Female-biased Size Dimorphism Research Articles

Does female control and male mating system predict courtship investment and mating outcomes? A comparative study in five widow spider species (genus Latrodectus) tested under similar laboratory conditions

BackgroundMale courtship investment may evolve in response to the male’s expectation of future mating opportunities or the degree of female control during mating interactions. We used a comparative approach to test this hypotheses by assessing the courtship and mating behaviors of five widow spider species (genus Latrodectus) under common laboratory conditions. We predicted male investment in courtship would be higher in species where males mate only once because of high cannibalism rates (monogyny, L. geometricus, L. hasselti, L. mirabilis), compared to species with rare cannibalism (L. mactans, L. hesperus) in which males should reserve energy for future mating opportunities. Increased male investment, measured as courtship duration, might also evolve with increased female control over mating outcomes if females prefer longer courtships. We tested this by assessing the frequency of copulations, timing of sexual cannibalism, and the degree of female-biased size dimorphism, which is expected to be negatively correlated with the energetic cost of rebuffing male mating attempts.ResultsCopulation frequency was consistently lower in species with extreme female-skewed size dimorphism, and where sexual cannibalism was more prevalent, suggesting the importance of female control for mating outcomes. We confirmed significant interspecific variation in average courtship duration, but contrary to predictions, it was not predicted by male mating system, and there was no consistent link between courtship duration and sexual size dimorphism.ConclusionWe show that the degree of sexual dimorphism is not only correlated with sexual cannibalism, but also with mating success since restriction of male copulation frequency by female Latrodectus affects paternity. However, predictions about male mating system or female control affecting courtship duration were not supported. We propose that the form of female control over mating and cannibalism, and male responses, might be more informative for understanding the evolution of courtship duration. For example, male tactics to avoid female aggression may drive lower courtship duration in species like L. mirabilis. Nonetheless, our results differ from inferences based on published studies of each species in isolation, illuminating the need for standardized data collection for behavioural comparative studies.

Causes and consequences of pre-hibernation body mass in golden-mantled ground squirrels (Callospermophilus lateralis)

Abstract Mammals that cope with harsh environmental conditions by hibernating show seasonal variation in body mass, as fat reserves are accumulated during summer and depleted during winter. In hibernating species, pre-hibernation mass is often considered a key phenotypic trait that can influence future survival and reproductive success—yet we know little about how variation in environmental conditions and seasonality affect pre-hibernation mass. We used 30 years of demographic data from a high-elevation population of golden-mantled ground squirrels (Callospermophilus lateralis) to identify life history and environmental variables that influenced pre-hibernation mass in females. Additionally, we determined if pre-hibernation mass influenced overwinter survival or subsequent reproduction. Growth curves revealed female-biased size dimorphism in body mass; adult mass was not attained until age 2 years, so we considered 3 age classes of females: juveniles, yearlings, and adults (≥2 years). We found that earlier snow melt during spring is associated with higher subsequent pre-hibernation body mass for yearlings and adults, and that juveniles emerging earlier from their natal burrows had higher pre-hibernation masses than later-emerging juveniles. Estimates of food quantity or quality did not affect pre-hibernation mass for any age class, nor was there an effect of reproductive status. Juvenile females with greater pre-hibernation mass were more likely to survive overwinter and reproduce as yearlings, but we found no such effect for yearlings or adult females. We did not find an effect of snow depth or winter duration on either survival or reproduction. Our results indicate that time appears to be the key variable for golden-mantled ground squirrels to acquire sufficient body mass to survive and reproduce, but the effects vary by age class.

Evolution of sexual dimorphism in the plateau brown frog fails to obey Rensch’s rule

Rensch’s rule describes sexual size dimorphism (SSD) that decreases with increasing body size when females are larger than males and SSD that increases when males are larger than females. The plateau brown frog Rana kukunoris, a species endemic to the eastern Tibetan Plateau, exhibits female-biased size dimorphism. Using data on body size from 26 populations and age from 21 populations, we demonstrated that SSD did not increase with increasing mean female snout-vent length (SVL) when controlling for sex-specific age structure, failing to support the Rensch’s rule. Thus, we suggest that fecundity selection (favouring large female size) balances out sexual selection (favouring large male size), which results in a similar divergence between males and females body size. In addition, sex-specific age differences explained most of the variation of SSD across populations.

