Postcopulatory fertilization bias as a form of cryptic sexual selection.

Abstract

Males and females share most of their genetic material yet often experience very different selection pressures. Some traits that are adaptive when expressed in males may therefore be maladaptive when expressed in females. Recent studies demonstrating negative correlations in fitness between parents and their opposite-sex progeny suggest that natural selection may favor a reduction in trait correlations between the sexes to partially mitigate intralocus sexual conflict. We studied sex-specific forms of selection acting in Anolis lizards in the Greater Antilles, a group for which the importance of natural selection has been well documented in species-level diversification, but for which less is known about sexual selection. Using the brown anole (Anolis sagrei), we measured fitness-related variation in morphology (body size), and variation in two traits reflecting whole animal physiological condition: running endurance and immune function. Correlations between body size and physiological traits were opposite between males and females and the form of natural selection acting on physiological traits significantly differed between the sexes. Moreover, physiological traits in progeny were correlated with the body-size of their sires, but correlations were null or even negative between parents and their opposite-sex progeny. Although results based on phenotypic and genetic correlations, as well as the action of natural selection, suggest the potential for intralocus sexual conflict, females used sire body size as a cue to sort sperm for the production of either sons or daughters. Our results suggest that intralocus sexual conflict may be at least partly resolved through post-copulatory sperm choice in A. sagrei.

Full text

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ORIGINAL ARTICLE
doi:10.1111/j.1558-5646.2008.00356.x
POSTCOPULATORY FERTILIZATION BIAS AS A
FORM OF CRYPTIC SEXUAL SELECTION
Ryan Calsbeek1,2and Camille Bonneaud3,4
1Department of Biological Sciences, 401 Gilman Hall, Dartmouth College, Hanover, North Hampshire 03755
2E-mail: ryan.calsbeek@dartmouth.edu
3Department of Ecology and Evolutionary Biology, University of California, Los Angeles, California 90095
Received October 11, 2007
Accepted January 27, 2008
Males and females share most of their genetic material yet often experience very different selection pressures. Some traits that are
adaptive when expressed in males may therefore be maladaptive when expressed in females. Recent studies demonstrating neg-
ative correlations in fitness between parents and their opposite-sex progeny suggest that natural selection may favor a reduction
in trait correlations between the sexes to partially mitigate intralocus sexual conflict. We studied sex-specific forms of selection
acting in Anolis lizards in the Greater Antilles, a group for which the importance of natural selection has been well documented
in species-level diversification, but for which less is known about sexual selection. Using the brown anole (Anolis sagrei), we
measured fitness-related variation in morphology (body size), and variation in two traits reflecting whole animal physiological
condition: running endurance and immune function. Correlations between body size and physiological traits were opposite be-
tween males and females and the form of natural selection acting on physiological traits significantly differed between the sexes.
Moreover, physiological traits in progeny were correlated with the body-size of their sires, but correlations were null or even
negative between parents and their opposite-sex progeny. Although results based on phenotypic and genetic correlations, as well
as the action of natural selection, suggest the potential for intralocus sexual conflict, females used sire body size as a cue to sort
sperm for the production of either sons or daughters. Our results suggest that intralocus sexual conflict may be at least partly
resolved through post-copulatory sperm choice in A. sagrei.
KEY WORDS: Island, lizard, physiology, selection, sexual conflict, sperm.
Divergenceinthereproductiveinterestsofmalesandfemalesmay
lead to sexual antagonism in mating systems (Losos et al. 1997;
Chippindale et al. 2001; Leal and Fleishman 2002; Ogden and
Thorpe 2002; Ebehard 2005). Interlocus sexual conflict, charac-
terized by sex-specific expression of antagonistic traits, may gen-
erate an evolutionary chase in which the evolution of a trait in one
sex is detrimental to the other sex and is countered by the evo-
lution of another trait (Parker 1979). For example, in Drosophila
melanogaster, seminal fluid proteins reducing female remating
4Present address: Department of Organismic and Evolutionary
Biology, Harvard University 26 Oxford Street, Cambridge, Mas-
sachusetts 02138
rates to increase paternity assurance are countered by the evo-
lution of female resistance to sperm toxicity (Rice 1996; Rice
and Holland 1997). In contrast to interlocus conflict, intralocus
sexual conflict arises when traits expressed in both males and fe-
malesdifferinfitnessoptimabetweenthesexes,therebyconstrain-
ing the evolution of the traits and preventing males and females
from reaching their fitness optima (Chippindale et al. 2001; Rice
and Chippindale 2001; Parker 2006). Strong empirical evidence
for intralocus conflict comes again from studies on Drosophila,
where elimination of female-driven selection pressures generated
populations in which males enjoyed enhanced fitness, whereas
females suffered diminished fitness (Pischedda and Chippindale
2006).
