological processes. Other than
costs of production, signal honesty also may be the result of
costs of ornament maintenance. For instance, if the expression of reproductive color increases females’ susceptibility to
predation or otherwise jeopardizes survival, only high-quality
individuals may be able to risk the expression of large or
bright color patches (Zahavi 1975).
In summary, female S. virgatus express a condition-dependent
ornament that can reliably predict female size, condition,
mite load, and average egg mass. As previous work shows
that male conspecifics preferentially associate with moreorange females, the female trait may be maintained via sexual
selection. Although more attention is being paid to female
ornaments in the recent literature, few studies have focused
on female-specific traits where hypotheses of genetic correlation can be excluded, and few have focused on nonavian
species. Only by examining a broad collection of species can
we establish the general principles guiding the evolution of
such traits.
I thank the American Museum of Natural History’s (AMNH) SWRS
for logistical support, numerous SWRS volunteers for help with lizard
collection, and V. Manteuffel for assistance with data collection of
clutch characteristics. Color spectra of Munsell color chips were provided by Munsell, a Division of GretagMacbeth, LLC (Regensdorf,
Switzerland). Funding was provided by National Science Foundation,
AMNH, the Animal Behavior Society, Sigma Xi, the North Carolina
Academy of Science, and Duke University Graduate School. The
manuscript benefited from the statistical advice of D. Burdick and
S. Carroll, as well as from helpful comments provided by D. Kabelik,
S. Nowicki, D. Painter, R. Papke, D. Pope, and T. Windfelder. Scientific
collecting permits were obtained from Arizona Game and Fish Department. Animal care and use conformed to Duke University and
AMNH guidelines and complied with the current laws of the United
States of America.
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