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Sexual conflict at loci influencing traits shared between the sexes occurs when sex-specific selection pressures are antagonistic relative to the genetic correlation between the sexes. To assess whether there is sexual conflict over shared traits, we estimated heritability and intersexual genetic correlations for highly sexually dimorphic traits (horn volume and body mass) in a wild population of bighorn sheep (Ovis canadensis) and quantified sex-specific selection using estimates of longevity and lifetime reproductive success. Body mass and horn volume showed significant additive genetic variance in both sexes, and intersexual genetic correlations were 0.24+/-0.28 for horn volume and 0.63+/-0.30 for body mass. For horn volume, selection coefficients did not significantly differ from zero in either sex. For body weight, selection coefficients were positive in females but did not differ from zero in males. The absence of detectable sexually antagonistic selection suggests that currently there are no sexual conflicts at loci influencing horn volume and body mass.
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Quantitative genetics and sex-specific selection
on sexually dimorphic traits in bighorn sheep
Jocelyn Poissant
1
, Alastair J. Wilson
2
, Marco Festa-Bianchet
3
,
John T. Hogg
4
and David W. Coltman
1,
*
1
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9
2
Institute of Evolutionary Biology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK
3
De
´
partment de biologie, Universite
´
de Sherbrooke, Que
´
bec, Canada J1K 2R1
4
Montana Conservation Science Institute, 5200 Upper Miller Creek Road, Missoula, MT 59803, USA
Sexual conflict at loci influencing traits shared between the sexes occurs when sex-specific selection
pressures are antagonistic relative to the genetic correlation between the sexes. To assess whether there is
sexual conflict over shared traits, we estimated heritability and intersexual genetic correlations for highly
sexually dimorphic traits (horn volume and body mass) in a wild population of bighorn sheep (Ovis canadensis)
and quantified sex-specific selection using estimates of longevity and lifetime reproductive success. Body mass
and horn volume showed significant additive genetic variance in both sexes, and intersexual genetic
correlations were 0.24G0.28 for horn volume and 0.63G0.30 for body mass. For horn volume, selection
coefficients did not significantly differ from zero in either sex. For body weight, selection coefficients were
positive in females but did not differ from zero in males. The absence of detectable sexually antagonistic
selection suggests that currently there are no sexual conflicts at loci influencing horn volume and body mass.
Keywords: animal model; genetic correlation; heritability; lifetime reproductive success; selection;
sexual conflict
1. INTRODUCTION
The widespread occurrence of sexual dimorphism
suggests that optimal trait values often differ between the
sexes ( Fairbairn 2007). Because traits shared by the sexes
are typically influenced by the same genes (Roff 1997),
sexual conflicts at loci influencing shared traits (intralocus
sexual conflicts; Arnqvist & Rowe 2005) may be common.
While negative cross-sex genetic correlations for fitness in
many laboratory and wild populations (Chippindale et al.
2001; Brommer et al. 2007; Foerster et al. 2007) suggest
that such sexual conflicts may be common (Arnqvist &
Rowe 2005), they have very rarely been studied in nature
(Arnqvist & Rowe 2005; Rowe & Day 2006).
Since Darwin’s (1871) suggestion that certain con-
spicuous male traits may have evolved through male–male
combat, the massive sexually selected horns of male
bighorn sheep (Ovis canadensis; figure 1) have attracted
much attention from evolutionary biologists (Geist 1966;
Fitzsimmons et al. 1995; Coltman et al. 2002, 2003, 2005;
Festa-Bianchet et al. 2004). On the other hand, the smaller
horns of females have almost never been studied and have
no clearly known fitness benefit. The presence of horns in
females could result from a genetic correlation with male
horns. Alternatively, horns may be useful to both sexes but
differ in size if they have different functions. For example,
female horns may play an important role in defence
against predators and intraspecific competition (Packer
1983; Roberts 1996).
The aim of this study was to test for the presence of
sexual conflict at loci influencing horn size and body
weight in a pedigreed population of wild bighorn sheep
studied extensively for over 35 years (Coltman et al. 2005).
Because a sexual conflict at the genetic level requires
heritable traits, we first quantified additive genetic
variance in both sexes. We then assessed the importance
of genetic constraints on the evolution of sexual dimorph-
ism by estimating intersexual genetic correlations (r
g
).
Finally, we quantified sex-specific selection using field
estimates of longevity and reproductive success. Signi-
ficant heritability in both sexes for a shared trait could lead
to sexual conflict at the genetic level if it was combined
with sexually antagonistic selection and an intersexual
r
g
O0. Conflict would also be present when selection is in
the same direction in both sexes but where r
g
!0. We
included body mass in our analyses not only to control for
the influence of body size on horn size, but also to contrast
quantitative genetic parameters and selection at traits
varying in their degree of sexual dimorphism (horn size
being much more dimorphic than body mass). This study
represents a rare test of sexual conflict at loci influencing
shared traits (Arnqvist & Rowe 2005; Rowe & Day 2006)
and provides much needed information on the importance
of genetic constraints on the evolution of sexual dimorph-
ism in nature (Rice & Chippindale 2001; Fairbairn 2007).
2. MATERIAL AND METHODS
(a) Study site and data collection
The study population inhabits Ram Mountain, Alberta,
Canada (528 N, 1158 W, elevation 1080–2170 m). Techniques
used to capture, mark, measure and monitor individuals are
Proc. R. Soc. B (2008) 275, 623–628
doi:10.1098/rspb.2007.1361
Published online 23 January 2008
One contribution of 18 to a Special Issue ‘Evolutionary dynamics of
wild populations’.
* Author for correspondence (dcoltman@ualberta.ca).
Received 7 November 2007
Accepted 28 November 2007
623 This journal is q 2008 The Royal Society
described in detail elsewhere (Jorgenson et al. 1993). The data
presented here were collected from 1970 to 2006. Briefly,
animals were captured in a corral trap baited with salt from late
May to September or early October each year. Almost all
animals were marked as lambs or yearlings, so that their exact
age was known. Individuals first captured as adults were aged
by counting horn growth rings. Marked sheep were monitored
throughout their lifetime.
Ewes and young rams are usually captured multiple times
each year, while rams 3 years and older are typically caught
only one to three times per season, usually in June or July. At
