Search

Male-female agonism varies throughout the primate order with males often dominating females, especially in sexually dimorphic species. While intersexual agonism has been attributed to sexually coercive contexts, it can also occur for other reasons and intersexual dominance may be influenced by the adult sex ratio. If the proportion of males is high, certain males will regularly lose against other males. Loser-effects may then pave the way for some females to dominate these males, an effect that has been described in a few primate species. Here we investigated the frequency, general style, and context of agonism among gray langurs (Semnopithecus spp.). Data were collected at two study sites, at Jodhpur, India (one group), and at Ramnagar, Nepal (two groups). The adult sex ratio varied between 0.077 and 1.000 males to females. At both sites, data on agonistic interactions (aggression and submission) were collected in continuous focal animal and ad libitum sampling techniques during 1,945 contact hours (including 1,220 focal animal hours, total). Although aggression intensity was low, high directional consistency and the rare occurrence of counteraggression suggested a despotic dominance style, a likely prerequisite for dominance effects based on adult sex ratio. Aggression by females against males was very rare and mainly occurred in the defense of offspring. We found little evidence for partial female dominance regardless of adult sex ratio. In a few cases in which a female had a higher dominance index than a male, she did not dominate this male in dyadic encounters. Agonism by males directed at females occurred mainly in a feeding context while male policing and a sexual context were both rare. The latter was mostly restricted to females after they had harassed a sexual interaction or after they had behaved proceptively toward a male. Our study suggests that across species the effect of adult sex ratio on female dominance might be more variable than previously suggested. The fact that most agonism between males and females occurred over food identifies intersexual feeding competition as a new research avenue with potentially important consequences for existing ideas on the costs and benefits of group life and composition.

Life history variables such as the age at first reproduction and the interval between consecutive births are measures of investment in growth and reproduction in a particular population or species. As such they allow for meaningful comparisons of the speed of growth and reproduction between species and between larger taxa. Especially in primates such life history research has far reaching implications and has led for instance to the "grandmother hypothesis". Other links have been proposed with respect to dietary adaptations: Because protein is essential for growth and one of the primary sources of protein, leaves, occurs much less seasonally than fruits, it has been predicted that folivorous primates should grow faster compared to frugivorous ones. However, when comparing folivorous Asian colobines with frugivorous Asian macaques we recently documented a longer, instead of a shorter gestation length in folivores while age at first reproduction and interbirth interval did not differ. This supports earlier findings for Malagasy lemurs in which all life history variables tested were significantly longer in folivores compared to frugivores. Wondering why these trends were not apparent sooner, we tried to reconstruct our results for Asian primates with data from four popular life history compilations. However, this attempt failed; even the basic, allometric relationship with adult female body mass that is typical for life history variables could not be recovered. This negative result hints at severe problems with data quality. Here we show that data quality can be improved significantly by standardizing the variables and by controlling for factors such as nutritional conditions or infant mortality. Ideally, in the future, revised primate life history data should be collated in a central database accessible to everybody. In the long run such an initiative should be expanded to include all mammalian species.

Dominance hierarchies based on resource holding potential (RHP) or age are central to the social structure of many group-living animals. Nonhuman primate females and some other mammals are unusual because ranks can depend on kin support or follow an inverse age-graded pattern independent of kin. Whereas the former hierarchies are mediated by frequent coalitions leading to long-term stability, the latter are unstable due to individualistic contests and occasional revolutionary coalitions. We use mathematical modeling to investigate this variation, based on the interaction of triads, subgroups of the group consisting of 3 individuals. This produces 4 key situations, namely that relationships can be either stable or unstable, and involving coalitionary support or not. Varying the parameter values of our model and applying it to real-world examples, we conclude that the emergence of different dominance hierarchies depends on 6 major factors. In addition to RHP, reproductive skew and demography often suggested previously, we stress the importance of 1) the variance in relatedness; 2) the costs and likelihood of interventions; and 3) the value of resources and rank in different life stages, which might be tightly linked to the life history of a species. [ABSTRACT FROM AUTHOR]

Feeding competition is suggested as a major factor constraining group size in social foragers. It has, however, been challenging to demonstrate consequences of reduced energy gain in terms of fitness, possibly because social foragers may compensate negative effects of scramble competition via adjustments in time budgets. Herbivorous animals are particularly interesting in this context because their fibrous diet and slow digestion process may make it difficult to adjust time budgets. Here we investigate infant development and reproductive rates in Phayres leaf monkeys (Trachypithecus phayrei) at Phu Khieo Wildlife Sanctuary, Thailand. The diet of the species consists of 39.0% leaves (maximum 81.2% per month). Our analysis is based on data for 3 groups (185 group months) of different sizes (mean 11.4, 18.3, and 25.8 individuals, respectively). Infant development was significantly slower in the large group, in which infants changed to the adult coat later than in the medium-sized group (20.3 vs. 26.3 weeks) and were older when weaned (18.3, 19.7, and 21.4 months, respectively). The interbirth interval after a surviving infant significantly increased with group size (21.3, 22.8, and 24.5 months) while rearing success did not differ (77.8%, 76.5%, and 82.4%, survival to 2 years). Thus, infants in the large group developed more slowly were weaned later and females reproduced more slowly. With similar infant survival rates, these different reproductive rates indicate fitness differences across groups. As in other herbivores, these group-specific differences may reflect scramble competition for food or differences in habitat quality. [ABSTRACT FROM AUTHOR]