When it comes to social hierarchy in wild baboons, top-ranking or “alpha” males have higher stress hormone levels than second-ranking males, suggesting that being at the very top of baboon society may be more costly than previously thought, reports a new study in the 15 July issue of the journal Science.
“We have long known that high rank confers major advantage in access to mates,” said Jeanne Altmann, senior author of the study and professor of ecology and evolutionary biology at Princeton University. “And we have shown that this advantage translates into high paternity success, sometimes very much higher for the top ranking than even for the next ranking, the beta male. But what are the costs?”
Two adult male baboons fighting at very close range. | Images courtesy of Jeanne Altmann; © Science/AAAS
Life at the top has many perks for alpha male baboons, including better access to resources like food and mates. Thus, it’s generally assumed that the advantages of top status outweigh the costs except in times of social instability, when alpha males usually have to fight to preserve their position or social group.
The advantages of top status are evident in previous non-human primate studies, where high-ranking males in stable groups tended to be less stressed than lower-ranked males. But a nine-year study of hormone levels in wild baboons in Amboseli, Kenya, shows there’s more to this story.
By testing hormone levels from baboon feces samples, Altmann and colleagues found that alpha males have higher stress hormone and lower testosterone levels than the second highest-ranked or beta males—even in times of stability.
An adult male mate-guarding a fertile female during a sexual consortship.This involves the male following the female closely, being vigilant and fighting off any males that might disrupt the consortship.
Surprisingly, alpha male stress levels are similar to those found in low-ranked males. The authors surmise that the causes of stress for these two male classes are probably different. Alpha male stress likely comes from having to continually preserve their status and engage in high levels of mating activity, while males in the lowest rank of society experience stress related to access to food and other resources.
The results suggest that the top position in animal (and possibly human) societies may have unique costs and benefits to be explored further.
“I think baboons can provide insights into identifying the ideal position in a complex society under different conditions,” said Altmann. “Humans also live in stratified societies, and social status is well-known to be associated with health outcomes. Yet, many of the precise effects have been difficult to pin down and could have their parallels in baboon society.”
Read the report “Life at the Top: Rank and Stress in Wild Male Baboons,” by Jeanne Altmann and colleagues.