Sexual selection on size and shape in Japanese beetles (Popillia japonica)

AbstractThe mobility hypothesis argues that species in which males compete for mates in scrambles often exhibit female-biased size dimorphism because smaller male body size should increase male mobility and success in searching for mates. Sexual dimorphism can be further exaggerated if fecundity or sexual selection concurrently selects for larger female size. Scramble competition can select for trait characteristics that optimize locomotion; for example, long and slender wings should be favored if aerial speed is important to mating success. I tested these predictions in the scrambling Japanese beetle (Popillia japonica), a female-biased size dimorphic insect pest that is invasive to North America. Multivariate selection analyses support the prediction that smaller body size and larger wings in males benefit their mating success. My analyses also revealed significant selection for larger wings in females but, contrary to prediction, direct sexual selection favors smaller body size in females. These results support the mobility hypothesis and partially explain the evolution of female-biased size dimorphism in this species. Sexual selection favored rounder bodies in females and more tapered bodies in males, whereas, in both sexes, the effect of wing shape appears less important to fitness than wing size.

Sexual, ontogenetic, and geographic variation of the Neotropical freshwater ostracod Cytheridella ilosvayi

BackgroundThe Neotropics are considered to represent one of the most biologically diverse areas on Earth. Nonetheless, many species are assumed to have widespread distributions and occur in the entire Neotropical range. However, many freshwater invertebrates such as ostracods challenge this contradiction since they live in discrete habitats and possess no active dispersal abilities. The freshwater ostracod Cytheridella is a prime example for this paradigm. From three extant species, only one is described to occur ubiquitously within the Neotropics.Examination of morphological variability is the prerequisite for identification of environmentally induced variations, estimation of inter- and intraspecific variability, and, ultimately, the distinction of species. This study focuses on the quantitative investigations of the appendages of Cytheridella from several living populations sampled in Florida, Mexico, Colombia, and Brazil.ResultsThe morphological traits including podomere ratios and limb-limb ratios, showed that the largest variability occurs at the antennae, and proved a geographical structure. Soft parts reflect the morphological divergence of regional Cytheridella priorly demonstrated by valves shape variability.Additionally, allometric coefficients, limb dimensions and body proportions revealed sexual and female biased size dimorphism. Large variability ranges of A-1 juveniles with reproductive anlagen could be the result of temporary deformations due to imminent moulting.ConclusionThe geographical structure in the morphological variability allows the conjecture how widespread (freshwater invertebrate) species in the Neotropics may have arisen. Passive dispersal via e.g., birds can constitute the maximum ranges of species. If a population has adapted to local ecological conditions and may have occupied all available niches it may impede colonisation through (occasionally) other species. Relatively recent speciation(s) could explain why morphological divergence is not recognizable in qualitative investigations.

Common Noctule Bats Are Sexually Dimorphic in Migratory Behaviour and Body Size but Not Wing Shape.

Within the large order of bats, sexual size dimorphism measured by forearm length and body mass is often female-biased. Several studies have explained this through the effects on load carrying during pregnancy, intrasexual competition, as well as the fecundity and thermoregulation advantages of increased female body size. We hypothesized that wing shape should differ along with size and be under variable selection pressure in a species where there are large differences in flight behaviour. We tested whether load carrying, sex differential migration, or reproductive advantages of large females affect size and wing shape dimorphism in the common noctule (Nyctalus noctula), in which females are typically larger than males and only females migrate long distances each year. We tested for univariate and multivariate size and shape dimorphism using data sets derived from wing photos and biometric data collected during pre-migratory spring captures in Switzerland. Females had forearms that are on average 1% longer than males and are 1% heavier than males after emerging from hibernation, but we found no sex differences in other size, shape, or other functional characters in any wing parameters during this pre-migratory period. Female-biased size dimorphism without wing shape differences indicates that reproductive advantages of big mothers are most likely responsible for sexual dimorphism in this species, not load compensation or shape differences favouring aerodynamic efficiency during pregnancy or migration. Despite large behavioural and ecological sex differences, morphology associated with a specialized feeding niche may limit potential dimorphism in narrow-winged bats such as common noctules and the dramatic differences in migratory behaviour may then be accomplished through plasticity in wing kinematics.

