Karen B. Strier

Fission-Fusion Dynamics: New Research Frameworks

Renewed interest in fission‐fusion dynamics is due to the recognition that such dynamics may create unique challenges for social interaction and distinctive selective pressures acting on underlying communicative and cognitive abilities. New frameworks for integrating current knowledge on fission‐fusion dynamics emerge from a fundamental rethinking of the term “fission‐fusion” away from its current general use as a label for a particular modal type of social system (i.e., “fission‐fusion societies”). Specifically, because the degree of spatial and temporal cohesion of group members varies both within and across taxa, any social system can be described in terms of the extent to which it expresses fission‐fusion dynamics. This perspective has implications for socioecology, communication, cognitive demands, and human social evolution.

Seasonal and Social Correlates of Fecal Testosterone and Cortisol Levels in Wild Male Muriquis (Brachyteles arachnoides)

Fecal testosterone and cortisol levels were analyzed from six wild male muriquis (Brachyteles arachnoides) over a 19-month period at the Estação Biológica de Caratinga in Minas Gerais, Brazil, to investigate the hormonal correlates of seasonal sexual behavior and environmental conditions. Group mean testosterone levels based on weekly samples from the six males did not differ between copulatory and noncopulatory periods or between rainy and dry seasons. Cortisol levels did change with copulatory periods, and were significantly higher during the second dry season, when mating continued following an exceptionally heavy rainy season, than during the first dry season, when mating ceased. Males exhibited individual variation in the timing of their hormone shifts relative to their sexual activity, but neither hormone levels nor sexual activity were related to male age. Despite individual differences in the timing of testosterone fluctuations around the onset and offset of the copulatory season, all males exhibited elevated cortisol concentrations following a slight increase in testosterone at the beginning of the copulatory season. Both the lack of significant changes in testosterone levels with the onset of the rainy and copulatory season and the lack of prebreeding increases in cortisol may be related to the low levels of overt aggression displayed by male muriquis over access to mates.

Adaptive radiation of the ateline primates

Abstract We propose a synthesis of anatomical, behavioral and ecological data in reconstructing the phylogeny and evolution of the ateline primates. The atelines are a monophyletic group divisible into an alouattin lineage, including Alouatta and the Miocene fossil Stirtonia , and an atelin lineage, including Lagothrix and its sister-group, the Ateles-Brachyteles clade. Body size increase was important in the origins of atelines and in their subsequent diversification. Larger body size perimitted an increased reliance on leaves (probably immature) by comparison with other platyrrhines, even in predominantly frugivorous forms. A novel locomotor pattern involving climbing, some forelimb climbing/hauling and hanging by the prehensile tail, which involved the evolution of many features derived for platyrrhines, was present in the first atelines, perhaps also connected with relatively large body size. The last common ancestor (LCA) of atelines, which morphologically resembled Lagothrix more than any other living form, probably lived in fluid multi-male polygynous groups characterized by female dispersal and had a moderate degree of sexual dimorphism. Alouattins became strongly committed to an energy minimizing adaptive strategy based upon a bulky, nutritionally poor diet composed mainly of leaves. Alouatta thus travels relatively little daily, uses a non-acrobatic style of deliberate quadrupedalism, defends resources inexpensively by long distance calling, lives in cohesive polygynous groups that influenced the development of high sexual dimorphism, and has a proportionately small brain size. These are derived characteristics which facilitate their exploitation of a wide variety of habitats, frequently in sympatry with atelins. Ancestral atelins were probably larger than the LCA. They engaged in more forelimb-dominated climbing and probably ranged widely in fluid, polygynous social groups. The least known modern form, Lagothrix , may occupy a hard-fruit feeding niche, in contrast to the soft-fruit specialist Ateles . A lithe, brachiating body plan that resembles hylobatids typifies Ateles and Brachyteles , suggesting a common ancestral foraging strategy involving rapid bursts of energy expenditure. The subsequent evolution of shearing postcanines in Brachyteles , which converges on Alouatta superficially, is interpretated as a derived adaptive compromise possibly related to the greater seasonality of the Atlantic Coastal Forest of southeastern Brazil, where the genus probably arose, in comparison to inland tropical forests. Energy saving features of Brachyteles include contracted home ranges and low intra-group aggression.

Aging in the Natural World: Comparative Data Reveal Similar Mortality Patterns Across Primates

Aging patterns in humans fall within the parameters of other primates in natural populations. Human senescence patterns—late onset of mortality increase, slow mortality acceleration, and exceptional longevity—are often described as unique in the animal world. Using an individual-based data set from longitudinal studies of wild populations of seven primate species, we show that contrary to assumptions of human uniqueness, human senescence falls within the primate continuum of aging; the tendency for males to have shorter life spans and higher age-specific mortality than females throughout much of adulthood is a common feature in many, but not all, primates; and the aging profiles of primate species do not reflect phylogenetic position. These findings suggest that mortality patterns in primates are shaped by local selective forces rather than phylogenetic history.

