Affiliative relations among male Japanese macaques (Macaca fuscata yakui) within and outside a troop on Yakushima Island

Male Japanese macaques (Macaca fuscata yakui) in a troop on Yakushima Island frequently groom other males. However, previous studies have not compared the social relations of troop males to those of non-troop males. I followed all troop males and non-troop males in and near a troop during a mating season and during the following non-mating season and recorded their neighbors, grooming, and agonistic interactions. Comparisons of the social relations of troop males and non-troop males with other troop members revealed that grooming and agonistic interactions with females during the mating season were similar between troop and non-troop males. However, troop males groomed each other more often and had fewer agonistic interactions among themselves than did non-troop males. Compared to what occurred in the mating season, troop males groomed females less often and exchanged grooming bouts more often with other troop males during the non-mating season. One non-troop male groomed females more frequently than did any troop male in both seasons, and this male groomed troop males more frequently than did any troop male in the non-mating season. This male immigrated into the troop during the following mating season. Regardless of their competition with respect to reproduction, male Japanese macaques on Yakushima Island maintain affiliative relations, probably to cooperatively defend fertile females from non-troop males.     

Male‐skewed adult sex ratio,survival, mating opportunity and annual productivity in the Snowy Plover Charadrius alexandrinus

Sex differences in adult mortality may be responsible for male‐skewed adult sex ratios and male‐skewed parental care in some birds. Because a surplus of breeding males has been reported in serially polyandrous populations of Snowy Plover Charadrius alexandrinus, we examined sex ratio, early‐season nesting opportunities, adult survival and annual reproductive success of a Snowy Plover population at Monterey Bay, California. We tested the hypotheses that male adult survival was greater than female survival and that a sex difference in adult survival led to a skewed adult sex ratio, different mating opportunities and different annual productivity between the sexes. Virtually all females left chicks from their first broods to the care of the male and re‐nested with a new mate. As a result, females had time to parent three successful nesting attempts during the lengthy breeding season, whereas males had time for only two successful attempts. Among years, the median population of nesting Plovers was 96 males and 84 females (median difference = 9), resulting in one extra male per eight pairs. The number of potential breeders without mates during the early nesting period each year was higher in males than in females. Adult male survival (0.734 ± 0.028 se) was higher than female survival (0.693 ± 0.030 se) in top‐ranked models. Annually, females parented more successful clutches and fledged more chicks than their first mates of the season. Our results suggest that in C. alexandrinus a sex difference in adult survival results in a male‐skewed sex ratio, which creates more nesting opportunities and greater annual productivity for females than for males.     

Long-term fitness benefits of polyandry in a small mammal, the bank vole Clethrionomys glareolus

Polyandry, i.e. mating with multiple males within one reproductive event, is a common female mating strategy but its adaptive function is often unclear. We tested whether polyandrous females gain genetic benefits by comparing fitness traits of monandrous (mated twice with a single male) and polyandrous (mated twice with two different males) female bank voles Clethrionomys glareolus. We raised the offspring in the laboratory until adulthood and measured their body size, before releasing them to outdoor enclosures to overwinter. At the onset of the breeding season in the following spring, we found that offspring of polyandrous females performed significantly better at reproduction than those of monandrous females. This was mainly due to sons of polyandrous females producing significantly more offspring than those of monandrous females. No significant differences were found for offspring body mass or winter survival between the two treatments. Our results appear to provide evidence that bank vole females gain long-term benefits from polyandry.     

Dominance is not always an honest signal of male quality,but females may be able to detect the dishonesty

Females prefer dominant males as mating partners in numerous species. Male dominance rank is considered as an honest signal of male quality, because only healthy males in good condition are thought to be able to win fights with other males. Here, we tested whether activation of the immune system influences the success of males in male–male competition and mating in the field cricket, Gryllus integer. We activated the immune system of males with a nylon monofilament (to mimic a parasitoid larva), and arranged fights between male pairs to assess male dominance and associated mating success. Activation of the immune system with nylon monofilament substantially enhanced the fighting success of males during male–male competition but had no effect on mating success. However, sham-manipulation (a wound only) did not have any effect on fighting success although females mated more often with dominant males. Our study suggests that when male crickets meet an apparent survival threat they may behave more dominantly, probably owing to terminal investment. Male success during male–male competition is not always an honest signal of males’ quality, but females may be able to detect this dishonesty.     