each capture, sheep are weighed to the nearest 250 g with a
Detecto spring scale. The horn length along the outside
curvature and the horn base circumference are measured to the
nearest millimetre for both horns using tape. Horn volume
(cm
3
) was calculated assuming a conical shape using the
average horn base circumference of both horns and the length
of the longest horn to reduce the influence of horn breakage.
(b) Pedigree information
The pedigree used in this study includes 764 maternal and
435 paternal links. It differs from the one in Coltman et al.
(2005) by the addition of individuals born between 2003 and
2006. Maternity was accurately determined from field
observations of suckling behaviour. Paternity was determined
using paternity test and half-sib reconstruction based on
genotypes at approximately 30 microsatellite loci for samples
collected from 1988 to 2006. The laboratory and statistical
methods are detailed in Coltman et al. (2005).
(c) Quantitative genetic analysis
Phenotypic variance in horn volume and body mass was
partitioned into additive genetic and other components using
an animal model and restricted maximum likelihood with the
program ASR
EML v. 2.0 (Gilmour et al.2006). The animal
model is a form of mixed model incorporating pedigree
information where the phenotype of each individual is
modelled as the sum of its additive genetic value and other
random and fixed effects. This method is particularly useful for
the study of natural populations because it optimizes the use of
information from complex and incomplete pedigrees when
estimating quantitative genetic parameters (Kruuk 2004).
Prior to analysis each trait for each age/sex class was
standardized to a standard deviation of unity. We then
partitioned the phenotypic variance left after taking into
account fixed effects into five components: additive genetic
(V
a
), permanent environmental (V
pe
), year (V
y
), year of birth
(V
yob
) and residual (V
r
). We also attempted to include a
maternal effect component but this often caused convergence
problems for bivariate models. Since the influence of maternal
effects for body size is known to be negligible by age 2 in the
study population (Wilson et al.2005), we decided not to
include maternal effects and to restrict our analysis to adult
sheep (2 years old and older). We also excluded animals older
than 5 years because the distribution of phenotypes in older
males is biased by trophy hunting (Coltman et al.2003;
Festa-Bianchet et al.2004) and most rams become vulnerable
to hunting at 5–7 years of age depending on their rate of horn
growth. Year and year of birth were fitted to account for the
influence of environmental variation (Postma 2006; Kruuk &
Hadfield 2007). Since different individuals were sampled at
different points within sampling seasons, we included day of
capture (continuous, second-order polynomial, with 24 May
as day 0) as a fixed effect. Since growth patterns differ between
age classes, we also fitted age (factor) and the age!date
interaction. We used bivariate models to estimate covariances
and correlations within and between the sexes. The signi-
ficance of (co)variance components was assessed using
likelihood ratio tests. Narrow sense heritability (h
2
) and other
ratios were calculated by dividing the appropriate variance
component by V
p
(e.g. V
a
/V
p
for h
2
), where V
p
ZV
a
CV
pe
C
V
y
CV
yob
CV
r
. The significance of ratios and correlations was
not explicitly tested but was instead inferred from the
significance of their associated (co)variance components.
Since a main objective of this study was to assess the
importance of genetic constraints, we also verified whether
genetic correlations were smaller than unity using likelihood
ratio tests. The number of individuals and measurements
included in the animal models are presented in table 1.
(d) Selection analysis
Our selection analyses were based on estimates of lifetime
reproductive success (LRS, number of lambs produced that
survived to weaning), longevity (in years) and mean
reproductive success (MRSZLRS!longevity
K1
). Separate
analyses were performed for males and females. We only
included animals that were born before 1996 so that every
individual had the opportunity to reach 10 years of age. For
LRS and MRS, we only included genotyped males that have
been DNA sampled and therefore included in paternity
analyses. Females that had received contraceptive implants
and individuals removed for translocations or research
purposes were excluded from the analysis. To account for
changes in density and environmental conditions, we fitted
year of birth as a factor in all models. Cohorts comprising a
single informative individual were therefore omitted (1968
and 1994 for male longevity, 1980 and 1994 for male
reproductive success and 1974 for female longevity and
reproductive success).
(a)
(b)
Figure 1. (a) Adult male and (b) female bighorn sheep from Ram Mountain, Alberta, Canada. Photos by Julien Martin.
624 J. Poissant et al. Sexual conflict in bighorn sheep
Proc. R. Soc. B (2008)
We estimated sex-specific standardized linear and quad-
ratic selection differentials and gradients using linear
regression (Lande & Arnold 1983). For phenotypic values,
we used body mass and horn volume at age 4 corrected to 5
June. These corrected values were obtained using individual
linear regressions for individuals sampled multiple times and
using mean population growth rate for individuals sampled
only once. The significance of coefficients was tested using
generalized linear models with negative binomial error for
LRS and Poisson error for longevity. For MRS, we used a
linear model with a square root transformation. Neither
quadratic nor interaction terms were statistically significant
and are therefore not shown. These analyses were performed
using S-P
LUS v. 7.0 ( Insightful).
3. RESULTS
(a) Quantitative genetic parameters
Body mass and horn volume showed significant additive
genetic variance in both sexes (table 2). The proportion of
phenotypic variance explained by additive genetic effects
after accounting for fixed effects ranged from 0.11G0.05
for female body mass (FBM) to 0.32G0.12 for male body
mass (MBM) and male horn volume (MHV; table 3). Year
and year of birth were also significant for all traits and
combined they explained 33–58% of the variation (tables
2 and 3). Finally, permanent environmental effects which
include non-additive genetic variance were also significant
for all traits and accounted for 14–27% of the variation
(tables 2 and 3).
Table 2. Additive genetic, year, year of birth and permanent environmental (co)variance components and correlations within
and between the sexes for body mass and horn volume in adult bighorn sheep. (Variance components are on the diagonal while
covariance components are below the diagonal and correlations are above the diagonal. Variance components were obtained
with sex-specific univariate animal models whereas covariances where obtained from bivariate models. Significance of
(co)variance components was tested with likelihood ratio tests.
p!0.05,

