Sex Differences In Costs Research Articles

Phenotypic plasticity in breeding plumage signals in both sexes of a migratory bird: responses to breeding conditions

Adaptive phenotypic plasticity may respond to present ambient conditions. Sexual and social signals in both sexes may express phenotype performance. Plumage signals that change discontinuously allow relating discrete variation to previous performance. Both sexes of the pied flycatcher Ficedula hypoleuca present white patches on the wings and on the forehead, which constitute sexual and social signals. Forehead patches are moulted together with body plumage in Africa, while wing patches are partly moulted in Africa and partly in the breeding area soon after breeding. We studied individual inter‐year changes (corrected for regression to the mean) in the size of forehead and wing patches of both sexes in seven years for females or six years for males in two nearby study areas in central Spain. We found that initial signal extent strongly delimits the possible subsequent changes negatively. There is a negative association of male age with forehead patch changes. Cold and rainy springs are associated in females with decreases in both patch areas and vice versa, while no association with climate is observed in male wing patch changes. Cold pre‐breeding conditions predict positive changes in female wing and male forehead patches. Breeding success is positively associated with forehead patch changes in females. Late‐breeding males experience more positive changes in forehead patch size than early‐breeding males. Some of these trends can be explained by variable costs of breeding in certain conditions for subsequent signal production and/or maintenance, while absence of trends in some cases may be explained by sex differences in costs of breeding and interactions with phenotypic quality of breeders.

The origin of female‐biased sex ratios in intertidal seagrasses (Phyllospadix spp.)

Flowering sex ratios of dioecious plants are commonly male-biased but rarely female-biased. While greater costs of reproduction from females have been repeatedly demonstrated and explain male biases, male reproductive costs almost never exceed female costs, making the origins of female biases enigmatic. I investigated the seagrasses Phyllospadix scouleri and P. serrulatus (surfgrasses), which have some of the most extreme female-biased sex ratios documented (>90% female), to identify the mechanisms driving sex ratio bias. I developed sex-linked amplified fragment length polymorphism (AFLP) markers and applied them to three P. scouleri life stages at four sites to determine when during the life cycle sex ratio bias arises. Sex ratios were even among seedlings but became more female-biased at later life stages, indicating that sex ratios were driven by male-biased mortality. To identify when during the life cycle sex ratio bias developed, I examined sex differences in survival among seedlings and three aspects of reproductive costs that could potentially generate biased sex ratios under field conditions. No differences in seedling survival between the sexes were detected, and there was no evidence of substantial sex differences in costs of reproduction. I found no support for a trade-off between current and future reproduction or between reproductive investment and growth. Thus, costs of reproduction appear unlikely to drive sex ratio bias in surfgrass. Instead, small sex differences in growth and survival spread across the life cycle appear to be responsible for female-biased sex ratios and suggest that life history trade-offs other than reproductive costs drive sex ratio bias.

Self‐Grooming Response of Meadow Voles to the Odor of Opposite‐Sex Conspecifics in Relation to the Dietary Protein Content of Both Sexes

AbstractMany animals self‐groom when they encounter the scent marks of opposite‐sex conspecifics. Self‐grooming transmits odiferous substances that contain information about the groomer’s condition, which is affected by its nutritional state. We tested the hypothesis that the amount of time that individuals self‐groom to opposite‐sex conspecifics is affected by the amount of protein in their diet and that of the scent donor. We did so by feeding meadow voles (Microtus pennsylvanicus) a diet containing 9%, 13%, or 22% dietary protein for 30 d and observing their self‐grooming behavior when they were exposed to bedding scented by an opposite‐sex conspecific (odor donor) fed one of the three diets, or fresh cotton bedding (control). The hypothesis was partially supported. We found that the protein content of the diet of male and female groomers did not affect the amount of time they self‐groomed. However, the protein content of the diet of male odor donors affected the amount of time that female voles spent self‐grooming. Female voles self‐groomed more in response to male odor donors fed a 22% protein‐content diet than to those produced by male odor donors fed either a 9% or a 13% protein‐content diet. Interestingly, the amount of time males self‐groomed was not affected by the protein content of the diet of the female odor donor. These results may, in part, be explained by the natural history of free‐living meadow voles, sex differences in costs associated with mate attraction and reproduction, and the direct or indirect benefits that females receive from males fed a diet high in protein content.

