Sex ratio variation in mammals

Parents will increase their fitness by varying the sex ratio of their progeny in response to differences in the costs and benefits of producing sons and daughters. Sex differences in energy requirements or viability during early growth, differences in the relative fitness of male and female offspring, and competition or cooperation between siblings or between siblings and parents might all be expected to affect the sex ratio. Although few trends have yet been shown to be consistent, growing numbers of studies have demonstrated significant variation in birth sex ratios in non-human mammals. These are commonly cited as evidence of adaptive manipulation of the sex ratio. However, several different mechanisms may affect the birth sex ratio, and not all of them are likely to be adaptive. Valid evidence that sex ratio trends are adaptive must be based either on the overall distribution of those trends or on cases in which the sex ratio can be shown to vary with the relative fitness of producing sons and daughters. The distribution of observed sex ratio trends does not conform closely to the predictions of any single adaptive theory. Some recent studies, however, indicate that, within species, the sex ratio varies with the costs or benefits of producing male or female offspring.     

Fluctuating populations and kin interaction in mammals

Kin selection in animals favors less aggressive interaction among related individuals. If the genetic relatedness among neighbors changes with population structure and density, behavioral interaction may also change according to the population structure. Charnov and Finerty proposed a hypothesis that kin selection in voles causes population cycles if the relatedness among neighbors decreases as density increases. Field experiments have recently tested this hypothesis. Furthermore, field studies of social interaction in voles have increased in number, so that the effects of kinship on reproductive success can be reviewed. These studies indicate that although kin interaction might be an important factor affecting social behavior and reproductive success in voles, the relationships both between kinship and degree of amicable behavior or reproductive rate, and between relatedness among neighbors and population density, are far less simple than had been supposed.     

Male parasitism and intrasexual competition in a burrowing barnacle

Summary In sexually dimorphic animals, large male body size is often associated with direct interference competition among males for access to females or resources used in reproduction. In constrast, small male body size may be associated with indirect scramble competition among males for temporal or spatial access to females. Minute, “parasitic” males of the acrothoracican barnacleTrypetesa lampas (Hancock) appear to compete with one another for permanent attachment sites on the external body of the female. Several spatial patterns suggest indirect male-male competition: 1) males were consistently aggregated on the anterior surface of the female ovarian disc; 2) the average distance from attached males to the site of insemination correlated positively with local male density; 3) average male body size on a female decreased as a function of male density; 4) the distribution of males on the left and right hand sides of the female ovarian disc was more even than expected, suggesting that males avoided crowded settlement sites. The number of males attached to a female increased with female body size and matched a null model in which males colonized female “targets” of differing areas. These results suggest that competition between males primarily affected settlement sites and male body sizes within, rather than among, females. Male parasitism may have evolved through both sexual selection for efficient access to females (Ghiselin 1974) and natural selection for reduced burrow density in a space-limited habitat (Turner and Yakovlev 1983).     

Hormonal contraceptive use lowers female intrasexual competition in pair-bonded women

The purpose of this study was to test the influence of hormonal contraceptive use on levels of female intrasexual competition. Twenty-eight women completed a scale for intrasexual competition on three occasions: when using hormonal contraceptives and when regularly cycling at a fertile and a non-fertile cycle stage. When using hormonal contraception, pair-bonded, but not single women, reported significantly lower levels of intrasexual competition than when regularly cycling at either fertile or non-fertile cycle stages. This effect remained significant when controlling for age, length of relationship and relationship satisfaction. Neither pair-bonded nor single women reported shifts in intrasexual competition across the menstrual cycle when fertile as compared to non-fertile. This study benefited from a within-subjects design and a more rigorous assessment of fertility status (transvaginal ultrasonography) than which is typical in the field. Results are discussed in consideration of the evolutionary literature on the stability of romantic relationships and fitness advantages associated with intrasexual competition.     