Differences in potential reproductive rates of male and female seahorses related to courtship roles

Dwarf seahorses, Hippocampus zosterae (Syngnathidae), are distinguished by extreme morphological specialization for paternal care, the formation of monogamous pair bonds and mating repeatedly over the course of a breeding season. To determine the potential reproductive rates of male and female dwarf seahorses, we measured (1) the maximum number of offspring produced per breeding cycle when sexually receptive mates were unlimited, and (2) the relative time each sex was unavailable for mating ('time out'). We paired sexually isolated males and females with sexually receptive partners and observed them from the day of introduction through to copulation, to determine the length of time it takes each sex to prepare to mate. We conducted additional experiments to determine the length of gestation, which when added to the time needed to prepare to mate and copulate gives an estimate of total reproductive cycle duration, T. We estimated potential reproductive rate by dividing the mean number of offspring produced per breeding cycle by the duration of the breeding cycle (T). We estimated reproductive 'time out' by identifying the period of time males and females were physiologically capable of mating ('time in', S) and subtracting time S from time T. When provided with sexually receptive partners, females took 2 days longer than males to complete courtship and copulation, but neither males nor females remated during gestation. Therefore, males could potentially produce 17% more offspring than females over the course of one breeding season. Females had reproductive 'times out' 1.2 times longer than did males, as they were only capable of mating during the 4 h directly preceding copulation. Thus, H. zosterae males have higher potential reproductive rates and shorter reproductive 'times out' compared with H. zosterae females. These results and previous work indicating that seahorses display traditional courtship roles support the prediction that the sex having the higher potential reproductive rate, or equivalently, the shorter 'time out', will compete more intensely for access to the opposite sex. Copyright 2000 The Association for the Study of Animal Behaviour.     

The complexity of mating decisions in stalk‐eyed flies

All too often, studies of sexual selection focus exclusively on the responses in one sex, on single traits, typically those that are exaggerated and strongly sexually dimorphic. They ignore a range of less obvious traits and behavior, in both sexes, involved in the interactions leading to mate choice. To remedy this imbalance, we analyze a textbook example of sexual selection in the stalk‐eyed fly (Diasemopsis meigenii). We studied several traits in a novel, insightful, and efficient experimental design, examining 2,400 male–female pairs in a “round‐robin” array, where each female was tested against multiple males and vice versa. In D. meigenii, females exhibit strong mate preference for males with highly exaggerated eyespan, and so we deliberately constrained variation in male eyespan to reveal the importance of other traits. Males performing more precopulatory behavior were more likely to attempt to mate with females and be accepted by them. However, behavior was not a necessary part of courtship, as it was absent from over almost half the interactions. Males with larger reproductive organs (testes and accessory glands) did not make more mating attempts, but there was a strong tendency for females to accept mating attempts from such males. How females detect differences in male reproductive organ size remains unclear. In addition, females with larger eyespan, an indicator of size and fecundity, attracted more mating attempts from males, but this trait did not alter female acceptance. Genetic variation among males had a strong influence on male mating attempts and female acceptance, both via the traits we studied and other unmeasured attributes. These findings demonstrate the importance of assaying multiple traits in males and females, rather than focusing solely on prominent and exaggerated sexually dimorphic traits. The approach allows a more complete understanding of the complex mating decisions made by both males and females.     

Survival,density and population composition of bellbirds (Anthornis melanura) on the Poor Knights Islands,New Zealand

Abstract

Annual survival by age, sex and population size and structure were estimated for bellbirds (Anthornis melanura) from a banding study undertaken on 66 ha Aorangi Island, the Poor Knights Islands, during the period 1978–86. The most parsimonious model showed that survival varied little with age and between years, and survival of males was significantly greater than that of females. Recapture probabilities of males and females were similar, but did vary with age and among years. There was a greater proportion of adult males than adult females and sub‐adult males and females in the population, and among adults the male bias in the population was greater during the breeding season than at other times of the year. The high density of bellbirds on the Poor Knights and larger size of males may explain the reduced survival of females relative to that of males.     

