Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.     

Sexual dimorphism of head morphology in three‐spined stickleback Gasterosteus aculeatus

This study examined sexual dimorphism of head morphology in the ecologically diverse three‐spined stickleback Gasterosteus aculeatus. Male G. aculeatus had longer heads than female G. aculeatus in all 10 anadromous, stream and lake populations examined, and head length growth rates were significantly higher in males in half of the populations sampled, indicating that differences in head size increased with body size in many populations. Despite consistently larger heads in males, there was significant variation in size‐adjusted head length among populations, suggesting that the relationship between head length and body length was flexible. Inter‐population differences in head length were correlated between sexes, thus population‐level factors influenced head length in both sexes despite the sexual dimorphism present. Head shape variation between lake and anadromous populations was greater than that between sexes. The common divergence in head shape between sexes across populations was about twice as important as the sexual dimorphism unique to each population. Finally, much of the sexual dimorphism in head length was due to divergence in the anterior region of the head, where the primary trophic structures were found. It is unclear whether the sexual dimorphism was due to natural selection for niche divergence between sexes or sexual selection. This study improves knowledge of the magnitude, growth rate divergence, inter‐population variation and location of sexual dimorphism in G. aculeatus head morphology.     

Environmental effects on sexual size dimorphism of a seed-feeding beetle

Sexual size dimorphism is widespread in animals but varies considerably among species and among populations within species. Much of this variation is assumed to be due to variance in selection on males versus females. However, environmental variables could affect the development of females and males differently, generating variation in dimorphism. Here we use a factorial experimental design to simultaneously examine the effects of rearing host and temperature on sexual dimorphism of the seed beetle, Callosobruchus maculatus. We found that the sexes differed in phenotypic plasticity of body size in response to rearing temperature but not rearing host, creating substantial temperature-induced variation in sexual dimorphism; females were larger than males at all temperatures, but the degree of this dimorphism was smallest at the lowest temperature. This change in dimorphism was due to a gender difference in the effect of temperature on growth rate and not due to sexual differences in plasticity of development time. Furthermore, the sex ratio (proportion males) decreased with decreasing temperature and became female-biased at the lowest temperature. This suggests that the temperature-induced change in dimorphism is potentially due to a change in non-random larval mortality of males versus females. This most important implication of this study is that rearing temperature can generate considerable intraspecific variation in the degree of sexual size dimorphism, though most studies assume that dimorphism varies little within species. Future studies should focus on whether sexual differences in phenotypic plasticity of body size are a consequence of adaptive canalization of one sex against environmental variation in temperature or whether they simply reflect a consequence of non-adaptive developmental differences between males and females.     

Factors influencing the induction of diapausing egg production in the calanoid copepod Diaptomus leptopus

Field and laboratory studies were carried out between 1995 and 1997 on four populations of Diaptomus leptopus found in seasonally temporary, occasionally temporary, and a permanent environment to assess the relative importance of photoperiod and temperature regimes versus other proximate local cues in inducing diapause egg production. Patterns of diapausing and subitaneous egg production were determined by observation of individual females bearing clutches that were produced in the field. A laboratory common-garden experiment was performed to assess the effects of four different regimes of temperature and photoperiod on the induction of diapause. Patterns of diapausing egg production differed among ponds: diapause occurred early in the seasonally temporary environment and occurred rarely in the permanent environment. In the common-garden experiment, populations exhibited substantial changes in the onset of diapause when compared to patterns found under field conditions. These results provide indirect evidence that the different populations respond to available cues of environmental change in different ways in nature, or that environmental cues vary among habitats.     