Sperm competition in mammals

Female promiscuity can lead to the spermatazoa of several males 'competing' to fertilize the ova of a single female. Such promiscuity is relatively common among mammals and has resulted in a suite of adaptations associated with sperm competition. In the last decade, laboratory scientists using experimental techniques have clarified the physiological and behavioural mechanisms that result from sperm competition. Field biologists have collected data on a variety of mammals to test predictions of sperm competition theory. Unfortunately, theories developed and tested in laboratory situations do not always explain variation in behaviour observed in field studies.     

Scent-marking and intrasexual competition in a cooperative carnivore with low reproductive skew

Most mammals scent-mark and a variety of hypotheses have been put forward to explain this behaviour. Differences in the main function of scent-marking between species are likely to be related to differences in social systems. Here, we investigate the functions of scent-marking in a cooperatively breeding carnivore. In the banded mongoose ( Mungos mungo ), individuals of both sexes commonly breed in their natal group and reproductive skew within groups is low. Using experimental scent-mark presentations, we tested predictions of the intrasexual competition, self-advertisement to potential mates and dominance assertion hypotheses. Both males and females responded more intensely to scent marks of same-sexed than of opposite-sexed individuals. Dominant individuals counter-marked more than subordinate ones and males showed higher counter-marking rates than females, but only marginally so. During oestrus, responses to scent marks were increased by both sexes. Our findings strongly indicate that scent-marking in the banded mongoose primarily serves a purpose in intrasexual competition both between and within groups. Unlike in other social herpestids and some solitary rodents, we found little evidence for self-advertisement. We suggest that the peculiar social system of the banded mongoose results in self-advertisement losing importance in this species, shifting the main function of scent-marking to intrasexual competition.     

Evolution of condition-dependent dispersal under kin competition

Dispersers often differ in body condition from non-dispersers. The social dominance hypothesis explains dispersal of weak individuals, but it is not yet well understood why strong individuals, which could easily retain their natal site, are sometimes exposed to risky dispersal. Based on the model for dispersal under kin competition by Hamilton and May, we construct a model where dispersal propensity depends on body condition. We consider an annual species that inhabits a patchy environment with varying patch qualities. Offspring body condition corresponds to the quality of the natal patch and competitive ability increases with body condition. Our main general result balances the fitness benefit from not dispersing and retaining the natal patch and the benefit from dispersing and establishing somewhere else. We present four different examples for competition, which all hint that dispersal of strong individuals may be a common outcome under the assumptions of the present model. In three of the examples, the evolutionarily stable dispersal probability is an increasing function of body condition. However, we found an example where, counterintuitively, the evolutionarily stable dispersal probability is a non-monotone function of body condition such that both very weak and very strong individuals disperse with high probability but individuals of intermediate body condition do not disperse at all.     

Nest site preference and intrasexual competition in female sockeye salmon,Oncorhynchus nerka

Many ecological circumstances present individuals with a conflict between the inherent benefits of a particular habitat and the costs incurred in acquiring or retaining use of the habitat in the face of competition. For example, the reproductive biology of female Pacific salmon (Oncorhynchus spp.) led us to hypothesize that female nest site choice should reflect a compromise between the benefits of obtaining a high quality nest site and the cost of competing for and defending it. To test this hypothesis we studied female sockeye salmon (O. nerka) spawning on beaches in Iliamna Lake, Alaska using a combination of snorkel surveys, tagging, behavior observations, and models. Females showed spatial preferences in nest site selection (for shallower water, where water circulation was higher), aggressive competition in preferred areas was higher, and there was evidence for costs associated with this increased competition. Over the course of the season, spawning activity shifted from shallower to deeper water, consistent with a tradeoff between benefits for embryo survival associated with shallow sites and the costs of competing for them. However, it was also consistent with date-specific optimal sites related to the probability of embryo mortality for eggs spawned in shallow water late in the season, due to annual cycles in lake level and temperature.     