Effects of testosterone treatment and season on the frequency of dewlap extensions during male-male interactions in the lizard Anolis sagrei

Lizards of the genus Anolis extend and retract a large and often brightly colored throat fan called a dewlap. The dewlap in most anoles is a sexually dimorphic structure. It is larger in males than females and males use the dewlap more frequently and in more contexts than do females. In the present study we investigated whether plasma testosterone (T) levels and season affect the frequency of dewlap use in male--male interactions in the brown anole, Anolis sagrei. We manipulated plasma T levels by implanting adult, castrated males with pellets that delivered no T, a moderate dose of T, or a high dose of T. In tests with stimulus males, castrated males that were treated with a moderate or with a high dose of T had a significantly higher frequency of dewlap extensions than did castrated males that received no T. However, the frequency of dewlap extensions in castrated males that received the high dose of T did not differ significantly from castrated males that received the moderate dose of T or from non-castrated control males. Males captured during the breeding season and tested in the laboratory had a significantly higher frequency of dewlap extensions than did males captured in the postbreeding season and tested in the same manner. These results suggest that plasma T levels affect how frequently males of A. sagrei extend their dewlaps in male--male interactions and that seasonal changes in male dewlap use may be due to seasonal differences in plasma T levels.     

Reproductive regulation in the female Japanese minnow, Pseudorasbora parva (Cyprinidae)

The Japanese minnow, Pseudorasbora parva, is a multiple spawner that lays eggs repeatedly during the spawning season. In laboratory aquaria, 19 of 63 females did not spawn, whereas 44 other females laid 167 to 6285 eggs in 1 to 14 mating sequences. Neither fish density nor sex ratio (number of males per female) affected fecundity, but the increase in fish density significantly decreased the growth rate of females. The increase in male length significantly increased fecundity in aquaria which contained only a single male, whereas it decreased female growth rate. These relationships were not evident in aquaria housing three or ten males. Since larger males were more dominant and had larger testes than smaller males, the correlation between fecundity and male size strongly suggests that individual females regulated fecundity to increase their reproductive success.     

The sexual selection paradigm: have we overlooked other mechanisms in the evolution of male ornaments?

Extravagant male ornaments expressed during reproduction are almost invariably assumed to be sexually selected and evolve through competition for mating opportunities. Yet in species where male reproductive success depends on the defence of offspring, male ornaments could also evolve through social competition for offspring survival. However, in contrast to female ornaments, this possibility has received little attention in males. We show that a male ornament that is traditionally assumed to be sexually selected—the red nuptial coloration of the three-spined stickleback—is under stronger selection for offspring survival than for mating success. Males express most coloration during parenting, when they no longer attract females, and the colour correlates with nest retention and hatching success but not with attractiveness to females. This contradicts earlier assumptions and suggests that social selection for offspring survival rather than for sexual selection for mating success is the main mechanism maintaining the ornament in the population. These results suggest that we should consider other forms of social selection beyond sexual selection when seeking to explain the function and evolution of male ornaments. An incorrect assignment of selection pressures could hamper our understanding of evolution.     

Mating patterns in late-maturing female Mediterranean tarantulas may reflect the costs and benefits of sexual cannibalism

