Chimpanzee Violence is a Serious Topic. A Response to Sussman and Marshack's Critique of Demonic Males: Apes and the Origins of Human Violence

Sussman and Marshack criticize Demonic Males as being wrong in its generalizations about chimpanzee behavior, and flawed in its theoretical interpretations. I show that studies of chimpanzees conducted since Demonic Males was published (in 1996) have amply supported the claim that coalitionary killing is an important feature of chimpanzee life. It therefore demands to be explained. The theory developed in Demonic Males remains useful, and can help in the development of nonviolent strategies.Human Evolutionary Biolog

Human Adaptation to the Control of Fire

Charles Darwin attributed human evolutionary success to three traits. Our social habits and anatomy were important, he said, but the critical feature was our intelligence, because it led to so much else, including such traits as language, weapons, tools, boats, and the control of fire. Among these, he opined, the control of fire was “probably the greatest ever [discovery] made by man, excepting language.” Despite this early suggestion that the control of fire was even more important than tool use for human success, recent anthropologists have made only sporadic efforts to assess its evolutionary significance. Here we use recent developments in understanding the role of cooked food in human diets to support the spirit of Darwin's offhand remark. We first consider the role of fire in increasing the net caloric value of cooked foods compared to raw foods, and hence in accounting for the unique pattern of human digestion. We then review the compelling evidence that humans are biologically adapted to diets that include cooked food, and that humans have a long evolutionary history of an obligate dependence on fire. Accordingly, we end by considering the influence of fire on various aspects of human biology. We pay particular attention to life history, and also briefly discuss effects on anatomy, behavior, and cognition.Human Evolutionary Biolog

Sex Differences in Object Use by Young Chimpanzees Resemble Those of Children

Human Evolutionary Biolog

Diet and Reproductive Function in Wild Female Chimpanzees (Pan Troglodytes Schweinfurthii)at Kibale National Park, Uganda

Human female reproductive function is highly sensitive to current energetic condition, indicating adaptation to modulate reproductive effort in accordance with changing ecological conditions that might favor or disfavor the production of offspring. Here, we test the hypothesis that reproductive capacity in female chimpanzees is likewise limited by current energetic condition. We used 12 years of data on wild chimpanzees (Pan troglodytes schweinfurthii) in the Kanyawara community of Kibale National Park, Uganda, to examine the relationship of dietary quality, as assessed by fruit components of the diet, to the occurrence of sexually receptive females, concentrations of ovarian steroid hormones, and timing of conception. We found that the frequency of females having sexual swellings was positively related to the consumption of drupe fruits. Estrogen levels of both cycling and noncycling females increased during seasonal peaks in the consumption of drupe fruits. When average fruit consumption remained high across months, females conceived more quickly. These results support the hypothesis that cycling and conception in chimpanzees are contingent upon high energy balance, and they indicate that the availability of fruit is a key variable limiting reproductive performance in chimpanzees. Chimpanzees appear to share with humans a reproductive system that is primed to respond to proximate levels of energy acquisition.Anthropolog

The Energetic Significance of Cooking

While cooking has long been argued to improve the diet, the nature of the improvement has not been well defined. As a result, the evolutionary significance of cooking has variously been proposed as being substantial or relatively trivial. In this paper, we evaluate the hypothesis that an important and consistent effect of cooking food is a rise in its net energy value. The pathways by which cooking influences net energy value differ for starch, protein and lipid, and we therefore consider plant and animal foods separately. Evidence of compromised physiological performance among individuals on raw diets supports the hypothesis that cooked diets tend to provide energy. Mechanisms contributing to energy being gained from cooking include increased digestibility of starch and protein, reduced costs of digestion for cooked versus raw meat, and reduced energetic costs of detoxification and defense against pathogens. If cooking indeed consistently improves the energetic value of foods through such mechanisms, its evolutionary impact depends partly on the relative energetic benefits of non-thermal processing methods used prior to cooking. We suggest that if non-thermal processing methods, such as pounding, were used by Lower Paleolithic Homo, they likely provided an important increase in energy gain over unprocessed raw diets. However, cooking has critical effects not easily achievable by non-thermal processing, including the relatively complete gelatinization of starch, efficient denaturing of proteins, and killing of foodborne pathogens. This means that however sophisticated the non-thermal processing methods were, cooking would have conferred incremental energetic benefits. While much remains to be discovered, we conclude that the adoption of cooking would have led to an important rise in energy availability. For this reason, we predict that cooking had substantial evolutionary significance.AnthropologyHuman Evolutionary Biolog

