The maneless lions had no trouble attracting a healthy harem of females—the Tsavo prides were surprisingly large, with seven or eight females, and were ruled by a single male lion. By contrast, Serengeti prides were slightly smaller, with six or seven females, and were ruled by a consortium of two to four males.
"None of the Tsavo prides had more than one male, which makes them very distinctive," said Patterson.
But the social scene gets even stranger.
It seems that there are far more nomadic males in Tsavo than have been observed in other places.
"Coalitions of three or four males live and hunt together—this is an alternative social structure that we have never seen," said Patterson. What surprised the scientists was that these coalitions were not able to displace the "pridemaster" and take over a pride.
The results of the study were published online on April 11, 2002, in the Canadian Journal of Zoology.
Hormones Gone Awry?
Patterson's theory is that the social structure and the absence of the mane all boils down to hormones—testosterone, to be specific. He suggests that the coalitions are transient social groups made up of adolescent males whose testosterone levels have not yet peaked.
"By the time testosterone peaks at around age six or seven, intolerance also peaks," said Patterson. This is when the coalition tends to break apart and the males go their separate ways.
Manelessness might also be due to hormone levels, Kays said. High testosterone levels inhibit hair growth in human males and leads to male pattern baldness. Similarly, Tsavo male lions may be genetically predisposed to high levels of testosterone. The high hormone levels may also explain the heightened aggressive tendencies seen in these lions.
Both authors are travelling back to Kenya this month to investigate how maneless lions in Tsavo East interact with maned lions in Tsavo West. The team will collect fecal and hair samples for DNA analyses that should reveal the family structure of lions in the area. The sample will also be used to measure hormone levels. The researchers plan to put satellite collars on lions to track where, when, and with whom the animals roam throughout the year.
Patterson and Kays' work will help determine whether the Tsavo lions are actually a genetically unique population, said John Gittleman, a biologist at the University of Virginia. "These studies are critically important for long-range conservation of the entire species," he added.
The role of the mane has never been firmly established. It was thought the manes were selected for during evolution because they attracted females, intimidated other males, and protected the neck regions during fights. But the findings in Tsavo indicate that the maneless males are certainly not starved for female attention and actually command larger prides than maned lions.
The environmental cost of having a mane in Tsavo may be greater than in other areas, suggests Kays.
It seems that "the Tsavo lions have a lot of tricks up their sleeve that will only become obvious as we study their behavior and ecology," Patterson added.
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