More aggression between individuals is measured in wet years, including extreme combat such as this, where an individual could be mortally wounded. (Photos: Caitlin O'Connell and Timothy Rodwell)

(PhysOrg.com) -- The lone bull elephant is an image as iconic to the African savanna as the lonesome cowboy on horseback is to the American West. Although female elephants form tightly knit groups guided by a matriarch, males are usually thought to be solitary wanderers. Now a striking exception to the notion of bulls always being "high savanna drifters" has been discovered in a study led by Stanford University researchers.

The study, conducted at a waterhole in Namibia, shows that in years of low rainfall, when resources are scarce, some male elephants band together into a social group with a clearly defined hierarchy, much the way females do. Group members associate in wet years as well, but with fewer individuals and a dominance hierarchy that is not as clearly defined.

"This is the first time this social structure has been documented in male elephants," said Caitlin O'Connell-Rodwell, an ecologist with Stanford's Center for Conservation Biology, who led the study.

"What is also really striking is that in wet years, when there are a lot of resources, the whole thing collapses and you don't have this linear hierarchy."

But when another dry year rolls around, the group resurrects itself, with each individual resuming the place he held in the hierarchy during the previous dry spell.

O'Connell-Rodwell is the lead author of a paper describing the study, published this week in Ethology Ecology & Evolution.

Social animals are thought to form linear hierarchies  in which each individual occupies its own rung on the ladder and the most dominant one sits at the top  to minimize conflict over resources. The theory is that a clear pecking order among the animals reduces the amount of energy squandered in squabbling, thereby enhancing everyone's chance of survival.

The study was conducted in the semi-desert environment of Etosha National Park in Namibia during the dry seasons of four consecutive years. Starting in 2005, the researchers observed all the activity  day and night  at a remote waterhole in an area closed to tourists.

Each night groups of female elephants came to drink, while the days belonged to the males.

"Because the females come at night, when there is a greater danger from lions, they tend to be nervous and don't spend much time at the waterhole," O'Connell-Rodwell said.

But the bulls  like males of a certain hominid species tend to do  spend a lot of time hanging out by the old watering hole.

"They spend hours there during the day and you can really get a sense of their social structure. Plus, the same ones come back every year, so you get a good idea of the stability of these hierarchies."

In the course of the study, the team observed over 150 different bulls at the waterhole. During the first year, which was drier than normal for the area, they noticed a dozen males of different ages, who consistently arrived in a group. Usually they trooped up to the waterhole in single file.

The dominant bull, christened Greg, always led the way both coming and going.

If the line was not ordered by rank when they emerged into the clearing around the waterhole, it usually fell into the consistent ranked order as the elephants neared the water.

The following year, Etosha experienced its greatest rainfall in 30 years, resulting in a larger than normal number of watering holes available during the dry season. The group structure seen the previous year was never observed in 2006. Two or three bulls would come in together, but never a procession of eight to 12.

The pecking order was similar to the previous year  as evidenced by which bulls asserted dominance over which other bulls in one-on-one encounters  but many more challenges were observed.

In 2007, a dry year, the researchers saw the same hierarchy they had observed in 2005 appear again and persist throughout the dry season.

In 2008, heavy rains caused a 50-year flood in Etosha and the group dissolved again.

Wet year rainfalls averaged 25 ½ inches, roughly 160 percent of the dry year rainfalls, which averaged 16 inches.

Low-ranking bulls Jack and Luke wrap trunks in a bonding moment during the dry year of 2007, when such affiliative behaviors made up much of the interactions between group members. (Photos: Caitlin O'Connell and Timothy Rodwell)

"Our study just happened to fall into four very unusual years, where there were two incredibly wet years alternating with two fairly dry years," O'Connell-Rodwell said. "That pattern of conditions showed us the kinds of behavior that can happen at the extremes, where much more aggression was observed in wet years and more affiliative behaviors in dry years."

O'Connell-Rodwell suspects that studies showing that bulls tend to be loners reached that conclusion because they were conducted in wetter locales, where competition for water never reaches the intensity seen in Etosha.

"In a very dry climate such as you have in Namibia, the social structure is very different from, for example, Amboseli National Park in Kenya, where rivers flow year-round," she said. "There each bull tends to have one close buddy, not three to five, or even seven consistently close buddies like we see at our field site in Etosha."

Even in the semi-desert of Etosha, not every bull that came to the waterhole joined the hierarchy. The researchers observed some bulls visit the site that clearly knew the bulls in the group and interacted with them, but came and went alone. Such "satellite" bulls were the exception, not the norm.

There was occasional testing of the order in the hierarchy of the group, particularly in the middle and lower ranks, where younger bulls are seeking to move up. But overall, the pecking order held.

O'Connell-Rodwell said it is difficult to project how stable the hierarchies are likely to be in the long term, as droughts in Africa tend to run in 10 to 20 year cycles.

"When the hierarchy reforms after that long a time, you'll probably get the second most dominant bull being able to supplant the one that used to be dominant, just because of age and fitness," she said. "But in a shorter term period, the dominant bull remains the dominant bull. The hierarchy is resilient."

The behavioral changes of the younger bulls suggest that being part of a structured hierarchy with mature males might help moderate aggressiveness in younger males, O'Connell-Rodwell said. That pattern has implications for the health of other male societies, including humans.

O'Connell-Rodwell is also an instructor in the otolaryngology  throat and neck surgery department at the Stanford School of Medicine. She teaches an online science writing class for Stanford with David Corcoran, editor of "Science Times" at the New York Times. Her latest book, An Elephant's Life, is due out this fall and offers an illustrated look at elephant societies for lay readers.

Robert Sapolsky, a professor of biology at Stanford, is a coauthor of the paper in Ethology Ecology & Evolution.