Morphology, ecology, and behaviour of Hylarana intermedia , a Western Ghats frog

Despite being common in the Western Ghats-Sri Lanka biodiversity hotspot, Golden-backed frogs ( Hylarana , Ranidae) remain poorly studied. In this paper, we present some preliminary data on the morphology, behaviour, and habitat use of Hylarana intermedia , a member of the Hylarana aurantiaca species group. We find evidence for female biased size dimorphism, as well as potential shape differences between the sexes in this species. Additionally, we investigate the relationships between traits that may contribute to male conspicuousness (call rates, dorsal coloration, and body size) in two breeding habitats, a paddy field and a trench. Our results suggest both size-dependent and environment-dependent variation in call rate and colour in this species. Specifically, we find evidence against the adoption of sneaker mating strategies by small males in H. intermedia , and instead find variation both within and between populations in traits contributing to male conspicuousness. We conclude by proposing future directions for research on this common frog species.

Effect of size on fighting and mating in a brachyuran crab with female-biased size dimoprohism, Ilyograpsus nodulosus(Macrophthalmidae)

The effect of body size on fighting and mating were examined in both sexes of the rare, Japan-endemic crab Ilyograpsus nodulosus (Sakai, 1983) (Macrophthalmidae), a species in which females are larger than males (female-biased size dimorphism). We found that male-male fights were more common than female-female fights; and that fights between males that were closely size-matched were more common than those between males that were disparate in size. When there was a small size difference between the fighting males, larger and smaller crabs were equally likely to win. When there was a large size difference between the fighting males, the larger crab was more likely to win. Neither male nor female body size predicted the occurrence of copulation. In copulating pairs, male size was positively correlated with female size. Copulation duration decreased with female size. In a female choice experiment, female more often copulated with the larger of the two presented males. In a male choice experiment, males had no preference for the larger or the smaller female. When a single female was placed in a tank with four different sized males, males of all sizes first approached the female and males of all sized copulated, indicating that there was no advantage to large male size in scramble competition. There appears to be a small overall benefit to being a large female and no benefit to being a large male in this species. Male mating success appears to be based on encountering females rather than being successful in male-male competitions or being preferred by females.

Sexual Dimorphism and Seasonal Variation of Reproductive Hormones in the Pascagoula Map Turtle,Graptemys gibbonsi

The Pascagoula Map Turtle (Graptemys gibbonsi) is a narrowly endemic species found only in the Pascagoula River drainage in Mississippi. It is among the most poorly known turtle species because of research taxonomic biases and this species' relatively recent recognition as a unique taxon. A recent petition requested protective status for G. gibbonsi under the U.S. Endangered Species Act. We describe population parameters, quantitatively assess sexual dimorphism of G. gibbonsi, and document hormone secretion patterns from the Chickasawhay and Leaf rivers in Mississippi. We demonstrate a significant male-skewed sex ratio and a female-biased size dimorphism in both carapace length and height. Males showed a bimodal peak of plasma testosterone in fall and spring, consistent with the pattern shown by many other southeastern turtles with late summer-fall spermatogenesis and mating during spring and fall. Females did not show seasonal variation in estradiol secretion, an unexpected result that was possibly due to our small sample size of females, none of which were gravid when captured. Although this observation may be due to our limited capacity to sample females, given the reproductive issues reported for Graptemys flavimaculata from the same drainage (e.g., reproductive hormone abnormalities, low nesting frequency and success), this finding warrants concern and necessitates additional research. Finally, in order to put our hormone data in context, we briefly review hormone and reproductive patterns in southeastern U.S. turtles. Our review includes the timing of follicular enlargement, ovulation and nesting, clutch frequency, and estradiol cycles. The review for male turtles includes details on the spermatogenic cycle, spermiation, and the timing and frequency of testosterone peaks.

