Smithsonian readers may recognize the name Caitlin O’Connell-Rodwell; she’s the Stanford University ecologist who wrote last November’s fascinating story about surprisingly sociable male elephants:

Male elephants have a reputation as loners. But in Amboseli National Park in Kenya, where the longest-running studies on male elephants have been conducted, bulls have been observed to have a best friend with whom they associate for years. Another study, in Botswana, found that younger males seek out older males and learn social behaviors from them. In my previous field seasons at Mushara, I’d noticed that males had not just one close buddy but several, and that these large groups of males of mixed ages persisted for many years. Of the 150 bulls that we were monitoring, the group I was particularly interested in, which I called the “boys’ club,” comprised up to 15 individuals—a dominant bull and his entourage. Bulls of all ages appeared remarkably close, physically demonstrating their friendship.

One aspect of male elephant society that O’Connell-Rodwell has been studying is the possibility that they form a hierarchy, something that so far has been observed only among female elephants, as she explained in her story:

Female elephants live much of their lives apart from males, in family groups led by a matriarch. A mother, grandmother and maybe even a great-grandmother live together with daughters, nieces, granddaughters and their offspring—on average, about 15 individuals. Young males leave the group when they are between 12 and 15 years old; the females stay together as long as they live, which can be up to 70 years. The matriarch, usually the oldest in the group, makes decisions about where and when to move and rest, on both a daily and seasonal basis.

Male elephants grow up within this female social group, but they leave their families when they reach sexual maturity, around age 14, and spend most of their adult lives hanging out with other male elephants. Researchers have previously found that there are dominant individuals among these groups of males, but they hadn’t observed any kind of linear hierarchy.

O’Connell-Rodwell and her colleagues, however, hypothesized that a linear dominance hierarchy would form in dry times, when resources were limited. A hierarchy, they reasoned, would help the elephants avoid injuries that could result from competing for water. And so they set out to test their hypothesis by observing male elephants around a remote permanent waterhole for four years in Etosha National Park in Namibia. (The results of the study appear in Ethology Ecology & Evolution.)

The researchers found that during drier years the males did, in fact, form a linear hierarchy, and that aggressive acts—such as charging, lunging and throwing a trunk towards another male—were less frequent than in wetter years. O’Connell-Rodwell and her colleagues think that the linear hierarchy may be moderating aggression among younger males in the group as the increase in aggression in the wetter years was primarily among subordinate individuals who tend to be younger.

This finding “highlight the potential benefit of structure that a hierarchy may provide for younger males,” the scientists write. “This seems particularly apparent as younger males are highly social and appear to choose the company of elders, suggesting the importance of mature males in society, a pattern that has implications for other healthy male societies including humans.”