p!0.01 and

p!0.001. The significance of
genetic correlations (in italics when different from zero) was inferred from the significance of associated covariance components.
Identifies genetic correlations significantly smaller than unity (
p!0.05 and
††
p!0.01). Standard errors generated by ASREML
are also presented. MBM, male body mass; MHV, male horn volume; FBM, female body mass and FHV, female horn volume.)
MBM MHV FBM FHV
additive genetic
MBM 0.19 (0.07)

0.74 (0.15) 0.63 (0.30) 0.27 (0.30)
MHV 0.15 (0.07)
0.22 (0.09)

0.02 (0.29)
††
0.24 (0.28)
FBM 0.08 (0.04)
0.00 (0.05) 0.10 (0.04)

0.63 (0.20)
FHV 0.06 (0.06) 0.06 (0.07) 0.10 (0.05)
0.25 (0.10)

year
MBM 0.08 (0.02)

0.70 (0.11) 0.51 (0.16) 0.53 (0.15)
MHV 0.06 (0.02)

0.10 (0.03)

K0.56 (0.14) K0.23 (0.20)
FBM 0.07 (0.03)
K0.11 (0.04)
0.28 (0.08)

0.90 (0.04)
FHV 0.05 (0.02)
K0.02 (0.02) 0.19 (0.06)

0.11 (0.03)

year of birth
MBM 0.12 (0.05)

0.96 (0.04) 0.10 (0.26) 0.30 (0.25)
MHV 0.15 (0.06)

0.18 (0.06)

K0.10 (0.23) 0.37 (0.22)
FBM 0.02 (0.04) K0.03 (0.06) 0.26 (0.08)

0.93 (0.04)
FHV 0.05 (0.04) 0.08 (0.06) 0.29 (0.09)

0.26 (0.09)

permanent environment
MBM 0.12 (0.06)

0.75 (0.20) ——
MHV 0.10 (0.06)
0.14 (0.08)
——
FBM 0.13 (0.04)

0.32 (0.17)
FHV 0.06 (0.04) 0.28 (0.08)

