Male-immature relationships in multi-male groups of mountain gorillas (Gorilla beringei beringei).

We examined the pattern and possible functions of social interactions between adult males and immatures in three free-ranging, multi-male groups of mountain gorillas (Gorilla beringei beringei). Previous studies conducted during the 1970s when groups contained one to three adult males concluded that male-immature relationships were likely to be a form of low-cost paternal investment [Stewart, Mountain gorillas: three decades of research at Karisoke. Cambridge, UK: Cambridge University Press, 2001]. We evaluated whether this hypothesis still held in groups containing six to nine adult males, or if male-immature relationships might serve other functions (e.g. mating effort, kin selection, or alliance building). Overall, we found that immatures spent the most time near, and interacted most with, the alpha silverback. These behaviors peaked during the period when infants were still quite vulnerable but increasing their independence from their mothers. Such findings suggest that parenting effort remains the primary function of male-immature relationships; however, there is some evidence for the mating effort hypothesis as well.

The responses of female baboons (Papio cynocephalus ursinus) to anomalous social interactions: evidence for causal reasoning?

Baboons' (Papio cynocephalus ursinus) understanding of cause-effect relations in the context of social interactions was examined through use of a playback experiment. Under natural conditions, dominant female baboons often grunt to more subordinate mothers when interacting with their infants. Mothers occasionally respond to these grunts by uttering submissive fear barks. Subjects were played causally inconsistent call sequences in which a lower ranking female apparently grunted to a higher ranking female, and the higher ranking female apparently responded with fear barks. As a control, subjects heard a sequence made causally consistent by the inclusion of grunts from a 3rd female that was dominant to both of the others. Subjects responded significantly more strongly to the causally inconsistent sequences, suggesting that they recognized the factors that cause 1 individual to give submissive vocalizations to another.

Human adoption in evolutionary perspective.

Exploitation is a fundamental element of the parental strategies of many species of birds. Cuckoos, for example, lay their eggs in the nest of other birds, who often unwittingly rear the alien nestlings as their own. Nest parasitism is an efficient reproductive strategy for cuckoos, who do not have to worry about building a nest, incubating their eggs, or feeding their nestlings. But not all hosts respond passively to such intrusions. In response to parasitic cowbirds, for example, robins have evolved the ability to detect and selectively eject alien young from their nests. Human parenting strategies differ sharply from the strategies of cuckoos and robins. Unlike cuckoos, we are reluctant to allow our children to be raised by others. Unlike robins, we knowingly rear strange young. What makes human behavior toward children so different from that of cuckoos and robins? Humans seem to share a number of predispositions that facilitate successful adoptive relationships, and the desire to raise children seems to be pervasive among modern humans. Despite these commonalities, patterns of adoption transactions vary greatly among contemporary human societies. This paper considers the origins and causes of cross-cultural variation in human adoptive behavior from an evolutionary perspective.

Making amends : Adaptive perspectives on conflict remediation in monkeys, apes, and humans.

Conflict is an integral, and potentially disruptive, element in the lives of humans and other group-living animals. But conflicts are often settled, sometimes within minutes after the altercation has ended. The goal of this paper is to understand why primates, including humans, make amends. Primatologists have gathered an impressive body of evidence which demonstrates that monkeys and apes use a variety of behavioral mechanisms to resolve conflicts. Peaceful post-conflict interactions in nonhuman primates, sometimes labeled "reconciliation," have clear and immediate effects upon former adversaries, relieving uncertainty about whether aggression will continue, reducing stress, increasing tolerance, and reducing anxiety about whether aggressors will resume aggression toward former victims. However, the long-term effects of these interactions are less clearly established, leaving room to debate the adaptive function of conflict resolution strategies among primates. It is possible that reconciliatory behavior enhances the quality of valued, long-term social relationships or that reconciliatory interactions are signals that the conflict has ended and the actor's intentions are now benign. Both of these hypotheses may help us to understand how and why monkeys, apes, and humans make amends.