Sexual selection, phenotypic variation, and allometry in genitalic and non-genitalic traits in the sexually size-dimorphic stick insectMicrarchus hystriculeus

The mobility hypothesis could explain the evolution of female-biased size dimorphism if males with a smaller body size and longer legs have an advantage in scramble competition for mates. This hypothesis is tested by performing a selection analysis in the wild on Micrarchus hystriculeus (Westwood) (Phasmatodea), a sexually size dimorphic stick insect endemic to New Zealand. This analysis examined the form and strength of sexual selection on body size, leg lengths (front, mid and hind), and clasper size (a genitalic trait), and also quantified the degree of phenotypic variation and the allometric scaling pattern of these traits. By contrast to the mobility hypothesis, three lines of evidence were found to support significant stabilizing sexual selection on male hind leg length: a significant nonlinear selection gradient, negative static allometry, and a low degree of phenotypic variation. Hind leg length might be under stabilizing selection in males if having average-sized legs facilitates female mounting or improves a male’s ability to achieve the appropriate copulation position. As predicted, a negative allometric scaling pattern and low phenotypic variation of clasper size is suggestive of stabilizing selection and supports the ‘one-size-fits-all’ hypothesis. Opposite to males, the mid and hind leg lengths of females showed positive static allometry. Relatively longer mid and hind leg lengths in larger females might benefit individuals via the better support of their larger abdomens. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, , ••‐••.

A test of Rensch's rule in greater horseshoe bat (Rhinolophus ferrumequinum) with female-biased sexual size dimorphism.

Sexual size dimorphism (SSD) is widespread within the animal kingdom. Rensch’s rule describes a relationship between SSD and body size: SSD increases with body size when males are the larger sex, and decreases with body size when females are the larger sex. Rensch’s rule is well supported for taxa that exhibit male-biased SSD but patterns of allometry among taxa with female-biased size dimorphism are mixed, there is evidence both for and against the rule. Furthermore, most studies have investigated Rensch’s rule across a variety of taxa; but among-population studies supporting Rensch’s rule are lacking, especially in taxa that display only slight SSD. Here, we tested whether patterns of intraspecific variation in SSD in greater horseshoe bats conform to Rensch’s rule, and evaluated the contribution of latitude to Rensch’s rule. Our results showed SSD was consistently female-biased in greater horseshoe bats, although female body size was only slightly larger than male body size. The slope of major axis regression of log10 (male) on log10 (female) was significantly different from 1. Forearm length for both sexes of greater horseshoe bats was significantly negatively correlated with latitude, and males displayed a slightly but nonsignificant steeper latitudinal cline in body size than females. We suggest that variation in patterns of SSD among greater horseshoe bat populations is consistent with Rensch’s rule indicating that males were the more variable sex. Males did not have a steeper body size–latitude relationship than females suggesting that sex-specific latitudinal variation in body size may not be an important contributing factor to Rensch’s rule. Future research on greater horseshoe bats might best focus on more comprehensive mechanisms driving the pattern of female-biased SSD variation.

