The effect of new alpha males on female stress in free-ranging baboons

In chacma baboons, Papio hamadryas ursinus, young adult males often rise to the top of the dominance hierarchy shortly after immigrating to a new group. Such events are potentially disruptive for pregnant and lactating females because high-ranking immigrant males often commit infanticide. In this preliminary study, we assessed the effects of upheavals in the male hierarchy on the physiology of 18 females in a baboon group living in the Moremi Game Reserve, Botswana. We collected behavioural and hormonal data to examine the effects of two separate events, a natal male take-over and an immigrant male take-over, on female faecal glucocorticoids (fGC). While few females had elevated fGC concentrations in response to the natal male take-over, following the immigrant male take-over there was a significant rise in fGCs, but only among lactating and pregnant females. Analysis of behavioural data indicated that elevated fGC concentrations were unrelated to male aggression towards females, female–female aggression, or rates of female–female grooming. Furthermore, lactating females with a male ‘friend’ during the immigrant male take-over period had a less marked increase in fGCs and lower fGC concentrations overall than females without a male friend. Taken together, these results suggest that male social instability itself does not necessarily elicit a stress response from females. Rather, it is the specific male that rises to the alpha position that prompts a stress response, and only from the females at risk for infanticide. Finally, females with a male friend may perceive themselves to be at a reduced risk of infanticide.