The influence of virginity and induced sterility on Drosophila melanogaster females and males life span

Reproduction and life span are negatively interrelated in both sexes, however sex differences in costs of reproduction are poorly understood. It was shown that mating cost has a main contribution to costs of reproduction in both sexes. molecular signals from gonads exceed cost of gametes production.

Sex and social costs of escaping in the striped plateau lizard Sceloporus virgatus

As a predator approaches, prey base decisions about when to flee on a balance between degree of predation risk and costs of escaping. Lost opportunities to perform activities that may increase fitness are major escape costs. Paramount among these are chances to increase fitness by courting and mating and by driving away sexual rivals. Because sexual selection imposes different social demands on the sexes, social opportunities can have different consequences for males and females, but effects of sex differences in social opportunity costs on escape behavior are unknown. We conducted a field experiment showing that male striped plateau lizards (Sceloporus virgatus) given the opportunity to court or perform aggressive behavior permit closer approach before fleeing, but females do not. Males allowed a simulated predator to approach closer before initiating escape if a tethered male or female rather than a control stimulus was introduced to them, but females initiated escape at similar distances in all conditions. For males, a trade-off between the greater predation risk accepted before fleeing due to the likelihood of enhancing fitness by sexual or aggressive behavior accounts for closer approach allowed in the presence of conspecifics. Mating opportunities are not limiting for females in most species and females often have little to gain by interacting aggressively with other females. Therefore, presence of a conspecific male or female may not justify taking greater risk. Results confirm the prediction of optimal escape theory that flight initiation decreases as cost of escaping increases. The sex difference in effect of presence of conspecifics on flight initiation distance is a consequence of the sex difference in costs of escaping. Key words: escape costs, flight initiation distance, predation risk, Sceloporus virgatus, sexual selection. [Behav Ecol]

Tardy females, impatient males: protandry and divergent selection on arrival date in the two sexes of the barn swallow

Protandry reflects the earlier arrival of males than females to the site of reproduction. Such protandry is hypothesised to arise from sex differences in costs and benefits of early arrival. I investigated temporal patterns of arrival date of male and female barn swallows Hirundo rustica and temporal patterns of selection to test the hypothesis that sex differences in selection account for sex differences in arrival date. Mean arrival date of male barn swallows but not of females advanced during the last 33 years, giving rise to an increasing sex difference in arrival date. Early arrival was favoured by increasingly better survival in males, while females showed an opposite pattern that did not reach significance, although the effect differed between sexes. Early arrival increased fecundity in both sexes equally.The sex difference in viability selection in relation to arrival date changed from positive to negative as the degree of protandry increased in recent years, although there was no similar significant relationship for fecundity selection. Furthermore, sex differences in viability selection in a given year affected the degree of protandry in the following year through differential survival of certain phenotypes over others. Finally, temporal changes in sex difference in viability selection and protandry were related to an increase in the interval between first and second clutches, as the duration of the breeding season increased because of climatic amelioration. These findings suggest that arrival date is under divergent selection in the two sexes, providing a mechanism for the evolution of protandry.

Sex-specific differences in site fidelity and the cost of dispersal in yellow-headed blackbirds