Table 1. Phenotypic data for body mass (kg) and horn volume (cm
3
) in bighorn sheep. (Number of individuals and observations
included in the animal models are indicated as well as age-specific trait means and variation (s.d.). Each sex/age class was
standardized (s.d. of unity) prior to analysis.)
trait/age
males females
23452345
body mass
individuals 203 169 142 119 235 222 199 177
observations 502 340 237 184 703 695 609 544
mean 56.6 69.1 77.3 83.5 48.6 56.3 60.0 62.5
s.d. 10.5 10.1 10.4 10.6 7.9 7.4 7.1 7.2
horn volume
individuals 201 169 145 121 225 210 189 164
observations 498 339 240 186 620 596 526 457
mean 486.8 1133.7 1877.8 2412.6 70.6 103.2 120.0 124.9
s.d. 237.4 431.1 597.6 592.2 24.2 25.7 27.2 25.6
Sexual conflict in bighorn sheep J. Poissant et al. 625
Proc. R. Soc. B (2008)
The r
g
estimates were relatively large and significantly
positive for three pairs of traits (table 2). These included r
g
for pairs of traits within each sex (body mass versus horn
volume) and between male and FBM. On the other hand,
intersexual r
g
involving horn volume was all relatively
small and significantly smaller than unity (table 2).
With the exception of covariance between MHV and
female traits, year and year of birth appeared to affect pairs
of traits similarly (table 2). In particular, year and year of
birth correlations were close to unity for pairs of traits
within each sex. The within-sex correlation for permanent
environmental effects was close to unity in males (0.75G
0.20) and negligible in females (0.06G0.04; table 2).
(b) Selection analysis
Selection coefficients were relatively small in both sexes
(table 4). In males, none of the selection coefficients for
body mass and horn volume were significant. However,
MHV showed a non-significant trend for a negative
association with longevity after accounting for selection
on body mass (K0.11G0.06, pZ0.13; table 4). In females,
selection differentials and gradients for body mass were all
positive and significant. There was no evidence for
directional selection on female horn volume (FHV).
4. DISCUSSION
(a) Quantitative genetic parameters
Body mass and horn volume showed significant additive
genetic variance in both sexes. Quantitative genetic
parameters had previously been estimated for FBM and
male traits (Re
´
ale et al. 1999; Coltman et al. 2003, 2005;
Pelletier et al.2007) but not for female horn size.
Heritability of horn volume in females was comparable
with the male estimate (h
2
Z0.24G0.09 versus 0.32G
0.12, respectively).
Our estimates of the genetic correlation between horn
size and body mass in females were significantly smaller
than unity. This is important because it suggests that horn
volume can evolve relative to body size in that sex. In
contrast, the same genetic correlation was not significantly
smaller than unity in males (0.74G0.15, pZ0.11). This is
consistent with the results of Coltman et al.(2003, 2005)
and suggests that the evolution of horn size relative to
body mass may be more constrained in males.
One of our main goals was to evaluate the importance
of genetic constraints on the evolution of sexual dimorph-
ism in bighorn sheep. As previously shown (Coltman et al.
2003, 2005), we found that the evolution of body size
sexualdimorphismmaybeconstrainedbyalarge
intersexual r
g
(0.63G0.30). On the other hand, r
g
was
smaller than unity for many other pairs of traits, which
suggests that horn volume should be able to evolve partly
independently in each sex and that sex-specific optima
could be reached more readily (Lande 1980). In
particular, the intersexual r
g
for horn volume was quite
small (0.24G0.28) and similar to estimates reported for
other highly sexually dimorphic traits in other species (e.g.
fat deposition in humans, Comuzzie et al. 1993; antenna
length in the fly Prochyliza xanthostoma, Bonduriansky &
Rowe 2005). This is consistent with the prediction that
sexual dimorphism and intersexual r
g
should be negatively
correlated in response to sexually divergent selection
(Bonduriansky & Rowe 2005; Fairbairn & Roff 2006).
(b) Selection analysis
None of the selection coefficients differed significantly
from zero in males. However, rams with fast-growing
Table 4. Sex-specific standardized directional selection differentials (S
0
i
) and gradients (b
0
i
) for body mass and horn volume in
bighorn sheep. (Male and female data were analysed separately. Analyses were based on phenotypic values on 5 June at 4 years
old. Fitness was defined as LRS (number of lambs produced that survived to weaning over an individual’s lifetime), longevity (in
years) and mean reproductive success (MRS, LRS!longevity
K1
).) Significant coefficients ( p!0.05) are italicized.
trait fitness metric n S
0
i
p b
0
i
p
male body mass LRS 72 K0.09 (0.25) 0.68 K0.12 (0.36) 0.87
longevity 129 K0.02 (0.04) 0.72 0.04 (0.05) 0.49
MRS 72 0.03 (0.21) 0.99 K0.02 (0.29) 0.91
male horn volume LRS 72 K0.05 (0.26) 0.50 0.03 (0.38) 0.58
longevity 128 K0.08 (0.05) 0.15 K0.11 (0.06) 0.13
MRS 72 0.06 (0.21) 0.89 0.07 (0.31) 0.86
female body mass LRS 137 0.13 (0.06) !0.05 0.16 (0.07) !0.01
longevity 137 0.09 (0.04) !0.05 0.11 (0.04) !0.05
MRS 137 0.08 (0.04) !0.05 0.10 (0.05) !0.05
female horn volume LRS 133 0.06 (0.05) 0.29 0.01 (0.06) 0.97
longevity 133 0.03 (0.03) 0.39 K0.01 (0.04) 0.87
MRS 133 0.01 (0.04) 0.73 K0.02 (0.04) 0.22
Table 3. Sex-specific proportions of phenotypic variance explained by additive genetic (h
2
), year, year of birth and permanent
environmental effects. (Standard errors generated by ASR
EML are also presented. MBM, male body mass; MHV, male horn
volume; FBM, female body mass and FHV, female horn volume.)
trait h
2
year year of birth perm. env.
MBM 0.32 (0.12) 0.13 (0.04) 0.20 (0.07) 0.21 (0.11)
MHV 0.32 (0.12) 0.14 (0.04) 0.25 (0.07) 0.20 (0.11)
FBM 0.11 (0.05) 0.30 (0.06) 0.28 (0.07) 0.14 (0.04)
FHV 0.24 (0.09) 0.11 (0.03) 0.25 (0.07) 0.27 (0.08)
626 J. Poissant et al. Sexual conflict in bighorn sheep
Proc. R. Soc. B (2008)
horns are artificially selected against by trophy hunters
in the study population (Coltman et al. 2003; Festa-
Bianchet et al.2004). Each year approximately 40% of
rams with horns that satisfy the legal definition of a
harvestable ram are shot. The trend towards a negative
association between horn volume and longevity after
controlling for selection on body mass (K0.11G0.06,
pZ 0.13) probably results from hunting pressure. A
similar negative relationship between horn volume and
longevity was documented in Soay sheep where it
probably results from the cost of growing and carrying
large horns (Robinson et al.2006). In our study
population, any natural selection against large horns is
unlikely to be expressed because of trophy hunting
(Coltman et al.2003; Festa-Bianchet et al.2004). It may
also be that artificial selection more effectively targets total
horn length or morphology rather than horn volume in
bighorn sheep. For example, harvest restrictions are based
on horn length and shape, not on horn volume. Similarly,
horn length is a good correlate of mating success in rams
after accounting for age (Coltman et al. 2002). Horn
volume may reflect the metabolic costs of growing and
carrying horns, however, total horn length may be more
relevant in terms of artificial and sexual selection.
Selection differentials and gradients for body mass
were all significantly positive in females. Coltman et al.
(2005) and Pelletier et al.(2007)also observed positive
relationships between body mass in June and female
fitness. On the other hand, horn volume does not appear
to be under directional selection in females. This
contrasts with the negative association observed between
horn size and LRS in female Soay sheep (Robinson et al.
2006). It may be that female horns in bighorn sheep are
so small relative to body size that they do not incur an
easily detectable fitness cost.
In summary, we tested for intralocus sexual conflict in a
wild population of bighorn sheep by estimated quan-
titative genetic parameters and selection coefficients for