1137
C ?2008 The Author(s). Journal compilation C ?2008 The Society for the Study of Evolution.
Evolution 62-5: 1137–1148

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R. CALSBEEK AND C. BONNEAUD
Fisher (1930) was the first to propose that because traits ex-
pressed in both males and females usually have a common ge-
netic basis, alternative fitness optima for those traits should lead
to antagonistic selection between the sexes. Lande (1980) formal-
ized this theory, and a recent extension by Day and Bonduriansky
(2004) suggested that sexual selection should favor reduced ex-
pression of the between-sex component of trait heritability (e.g.,
father–daughter or mother–son) to minimize possible intralocus
conflict. Several recent studies have built on this hypothesis, and
demonstrated sexual conflict by showing that the heritability of
fitness was negatively correlated between parents and their op-
posite sex progeny (Fedorka and Mousseau 2004; Pischedda and
Chippindale2006;Foersteretal.2007).Thesestudiesshowedthat
high fitness sires produced high fitness sons, and at the same time
produced low fitness daughters. Similarly, dams with high fitness
tended to produce high fitness daughters and low fitness sons. As
suggested by Chippindale et al. (2001), one could therefore ex-
pect females to at least partly mitigate the intralocus conflict by
selecting different sires for the production of sons and daughters.
Although these studies elegantly demonstrate correlations in
total fitness between parents and progeny, we still lack clear illus-
trations of sex differences in the action of selection on phenotypic
traitsinthewild(PreziosiandFairbairn2000;Fairbairn2007),and
howtheyinfluencethenatureofsexualconflictinmatingsystems.
Herewebuildonagrowingnumberofstudiesaimedatexamining
theimportanceofsexdifferencesinnaturalpopulations,usingthe
brownanole(Anolissagrei)asastudysystem.Previousstudiesof
Anolis lizards have indicated an important role for natural selec-
tion acting on morphology [e.g., limb length: (Losos et al. 1997)
body size: (Calsbeek and Smith 2007)] and physiology [e.g., im-
mune function: (Calsbeek et al., 2008) locomotor performance:
(Calsbeek and Irschick 2007)], although less is known about how
thesetraitsinfluencereproductivedecisions.Bodysize,animpor-
tant trait in the ecomorphological diversification of anoles, shows
extensive variation between the sexes in A. sagrei (Butler and
Losos 2002; Butler et al. 2007). In addition, natural selection has
previouslybeenfoundtoactonbothmaleandfemalebodysizein
this species (Calsbeek et al. 2007b), although explicit sex-based
comparisons were not made in that study. Because differences in
body size are thought to reflect differences in habitat use (Butler
and Losos 2002; Butler et al. 2007), we might expect variation in
this trait to be associated with differing life-history strategies, and
hencepossiblycorrelatedwithsexuallyantagonistictraits.Forex-
ample,sex-specificdifferencesinselectiononphysiologicaltraits
are predicted by “the susceptible male hypothesis” (Rolff 2002)
which suggests that compared to males, females should invest
more in longevity, and hence immune function.
We used experiments in the laboratory in combination with
surveys of natural populations to provide insight into the role of
natural selection in generating and/or maintaining intralocus sex-
ual conflict. We also investigated the potential resolution of con-
flict through female reproductive choice. First, we tested whether
expression of traits reflecting whole animal physiological con-
dition (Arnold 1983; Stoehr and Kokko 2006), namely stamina
and immune function, differed between male and females, and
whether the action of selection on these traits also differed be-
tween the sexes. Given that an organism’s limited resources can
generate trade-offs between life-history traits (Bonneaud et al.
2003;StoehrandKokko2006)andsincetheoptimalcombination
of life-history traits over the life of an individual may differ be-
tweenthesexes(Bateman1948),wealsotestedwhetherthecorre-
lations between stamina and immune function were sex-specific.