Evolution of parental roles in provisioning birds: diet determines role asymmetry in raptors

Raptors are known for asymmetric parental roles, with the female brooding and feeding offspring, and the male providing food, usually assisted by the female from the latter part of the rearing period. Yet, the evolution of this role asymmetry is poorly understood. From filming prey delivery and handling in 61 nests of 9 raptor species, we show that the female role is related to prey size and prey type. Within species, nestlings were less likely to ingest larger prey unassisted, independent of increasing ability of unassisted feeding with increasing age. Feeding time increased with prey size and was longer for avian than for mammalian prey of the same size when nestlings fed unassisted. Across species, the female partitioned prey and fed offspring for a longer portion of the rearing period as prey size increased and as the diet contained more birds. Providing for the family selects for small body size; hence, extended female confinement as sedentary food processor for offspring would leave greater potential for differential selection on male and female body size. This may explain the female-biased size dimorphism among raptors, which becomes larger as diet changes from insects via reptiles and mammals to birds and as relative prey size increases. Symmetric parental roles and no female-biased size dimorphism would be expected in birds providing insects or other food that nestlings swallow without maternal assistance. Prey type and prey size would be important also for sexual conflicts and evolution of polygamy and mate desertion in birds providing for offspring.

Evolution of Sexual Size Dimorphism in a Frog Obeys the Inverse of Rensch’s Rule

Rensch’s rule refers to a pattern in sexual size dimorphism (SSD) in which SSD increases with body size when males are the larger sex and decreases with body size when females are the larger sex. Using data on body size from 40 populations and age from 31 populations of the rice frog Rana limnochari with female-biased size dimorphism, I tested the consistency of allometric relationships between males and females with Rensch’s rule and evaluated the hypothesis that SSD was largely a function of age differences between the sexes. Statistical comparisons of body sizes between the sexes showed the evidence for the inverse of Rensch’s rule, indicating the level of SSD increased with increasing mean body size. One of the explanations for the occurrence of the inverse of Rensch’s rule may be the fecundity selection hypothesis assuming increased reproductive output in large females. However, differences in age between males and females among populations could explain mildly the variation in SSD.

Ecology of the Flap-Necked Chameleon Chamaeleo dilepis In Southern Africa

Abstract We quantified sexual size dimorphism, reproduction, and diet in the flap-necked chameleon, Chamaeleo dilepis, using museum specimens. Females were larger than males in both snout-vent length (SVL) and pelvic width. The smallest sexually reproductive female was 80 mm SVL, whereas the smallest mature male was 60 mm. Female body size also correlated with clutch size (mean: 44.2, range: 19–74) and volume, suggesting the female-biased size dimorphism may be the product of fecundity selection. Males and females have slightly asynchronous reproductive cycles but breed during spring–summer. Chamaeleo dilepis feeds on a range of arthropods, but their diet is dominated both numerically and volumetrically by orthopterans, followed by coleopterans.

Inverse Rensch's rule in a frog with female-biased sexual size dimorphism

Rensch's rule claims that sexual size dimorphism (SSD) increases with body size when males are larger but decreases with body size when males are smaller. Chinese wood frog Rana chensinensis is a medium-sized species with female-biased size dimorphism. Using data on body size and age in 27 populations covering the full known size range of the species, we tested the consistency of allometric relationships between the sexes with Rensch's rule and evaluated the hypothesis that SSD is largely a function of age differences between the sexes. The results showed that level of female-biased SSD increased with increasing mean size, supporting the inverse of Rensch's rule. Moreover, most of the variation in SSD can be explained in terms of differences in age between the sexes in populations.

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.

The genetic mating system of a sea spider with male-biased sexual size dimorphism: evidence for paternity skew despite random mating success.

Male-biased size dimorphism is usually expected to evolve in taxa with intense male–male competition for mates, and it is hence associated with high variances in male mating success. Most species of pycnogonid sea spiders exhibit female-biased size dimorphism, and are notable among arthropods for having exclusive male parental care of embryos. Relatively little, however, is known about their natural history, breeding ecology, and mating systems. Here we first show that Ammothella biunguiculata, a small intertidal sea spider, exhibits male-biased size dimorphism. Moreover, we combine genetic parentage analysis with quantitative measures of sexual selection to show that male body size does not appear to be under directional selection. Simulations of random mating revealed that mate acquisition in this species is largely driven by chance factors, although actual paternity success is likely non-randomly distributed. Finally, the opportunity for sexual selection (I s), an indirect metric for the potential strength of sexual selection, in A. biunguiculata males was less than half of that estimated in a sea spider with female-biased size dimorphism, suggesting the direction of size dimorphism may not be a reliable predictor of the intensity of sexual selection in this group. We highlight the suitability of pycnogonids as model systems for addressing questions relating parental investment and sexual selection, as well as the current lack of basic information on their natural history and breeding ecology.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-011-1170-x) contains supplementary material, which is available to authorized users.