During courtship and mating, males of some invertebrate predators risk being killed and consumed by females, who in turn can obtain a foraging benefit from feeding on males. In these invertebrates, the sex ratio at the end of the mating season is usually female biased, probably due to sexual cannibalism and other sources of male mortality. Thus, at the end of the mating season males can be a limited resource to females as both mates and prey. Because of the high risk incurred when approaching females, males should show mate choice. To date there are little data on the costs and benefits of sexual cannibalism in natural populations. For one month we followed the mating patterns of 60 late-maturing Mediterranean tarantula, Lycosa tarentula L., females in a desert grassland population. The later a female matured, the shorter was her cohabitation time with males and the lower her probability of cohabiting with a male at all, suggesting that late-maturing females may be limited in their access to males as mates. At the end of the mating season, nonsexually cannibalistic late-maturing females also had poorer body conditions than did both sexually cannibalistic late-maturing females and early-maturing females, suggesting that late-maturing females may be also limited in their access to males as food. Females had higher mating success if they were smaller or in better condition (better fed). This pattern may reflect either male choice, or the possibility that small, well-fed females have higher mating success because they are less aggressive towards males. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Within‐Species Mate Preferences Do Not Contribute to the Maintenance of Sexually Monomorphic Mating Signals in Green Lacewings

We know much less about the evolutionary forces and constraints that maintain similar mating displays in females and males than we do about sexually dimorphic mating displays. Both female and male green lacewings have sexually monomorphic vibrational mating signals and are equally choosy against heterospecific mating signals. This similarity in between‐species sex roles may explain a large part of the presence of species‐specific female signals in these species, but does not necessarily predict why female and male signals are similar. We tested for within‐species sex‐specific similarities in mate preferences in Chrysoperla lucasina that may contribute to the maintenance of sexually monomorphic mating signals in this species. We found weak preferences and low levels of discrimination for signals with varying fine‐scale temporal features (volley duration, period, and volley‐duty cycle). The longer signals that both sexes produced in response to playback were sexually monomorphic, but some females and most males also produced shorter signals with significantly reduced volley durations and periods. Notably, all of these signals had indistinguishable volley‐duty cycles, the ratio of volley duration to volley period. We propose that mating signals in C. lucasina are maintained in both sexes because of similar between‐species mate preferences, but the sexually monomorphic mating signals cannot be attributed to significant within‐species mate preferences. What differences are present in within‐species sex roles may be resolved by a male‐biased signal polymorphism, in long and short signals that are hypothesized to have distinct functions during mate calling and courtship.     

The Sicker Sex: Understanding Male Biases in Parasitic Infection,Resource Allocation and Fitness

The “sicker sex” idea summarizes our knowledge of sex biases in parasite burden and immune ability whereby males fare worse than females. The theoretical basis of this is that because males invest more on mating effort than females, the former pay the costs by having a weaker immune system and thus being more susceptible to parasites. Females, conversely, have a greater parental investment. Here we tested the following: a) whether both sexes differ in their ability to defend against parasites using a natural host-parasite system; b) the differences in resource allocation conflict between mating effort and parental investment traits between sexes; and, c) effect of parasitism on survival for both sexes. We used a number of insect damselfly species as study subjects. For (a), we quantified gregarine and mite parasites, and experimentally manipulated gregarine levels in both sexes during adult ontogeny. For (b), first, we manipulated food during adult ontogeny and recorded thoracic fat gain (a proxy of mating effort) and abdominal weight (a proxy of parental investment) in both sexes. Secondly for (b), we manipulated food and gregarine levels in both sexes when adults were about to become sexually mature, and recorded gregarine number. For (c), we infected male and female adults of different ages and measured their survival. Males consistently showed more parasites than females apparently due to an increased resource allocation to fat production in males. Conversely, females invested more on abdominal weight. These differences were independent of how much food/infecting parasites were provided. The cost of this was that males had more parasites and reduced survival than females. Our results provide a resource allocation mechanism for understanding sexual differences in parasite defense as well as survival consequences for each sex.     

Male‐mimicking females increase male‐male interactions,and decrease male survival and condition in a female‐polymorphic damselfly