From Local Adaptation to Ecological Speciation in Copepod Populations from Neighboring Lakes

Continental copepods have been derived from several independent invasive events from the sea, but the subsequent evolutionary processes that account for the current diversity in lacustrine environments are virtually unknown. Salinity is highly variable among lakes and constitutes a source of divergent selection driving potential reproductive isolation. We studied four populations of the calanoid copepod Leptodiaptomus cf. sicilis inhabiting four neighboring lakes with a common history (since the Late Pleistocene) located in the Oriental Basin, Mexico; one lake is shallow and varies in salinity periodically (1.4–10 g L^-1), while three are deep and permanent, with constant salinity (0.5, 1.1 and 6.5 g L^-1, respectively). We hypothesized that (1) these populations belong to a different species than L. sicilis sensu stricto and (2) are experiencing ecologically based divergence due to salinity differences. We assessed morphological and molecular (mtDNA) COI variation, as well as fitness differences and tests of reproductive isolation. Although relationships of the Mexican populations with L. sicilis s.s. could not be elucidated, we identified a clear pattern of divergent selection driven by salinity conditions. The four populations can still be considered a single biological species (sexual recognition and hybridization are still possible in laboratory conditions), but they have diverged into at least three different phenotypes: two locally adapted, specialized in the lakes of constant salinity (saline vs. freshwater), and an intermediate generalist phenotype inhabiting the temporary lake with fluctuating salinity. The specialized phenotypes are poorly suited as migrants, so prezygotic isolation due to immigrant inviability is highly probable. This implication was supported by molecular evidence that showed restricted gene flow, persistence of founder events, and a pattern of allopatric fragmentation. This study showed how ecologically based divergent selection may explain diversification patterns in lacustrine copepods.     

Geographic variation in body size, sexual size dimorphism and fitness components of a seed beetle: local adaptation versus phenotypic plasticity

Variation in body size, growth and life history traits of ectotherms along latitudinal and altitudinal clines is generally assumed to represent adaptation to local environmental conditions, especially adaptation to temperature. However, the degree to which variation along these clines is due to adaptation vs plasticity remains poorly understood. In addition, geographic patterns often differ between females and males – e.g. sexual dimorphism varies along latitudinal clines, but the extent to which these sex differences are due to genetic differences between sexes vs sex differences in plasticity is poorly understood. We use common garden experiments (beetles reared at 24, 30 and 36°C) to quantify the relative contribution of genetically‐based differentiation among populations vs phenotypic plasticity to variation in body size and other traits among six populations of the seed‐feeding beetle Stator limbatus collected from various altitudes in Arizona, USA. We found that temperature induces substantial plasticity in survivorship, body size and female lifetime fecundity, indicating that developmental temperature significantly affects growth and life history traits of S. limbatus. We also detected genetic differences among populations for body size and fecundity, and genetic differences among populations in thermal reaction norms, but the altitude of origin (and hence mean temperature) does not appear to explain these genetic differences. This and other recent studies suggest that temperature is not the major environmental factor that generates geographic variation in traits of this species. In addition, though there was no overall difference in plasticity of body size between males and females (when averaged across populations), we did find that the degree to which dimorphism changed with temperature varied among populations. Consequently, future studies should be extremely cautious when using only a few study populations to examine environmental effects on sexual dimorphism.     

Variation in sexual size dimorphism among populations: testing the differential‐plasticity hypothesis

Sexual size dimorphism (SSD) is a common phenomenon in animals and varies widely among species and among populations within species. Much of this variation is likely due to variance in selection on females vs. males. However, environmental variables could have different effects on females vs. males, causing variation in dimorphism. In this study, we test the differential‐plasticity hypothesis, stating that sex‐differential plasticity to environmental variables generates among‐population variation in the degree of sexual dimorphism. We examined the effect of temperature (22, 25, 28, and 31 °C) on sexual dimorphism in four populations of the cockroach Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), collected at various latitudes. We found that females were larger than males at all temperatures and the degree of this dimorphism was largest at the highest temperature (31 °C) and smallest at the lowest temperature (22 °C). There is variation in the degree of SSD among populations (sex*population interaction), but differences between the sexes in their plastic responses (sex*temperature interaction) were not observed for body size. Our results indicated that sex‐differential plasticity to temperature was not the cause of differences among populations in the degree of sexual dimorphism in body size.     