Male migratory birds tend to be more faithful than females to previous breeding sites, suggesting sex differences in costs or benefits of dispersal. In Illinois, greater site fidelity by male yellow-headed blackbirds (Xanthocephalus xanthocephalus) was associated with reduced reproductive success the following year for dispersers relative to non-dispersers. Dispersing females suffered no reduction in reproductive success the following year. Males that attracted few social mates, and thus had low reproductive success, were more likely to disperse, whereas females dispersed in response to low-patch reproductive success, regardless of their individual performance. Males that dispersed appeared to be successful acquiring territories because none was observed as a floater. The rate of dispersal by males in this low-density population was greater than in more dense populations where dispersing males may be less successful at acquiring territories. Despite success at obtaining territories, males that dispersed acquired territories on the periphery of wetlands where fewer females nested, resulting in lower reproductive success. In the second year after dispersing, however, males moved onto more central territories where they acquired larger harems. Thus, dispersal by males may be a long-term strategy requiring at least 2 years for benefits to be realized. Long-term success was enhanced because dispersing males moved to wetlands on which reproductive success was higher than on the wetlands they left. In addition to demonstrating that both individual and patch reproductive success affect dispersal decisions, these data indicate that when evaluating costs and benefits of dispersal, researchers should use a time frame beyond 1 year.

Sex differences in the diving behaviour of a size-dimorphic capital breeder: the grey seal

Both body size dimorphism and sex differences in the relative costs and benefits associated with acquiring energy for reproduction have been advanced to explain the evolution of sex differences in foraging behaviour. We examined the extent to which these factors influenced sex differences in the diving behaviour of a size-dimorphic, capital breeder, the grey seal, Halichoerus grypus. Using time-depth data loggers, we examined the diving behaviour of 46 male and 49 female grey seals for 7 months before parturition and mating. Males and females showed significantly different seasonal patterns in the characteristics of individual dives and dive effort. Compared with males, females showed significantly higher levels of dive effort immediately following moult and in the 3 months before parturition. Females also had longer dives (5.5 versus 4.9 min) and spent more time at depth (3.4 versus 2.7 min), whereas males dived deeper (57 versus 49 m). Males dived consistently throughout the day, whereas females showed strong diurnal patterns in dive depth, duration and frequency. The diving behaviour and rates of mass gain by females suggested a pattern of foraging consistent with early accumulation of body energy to support pregnancy and the subsequent lactation period during which females fast. Males, on the other hand, showed diving behaviour and rates of mass gain consistent with a more gradual accumulation of energy stores. Our results suggest that sex differences in the seasonal patterns of diving behaviour reflect sex differences in the costs and benefits of stored energy for reproduction rather than the influence of body size dimorphism alone.

MUTUAL MATE CHOICE AND SEX DIFFERENCES IN CHOOSINESS.

Sexual competition is associated closely with parental care because the sex providing less care has a higher potential rate of reproduction, and hence more to gain from competing for multiple mates. Sex differences in choosiness are not easily explained, however. The lower-caring sex (often males) has both higher costs of choice, because it is more difficult to find replacement mates, and higher direct benefits, because the sex providing more care (usually females) is likely to exhibit more variation in the quality of contributions to the young. Because both the costs and direct benefits of mate choice increase with increasing parental care by the opposite sex, general predictions about sex difference in choosiness are difficult. Furthermore, the level of choosiness of one sex will be influenced by the choosiness of the other. Here, we present an ESS model of mutual mate choice, which explicitly incorporates differences between males and females in life history traits that determine the costs and benefits of choice, and we illustrate our results with data from species with contrasting forms of parental care. The model demonstrates that sex differences in costs of choice are likely to have a much stronger effect on choosiness than are differences in quality variation, so that the less competitive sex will commonly be more choosy. However, when levels of male and female care are similar, differences in quality variation may lead to higher levels of both choice and competition in the same sex.

COSTS OF REPRODUCTION INFLUENCE THE EVOLUTION OF SEXUAL SIZE DIMORPHISM IN SNAKES.