Pairing and insemination patterns in a giant weta (Deinacrida rugosa: Orthoptera; Anostostomatidae)

Positive size assortative mating can arise if either one or both sexes prefer bigger mates or if the success of larger males in contests for larger females leaves smaller males to mate with smaller females. Moreover, body size could not only influence pairing patterns before copulation but also the covariance between female size and size of ejaculate (number of spermatophores) transferred to a mate. In this field study, we examine the pre-copulatory mate choice, as well as insemination, patterns in the Cook Strait giant weta (Deinacrida rugosa). D. rugosa is a large orthopteran insect that exhibits strong female-biased sexual dimorphism, with females being nearly twice as heavy as males. Contrary to the general expectation of male preference for large females in insects with female-biased size dimorphism, we found only weak support for positive size assortative mating based on size (tibia length). Interestingly, although there was no correlation between male body size and the number of spermatophores transferred, we did find that males pass more spermatophores to lighter females. This pattern of sperm transfer does not appear to be a consequence of those males that mate heavier females being sperm depleted. Instead, males may provide lighter females with more spermatophores perhaps because these females pose less of a sperm competition risk to mates.

Small-male mating advantage in a species of Jerusalem cricket (Orthoptera: Stenopelmatinae: Stenopelmatus)

Abstract Scientific literature often touts the many advantages of large body size, but seldom addresses the value of small body size. Yet selection for large size must be counterbalanced by selection for small size, otherwise, all animals would be large. In this paper, we demonstrate female-biased size dimorphism and a strong copulatory advantage for small males in a Jerusalem cricket (JC) (Stenopelmatus) from central California. We selectively paired male and female JCs of diverse body sizes and recorded their ability to copulate. All copulations were successful for males smaller or equal in size to females. In contrast, when the male was 6.1 mm longer than the female, copulation had only a 50% chance of occurring successfully. In general, as the difference between male and female body length increased (i.e., as males became longer than their mates), the probability of successful copulation decreased. These patterns of mating resulted in net selection for small male size and large female size. We also de...

Parental care and sexual size dimorphism in wasps and bees

Sexual size dimorphism, in which one sex is larger than the other, occurs when body size has differential effects on the fitness of males and females. Mammals and birds usually have male-biased size dimorphism, probably because of strong sexual competition among males. Invertebrates usually have female-biased size dimorphism, perhaps because their inflexible exoskeletons limit ovary size, leading to a strong correlation between female body size and fecundity. In this paper, we test whether an additional factor, the type of parental care provided, affects the degree of sexual size dimorphism. Among wasps and bees, there is a contrast between provisioning taxa, in which females must gather and transport heavy loads of provisions to nests they have constructed, and non-provisioning taxa, in which females lay eggs but do not construct nests or transport provisions. Males have no role in parental care in either case. An analysis of British wasps and bees shows that provisioning taxa have significantly more female-biased size dimorphism than non-provisioning taxa. This is true for simple cross-species comparisons and after controlling for phylogeny. Our data imply that the demands of carrying provision loads are at least part of the explanation for this pattern. Thus, sexual size dimorphism is greatest in pompilid wasps, which carry the heaviest prey items. Bees, which transport minute pollen grains, exhibit the least dimorphism. We also find that cavity-nesting species, in which nest construction costs may be minimized, exhibit reduced dimorphism, but this was not significant after controlling for phylogeny.