Second, we tested whether between-sex heritability estimates of
size-related traits appeared to be reduced in anoles using the in-
tersexual correlation coefficient rMF(Fisher 1930). Finally, we
captured males and gravid females from the wild and reared their
offspring to assess natural variation in male siring success, and
whether females would engage in adaptive mate-choice to reduce
intralocus sexual conflict (Chippindale et al. 2001).
Methods
Anolissagreiisasmall(40–70mmsnout-vent-length;SVL)semi-
arboreal lizard with a broad tropical and sub-tropical distribution.
It is the most common anole in the Bahamas and a member of the
“trunk-ground” ecomorph in the Greater Antilles adaptive radi-
ation. We studied wild populations of A. sagrei in the Bahamas
from2004to2005.Initiallizardcapturestookplaceinspringdur-
ingMayandJuneonKiddcay,anoffshoreislandconnectedtothe
mainlandbya>80mcementcausewaysupportingnolizardhabi-
tat. The island is small (∼0.084 km2) and near Great Exuma, Ba-
hamas(23◦31?N,75◦49.5?W).Itcontainsbothbroaddiameterveg-
etation including palm trees (Pseudophoenix spp.) and Australian
pine trees (Casuarina equisetifolia) (mean perch diameter =
230 mm), and narrow diameter vegetation such as sea-grape
(Coccoloba uvifera), sea hibiscus (Hibiscus tiliaceus), and but-
tonwood (Conocarpus erectus) (mean perch diameter = 25 mm).
All lizards were sexed, weighed with a Pesola spring scale
(nearest g), and measured for snout-vent-length (SVL; nearest
mm). Hind and forelimb lengths were measured with dial calipers
from the point of insertion into the abdomen to the femoral-tibial
and humero-radio-ulnar joints. Lizards were marked with unique
combinations of nontoxic colored elastomer dye, injected in the
ventral side of the hind and forelimbs (Nauwelaerts et al. 2000).
Tags are not visible to predators and serve as permanent identifi-
cation in the wild, allowing us to track the fate of every individual
overthecourseofthestudy(CalsbeekandIrschick2007;Calsbeek
and Smith 2007).
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CRYPTIC MATE CHOICE
BREEDING EXPERIMENTS AND GENOTYPING
We raised field-caught lizards and their progeny in the laboratory
to measure the heritability of morphology, and to assess female
matechoiceasafunctionofmalemorphology.DuringJune2004,
wecollectedmaleandfemaleA.sagreifromoneofourstudypop-
ulations near Georgetown Exuma, Bahamas. We captured lizards
during the middle of the breeding season, ensuring that females
had mated prior to capture. To determine paternity, we captured
andsampledtissue(tailtips)fromallneighboringmalesthatmight
have mated with the females (Calsbeek et al. 2007a). All lizards
were weighed and measured as above, and morphological mea-
sures were used to assess cryptic female choice (below). Each
female was housed in a separate 10-gallon terrarium and was pro-
vided with full spectrum lighting, and ad libitum food (Achaeta
crickets) and water. A potted plant was also provided in which the
female was allowed to lay her eggs.
Eggs were left undisturbed to incubate in the potted plants
untilhatching.WeraisedF1progenyfromknowndamsinthelabo-
ratoryandassignedpaternityusingalibraryofeightmicrosatellite
loci (Bardelbeden et al. 2004). We genotyped all progeny, dams,
andcandidatesires,andassignedpaternitywiththesoftwarepack-
age CERVUS. Males were assigned as sire if the confidence of
paternity exceeded 95%. Because females were housed individu-
ally, maternity of all offspring was known with certainty. Progeny
were sexed at hatching (males have enlarged postanal scales).
Siblings were randomized into separate terraria until maturity in
an attempt to minimize the potentially confounding influence of
family origin on development.
Given the outcome of paternity results from 2004 (see Re-
sultssectionbelow)webeganbreedingF1lizardsinthelaboratory
during 2005, but experimentally controlled the females’ access to
sires of differing body size. During 2005, 17 virgin females were
each housed in separate tanks with one large and one small male
[Mean(SE)SVLLARGE=58.53mm(±2.69);SVLSMALL=48.63
mm (± 3.49)], and allowed to mate over the course of two weeks.