two sexually dimorphic traits. Because all traits showed
significant additive genetic variance and all genetic
correlations were positive, sexual conflicts at the genetic
level are possible in the presence of sexually antagonistic
selection. However, the absence of detectable sexually
antagonistic selection suggests that there are currently no
such conflicts.
This research was funded by the Alberta Conservation
Association, the Yukon Department of Environment, the
Natural Environment Research Council (UK), the Natural
Sciences and Engineering Research Council (NSERC,
Canada), Sustainable Resource Development (Alberta),
Alberta Ingenuity, the Charles Engelhard Foundation,
Eppley Foundation for Research, Juniper Hill, Inc., National
Geographic Society and the Tim and Karen Hixon Foun-
dation. We are grateful for the logistic support of the Alberta
Forest Services. J.P. was supported by graduate scholarships
from the University of Alberta, Alberta Ingenuity and
NSERC. A.J.W. was supported by the Natural Environment
Research Council (UK). D.W.C. is an Alberta Ingenuity
Scholar. We would like to thank Loeske Kruuk, Bill Hill,
Fanie Pelletier and anonymous referees for their comments
on previous versions of this manuscript. We also thank the
many students, colleagues, volunteers and assistants who
contributed to this research. In particular, Jon Jorgenson
provided invaluable help and expertise for over 30 years.
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... Cette différence de masse élevée à l'âge adulte suggère que la masse joue probablement un rôle plus important chez les mâles que les femelles. En revanche, le dimorphisme sexuel au niveau des cornes, bien que présent, est beaucoup plus faible (Figure 1.1), suggérant que les cornes chez les mâles n'aient pas un aussi grand rôle pour la reproduction (Côté et al. 1998a) que chez d'autres espèces d'ongulés où le dimorphisme est beaucoup plus apparent, tel que chez le mouflon d'Amérique (Poissant et al 2008). ...
... Recently, Calsbeek & Bonneaud (2008) have shown in the brown anole {Anolis sagrei), a sexually dimorphic lizard, that intralocus sexual conflict could explain the négative corrélation between the body size of sires and that of their daughters, and the positive corrélation with the body size of their sons. Poissant et al. (2008), however, did not find évidence of sexual antagonism for body mass and horn size in the highly sexually dimorphic bighorn sheep (Ovis canadensis), which would argue in this case against gender-specific paternal (or maternai) genetic effects. Therefore, additional studies are needed to investigate whether sexual conflict for shared traits is common in the wild or if such results could be confounded by other factors such as environmental conditions at birth or biased maternai investment. ...
... L'ajout de liens de parenté additionnels combiné à l'utilisation du «modèle animal » (Kruuk 2004) permettra probablement de mieux départager les effets génétiques de ceux qui ne le sont pas. Récemment, Poissant et al. (2008) ont utilisé cette approche et n'ont pas trouvé d'effets génétiques antagonistes liés au genre chez le mouflon d'Amérique pour la masse, indiquant que d'autres études approfondies devront être menées pour mieux évaluer la présence ou l'absence de « conflits sexuels » chez des gènes codant pour des traits retrouvés chez les deux genres. Chez les chèvres, il sera également intéressant d'examiner dans le futur si les femelles s'accouplant avec un mâle affichant une masse élevée produiront davantage de fils que de filles par rapport à celles s'accouplant avec des mâles de faible masse, tel que récemment démontré chez le renne (Rangifer tarandus) par R0ed et al. (2007). ...
Thesis
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Les études à long terme offrent une opportunité extraordinaire d'étudier l'écologie des espèces fauniques dans un contexte évolutif en plus d'améliorer nos connaissances pour leur gestion et leur conservation. Ici, nous avons combiné l'utilisation de marqueurs moléculaires et d'observations comportementales au suivi à long terme d'une population de chèvres de montagne (Oreamnos americanus) marquées située à Caw Ridge, en Alberta, afin d'étudier des aspects méconnus de la reproduction de cet ongulé. Plus spécifiquement, nous nous sommes intéressés à la variabilité génétique individuelle et ses effets sur les composantes biodémographiques ainsi qu'aux stratégies de reproduction des mâles en fonction de leurs caractéristiques individuelles. De plus, pour une des rares fois chez un mammifère à l'état sauvage, nous avons examiné l'effet des caractéristiques paternelles sur la qualité phénotypique des jeunes. D'abord, nous avons démontré que la variabilité génétique de cette population était faible, et ce, tant à des gènes neutres que fonctionnels, probablement dû à son historique favorisant la consanguinité. Les juvéniles affichant une diversité génétique réduite, résultant apparemment d'unions consanguines, survivaient moins bien que ceux affichant une diversité élevée, suggérant de la dépression de consanguinité. Au niveau de la reproduction des mâles, nous avons montré que l'âge, la masse et le rang social étaient tous des déterminants de l'effort reproducteur ou de l'accès aux femelles en oestrus par l'entremise de tactiques alternatives de reproduction. Ces observations ont été validées par l'étude du succès reproducteur chez les mâles établi par assignations génétiques. La masse corporelle était le facteur le plus déterminant du succès reproducteur et seulement quelques individus étaient pères de la majorité des chevreaux, confirmant un système d'accouplement hautement polygyne. Les pères de masse élevée engendraient des fils affichant une masse élevée, mais des filles de faible masse, suggérant que des gènes paternels puissent avoir des effets antagonistes selon le genre du chevreau. Nos résultats permettent de mieux comprendre le système de reproduction de la chèvre de montagne en plus d'avoir des applications en conservation et en biologie évolutive chez d'autres populations de vertébrés.
... Several studies have attributed this difference in growth between sexes to the influence of sex chromosomes in different species (Lindholm and Breden, 2002;Saether et al., 2007); however, these results lack a common pattern. In addition, increasing evidence supports the function of autosomes in SSD (Mank et al., 2007;Poissant et al., 2008). These discordances imply a complex mechanism governing SSD in fish than that proposed by current models. ...
Article
Full-text available
Certain members of the Actinopterygii class are known to exhibit sexual dimorphism (SD) that results in major phenotypic differences between male and female fishes of a species. One of the most common differences between the two sexes is in body weight, a factor with a high economic value in aquaculture. In this study, we used RNA sequencing (RNA-seq) to study the liver and brain transcriptomes of Ancherythroculter nigrocauda , a fish exhibiting SD. Females attain about fourfold body weight of males at sexual maturity. Sample clustering showed that both sexes were grouped well with their sex phenotypes. In addition, 2,395 and 457 differentially expressed genes (DEGs) were identified in the liver and brain tissues, respectively. The gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses predicted the association of PPAR signaling, cytochrome P450, and steroid hormone biosynthesis to the differences in sexual size. In addition, weighted gene co-expression network analyses (WGCNA) were conducted, and the green module was identified to be significantly correlated with sexual size dimorphism (SSD). Altogether, these results improve our understanding of the molecular mechanism underlying SSD in A. nigrocauda .
... Ceratopsian horns developed late in ontogeny and have been hypothesized to be involved in mate competition (Sampson et al., 1997). Triceratops and Centrosaurus show variation in their horns that mirrors sexual variation in many modern bovids (Poissant et al., 2008). In these bovids, males have large, highly curved horns with wide bases whose tips point back towards the skull to allow for nonlethal sparring/head-butting and withstanding associated forces. ...