Biologists are still discovering diverse and powerful ways sexual conflicts shape biodiversity. The present study examines how the proportion of females in a population that exhibit male mimicry, a mating resistance trait, influences conspecific males’ behavior, condition, and survival. Like most female‐polymorphic damselflies, Ischnura ramburii harbors both “andromorph” females, which closely resemble males, and sexually dimorphic “gynomorph” counterparts. There is evidence that male mimicry helps andromorphs evade detection and harassment, but males can also learn to target locally prevalent morph(s) via prior mate encounters. I hypothesized that the presence of male mimics could therefore predispose males to mate recognition errors, and thereby increase rates of costly male‐male interactions. Consistent with this hypothesis, male‐male interaction rates were highest in mesocosms containing more andromorph (vs. gynomorph) females. Males in andromorph‐biased mesocosms also had lower final body mass and higher mortality than males assigned to gynomorph‐majority treatments. Male survival and body mass were each negatively affected by mesocosm density, and mortality data revealed a marginally significant interaction between andromorph frequency and population density. These findings suggest that, under sufficiently crowded conditions, female mating resistance traits such as male mimicry could have pronounced indirect effects on male behavior, condition, and survival.     

Sexual abstinence and the cost of reproduction in adult male water snakes, Nerodia sipedon

Low frequency of reproduction among iteroparous organisms is most often observed among female ectothermic vertebrates and is thought to be a strategy used to defer reproductive costs. We assessed reproductive costs of male water snakes ( Nerodia sipedon ) to determine why half of adult males abstain from reproduction each year. There was no evidence of a short-term energetic cost of reproduction. Change in mass did not differ between reproductive and non-reproductive males during the one-month mating season or during the entire four-month activity season. Changes in mass of reproductive males were similar at two sites in which the spatial distribution of females differed. However, there were size-specific differences in growth and survival between reproductive and non-reproductive males. Among reproductive males growth rate decreased with body size at a lower rate than among non-reproductive males. Survival increased with body size for reproductive males, but decreased with body size among non-reproductive males. Most of the differential survival between reproductive and non-reproductive males did not occur during the mating season but rather during hibernation. Size-related differences between reproductive and non-reproductive males may reflect selection having eliminated low quality males from the larger size classes. Overall our results appear most consistent with there being high variance in male quality, such that the best males can bear the cost of reproducing and still grow and survive as well or better than low quality males that abstain from reproduction.     

Sex allocation predicts mating rate in a simultaneous hermaphrodite

Sexual selection theory for separate-sexed animals predicts that the sexes differ in the benefit they can obtain from multiple mating. Conventional sex roles assume that the relationship between the number of mates and the fitness of an individual is steeper in males compared with females. Under these conditions, males are expected to be more eager to mate, whereas females are expected to be choosier. Here we hypothesize that the sex allocation, i.e. the reproductive investment devoted to the male versus female function, can be an important predictor of the mating strategy in simultaneous hermaphrodites. We argue that within-species variation in sex allocation can cause differences in the proportional fitness gain derived through each sex function. Individuals should therefore adjust their mating strategy in a way that is more beneficial to the sex function that is relatively more pronounced. To test this, we experimentally manipulated the sex allocation in a simultaneously hermaphroditic flatworm and investigated whether this affects the mating behaviour. The results demonstrate that individuals with a more male-biased sex allocation (i.e. relatively large testes and small ovaries) are more eager to mate compared with individuals with a more female-biased sex allocation (i.e. relatively small testes and large ovaries). We argue that this pattern is comparable to conventional gender roles in separate-sexed organisms.     

Social implications of the battle of the sexes: sexual harassment disrupts female sociality and social recognition

Across sexually reproducing species, males and females are in conflict over the control of reproduction. At the heart of this conflict in a number of taxa is male harassment of females for mating opportunities and female strategies to avoid this harassment. One neglected consequence that may result from sexual harassment is the disruption of important social associations. Here, we experimentally manipulate the degree of sexual harassment that wild female guppies (Poecilia reticulata) experience by establishing replicated, semi-natural pools with different population sex ratios. We quantify the effects of sexual harassment on female social structure and the development of social recognition among females. When exposed to sexual harassment, we found that females had more disparate social networks with limited repeated interactions when compared to females that did not experience male harassment. Furthermore, females that did not experience harassment developed social recognition with familiar individuals over an 8-day period, whereas females that experienced harassment did not, an effect we suggest is due to disruption of association patterns. These results show that social network structure and social recognition can be affected by sexual harassment, an effect that will be relevant across taxonomic groups and that we predict will have fitness consequences for females.     