The role of fecundity and sexual selection in the evolution of size and sexual size dimorphism in New World and Old World voles (Rodentia: Arvicolinae)

Evolutionary ecologists dating back to Darwin (1871) have sought to understand why males are larger than females in some species, and why females are the larger sex in others. Although the former is widespread in mammals, rodents and other small mammals usually exhibit low levels of sexual size dimorphism (SSD). Here, we investigate patterns of sexual dimorphism in 34 vole species belonging to the subfamily Arvicolinae in a phylogenetic comparative framework. We address the potential role of sexual selection and fecundity selection in creating sex differences in body size. No support was found for hyperallometric scaling of male body size to female body size. We observed a marginally significant relationship between SSD and the ratio of male to female home range size, with the latter being positively related to the level of intrasexual competition for mates. This suggests that sexual selection favours larger males. Interestingly, we also found that habitat type, but not mating system, constitutes a strong predictor of SSD. Species inhabiting open habitats – where males have extensive home ranges in order to gain access to as many females as possible – exhibit a higher mean dimorphism than species inhabiting closed habitats, where females show strong territoriality and an uniform distribution preventing males to adopt a territorial strategy for gaining copulations. Nonetheless, variation in the strength of sexual selection is not the only selective force shaping SSD in voles; we also found a positive association between female size and litter size across lineages. Assuming this relationship also exists within lineages (i.e. fecundity selection on female size), this suggests an additional role for variation in the strength of fecundity selection shaping interspecific differences in female size, and indirectly in SSD. Therefore our results suggest that different selective processes act on the sizes of males and females, but because larger size is favoured in both sexes, SSD is on average relatively small.     

Genetic variation for sexual dimorphism in flower size within and between populations of gynodioecious Thymus vulgaris

There has been very little empirical study of quantitative genetic variation in flower size in sexually dimorphic plant species, despite the frequent occurrence of flower size differences between sexual phenotypes. In this study we quantify the nature of quantitative flower size variation in females and hermaphrodites of gynodioecious Thymus vulgaris. In a field study, females had significantly smaller flowers than hermaphrodites, and the degree of flower size dimorphism varied significantly among populations. To quantify the genetic basis of flower size variation we sampled maternal progeny from 10 F₀ females in three populations (across the range of variation in flower size in the field), performed controlled crosses on F₁ offspring in the glasshouse and grew F₂ progeny to flowering in uniform field conditions. A significant population * sex interaction was again observed, hence the degree of sexual dimorphism shows genetic variation among populations. A significant family * sex interaction was also observed, indicating that the degree of sexual dimorphism shows genetic variation among families. Females showed significantly greater variation among populations and among families than hermaphrodites. Female flower size varied significantly depending on the degree of stamen abortion, with morphologically intermediate females having flowers more similar to hermaphrodites than to other females. The frequency of female types that differ in the degree of stamen abortion varied among populations and families and mean family female flower size increased as the proportion of intermediate female types increased across families. Variation in the degree of flower size dimorphism thus appears to be a result of variation in the degree of stamen abortion in females, the potential causes of which are discussed.     

Female‐biased dimorphism in size and age at maturity is reduced at higher latitudes in lake whitefish Coregonus clupeaformis

Female‐biased sexual dimorphism in size at maturity is a common pattern observed in freshwater fishes with indeterminate growth, yet can vary in magnitude among populations for reasons that are not well understood. According to sex‐specific optimization models, female‐biased sexual size dimorphism can evolve due to sexual selection favouring earlier maturation by males, even when sexes are otherwise similar in their growth and mortality regimes. The magnitude of sexual size dimorphism is expected to depend on mortality rate. When mortality rates are low, both males and females are expected to mature at older ages and larger sizes, with size determined by the von Bertalanffy growth equation. The difference between size at maturity in males and females becomes reduced when maturing at older ages, closer to asymptotic size. This phenomenon is called von Bertalanffy buffering. The predicted relationship between the magnitude of female‐biased sexual dimorphism in age and size at maturity and mortality rate was tested in a comparative analysis of lake whitefish Coregonus clupeaformis from 26 populations across a broad latitudinal range in North America. Most C. clupeaformis populations displayed female‐biased sexual dimorphism in size and age at 50% maturity. As predicted, female‐biased sexual size dimorphism was less extreme among lower mortality, high‐latitude populations.     

