Determinants of Harem Size in a Polygynous Primate: Reproductive Success and Social Benefits

We used long-term data on the variation in harem size in Yunnan snub-nosed monkeys to research the effects of harem size on reproductive success and the ratio of grooming received to given (RGRG). The results suggest that harem holders derive reproductive benefits commensurate with harem size, whereas the females’ reproductive success is unaffected by harem size. Males of larger harems groomed less and had higher RGRG than males of smaller harems. In the case of females, grooming given increased, and RGRG decreased with an increase in harem size. The males’ reproductive success seems to be a driver of harem size maximization. From the females’ perspective, dwindling social benefits appear to set the upper limit for harem enlargement. We also showed that males of monogamous units (‘single-female harems’) invested more into grooming their female, presumably to prevent unit disintegration and loss of mating privileges.

The Sex Ratio Influence on the Dynamics of Structured Population

This article develops and investigates a simple mathematical model that simultaneously studies influence of age and sex structure formation and sex ratio on demographic and evolutionary processes. We assume fertility depends on the population sex ratio and is described by the modified harmonic mating function with switching. The population size is regulated by limiting the juvenile survival rate when immature individual survival decreases with growth of sex-age class sizes. We received the conditions for sustainable development of the population in the parametric space of the model. The relationships between the group sizes of newborns and mature females and males are analyzed. The growth of the harem size is shown to result in higher the newborn group size in case of female numerical dominance. The offspring number demonstrates the same tendency as the sex group with the smallest number in the cases of higher female survival rate and higher newborn female proportion, and lower male survival rate. Excessive asymmetry of the sexes is shown to lead to a decrease in reproduction of polygamous species. Complex scenarios of population dynamics are studied. Transitions between different dynamic modes are caused by changes in both population parameters determining birth, survival and self-regulation rates, and the formation process of the mating pairs. The model parametric space is shown to have multistability areas in which the initial condition variation can lead to the realization of one dynamic mode or the other. The multistability is the result of both the system nonlinearity and the complex bifurcation mechanisms, and the changing pair formation principle. Consequently, even a small variation of the current population size changing the sex ratio complicates the population behavior and can give significant change in the dynamic mode observed.

Mating success and body condition not related to foraging specializations in male fur seals

Individual specialization is widespread among wild populations. While its fitness consequences are central in predicting the ecological and evolutionary trajectories of populations, they remain poorly understood. Long-term individual foraging specializations occur in male Antarctic ( Arctocephalus gazella ) and Australian ( A. pusillus doriferus ) fur seals. Strong selective pressure is expected in these highly dimorphic and polygynous species, raising the question of the fitness payoffs associated with different foraging strategies. We investigated the relationship between individual isotopic niche (a proxy of foraging specialization), body size and condition, and an index of reproductive success (harem size) in territorial males. Individuals varied greatly in their skin and fur isotopic values reflecting a range of foraging strategies within the two populations. However, in both species, isotopic niche was not correlated to body size, condition or mating success ( R 2 / ρ < 0.06). Furthermore, no foraging niche was predominant in either species, which would have indicated a substantial long-term fitness benefit of a particular strategy via a higher survival rate. These results suggest that the fitness consequences of a foraging strategy depend not only on the quality of prey and feeding habitat but also on an individual's hunting efficiency and skills.

Early life expenditure in sexual competition is associated with increased reproductive senescence in male red deer

The evolutionary theories of senescence predict that investment in reproduction in early life should come at the cost of reduced somatic maintenance, and thus earlier or more rapid senescence. There is now growing support for such trade-offs in wild vertebrates, but these exclusively come from females. Here, we test this prediction in male red deer ( Cervus elaphus ) using detailed longitudinal data collected over a 40-year field study. We show that males which had larger harems and thereby allocated more resources to reproduction during early adulthood experienced higher rates of senescence in both harem size and rut duration. Males that carried antlers with more points during early life did not show more pronounced declines in reproductive traits in later life. Overall, we demonstrate that sexual competition shapes male reproductive senescence in wild red deer populations and provide rare empirical support for the disposable soma theory of ageing in males of polygynous vertebrate species.

Testosterone is associated with harem maintenance ability in free-ranging grey-headed flying-foxes, Pteropus poliocephalus

Males of many vertebrate species aggressively defend their reproductive interests by monopolizing females, and the ‘challenge hypothesis’ predicts that testosterone levels in reproductive contexts rise to facilitate males' competitive behaviours necessary for meeting social challenges. The hypothesis is successful in explaining patterns of testosterone secretion in many avian species, but remains comparatively unexplored in mammals. ‘Circulating plasma testosterone levels (T)’ were studied in relation to harem maintenance in grey-headed flying-foxes, Pteropus poliocephalus . In this species, harems provide mating opportunities and so a male's ability to maintain a harem is likely to correlate with his fitness. We hypothesized that if T reflect a male's ability to withstand challenges from competitors, then T should be linked to successful harem maintenance. To test this, we temporarily removed males from their territories prior to and during the short mating period, recording their harem sizes both before removal and after reintroduction. Most males successfully reclaimed their territory and a harem, but during the mating period, males with higher T had harems closer to their original size, and males with lower T suffered reduction in harem size. Our findings highlight the role of T in harem maintenance in a major mammalian taxon with complex forms of social organization.