Most explanations for sex differences in adult body size have focused on differences between the two sexes in the degree to which larger body size enhances reproductive success. For example, the general tendency for females to grow larger than males has been attributed to production of larger litters by larger females (Darwin 1871; Trivers 1972; but see Shine 1988), and the correlation between large male size and mating systems with male-male competition has led many authors to conclude that larger male size enhances male reproductive success in such systems (Darwin 1871; Clutton-Brock et al. 1977; Shine 1978; Berry and Shine 1980). Because each of these hypotheses deals only with the relationship between body size and reproductive success within one sex, no prediction can be made about the relative sizes of the two sexes (Greenwood and Adams 1987; Shine 1987). The degree of size dimorphism is presumably determined by the relative intensities of selection for large size in the two sexes, as well as the degree of genetic correlation between the sexes for alleles associated with body size (Arnold 1985). It is entirely likely that large size enhances reproductive success in both sexes, but more strongly in one than in the other. In order to predict the actual direction or degree of sexual size dimorphism within a population, we need data on the effects of body size on reproductive success in both sexes. Such data are rarely available (Clutton-Brock 1988), so the balance between selective forces acting on conspecific males and females has rarely been evaluated. A comparative analysis of sexual size dimorphism and mating systems in snakes reveals that females grow larger than males in most species, but not in species that show male-male combat behavior (Shine 1978). The same pattern has been noted in many other animals, including other reptiles, and generally has been attributed to sexual selection for large male body size in taxa with male-male combat (Shine 1979; Berry and Shine 1980; Fitch 1981). Nonetheless, there are many exceptions to this generalization, and the most puzzling of these are species in which male-male combat occurs but in which females attain larger sizes than males. Species that fail to conform to the usual pattern may tell us a great deal about the factors responsible for those general patterns. In this paper, we use detailed demographic and ecological data on one of these atypical species (the adder Vipera berus) to test among potential explanations for female-larger dimorphism in a species with male-male combat. Our analyses suggest that sex differences in costs of reproduction may strongly influence optimal body sizes in each sex, and hence may affect the direction of sexual size dimorphism. We focus here on the potential costs and benefits of larger body size in terms of reproductive success, especially, fecundity selection in females and sexual selection in males.

Ecological Correlates of Labile Sex Expression in the Orchid Catasetum Viridiflavum

Patterns of sex expression were studied in a natural population of the epiphytic orchid Catasetum viridiflavum on Barro Colorado Island (BCI), Panama. Data on gender and flowering frequency were collected for 2 yr from 186 orchids occurring in both young (70—100 yr old) and mature forests. The distribution of phenotypic gender ratio was strongly bimodal in the population, and sex ratios were significantly affected by forest age. In the young forest was statistically indistinguishable from 1:1. Gender in C. viridiflavum is largely determined by light intensity. Females occurred most frequently in open canopies,and males predominated in closed canopies. Experimental shading of developing inflorescences of well—illuminated plants duplicated this pattern, suggesting sex expression is controlled by cues received by the developing inflorescence. Small, but significant effects of plant size on gender were also detected. Forty—two percent of plants occurred on dead, decaying wood (mostly standing dead trees), while the remainder occurred on live trees. Substrate type and canopy openness are correlated because dead trees present no canopy to shade epiphytes on them. Differences in sex ratio due to forest type resulted from a more frequent occurrence of plants on standing dead trees in the young vs. the mature forest. Theory for the evolution of environmental sex determination (ESD) assumes that environment affects reproductive success (RS), but that the relationship between RS and environment is different for males and females. Light availability and substrate were investigated for their effects on two components of RS, flowering frequency and vegetative growth. C. viridiflavum on dead, decaying wood flowered more often than those on live trees, but no significant effect due to canopy openness was detected. The influence of substrate and light availability on estimated rates of vegetative growth varied from year to year, but increased rates were associated with either dead, decaying wood or high light intensities in at least 1 yr. Thus, explanations for the evolution of ESD in C. viridiflavum must include potential effects of both substrate type and light environment on male and female RS. Plants growing on dead, decaying wood in an open habitat that produced fruit exhibited reduced vegetative growth compared to nonfruiting plants, indicating there is long—term cost to fruit production in C. viridiflavum. In forest habitats, females setting fruit on live trees did not flower in the following year. Most nonfruiting females on this substrate type, as well as fruiting and nonfruiting plants on dead trees, flowered in the following year. Although sample sizes supporting this pattern were small, it is consistent with the hypothesis that sex differences in costs of reproduction in response to changes in substrate (and correlated changes in light availability) may explain the evolution of ESD in C.viridiflavum.