When do meadow vipers (Vipera ursinii) become sexually dimorphic? – ontogenetic patterns of sexual size dimorphisms

Contrary to an increasing number of papers that document sexual dimorphism in size (and/or shape) in adults, studies dealing with sex differences in newborn and juvenile snakes are surprisingly scarce. Data about ontogenetic shifts in sexual dimorphism are generally lacking and hence, it is unclear whether sex differences are set at birth or arise post‐natally. In this study, we analyzed patterns of sexual dimorphism in body size, head dimensions and tail length (TL) among newborn, subadult and adult meadow vipers (Vipera ursinii) from the Bjelasica Mt. in Montenegro. Patterns of sexual size dimorphisms differed among traits. There was no significant difference in head dimension of males and females, but adult snakes were sexually dimorphic in body size. Sexual differences in TL were evident since birth but changed in degree throughout ontogeny. Neonate meadow vipers presented highly significant inter‐litter variation in the sexual dimorphism of all traits we have measured. Such family effects may have an important influence on extent of inter‐sexual differences in snakes and should be included in analyses of sexual dimorphism.     

Morphological differences between an artificial lentic and adjacent lotic environments in a characid species

Dam constructions cause fundamental changes in the natural landscape, creating new ecological and evolutionary challenges for aquatic organisms. In some cases, such water impoundments have been related with morphological changes in organisms. Understanding how populations respond to rapid environmental changes imposed by dams is the first step to elucidate the consequences that disturbed habitats may have on species evolution. In this work, we analyzed shape and size variation in Bryconamericus iheringii Boulenger 1887 from the Chasqueiro stream basin, south of Brazil, which was recently dammed. We used linear measurements and geometric morphometrics to identify morphological differences among specimens from the reservoir (lentic habitat) compared to the habitat upstream and downstream of the dam (lotic habitats). We also tested for size- and shape-related sexual dimorphism to determine whether variations observed were the same for both sexes. We found that B. iheringii from the artificial reservoir were distinct in shape and size to those from their natural habitat in the stream. The size variation between environments was the same for both sexes, but the shape variation differed between males and females. Regarding the linear measurements, lotic populations were larger (greater body length, width, pectoral fin base length and caudal peduncle length), probably in response to increased swimming activity. Regarding body shape, we found that both sexes have a more fusiform body in lotic habitats than in the reservoir. In addition, females showed an altered mouth position that was distinct between these environments. This work indicates that the water reservoir seems to be an important factor influencing morphological variation in B. iheringii, a species with sexual shape dimorphism.

     

Sexual dimorphism in the size and shape of the os coxae and the effects of microevolutionary processes

Sexual dimorphism in the human pelvis has been studied widely for forensic purposes, but it is still unclear to what extent it varies among human populations. There is evidence that microevolutionary processes, both neutral (i.e., population history) and selective (e.g., thermoregulatory adaptation and size‐related obstetrical constraints) contribute to explain pelvic variation among populations, but the extent to which these factors affect pelvic sexual dimorphism is unknown. In this study, I analyze sexual dimorphism of the os coxae in 20 globally distributed human populations, using 3D morphometric data to separate the size and shape components of sexual differences. After evaluating population differences in the degree and pattern of sexual dimorphism, I test for the effect of population history, climate, and body size in shaping global diversity. The results show that size and shape dimorphism follow different patterns. Coxal size dimorphism is generally quite consistent through populations, with males bigger than females, but it appears to be reduced in small‐bodied populations, possibly in relation to obstetrically‐related selective pressures for a spacious birth canal. Beyond a general species‐wide pattern of shape dimorphism, commonly used for forensic sex determination, other aspects of sexual differences in coxal shape vary among human populations, reflecting the effects of neutral demographic processes and climatic adaptation. Am J Phys Anthropol 153:167–177, 2014. © 2013 Wiley Periodicals, Inc.     