Article
Despite reports of sexual dimorphism in extinct taxa, such claims in non-avian dinosaurs have been rare over the last decade and have often been criticized. Since dimorphism is widespread in sexually reproducing organisms today, under-reporting in the literature might suggest either methodological shortcomings or that this diverse group exhibited highly unusual reproductive biology. Univariate significance testing, especially for bimodality, is ineffective and prone to false negatives. Species recognition and mutual sexual selection hypotheses, therefore, may not be required to explain supposed absence of sexual dimorphism across the grade (a type II error). Instead, multiple lines of evidence support sexual selection and variation of structures consistent with secondary sexual characteristics, strongly suggesting sexual dimorphism in non-avian dinosaurs. We propose a framework for studying sexual dimorphism in fossils, focusing on likely secondary sexual traits and testing against all alternate hypotheses for variation in them using multiple lines of evidence. We use effect size statistics appropriate for low sample sizes, rather than significance testing, to analyse potential divergence of growth curves in traits and constrain estimates for dimorphism magnitude. In many cases, estimates of sexual variation can be reasonably accurate, and further developments in methods to improve sex assignments and account for intrasexual variation (e.g. mixture modelling) will improve accuracy. It is better to compare estimates for the magnitude of and support for dimorphism between datasets than to dichotomously reject or fail to reject monomorphism in a single species, enabling the study of sexual selection across phylogenies and time. We defend our approach with simulated and empirical data, including dinosaur data, showing that even simple approaches can yield fairly accurate estimates of sexual variation in many cases, allowing for comparison of species with high and low support for sexual variation.
... Compared with ornaments, relatively few studies have quantified selection acting on weapons in the wild, and results to date vary (Conner 1988;Zeh et al. 1992;Wellborn 2000;Coltmann et al. 2002;Kruuk et al. 2002;Kelly 2004;Hongo 2007;Robinson et al. 2008;Vanpé et al. 2010;LeGrice et al. 2019;reviewed in O'Brien et al. 2017), with many studies failing to detect evidence of selection on weapons in contemporary populations (e.g., Poissant et al. 2008;Kim et al. 2011;. This suggests that sexual selection on weapons is episodic, or that costs of these structures sometimes offset benefits, resulting in balancing net selection on weapon form (reviewed in O'Brien et al. 2017). ...
Article
Exaggerated weapons of sexual selection often diverge more rapidly and dramatically than other body parts, suggesting that relevant agents of selection may be discernible in contemporary populations. We examined the ecology, reproductive behavior, and strength of sexual selection on horn length in five recently diverged rhinoceros beetle (Trypoxylus dichotomus) populations that differ in relative horn size. Males with longer horns were better at winning fights in all locations, but the link between winning fights and mating success differed such that selection favored large males with long horns at the two long‐horned populations, but was relaxed or nonexistent at the populations with relatively shorter horns. Observations of local habitat conditions and breeding ecology point to shifts in the relative abundance of feeding territories as the most likely cause of population differences in selection on male weapon size in this species. Comparisons of ecological conditions and selection strength across populations offer critical first steps toward meaningfully linking mating system dynamics, selection patterns, and diversity in sexually selected traits.
... Ceratopsian horns developed late in ontogeny and have been hypothesized to be involved in mate competition (Sampson et al., 1997). Triceratops and Centrosaurus show variation in their horns that mirrors sexual variation in many modern bovids (Poissant et al., 2008). In these bovids, males have large, highly curved horns with wide bases whose tips point back towards the skull to allow for nonlethal sparring/head-butting and withstanding associated forces. ...
Article
Full-text available
Despite reports of sexual dimorphism in extinct taxa, such claims in non-avian dinosaurs have been rare over the last decade and have often been criticized. Since dimorphism is widespread in sexually reproducing organisms today, under-reporting in the literature might suggest either methodological shortcomings or that this diverse group exhibited highly unusual reproductive biology. Univariate significance testing, especially for bimodality, is ineffective and prone to false negatives. Species recognition and mutual sexual selection hypotheses, therefore, may not be required to explain supposed absence of sexual dimorphism across the grade (a type II error). Instead, multiple lines of evidence support sexual selection and variation of structures consistent with secondary sexual characteristics, strongly suggesting sexual dimorphism in non-avian dinosaurs. We propose a framework for studying sexual dimorphism in fossils, focusing on likely secondary sexual traits and testing against all alternate hypotheses for variation in them using multiple lines of evidence. We use effect size statistics appropriate for low sample sizes, rather than significance testing, to analyse potential divergence of growth curves in traits and constrain estimates for dimorphism magnitude. In many cases, estimates of sexual variation can be reasonably accurate, and further developments in methods to improve sex assignments and account for intrasexual variation (e.g. mixture modelling) will improve accuracy. It is better to compare estimates for the magnitude of and support for dimorphism between datasets than to dichotomously reject or fail to reject monomorphism in a single species, enabling the study of sexual selection across phylogenies and time. We defend our approach with simulated and empirical data, including dinosaur data, showing that even simple approaches can yield fairly accurate estimates of sexual variation in many cases, allowing for comparison of species with high and low support for sexual variation.
... In a variety of species, there is evidence for many genes across the genome with sex-biased expression and many genes with sex-specific isoforms due to differential splicing (35)(36)(37). These sex-specific isoforms can contribute to sex-specific phenotypes (21), such as pigmentation in Drosophila (Diptera: Drosophilidae) (Fallen), horn volume and body mass in bighorn sheep, facial adornments in primate species, and many other traits are classic examples of sexual dimorphism (38)(39)(40). ...
Article
Blow flies (Calliphoridae) are important medically and economically and are commonly used in forensics as temporal markers in death investigations. While phenotypic traits in adult flies can be sexually dimorphic, sex identification in immatures is difficult. Consequently, little is known about how sex may result in developmental disparities among sexes even though there are indications that they may be important in some instances. Since genetic mechanisms for sex are well studied in model flies and species of agricultural and medical importance, we exploit the sex‐specifically spliced genes transformer (tra ) and doublesex (dsx ) in the sex determination pathway to optimize a sex identification assay for immatures. Using known primer sets for tra and with a novel one for dsx , we develop PCR assays for identifying sex in four forensically relevant Calliphoridae species: Lucilia sericata (Meigen), Lucilia cuprina (Wiedemann), Cochliomyia macellaria (Fabricius), and Chrysomya rufifacies (Macquart) and evaluated their performance. Band detection rates were found to range from 71 to 100%, call rates ranged from 90 to 100%, and no error was found when bands could be called. Such information is informative for purposes of testimony and in preparation for development studies. The developed assays will assist in further differentiating sexually dimorphic differences in development of the Calliphoridae and aid in more accurately estimating insect age when age predictive markers (size, development time, molecular expression) are sexually dimorphic.