Twenty-fiveadditionalvirginfemaleswereallowedtomatewitha
single male that was assigned at random. All pairings of F1 males
with virgin females were made to prevent any inbreeding effects
(i.e., no pairings of full or half-siblings and no first cousins). Pa-
ternity analyses were conducted on F2 progeny as in 2004 except
in the case of monogamous females for whose progeny paternity
was already known.
PHYSIOLOGICAL TRAITS
We measured variation in two physiological traits: stamina
(Arnold1983)andimmunefunction(Blountetal.2003)onlizards
in the field during spring 2005. Running time to exhaustion (i.e.,
stamina) is the cumulative result of many underlying physiologi-
calprocesses,andlikelyindicatesoverallvigor(Arnold1983).We
estimated stamina for all lizards by running them to exhaustion
on an electrical treadmill (0.4 km/hr). Because anoles do not run
well on level surfaces (Perry et al. 2004), the treadmill was set at
a 20◦incline. Lizards were motivated to run by manually tapping
the hind limb (Le Galliard et al. 2004). Fatigue was considered
complete following three consecutive failed attempts to induce
running, and/or the loss of the righting response.
We estimated immune function (>2 weeks poststamina) for
all lizards in the field and laboratory (F1 and F2) by using a PHA
assay (Goto et al. 1978) (phytohemaglutinin-PHA-P; Sigma). Re-
sponse to PHA is a standard estimate of immune function in wild
individuals (Martin et al. 2006), but we do not attempt to discuss
here whether a higher response to PHA is more or less adaptive
(Kennedy and Nager 2006). We challenged males with 0.20 mg
PHA in 0.02 mL phosphate buffered saline (PBS) and females
with 0.10 mg PHA in 0.01 mL PBS, injected in the left hind-foot
pad. We injected the same volume of PBS in the right hind-foot
pad as a control. We measured the thickness of each foot pad with
dial calipers (± 0.01 mm) at the injection site, prior to and 24 h
post-injection, and assessed the intensity of the response as the
difference in swelling between the PHA-injected and the control
foot pad. Lizards were held overnight in separate containers dur-
ing the PHA test. Because animals that were measured in the wild
hadtoberecapturedtwoweeksafterstaminatrials,notallanimals
were tested and sample sizes differ for statistical tests.
VIABILITY STUDIES
Tomeasurenaturalselectiononphysiologicaltraits,weestimated
sex-specific differences in viability as a function of stamina and
immune function. During spring 2005, we captured all of the
112 male and 128 female sub-adult A. sagrei that were naturally
present on Kidd cay. To supplement sample size for our studies
of natural selection (below), we also captured an additional 300
lizards from an adjacent site (250 m away) and released them
to two additional offshore cays (92 females and 110 males to
Flamingo bay cay; 98 males to Nightmare cay). We have been
studying natural selection on these cays for several years; de-
tails are available elsewhere (Calsbeek and Irschick 2007; Cals-
beek and Smith 2007). All lizards were uniquely marked, mea-
sured,andreleasedattheiroriginalpointofcapture.Wemeasured
viability selection on 540 lizards over the four-month period from
first capture in spring to our population censuses conducted in
autumn. This time frame encompasses survival to sexual matu-
rity and the end of the first breeding season and is an estimate of
one component of fitness (survival). Each day we walked mul-
tiple transects over the entire study site and captured surviving
lizards. Lizards were marked with a small spot of white paint to
preventrecapture.Censusescanbeconsideredexhaustivebecause
although the majority of surviving lizards were captured within
the first two days, censuses continued for two to three weeks, or
untilthreeconsecutivedaysofsearchingturnedupnonewmarked
EVOLUTION MAY 2008
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R. CALSBEEK AND C. BONNEAUD
lizards. Lizards that were not recovered during our censuses were
considered dead; a reasonable assumption given that emigration
fromislandsisrare,exceptduringhurricanes(CalsbeekandSmith
2003), none of which affected our study islands during this study.
Recapture efficiencies were ∼97% based on regression of the
numberoflizardscapturedeachday(logtransformed)againstthe
cumulative days of capture effort.
We measured the strength of linear (i.e., directional, ?) and
nonlinear (i.e., stabilizing/disruptive ?1,1or correlational ?1,2)
selection using separate regressions of morphological and phys-
iological traits against survival to the fall census. We calculated
relative fitness (standardized by the population mean) separately
foreachsex.Alltraitdistributionswerestandardizedtomeanzero
and unit standard deviation. Because the dependent variable “sur-
vival”wasbinomiallydistributed(live/die),wereportsignificance
values of each selection gradient from a logistic regression that
accounts for binomial error variance (Janzen and Stern 1998).