Hamilton's rule and kin competition: the Kipsigis case

Evolutionary studies of human behavior have emphasized the importance of kin selection in explaining social institutions and fitness outcomes. Our relatives can nevertheless be competitors as well as sources of altruism. This is particularly likely when there is local competition over resources, where conflict can lead to strife among nondispersing relatives, reducing or even negating the effects of relatedness on promoting altruism. Here, I present demographic data on a land-limited human population, utilizing large within-population variation in land ownership to determine the interactions between local resource competition and the benefits of kin in enhancing child survival, a key component of fitness in this population. As predicted, wealth affects the extent of kin altruism, in that paternal relatives (specifically father's brothers) appear to buffer young children from mortality much more effectively in rich than in poor households. This interaction effect is interpreted as evidence that the extent of nepotism among humans depends critically on resource availability. Further unanticipated evidence that maternal kin play a role in buffering children from mortality in situations where paternal kin control few resources speaks to the important role that specific local circumstance plays in shaping kin contributions to child welfare.     

Sex chromosome specialization and degeneration in mammals

Sex chromosomes--particularly the human Y--have been a source of fascination for decades because of their unique transmission patterns and their peculiar cytology. The outpouring of genomic data confirms that their atypical structure and gene composition break the rules of genome organization, function, and evolution. The X has been shaped by dosage differences to have a biased gene content and to be subject to inactivation in females. The Y chromosome seems to be a product of a perverse evolutionary process that does not select the fittest Y, which may cause its degradation and ultimate extinction.     

Sex biases in kin shoaling and dispersal in a cichlid fish

Animal dispersal is associated with diverse costs and benefits that vary among individuals based on phenotype and ecological conditions. For example, females may disperse when males benefit more from defending territories in familiar environments. Similarly, size differences in dispersal propensity may occur when dispersal costs are size-dependent. When individuals do disperse, they may adopt behavioral strategies that minimize dispersal costs. Dispersing fish, for example, may travel within shoals to reduce predation risks. Further, kin shoaling may augment inclusive fitness by reducing predation of relatives. However, studies are lacking on the role of kin shoaling in dispersal. We explored how sex and size influence dispersal and kin shoaling in the cichlid Neolamprologus caudopunctatus. We microsatellite genotyped over 900 individuals from two populations separated by a potential dispersal barrier, and documented patterns of population structure, migration and within-shoal relatedness. Genetic differentiation across the barrier was greater for smaller than larger fish, suggesting larger fish had dispersed longer distances. Females exhibited weaker genetic differentiation and 11 times higher migration rates than males, indicating longer-distance female-biased dispersal. Small females frequently shoaled with siblings, possibly offsetting dispersal costs associated with higher predation risks. In contrast, small males appeared to avoid kin shoaling, possibly to avoid local resource competition. In summary, long-distance dispersal in N. caudopunctatus appears to be female-biased, and kin-based shoaling by small females may represent a behavioral adaptation that reduces dispersal costs. Our study appears to be the first to provide evidence that sex differences in dispersal influence sex differences in kin shoaling.     

Sperm competition and brain size evolution in mammals

The ‘expensive tissue hypothesis’ predicts a size trade‐off between the brain and other energetically costly organs. A specific version of this hypothesis, the ‘expensive sexual tissue hypothesis’, argues that selection for larger testes under sperm competition constrains brain size evolution. We show here that there is no general evolutionary trade‐off between brain and testis mass in mammals. The predicted negative relationship between these traits is not found for rodents, ungulates, primates, carnivores, or across combined mammalian orders, and neither does total brain mass vary according to the level of sperm competition as determined by mating system classifications. Although we are able to confirm previous reports of a negative relationship between brain and testis mass in echolocating bats, our results suggest that mating system may be a better predictor of brain size in this group. We conclude that the expensive sexual tissue hypothesis accounts for little or none of the variance in brain size in mammals, and suggest that a broader framework is required to understand the costs of brain size evolution and how these are met.     