Female choice, male interference, and sperm precedence in the red-spotted newt

Darwin first identified female choice and male—male competitionas forms of sexual selection resulting in the evolution of conspicuoussexual dimorphism, but it has proven challenging to separatetheir effects. Their effects on sexual selection become evenmore complicated when sperm competition occurs because spermprecedence may be either a form of cryptic female choice ora form of male—male competition. We examined the effectsof tail height on male—male competition and female choiceusing the sexually dimorphic red-spotted newt (Notophthalmusviridescens viridescens). Experiment 1 examined whether maletail height influenced male mating success. Males with deeptails were more successful at mating with females than thosewith shallow tails. Successful, deep-tailed males also were bigger(snout-vent length; SVL) than unsuccessful, shallow-tailed males,but they did not vary in tail length or body condition. Of these,only tail height and tail length are sexually dimorphic traits.Experiment 2 tested the hypothesis that the differential successof males with deeper tails was due to female choice by examiningboth simultaneous female preference for association and sequentialfemale choice. We found no evidence of female choice. When maleswere not competing to mate with females, tail height did notinfluence male mating success. Successful males did not havedifferent SVL and tail lengths than unsuccessful males. Thus,tail height in male red-spotted newts appears to be an intrasexuallyselected secondary sexual characteristic. Experiment 3 usedpaternity exclusion analyses based on molecular genetic markersto examine the effect of sperm precedence on sperm competitionin doubly-mated females. Sperm precedence likely does not havea pervasive and consistent effect on fertilization success becausewe found evidence of first, last, and mixed sperm usage.     

Effects of reproductive and social variables on fecal glucocorticoid levels in a sample of adult male ring-tailed lemurs (Lemur catta) at the Beza Mahafaly Reserve, Madagascar

Glucocorticoids, a group of adrenal hormones, are secreted in response to stress. In male primates, variables such as breeding seasonality, dominance hierarchy stability, and aggressive and affiliative interactions can affect glucocorticoid levels. In this study, we examined interindividual differences in mean fecal glucocorticoid (fGC) levels among males in three groups of wild ring-tailed lemurs to better understand the physiological costs of group living for males in a female-dominant species that exhibits strict reproductive seasonality. Fecal and behavioral data samples were collected during one mating and two postmating seasons (2001 and 2003). The mean fGC levels were examined in relation to reproductive season, male rank, number of resident males, intermale and female-male agonism, and affiliative behavior with females. The mean fGC levels were not significantly elevated during mating season compared to the postmating period. During the mating season, male dominance hierarchies broke down and rank effects could not be tested; however, there was no relationship between male rank and fGC levels in the postmating periods. In 2001, males that resided in the group with the fewest males exhibited lower fGC levels during the postmating period. They also affiliated more with females than did males in the other groups. During the mating season of 2003, males engaged in more affiliative behaviors with females compared to the postmating season, but female-male agonism did not differ by season. However, rates of intermale agonism were significantly higher during mating compared to postmating periods, but such heightened agonism did not translate to a higher stress response. Thus, neither male-male competition for mates nor heightened agonism between males during the breeding season affected male fGC levels. Fewer males residing in a group, however, did have some effect on male-female affiliation and male fGC levels outside of the mating period. Males that live in a group with only a few (two or three) males may experience less physiological stress than those that live in groups with more males.     

Male fish use prior knowledge about rivals to adjust their mate choice

Mate choice as one element of sexual selection can be sensitive to public information from neighbouring individuals. Here, we demonstrate that males of the livebearing fish Poecilia mexicana gather complex social information when given a chance to familiarize themselves with rivals prior to mate choice. Focal males ceased to show mating preferences when being observed by a rival (which prevents rivals from copying mating decisions), but this effect was only seen when focal males have perceived rivals as sexually active. In addition, focal males that were observed by a familiar, sexually active rival showed a stronger behavioural response when rivals were larger and thus, more attractive to females. Our study illustrates an unparalleled adjustment in the expression of mating preferences based on social cues, and suggests that male fish are able to remember and strategically exploit information about rivals when performing mate choice.