Local resource competition and the evolution of male-biased sex ratios.

In many mammalian species, maturing males disperse from their natal groups, while females remain near their places of birth. If sources are limited locally, related females may be forced to compete with one another for access to resources, while mature males will compete primarily with unrelated individuals in non-natal groups. Clark (1978) suggested that under such circumstances the extent of competition among females could be reduced if females uniformly skewed the sex ratio of their offspring in favor of males. Skewed secondary sex ratios and investment patterns in galagos and red deer appear to be consistent with this hypothesis. Several authors have recently demonstrated that the evolution of skewed sex ratios may be influenced by population structure, and Wilson & Colwell (1981) have shown that male-biased sex ratios can evolve in structured haplodiploid populations. Here, I construct an intrademic group selection model to simulate the effects of density-dependent mortality upon diploid populations structured much as Clark describes: males disperse, females remain near their female relatives, and local resource competition limits the size of local groups. The results confirm that male-biased sex ratios can evolve in a diploid species, even when groups are large and the skew in sex ratio is substantial.

Feeding, foraging, and food sharing behavior of immature chimpanzees.

Observations of chimpanzees under naturalistic conditions show that nutritional autonomy increases as young individuals mature. Substantial proportions of feeding time are spent eating food obtained through solicitation. Food sharing at SOPF occurs among mother and infant pairs most frequently; less frequently between adult males and infants or juveniles, and least frequently between adult females and immature individuals. Mothers tend to share what their offspring cannot obtain independently while others primarily share what is easiest to replace. These patterns of solicitation and distribution of food are consistent with predictions based upon benefits to individuals through kin selection and/or energetic efficiency.

Patterns of food sharing among mother and infant chimpanzees at Gombe National Park, Tanzania.

The patterns of food sharing among mother and infant chimpanzees at the Gombe National Park, Tanzania, are primarily influenced by the infants' ability to procure and process foods independently. Foods infants could find and process on their own were not shared by mothers as frequently as foods infants could not obtain independently. Temporal changes were observed in the patterns of interactions as infants matured. It is suggested that food sharing may be considered a behavioral strategy which facilitates the infant's transition from dependent suckling to independent foraging.

Sources of variation in interbirth intervals among captive bonnet macaques (Macaca radiata).

Female fitness is a function of variation in the length of females' reproductive careers, the viability of their offspring, and the frequency with which they give birth. Infant loss shortens interbirth intervals in most primate species, but we know considerably less about other factors that contribute to variation in the length of interbirth intervals within groups. In one large captive group of bonnet macaques, maternal parity, age, experience, family size, and recent reproductive history are all associated with variation in the length of intervals that follow the birth of surviving infants. Primiparous females have the longest interbirth intervals, while multiparous females who have produced surviving infants in the past and have raised their last infant successfully have the shortest interbirth intervals. Infant sex and maternal rank have no direct effect upon the length of interbirth intervals. One of the underlying causes of variation in the length of interbirth intervals after surviving births seems to be variation in the timing of conceptions among females. Females who conceive early in the mating season tend to have shorter interbirth intervals than other females. However, females who are multiparous, experienced, and have recently raised infants have late conceptions and short interbirth intervals.

Empirical evaluation of preservation methods for faecal DNA.

We evaluate the relative effectiveness of four methods for preserving faecal samples for DNA analysis. PCR assays of fresh faecal samples collected from free-ranging baboons showed that amplification success was dependent on preservation method, PCR-product size, and whether nuclear or mitochondrial DNA was assayed. Storage in a DMSO/EDTA/Tris/salt solution (DETs) was most effective for preserving nuclear DNA, but storage in 70% ethanol, freezing at -20 degrees C and drying performed approximately equally well for mitochondrial DNA and short (< 200 bp) nuclear DNA fragments. Because faecal DNA is diluted and degraded, repeated extractions from faeces may be necessary and short nuclear markers should be employed for genotyping. A review of molecular scatology studies further suggests that three to six faeces per individual should be collected.