Male more than female infants imitate propulsive motion

Few experimental studies investigate the mechanisms by which young children develop sex-typed activity preferences. Gender self-labeling followed by selective imitation of same-sex models currently is considered a primary socialization mechanism. Research with prenatally androgenized girls and non-human primates also suggests an innate male preference for activities that involve propulsive movement. Here we show that before children can label themselves by gender, 6- to 9-month-old male infants are more likely than female infants to imitate propulsive movements. Further, male infants’ increase in propulsive movement was linearly related to proportion of time viewing a male model’s propulsive movements. We propose that male sex-typed behavior develops from socialization mechanisms that build on a male predisposition to imitate propulsive motion.Anthropolog

Bonobos Exhibit Delayed Development of Social Behavior and Cognition Relative to Chimpanzees

Phenotypic changes between species can occur when evolution shapes development. Here, we tested whether differences in the social behavior and cognition of bonobos and chimpanzees derive from shifts in their ontogeny, looking at behaviors pertaining to feeding competition in particular. We found that as chimpanzees (n = 30) reached adulthood they became increasingly intolerant of sharing food, whereas as adults, bonobos (n = 24) maintained high, juvenile levels of food-related tolerance. We also investigated the ontogeny of inhibition during feeding competition. In two different tests, we found that bonobos (n = 30) exhibited developmental delays relative to chimpanzees (n = 29) in the acquisition of social inhibition, with these differences resulting in less skill among adult bonobos. The results suggest that these social and cognitive differences between two closely related species result from evolutionary changes in brain development.Human Evolutionary Biolog

A Direct Comparison of Scan and Focal Sampling Methods for Measuring Wild Chimpanzee Feeding Behaviour

Focal sampling is the most accurate method for measuring primate activity budgets, but is sometimes impractical. An alternative is scan sampling, in which the behaviour of the group is recorded at regular intervals. The simplest technique is to record whether at least one animal is engaged in the behaviour of interest. By direct comparison with focal data collected simultaneously on the same population, we assess the validity of this simple group-level sampling method for studying chimpanzee (Pan troglodytes schweinfurthii) feeding behaviour. In a 13-month study at Kanyawara, Kibale National Park, Uganda, group-level scan sampling provided statistically similar measures of broad diet composition to those produced by focal data, despite considerable seasonal variation. Monthly means of the percentage of time spent consuming non-fig fruit calculated from group-level scan sampling were highly correlated with those from focal sampling. This validates previous methodology used to identify periods of high energy availability. However, group-level scans tended to overestimate the percentage of observation time spent feeding, particularly for adult males. We conclude that this method of group-level scan sampling provides valuable data for characterising broad diet choice in chimpanzees and other species, but may be of limited use for estimating individual feeding time.Human Evolutionary Biolog

Long-term inter-sexual association patterns among wild chimpanzees

The widely accepted socio-ecological model of primate sociality assumes that males and female chimpanzees do not exhibit differentiated social relationships. However, despite anecdotal evidence to the contrary, this assumption has never been explicitly tested. We used 14 years of data from the Kanyawara community in Kibale National Park, Uganda to describe inter-sexual association patterns among these chimpanzees. We calculated a composite index using temporal and spatial association data to characterize the relationships between data to characterize the relationships between 336 male-females dyads. We considered any dyad with a composite index greater than one standard deviation above the mean to be strongly associated. We found that: (1) while the majority of male-female dyads were not strongly associated, a subset of dyads showed greater than average association across several two-year time periods; (2) all but one of the maternal kin dyads (either mother-son or brother-sister) had differentiated relationships; and (3) the association preferences of some dyads remained consistent despite changes in the reproductive condition of the female over time. We used generalized linear models to determine the effect of reproductive state, rank and seasonality on patterning these long-term associations. Our finding that chimpanzees exhibit differentiated inter-sexual association patterns will have far-reaching effects on studies of other forms of male-female interaction such as aggression, and further our understanding of the evolution of human pair-bonding.AnthropologyHuman Evolutionary Biolog