... Sexual selection favoring increased trait size in males can result in a correlated response in females that reduces female fitness (Bonduriansky & Chenoweth, 2009). There was evidence of sexually antagonistic selection on horn size in Soay sheep Ovis aries (Robinson et al., 2006) but not bighorn sheep Ovis canadensis (Poissant, Wilson, Festa-Bianchet, Hogg, & Coltman, 2008). ...
Article
Full-text available
Abstract While all models of sexual selection assume that the development and expression of enlarged secondary sexual traits are costly, males with larger ornaments or weapons generally show greater survival or longevity. These studies have mostly been performed in species with high sexual size dimorphism, subject to intense sexual selection. Here, we examined the relationships between horn growth and several survival metrics in the weakly dimorphic Pyrenean chamois (Rupicapra pyrenaica). In this unhunted population living at high density, males and females were able to grow long horns without any apparent costs in terms of longevity. However, we found a negative relationship between horn growth and survival during prime age in males. This association reduces the potential evolutionary consequences of trophy hunting in male chamois. We also found that females with long horns tended to have lower survival at old ages. Our results illustrate the contrasting conclusions that may be drawn when different survival metrics are used in analyses. The ability to detect trade‐off between the expression of male secondary sexual traits and survival may depend more on environmental conditions experienced by the population than on the strength of sexual selection.
Article
A longstanding focus in evolutionary physiology concerns the causes and consequences of variation in maintenance metabolism. Insight into this can be gained by estimating the sex-specific genetic architecture of maintenance metabolism alongside other, potentially correlated traits on which selection may also act, such as body mass and locomotor activity. This may reveal potential genetic constraints affecting the evolution of maintenance metabolism. Here, we used a half-sibling breeding design to quantify the sex-specific patterns of genetic (co)variance in standard metabolic rate (SMR), body mass, and daily locomotor activity in Drosophila melanogaster. There was detectable additive genetic variance for all traits in both sexes. As expected, SMR and body mass were strongly and positively correlated, with genetic allometry exponents (bA ± se) that were close to 2/3 in females (0.66 ± 0.16) and males (0.58 ± 0.32). There was a significant and positive genetic correlation between SMR and locomotor activity in males, suggesting that alleles that increase locomotion have pleiotropic effects on SMR. Sexual differences in the genetic architecture were largely driven by a difference in genetic variance in locomotor activity between the sexes. Overall, genetic variation was mostly shared between males and females, setting the stage for a potential intralocus sexual conflict in the face of sexually antagonistic selection.
Article
Intra- and intersexual selection drives the evolution of secondary sexual traits, leading to increased body size, trait size and generally increased reproductive success in bearers with the largest, most attractive traits. Evolutionary change through natural selection is often thought of primarily in terms of genetic changes through mutations and adaptive selection. However, this view ignores the role of the plasticity in phenotypes and behaviour and its impact on accelerating or decelerating the expression of sexually selected traits. Here, we argue that sudden changes in selection pressures (e.g. predation pressure) may cause a cascade of behavioural responses, leading to a rapid change in the size of such traits. We propose that selective removal of individuals with the most prominent traits (such as large antlers or horns in male ungulates) induces behavioural changes in the surviving males, leading to a reduction in the growth of these traits (phenotypic expression). To test this idea, we used an individual-based simulation, parametrized with empirical data of male bighorn sheep, Ovis candensis. Our model shows that the expression (phenotype, not genotype) of the trait under selection (here horn size) can be negatively impacted, if the biggest, most dominant males in the population are removed. While the selective removal of prime males opens breeding opportunities for younger, smaller males, we predicted that it would come at the expense of growth and maintenance. As predicted, we observed a rapid decline in average male horn length in our model. We argue that this decline happens because smaller males, instead of allocating energy into growth, divert this energy towards participation in mating activities that are typically exclusively available to prime males. While our model deals with ecological life-history trade-offs, it cannot predict evolutionary outcomes. However, this nongenetic mechanism is important for the understanding of evolutionary processes because it describes how heritable traits can rapidly change because of behavioural plasticity, long before any genetic changes might be detectable.
Article
Full-text available
Sexual selection favours traits that increase reproductive success via increased competitive ability, attractiveness, or both. Male rhesus macaque (Macaca mulatta) morphological traits are likely to reflect the effects of multiple sexual selection pressures. Here, we use a quantitative genetic approach to investigate the production and maintenance of variation in male rhesus macaque morphometric traits which may be subject to sexual selection. We collected measurements of body size, canine length, and fat, from 125 male and 21 female free-ranging rhesus macaques on Cayo Santiago. We also collected testis volumes from males. We used a genetic pedigree to calculate trait heritability, to investigate potential trait trade-offs, and to estimate selection gradients. We found that variation in most male morphometric traits was heritable, but found no evidence of trait trade-offs nor that traits predicted reproductive success. Our results suggest that male rhesus macaque morphometric traits are either not under selection, or are under mechanisms of sexual selection that we could not test (e.g. balancing selection). In species subject to complex interacting mechanisms of selection, measures of body size, weaponry, and testis volume may not increase reproductive success via easily-testable mechanisms such as linear directional selection.
Book
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This book is an edited compendium of twenty chapters addressing the evolution, adaptive significance, and genetic and developmental basis of differences between the sexes in body size and morphology. General concepts and methodologies are introduced in Chapter 1, which also includes an overview of variation in sexual size dimorphism (SSD) with emphasis on extreme dimorphisms (i.e., dwarf males) and taxa not covered in subsequent chapters. Chapters 2-7 present new, comprehensive, comparative analyses of broad-scale patterns of SSD in mammals, birds, reptiles, amphibians, spiders, and insects, respectively. Chapters 8-15 comprise case studies of SSD within species or groups of closely related species. Flowering plants, insects, lizards, birds, and mammals are represented in this section. Chapters 16-20 emphasize proximate mechanisms underlying SSD and include theoretical explorations of anisogamy, genomic conflict, genomic imprinting, sex-linkage, and sex-specific gene expression, as well as experimental studies of sex-specific patterns of growth and development. Throughout the book, the emphasis is on testing hypotheses concerning the evolution and adaptive significance of SSD, and the importance of sexual selection on male size emerges as a common theme. However, this adaptationist approach is balanced by studies of proximate genetic, developmental, and physiological processes.