Selection surfaces were computed using nonparametric
cubic-spline regression (Schluter 1988; Schluter and Nychka
1994).Thismethodmakesnoaprioriassumptionsaboutdatadis-
tributions. To find the best-fit cubic spline, we searched a range
of possible smoothing parameters (?) to find the value of ? that
minimizedthegeneralizedcrossvalidation(GCV)score.Wethen
usedthis?toplotthebest-fitcubicsplinetosurvivaldata.Figures
showdatapointsintermediatebetweenzeroandone.Thesevalues
are the estimated (y-hats) values of survival that were calculated
from jackknife analyses (Schluter 1988; Schluter and Nychka
1994).
HERITABILITY AND PATERNITY ANALYSES
We estimated rMFof body size and limb-length using all four
parent–offspring regressions (i.e., father–daughter, mother–son,
mother–daughter, father–son). We used mean values for progeny
morphology (standardized by year) and performed weighted least
squares regression against standardized parent morphology (by
year) with family sizes as weights. Although the use of restricted
maximum likelihood methods (REML) for estimating variance
components has increased in recent years, its application is lim-
ited here (Lynch and Walsh 1998) by several characteristics of
our data. First, females lay one egg per clutch and this limits the
replication of sires that is needed for nested analyses of variance
(ANOVA).Second,therelationshipbetweensiresandtheirdaugh-
ters was negative (see Results) and negative variance components
cannot be measured using REML (Lynch and Walsh 1998). Gen-
erally,sire-offspringregressionsprovidereliableestimatesoftrait
heritability,asthesearefreefrommaternaleffects.Toprovidead-
ditional support for our estimates of heritability, we verified the
significance of these regressions with 1000 randomizations of the
sire-offspring regressions. Significance values were computed as
the fraction of iterations in which the randomized correlation was
at least as strong as that observed in our data. rMFwas calculated
following (Bonduriansky and Rowe 2005) as:
rMF=
?
h2
h2
FD· h2
MD· h2
MS
FS
,
where subscripts represent heritability estimates from Father–
Daughter (FD), Mother–Son (MS), Mother–Daughter (MD) and
Father–Son (FS) respectively. Because this quantity is derived
from the ratio of inter- to intrasexual heritability, it is predicted
to decrease as selection acts to reduce the costs of intralocus sex-
ual conflict (Fisher 1930; Lande 1980; Bonduriansky and Rowe
2005).Wepredictedsignificantcorrelationsonlybetweenparents
and their same-sex progeny.
We tested whether the sex of F1 progeny produced by field-
caught females depended on sire body size using multivariate
ANOVA, with sire size nested within females and the dependent
variables “numbers of sons and daughters.” In 2005, we used
controlledmatinginthelaboratoryandtestedthesamehypothesis
as above. We performed an additional test using a generalized
linear model with a logit link function to account for binomial
error variance in the dependent variable “progeny sex.” We note
that this is not an analysis of sex ratio biasing, but rather a test of
how females allocate progeny gender based on sire size.
Results
CORRELATIONS BETWEEN PHYSIOLOGICAL AND
MORPHOLOGICAL TRAITS
Raw values of stamina, and of swelling in response to PHA (im-
mune response), were both significantly dependent on body size
(SVL) (ANOVA; immune swelling: r2= 0.293, F1,78= 32.31,
P < 0.0001; stamina: r2= 0.29, F1,125= 51.07, P < 0.0001).
Males and females significantly differed in body size in
the field (ANOVA, F1,131 = 322.97, P < 0.0001; ¯Xmales =
55.16 mm ± 4.88 and¯Xfemales= 42.76 mm ± 2.89). Immune
response varied with body size in the field (males r2= 0.13,
F1,226= 5.88, P < 0.0001; quadratic effect F = 0.02, P = 0.88,
Fig.1A,females:r2=0.04,F1,108=4.74,P=0.03,quadraticef-
fectF1,107=2.57,P=0.01,Fig.1B)andtheserelationshipswere
significantly different (sex × SVL2F = 2.19, P = 0.03). Sim-
ilarly, stamina varied with body size, but only in males (males:
r2= 0.002, F1,205= 0.41, P = 0.52, quadratic effect F1,204=
−2.76, P = 0.006, Fig. 1C, females: r2= 0.003, F1,193= 0.59,
P = 0.44, Fig. 1D). The difference in the relationship between the
sexes was not significant (sex × SVL ANCOVA F = −1.78, P =
0.09).