New World primates, new frontiers: Insights from the woolly spider monkey, or muriqui (Brachyteles arachnoides)

Despite over 25 years of intensive research, much of our knowledge of primate behavior has been limited to a small number of Old World, semiterrestrial species. With the exception of chimpanzees, these species share consistent patterns of behavior, including aggressive competition between males, male dominance over females, male dispersal and female kin groups, that have come to characterize the Order Primates. However, as research has expanded to include a number of newly studied species, such generalizations about primate behavior are no longer appropriate. Data presented on the wooly spider monkey, or muriqui (Brachyteles arachnoides), provide examples of the ways in which traditional views of primate behavior are being reconsidered. In muriquis, and in several other New World monkeys, female kin groups do not occur because females disperse from their natal groups. In addition, aggressive competition between males is virtually absent; and, female muriquis appear to be codominant with males. Constraints on sexual dimorphism may help to explain why muriquis (and other New World primates) differ so fundamentally from the better known semi-terrestrial Old World monkeys.

Brotherhoods Among Atelins: Kinship, Affiliation, and Competition

U.S.A.) (With 3 Figures) (Ace. 20-VII-1994) Summary This paper examines differences in male affiliative associations and dominance relationships in atelin primates. Despite the occurrence of male philopatry in all three genera, male social relations varied considerably. The strength of male affiliations was attributed to differences in between group competition for access to groups of females, while the type of male dominance relationships was attributed to differences in within group competition for access to individual females. Female grouping patterns and the socionomic sex ratio appeared to be more important than group size in predicting the strength of between group competition and male affiliations. Male dominance over females appeared to be more important than estrus synchrony in predicting the type of within group competition and the presence of hierarchical or egalitarian relationships among males.

Social dynamics of male muriquis (Brachyteles arachnoides hypoxanthus)

We investigated patterns of sociality among wild male northern muriquis (Brachyteles arachnoides hypoxanthus) to explore some of the possible ways in which within-group scramble competition might shape their social and mating strategies. The 13 adult males in our study group spent an average of 54.5′4.2% of their time in proximity to at least one other adult male. They were more likely to associate with one another, but not with adult females, when they were resting than when they were feeding. Embrace rates were positively related to the proportion of time males spent in proximity during resting. Males that associated closely with one another tended to interact more often, although individual differences in male social styles and rates of interactions were apparent. Young adult males had significantly higher social maintenance rates than the oldest males in our sample, but no other effects of age were detected in any of our other behavioral comparisons. In roughly 20% of all dyads, one male valued the association significantly more than the other. Males tended to value associates with higher mating success than themselves, and to share access to the same females on the same days with their closest associates. Neither of the two pairs of maternal brothers in our study group were important associates to their kin, but brothers shared copulations in the same polyadic copulation partnerships with one another. Although still preliminary, our findings suggest that muriqui males differentiate among their possible social partners in ways that may minimize the variance in their mating success under the unusual conditions imposed by scramble competition for reproductive opportunities.

Primates and parasites: A case for a multidisciplinary approach

Examinations of primate parasitic infections can inform both primatologists and parasitologists about evolutionary and ecological relationships. Interspecific, intraspecific, and interindividual variation in parasitic infections may correlate with environmental, demographic, behavioral, and human variables. Understanding these relationships is particularly important for conservation management issues for endangered species. We describe techniques for the noninvasive collection and preservation of fecal samples from wild primates and the salvaging of parasitological information from primate hosts in the field.

Demography and social-structure of one group of muriquis (Brachyteles arachnoides)

We monitored one group of muriquis, or woolly spider monkeys (Brachyteles arachnoides), over a 9-year period at Fazenda Montes Claros, Minas Gerais, Brazil. The group grew from 22 to 42 individuals due to the births of 21 surviving infants. Eight immigrations involving immature females were offset by emigrations and disappearances. The home range of the group expanded as the group size increased. The group traveled as a cohesive unit during the first 6 years of the study, but recently it has begun to show greater tendencies to fission temporarily into smaller subgroups. Six adult males from the other muriqui group at this site have simultaneously increased their associations with the main study group. These observations indicate that the group is in a state of transition which may lead, ultimately, either to its division into two smaller units or to a more fluid social structure.

Reproductive aging patterns in primates reveal that humans are distinct

Women rarely give birth after ∼45 y of age, and they experience the cessation of reproductive cycles, menopause, at ∼50 y of age after a fertility decline lasting almost two decades. Such reproductive senescence in mid-lifespan is an evolutionary puzzle of enduring interest because it should be inherently disadvantageous. Furthermore, comparative data on reproductive senescence from other primates, or indeed other mammals, remains relatively rare. Here we carried out a unique detailed comparative study of reproductive senescence in seven species of nonhuman primates in natural populations, using long-term, individual-based data, and compared them to a population of humans experiencing natural fertility and mortality. In four of seven primate species we found that reproductive senescence occurred before death only in a small minority of individuals. In three primate species we found evidence of reproductive senescence that accelerated throughout adulthood; however, its initial rate was much lower than mortality, so that relatively few individuals experienced reproductive senescence before death. In contrast, the human population showed the predicted and well-known pattern in which reproductive senescence occurred before death for many women and its rate accelerated throughout adulthood. These results provide strong support for the hypothesis that reproductive senescence in midlife, although apparent in natural-fertility, natural-mortality populations of humans, is generally absent in other primates living in such populations.