Unique social system found in famous Tsavo lions

Male Tsavo lions are now known to be the only lions to rule individually a large group of females. But why they have no manes remains a mystery. Photo by John Weinstein, Courtesy of The Field Museum.

CHICAGO—Tsavo lions, famous for man-eating at the end of the 19th century, are also novel for being maneless. Now, the first scientific peer-reviewed study of the ecology of Tsavo lions reveals that they have a unique social system. Tsavo lions are the only lions known to live in large groups of females ruled by a single male.

The scientists surveyed the lion population of Tsavo East National Park in eastern Kenya, documenting the size and composition of each group and the condition of manes on males. The five resident groups of females they documented had an average of 7.4 adult females per group, large for prides in general. However, each was attended by only one male rather than a coalition of two-to-four males, typical of large prides elsewhere.

“Although the lion is one of the world’s best-studied mammals, no one has documented undisturbed groups of seven-to-eight adult females with a single adult male,” says Roland Kays, PhD, co-author of the research to be published online April 11 by NRC Research Press’ Canadian Journal of Zoology.

A maneless Tsavo lion with one of his cubs in the Tsavo East National Park in Kenya. Photo by Bruce Patterson, Courtesy of The Field Museum.

Most of what we know about lions comes from studies conducted in the thriving grasslands of the Serengeti Plains in Tanzania where lions are surrounded by teeming herds of game, and lone males are quickly deposed by coalitions of males that rule prides in unison. In George Schaller’s classic study, The Serengeti Lion, none of the 12 prides he studied had lone pride masters.

But the well-studied Serengeti lion may not be representative of the species as a whole. In fact, in historic times the lion ranged over an incredibly vast geographic area, from South Africa to the Balkans, from Morocco to western India. To exploit such a diverse array of habitats, lions must have adapted many different lifestyles and behavior patterns.

“How does Panthera leo behave in markedly different habitats and climates?” asks the study’s co-author Bruce Patterson, PhD, MacArthur curator of mammals at The Field Museum. “Studying lions in Tsavo indicates that the ‘king of the beasts’ has a lot of tricks up its sleeve that confuse and astound those who would type-cast it.”

Claims to fame
The Tsavo lions, which live in the dry, scrubby grasslands of East Africa, represent the only well documented population of maneless lions. They attracted international attention in 1996 with the release of the major motion picture The Ghost and the Darkness, which tells the story of two large Tsavo lions that killed and ate more than 130 railroad workers in 1898.

The workers were building a bridge for the British over the Tsavo River in southeastern Kenya when they faced a nine-month “Reign of Terror.” Construction was halted for several months until the lions were hunted down and killed. The two lions were stuffed and are now one of The Field Museum’s most popular displays (see “Tsavo lions” under Quick Links at (http://www.fieldmuseum.org/).

Ironically, manes are so symbolic of male lions that The Ghost and the Darkness used maned lions to represent the two maneless man-eaters of Tsavo. Scientists believe that lions evolved manes because they attract females, deter trespassers, intimidate nomadic males, give a visual sign of a territorial male’s control, and protect the vital head and neck regions in a fight. Manes serve similar functions as antlers in deer.

Most of what we know about lions comes from studies in the grasslands of the Serengeti Plains in Tanzania, where lions have majestic manes. But the Serengeti lion may not be representative. Photo by R. J. Patterson, Courtesy of The Field Museum.

But with so many advantages, why do Tsavo lions lack manes – something that this research confirmed scientifically for the first time? Some scientists have suggested that male Tsavo lions really do grow manes but that the region’s tangled thorn bushes tear away tufts of hair as the lions hunt and chase their prey. This theory was dispelled by observations of maneless male lions in arid grasslands lacking thorn bush. Some have even suggested that the Tsavo lions are a distinct species, but this possibility is unsubstantiated.

The new research, on the other hand, suggests that Tsavo lions have actually evolved to be maneless over many generations. This may be because of the high environmental cost of possessing a mane in the hot, dry Tsavo landscape. In addition, a mane would make it difficult to negotiate the thorns and bramble of Tsavo’s thick undergrowth. Thus, manes might put Tsavo lions at a disadvantage because they would retain heat, cost water, attract potential rivals and scare off prey.

“Comparing the ecology and social life of maned and maneless lions should shed light on the evolutionary significance of the mane,” says Kays, curator at the New York State Museum in Albany. Kays was a post-doc at The Field Museum when the research was conducted.

On-going research
Whatever the ultimate evolutionary purpose of manelessness, the potential mechanisms are especially intriguing because they might also explain how single male lions are able to control large prides on their own. Both their manelessness and novel social behavior may be related to hormones, the authors suggest.

One possibility is that Tsavo lions have elevated testosterone levels. Testosterone is thought to inhibit hair growth and cause balding on the scalp of genetically disposed human males. In addition, it is known to raise levels of aggression and is higher in territorial males than in nonterritorial males.

The two man-eating lions of Tsavo, who ate more than 130 railroad workers in Kenya in 1898 before they were finally hunted down and killed. Now on permanent display at The Field Museum in Chicago, their story was told in the major motion picture The Ghost and the Darkness. Photo by John Weinstein, Courtesy of The Field Museum.

“Tsavo lions are thought to be especially aggressive, and high levels of male hormones may simultaneously underlie this aggression, their unique social system, and manelessness,” Patterson says.

To test this theory, Patterson and Kays—together with Dr. Samuel Kasiki of the Kenya Wildlife Service and Dr. Julie Thornton of the University of Bradford in the U.K.—will survey genetic and hormonal attributes of the maneless lions starting this April. The team will be supported by the National Geographic Society and Earthwatch.

The scientists will also radio-collar lions on ranches fringing Tsavo to follow their behavior and ecology through an annual cycle.

“We hope to gain a better understanding of the developmental mechanism and physiological context of manelessness, as well as the economics of the Tsavo lions’ social system,” Patterson says.

Note: Photos available of maned and maneless lions in the wild today, as well as the man-eaters of Tsavo killed in 1898 and now on display at The Field Museum in Chicago.