May 14, 2010
A newly published study on African monkeys forges a surprising link among males, infants, and stress. An in-press, just-now-online paper from the journal Animal Behaviour by German scientist Dr. Stefanie Henkel and her colleagues makes a convincing case that male monkeys choose to carry infants as a way to cement good relationship with other male monkeys—and that they pay a price for doing so.
Carried out on Barbary macaques, a type of monkey that lives in large multi-male, multi-female groups, with the females staying put and the males transferring to live elsewhere at puberty, the study was not done in Africa but rather in a monkey park called La Foret des Singes in France. Previous work on this species had shown males to be fantastically interested in infants shortly after birth. Yet DNA fingerprinting indicated that they weren’t targeting their own offspring.
Why the interest, then? Scientists had tentatively linked male infant-carrying with the regulation of social relationships and the formation of coalitions. Henkel et al. have blazed a new path by assessing hormonal levels of the free-ranging macaques in the French park, while also applying social-network analysis to the animals’ relationships. Hypothesizing that infant-carrying might relieve the males’ stress as they go about negotiating their tense world, the researchers got a surprise.
During the birth season, male macaques who carried infants both had significantly stronger relationships with other macaques, and higher stress-hormone outputs (of glucocorticoids) than males who did not carry. Wending their way through a detailed and careful analysis, the authors conclude that there is likely a causal rather than merely correlative relationship between the babies and the males’ hormone bath.
Why should infant-carrying lead to better male-male relating? The scientists noticed that males who carry infants interact differently with other males than non-carriers do; only when carrying do males engage in elaborate greetings and nuzzling behaviors.
Three facets of this study interest me. First, it’s clearly a strategic choice of some males and not others to infant-carry. The data suggest that “the motivation to invest in infant carrying strongly depends on the position in the social network and the current constellation of relationships.”
Second, while it’s no great shock to human parents everywhere that babies cause stress, it’s intriguing to learn that male monkeys find their tiny passengers to be bothersome in some way (as shown by the hormone levels). Henkel et al. think it may be because primate infant crying is “an aversive stimulus.”
Third, what a statement this study makes about the pull of social relationships for primates! Stress, at least under some circumstances, may be worth enduring (at least in the short term) when we succeed in staying close to our preferred companions. Of course, macaques are not people, but strong physiological and behavioral continuities may exist across primate species.
Speaking of stress, another monkey research project offers a glimpse into how a frightening and potentially stressful event may turn into something quite different. Here I highlight the work of Dr. Peter Fashing, my former student at William and Mary and now anthropology professor at California State-Fullerton. Working with his wife, biologist Dr. Nga Nguyen (also at Fullerton), and his father, biologist Dr. Norman Fashing (at William & Mary), Peter recorded what happened when gelada monkeys in Ethiopia encountered a huge swarm of locusts. Initially fearful, the monkeys soon made excellent use of the insects. Watch the video and see what happened:
Geladas & Locusts