Book
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The past decade has seen a profound change in the scientific understanding of reproduction. The traditional view of reproduction as a joint venture undertaken by two individuals, aimed at replicating their common genome, is being challenged by a growing body of evidence showing that the evolutionary interests of interacting males and females diverge. This book demonstrates that, despite a shared genome, conflicts between interacting males and females are ubiquitous, and that selection in the two sexes is continuously pulling this genome in opposite directions. These conflicts drive the evolution of a great variety of those traits that distinguish the sexes and also contribute to the diversification of lineages. Göran Arnqvist and Locke Rowe present an array of evidence for sexual conflict throughout nature, and they set these conflicts into the well-established theoretical framework of sexual selection. The recognition of conflict between the sexes is transforming our theories for the evolution of mating systems and the sexes themselves. Written by two top researchers in the field, Sexual Conflict is the first book to describe this transformation. It is a must read for all scholars and students interested in the evolutionary biology of reproduction.
Article
In the current resurgence of interest in the biological basis of animal behavior and social organization, the ideas and questions pursued by Charles Darwin remain fresh and insightful. This is especially true of The Descent of Man and Selection in Relation to Sex, Darwin's second most important work. This edition is a facsimile reprint of the first printing of the first edition (1871), not previously available in paperback. The work is divided into two parts. Part One marshals behavioral and morphological evidence to argue that humans evolved from other animals. Darwin shoes that human mental and emotional capacities, far from making human beings unique, are evidence of an animal origin and evolutionary development. Part Two is an extended discussion of the differences between the sexes of many species and how they arose as a result of selection. Here Darwin lays the foundation for much contemporary research by arguing that many characteristics of animals have evolved not in response to the selective pressures exerted by their physical and biological environment, but rather to confer an advantage in sexual competition. These two themes are drawn together in two final chapters on the role of sexual selection in humans. In their Introduction, Professors Bonner and May discuss the place of The Descent in its own time and relation to current work in biology and other disciplines.
Article
The evolution of sexual dimorphism in quantitative characters under natural and sexual selection acting differently on the sexes is analyzed using population genetics models. The effects of genes when in males may be correlated with their effects when in females, producing correlated selective responses between the sexes, so that male and female phenotypes cannot evolve independently But under weak natural selection with constant relative fitnesses, in the absence of sexual selection, the joint evolution of the mean phenotypes of the sexes increases the mean fitness in a population, and if there is genetic variation for sexual dimorphism each sex eventually achieves a locally optimum phenotype. With sexual selection, fitnesses are generally frequency-dependent and evolution of the mean phenotypes does not maximize the mean fitness. Provided individual fitnesses exist, at equilibrium natural and sexual selection balance in each sex. A moderate intensity of sexual selection acting on a character under weak natural selection toward an intermediate optimum phenotype can produce a large deviation of the mean phenotype from the optimum and a substantial decrease of the mean fitness in a population, increasing the probability of extinction. When homologous characters in males and females vary similarly and are highly correlated genetically, the rate of evolution of sexual dimorphism may be one or more orders of magnitude slower than that for the average phenotype in a population. Methods for partitioning sexual dimorphism into contributions from natural and sexual selection are discussed, and genetic experiments are suggested for testing the involvement of non-equilibrium correlated selective responses between the sexes in observed cases of sexual dimorphism.
Article
A variance decomposition analysis using maximum likelihood methods was employed to examine the genetic architecture of sexual dimorphism in anthropometric traits in a large pedigreed sample of Mexican American individuals from San Antonio, Texas. For this analysis the magnitude of sexual dimorphism was viewed as arising from a special case of genotype by environment interaction (G × E), that of genotype by sex (G × S). Evidence indicates a marked G × S interaction for 9 of the 12 traits examined and 1 of the 4 indices, findings which are interpreted as indicators of a strong genetic component to the degree of sexual dimorphism expressed in these traits. Such results have important implications for the use and interpretation of these traits in an epidemiological as well as an evolutionary context. © 1993 Wiley-Liss, Inc.
Article
The hypothesis was tested, that horns of male mountain sheep evolved as display organs. Seven predictions were made from this hypothesis, of which six were investigated and verified. These are: Rams have a distinct horn display behavior; rams can distinguish horn sizes; large horned individuals dominated smaller horned rams irrespective of the latter's age; horn size conveyed a priori dominance to its bearer; large horned rams enjoyed a reproductive advantage. The prediction, that loss of much horn leads to a loss in dominance was not investigated. The above hypothesis in conjunction with the observation that breeding rams show higher mortality than nonbreeding rams, predicted that rams with good horn growth die younger than rams with poor horn growth. The prediction was verified. The horns of rams are not only important as weapons and shields in combat, but also as the major dominance-rank determinance, and as visual dominance-rank symbols.
Article
Bighorn sheep (Ovis canadensis) are restricted in distribution and numbers relative to presettlement conditions. Some populations have alledgedly suffered losses of fitness resulting from small, insular populations and a breeding system that reduces effective population size. Large horns in rams, which confer breeding superiority, are absent from some populations, and this absence may result in part from loss of genetic variability. We investigated the relationship among allozyme variability, population history, and horn growth in bighorn sheep from the Rocky Mountains. Heterozygosity was higher for bighorn sheep than has been reported for Dall sheep (O. dalli). Heterozygosity and allelic variability were marginally related to effective population size for the proceeding 15 years. Horn growth was significantly higher in more heterozygous than in less heterozygous rams for years 6, 7, and 8 of life. By the end of year 8, more heterozygous rams had 13% higher horn volumes than less heterozygous rams. Mos