Subliminally perceived odours modulate female intrasexual competition: an eye movement study

Background

Evidence suggests that subliminal odorants influence human perception and behavior. It has been hypothesized that the human sex-steroid derived compound 4,16-androstadien-3-one (androstadienone) functions as a human chemosignal. The most intensively studied steroid compound, androstadienone is known to be biologically relevant since it seems to convey information about male mate quality to women. It is unclear if the effects of androstadienone are menstrual cycle related.

Methodology/Principal Findings

In the first experiment, heterosexual women were exposed to androstadienone or a control compound and asked to view stimuli such as female faces, male faces and familiar objects while their eye movements were recorded. In the second experiment the same women were asked to rate the level of stimuli attractiveness following exposure to the study or control compound. The results indicated that women at high conception risk spent more time viewing the female than the male faces regardless of the compound administered. Women at a low conception risk exhibited a preference for female faces only following exposure to androstadienone.

Conclusions/Significance

We contend that a woman''s level of fertility influences her evaluation of potential competitors (e.g., faces of other women) during times critical for reproduction. Subliminally perceived odorants, such as androstadienone, might similarly enhance intrasexual competition strategies in women during fertility phases not critical for conception. These findings offer a substantial contribution to the current debate about the effects that subliminally perceived body odors might have on behavior.     

The number of subordinates moderates intrasexual competition among males in cooperatively breeding meerkats

For dominant individuals in cooperatively breeding species, the presence of subordinates is associated with both benefits (i.e. increased reproductive output and other group-living benefits) and costs (i.e. intrasexual competition on reproduction). The biological market theory predicts that dominant individuals are tolerant to same-sex group members when there are only a few subordinates, so as to maximize their own reproductive success. We investigated factors affecting aggression by dominant males and submission by subordinate males for a cooperatively breeding mammal, meerkats, Suricata suricatta. In this species, reproductive conflict occurs between the dominant male and the non-offspring males. As predicted, the number of subordinates in a group was positively associated with the aggression frequency by the dominant male and with the submission frequency by the subordinate males. Relative to the aggression frequency against male offspring, the frequency of aggression against non-offspring males was comparable in small groups, but was higher in large groups. These results indicate that reproductive conflict is present between the dominant male and the non-offspring males but is moderated in groups with small numbers of subordinates. This study provides an empirical data agreeing with the biological market theory in the context of intrasexual competition in cooperatively breeding species.     

Evolution of sex ratios in social hymenoptera: kin selection,local mate competition,polyandry and kin recognition

A model is constructed to study the effects of local mate competition and multiple mating on the optimum allocation of resources between the male and female reproductive brood in social hymenopteran colonies from the ‘points of view’ of the queen (parental manipulation theory) as well as the workers (kin selection theory). Competition between pairs of alleles specifying different sex investment ratios is investigated in a game theoretic frame work. All other things being equal, local mate competition shifts the sex allocation ratio in favour of females both under queen and worker control. While multiple mating has no effect on the queen’s optimum investment ratio, it leads to a relatively male biased investment ratio under worker control. Under queen control a true Evolutionarily Stable Strategy(ess) does not exist but the ‘best’ strategy is merely immune from extinction. A trueess exists under worker control in colonies with singly mated queens but there is an asymmetry between the dominant and recessive alleles so that for some values of sex ratio a recessive allele goes to fixation but a dominant allele with the same properties fails to do so. Under multiple mating, again, a trueess does not exist but a frequency dependent region emerges. The best strategy here is one that is guaranteed fixation against any competing allele with a lower relative frequency. Our results emphasize the need to determine levels of local mate competition and multiple mating before drawing any conclusions regarding the outcome of queen-worker conflict in social hymenoptera. Multiple mating followed by sperm mixing, both of which are known to occur in social hymenoptera, lower average genetic relatedness between workers and their reproductive sisters. This not only shifts the optimum sex ratio from the workers’ ‘point of view’ in favour of males but also poses problems for the kin selection theory. We show that kin recognition resulting in the ability to invest in full but not in half sisters reverts the sex ratio back to that in the case of single mating and thus completely overcomes the hurdles for the operation of kin selection.