Phenotypic plasticity and sexual dimorphism in size at post-juvenile metamorphosis: common-garden rearing of an intertidal gastropod with determinate growth

Proximate factors of the intraspecific variation in molluscan shell morphology have long received attention in biology. The intertidal gastropod Monetaria annulus (Mollusca; Gastropoda; Cypraeidae) is particularly suitable for the study of variation in body size, because this species is a determinate grower in the sense that soft-body size shows no further increase after the juvenile stage. Cross-sectional field surveys on post-juvenile individuals have indicated that the mean body size varies widely among populations and is larger in females than in males within populations. To examine whether these patterns are due to genetic differences, we conducted a common-garden rearing experiment with juvenile individuals collected from two populations on Okinawa Island. After adjusting for among-individual differences in initial degree of development, statistical analyses revealed that this species exhibits female-biased sexual size dimorphism mediated by a longer development time rather than by faster growth rates in females. Although wild individuals show a remarkable size difference between populations, no size difference was found between the populations in the individuals reared in a common-garden condition. This result suggests that the among-population size difference does not have a genetic basis and is caused by phenotypic plasticity based on environmental heterogeneity among habitats.     

Ecology of Diaptomus leptopus s. a. Forbes 1882 (Copepoda: Calanoida) under temporary pond conditions

An ecological study of the calanoid copepod Diaptomus leptopus under temporary pond conditions was undertaken from March 4 to July 24, 1970, in Windsor, Ontario, Canada. Twice weekly samples were taken from a single station in the pond and among the more important physical-chemical parameters measured were: temperature, precipitation, depth, dissolved oxygen, total alkalinity, total hardness and pH. Initially the desmid Closterium sp. was the dominant phytoplankter but subsequent to May 26, the diatoms, mainly Navicula sp. were most abundant. The dominant zooplankters were the copepods Diaptomus leptopus, Cylcops bicuspidatus thomasi and the cladocerans Daphnia schødleri, Daphnia pulex and Moina recticostris. A positive linear correlation, which was barely significant at the 5% level (r = 0.539) was found between total calanoida and total cladocera. Clutch size in Diaptomus leptopus appeared to decrease with increasing water temperature. A tendency in this species towards suppression of the reproductive activity was associated with increases in the total cladoceran population. Overall metasomal lengths of adult males were less than those reported in the literature. Possible mechanisms of overwintering of this species in the absence of winter eggs is discussed briefly.     

Sexual variation in assimilation efficiency: its link to phenotype and potential role in sexual dimorphism

Sex-specific variation in morphology (sexual dimorphism) is a prevalent phenomenon among animals, and both dietary intake and resource allocation strategies influence sexually dimorphic traits (e.g., body size or composition). However, we investigated whether assimilation efficiency (AE), an intermediate step between dietary intake and allocation, can also vary between the sexes. Specifically, we tested whether sex-based differences in AE can explain variation in phenotypic traits. We measured morphometric characteristics (i.e., body length, mass, condition, and musculature) and AE of total energy, crude protein, and crude fat in post-reproductive adult Children’s pythons (which exhibit a limited female-biased sexual size dimorphism) fed both low and high dietary intakes. Meal size was negatively related to AE of energy. Notably, male snakes absorbed crude protein more efficiently and increased epaxial (dorsal) musculature faster than females, which demonstrates a link between AE and phenotype. However, females grew in body length faster but did not absorb any nutrient more efficiently than males. Although our results do not provide a direct link between AE and sexual size dimorphism, they demonstrate that sexual variation in nutrient absorption exists and can contribute to other types of sex-based differences in phenotype (i.e., sexual dimorphism in growth of musculature). Hence, testing the broader applicability of AE’s role in sexually dimorphic traits among other species is warranted.     

A role for ecology in the evolution of colour variation and sexual dimorphism in Hawaiian damselflies

Variation in traits that are sexually dimorphic is usually attributed to sexual selection, in part because the influence of ecological differences between sexes can be difficult to identify. Sex‐limited dimorphisms, however, provide an opportunity to test ecological selection disentangled from reproductive differences between the sexes. Here, we test the hypothesis that ecological differences play a role in the evolution of body colour variation within and between sexes in a radiation of endemic Hawaiian damselflies. We analysed 17 Megalagrion damselflies species in a phylogenetic linear regression, including three newly discovered cases of species with female‐limited dimorphism. We find that rapid colour evolution during the radiation has resulted in no phylogenetic signal for most colour and habitat traits. However, a single ecological variable, exposure to solar radiation (as measured by canopy cover) significantly predicts body colour variation within sexes (female‐limited dimorphism), between sexes (sexual dimorphism), and among populations and species. Surprisingly, the degree of sexual dimorphism in body colour is also positively correlated with the degree of habitat differences between sexes. Specifically, redder colouration is associated with more exposure to solar radiation, both within and between species. We discuss potential functions of the pigmentation, including antioxidant properties that would explain the association with light (specifically UV) exposure, and consider alternative mechanisms that may drive these patterns of sexual dimorphism and colour variation.     

