Each year, the Evolutionary Anthropology Society (EAS) recognizes outstanding research presented at the American Anthropological Association Annual Meeting. Best Paper by a Student is awarded to an individual who was enrolled as a student at the time of paper submission, and Best Paper by a New Investigator is awarded to an individual who received their PhD within five years of submission.
This year, EAS recognized the contribution of PhD student Stephanie Fox (University of New Mexico) and postdoctoral researcher Joe Hackman (University of Utah).
Stephanie Fox’s presentation, “Social Relationship Quality and Coalition Formation among Adult Female Chimpanzees at Kanyawara, Kibale National Park, Uganda,” investigated the importance of relationship strength for partner choice in cooperation for adult female chimpanzees. In many gregarious primate species, females are philopatric and affiliative, cooperative relationships often form around kinship ties. However, among female dispersing chimpanzees, females rarely have access to kin and are notorious for affiliating with each other infrequently compared to male chimpanzees and compared to females in other primate species. Yet females do form differentiate relationships and occasionally form cooperative coalitions against other chimpanzees in their community. Using 10 years of behavioral data on wild chimpanzees, the researchers demonstrated that female chimpanzees tend to form coalitions with partners that they affiliate with more. This suggests that in the absence of kinship, females can leverage social ties for cooperation, even if those ties are relatively subtle. Additionally, these results highlight a difference between chimpanzees and bonobos, as patterns of affiliation fail to predict coalitionary support among female bonobos despite frequent affiliation and coalition formation. Thus, this new research on chimpanzee females is a valuable step in understanding how different patterns of cooperation arise among female apes. This presentation was coauthored with Martin Muller, Nicole Thompson González, Drew Enigk, Zarin Machanda, Emily Otali, Richard Wrangham, and Melissa Emery Thompson.
Joe Hackman’s presentation, “Fertility Variance and Inequality over the Course of the Demographic Transition,” coauthored with Karen Kramer, examined a unique case study of a Mayan community undergoing fertility decline while increasingly engaged in the regional market economy. Many evolutionary explanations of fertility decline imply that small family sizes will reap economic dividends, with families adopting a low fertility, high investment reproductive strategy that produces offspring capable of landing high status jobs in the wage-labor economy. However, in mixed economies, or places where traditional economic production is practiced alongside novel market-based opportunities, household livelihood diversification is a key risk-mitigating strategy that often requires larger families. The authors suggest that in contexts characterized by unequal, inconsistent or uncertain access to market opportunities, the economic benefits of reduced fertility may not outweigh the need to diversify livelihoods. Using longitudinal reproductive history data with detailed household composition and economic data the authors tested whether fertility decisions made in the early days of the demographic transition shaped household composition years later, and whether household composition had an impact on livelihood diversity and income. Predictions were tested with a structural equation model to account for the simultaneous effects of fertility on household size and household size on livelihood diversity and income.
The authors found that households with higher fertility tended to have more productive adults years later. Additionally, households with more adults were better able to diversify their livelihoods, which lead to an increase in income per capita. However, after controlling for livelihood diversity, larger households tended to have lower income. Results suggest that households seem to face a demographic balancing act, whereby too large a household may dilute available income, while too few adults may mean the households cannot diversify and may need to commit to a single, and perhaps uncertain, economic activity. Additionally, the persistence of mixed economies around the world, and the fertility dynamics associated with them, may help explain stalling or slowed demographic transitions where livelihood diversity is a viable economic strategy.