We detected similar, although not identical, relationships be-
tween traits correlated with body size in the laboratory. Both
immune function and stamina varied with body size in the
laboratory, and these relationships also differed between the
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CRYPTIC MATE CHOICE
0.1
0.3
0.5
0.7
0.9
1.1
1.3
40506070
Male Immune response (mm)
SVL (mm)
A
10
30
50
70
90
110
130
150
405060 70
Male Stamina (sec.)
SVL (mm)
C
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
3540 4550
SVL (mm)
Female Immune response (mm)
B
20
40
60
80
100
120
140
35404550
Female Stamina (sec.)
D
SVL (mm)
field
lab
Figure 1. Negative intersexual correlations in three components of fitness are consistent with the interpretation that conflict genes have
antagonistic fitness effects in males and females. Males of intermediate and large size had the highest response to PHA (immune function)
(A) but females of intermediate size had the lowest response (B). Body size and stamina were positively correlated in both males and
females (C and D) but the slope of this relationship was significantly greater for females than males resulting in a significant sex × body
size interaction (P < 0.0035; note the difference in scale on the x-axes to accommodate sexual size dimorphism). Physiological measures
are based on 76 male and 84 female progeny in the laboratory (light line, open symbols). Field measures (heavy line, filled symbols) of
immune function were based on 113 females and 231 males, and measures of stamina were based on 200 females and 211 males. Model
fits were compared by checking all possible model subsets and choosing parameters that minimized the Akaike information criterion
score (AIC).
sexes. For example, males of intermediate size had the high-
est immune function (quadratic effect of SVL F1,83 = 4.8,
P = 0.03 Fig. 1A), whereas females of intermediate size had
the lowest immune function (quadratic effect of SVL F1,92 =
4.8, P = 0.006) and the difference between the sexes was sig-
nificant (analysis of covariance [ANCOVA] sex × SVL, F1,154
= 1.91, P < 0.05; Fig. 1B). Stamina was positively related to
body size in both sexes (e.g., males r2= 0.08, P = 0.004;
females r2= 0.08, P = 0.007), and the slope of this relationship
wassignificantlydifferentbetweenmalesandfemales(ANCOVA
sex × SVL, F1,183= 8.81, P < 0.004; 1C, D). All interaction ef-
fects remained significant following Bonferroni corrections for
multiple comparisons.
Because stamina and immune swelling were both signifi-
cantly correlated with body size, we corrected these measures for
body size in all subsequent analyses by calculating the residuals
from the regressions of stamina and immune swelling on SVL.
All relevant analyses provided qualitatively similar results using
raw and residual trait values.
SEX DIFFERENCES IN PHYSIOLOGICAL TRAITS AND
IN SELECTION ON THOSE TRAITS IN THE FIELD
Stamina did not differ significantly between the sexes (ANOVA,
r2= 0.004, F1,125= 0.55, P = 0.46,¯Xmales= 71.13 sec. ± 17.14
and¯Xfemales=53.20sec.±13.95).Immunefunctiondidnotdiffer
betweenthesexeseither(¯Xmales=5.97mm.±2.26and¯Xfemales=
3.72 mm. ± 1.66), but was differently related to stamina between
the sexes (ANOVA, r2= 0.1355; sex: F1,70= 0.04, P = 0.847;
stamina: F1,70= 0.21, P = 0.648; sex × stamina: F1,70= 10.73,
P = 0.0016; Fig. 2). The traits were significantly related in both
sexes but in opposite directions: the relationship between stamina
andimmunefunctionwaspositiveinfemales(ANOVA,r2=0.13;
F1,35= 5.27, P = 0.028) but negative in males (ANOVA, r2=
0.14; F1,35= 5.61, P = 0.024), suggesting a trade-off in males
only (Stoehr and Kokko 2006) (Fig. 2).
We recaptured 50 of 220 females (23% survival) and 108 of
320 males (34% survival) during our fall censuses. Selection on
body size was directional in females, favoring larger body size
(? = 0.33 ± 0.13; ?2= 7.03, P = 0.008; Fig. 3), but was nearly
EVOLUTION MAY 2008
1141