Variation in guenon skulls (II): sexual dimorphism

Patterns of size and shape sexual dimorphism in adult guenons were examined using a large sample of skulls from almost all living species. Within species, sexual dimorphism in skull shape follows the direction of size-related shape variation of adults, is proportional to differences in size, and tends to be larger in large-bodied species. Interspecific divergence among shape trajectories, which explain within species sex differences, are small (i.e., trajectories of most species are nearly parallel). Thus, changes in relative proportions of skull regions that account for the distinctive shape of females and males are relatively conserved across species, and their magnitude largely depends on differences in size between sexes. A conservative pattern of size-related sexual dimorphism and a model of interspecific divergence in shape which strongly reflects size differences suggest a major role of size and size-related shape variation in the guenon radiation. It is possible that in the guenons, as in the neotropical primates (with whom they have obvious parallels), size has helped to determine morphological change along lines of least evolutionary resistance, influencing sexual dimorphism. In Miopithecus and Erythrocebus, the smallest and largest guenon genera, it is likely that the interaction of ecology and size contributes significantly to patterns of sexual dimorphism. The results of this study thus emphasise the need to consider allometry and size alongside ecology and behaviour when examining primate sexual dimorphism.     

Female‐biased size dimorphism in a diapausing caddisfly,Mesophylax aspersus: effect of fecundity and natural and sexual selection

1. The effect of mating success, female fecundity and survival probability associated with intra‐sex variation in body size was studied in Mesophylax aspersus, a caddisfly species with female‐biased sexual size dimorphism, which inhabits temporary streams and aestivates in caves. Adults of this species do not feed and females have to mature eggs during aestivation. 2. Thus, females of larger size should have a fitness advantage because they can harbour more energy reserves that could influence fecundity and probability of survival until reproduction. In contrast, males of smaller size might have competitive advantages over others in mating success. 3. These hypotheses were tested by comparing the sex ratio and body size of individuals captured before and after the aestivation period. The associations between body size and female fecundity, and between mating success and body size of males, were explored under laboratory conditions. 4. During the aestivation period, the sex ratio changed from 1 : 1 to male biased (4 : 1), and a directional selection on body size was detected for females but not for males. Moreover, larger clutches were laid by females of larger size. Finally, differences in mating success between small and large males were not detected. These results suggest that natural selection (i.e. the differential mortality of females associated with body size) together with possible fecundity advantages, are important factors responsible of the sexual size dimorphism of M. aspersus. 5. These results highlight the importance of taking into account mechanisms other than those traditionally used to explain sexual dimorphism. Natural selection acting on sources of variation, such as survival, may be as important as fecundity and sexual selection in driving the evolution of sexual size dimorphism.     

Causes of sex‐biased adult survival in ungulates: sexual size dimorphism,mating tactic or environment harshness?

Using both a conventional and a phylogenetic approach, we tested whether sexual size dimorphism, mating tactic and environmental conditions influenced the between-sex differences in adult survival among 26 populations of polygynous ungulates. As a general rule, male survival was both lower and more variable among species than female survival. Whatever the method we used, sexual size dimorphism had no direct influence on male-biased mortality. In food-limited environments, the survival of males relative to that of females was lower than in good environments, suggesting a cost of large size for males facing harsh conditions. On the other hand, the survival of males relative to that of females tended to increase with sexual size dimorphism in good environments, indicating that large size may be profitable for males facing favourable conditions. Lastly, we found that the between-sex differences in adult survival did not vary with sexual size dimorphism in harem-holding or tending species, but tended to increase with sexual size dimorphism in territorial species. Our analyses indicate that sexual size dimorphism does not lead directly to a decrease in male survival compared to that of females. Thus, environmental conditions rather than the species considered could shape between-sex differences in adult survival observed in ungulate populations.