John Patterson，a hunter and a British Colonel，was one of the first to document manelessness in Tsavo's famous man­eaters. Ever since，naturalists have developed evolutionary descriptions. Some researchers suggested that lions lost their manes because they were caught too many times in Tsavo's thorn bushes everywhere. Others argued that Tsavo's aggressive lions have unusually high testosterone (雄激素) levels，known to cause male pattern baldness (秃顶) in humans. Still others supposed that Tsavo's lions were a distinct group or were a related to a lion pictured in prehistoric caves，which no longer existed.

　　But expert Thomas Gnoske at the Field Museum in Chicago，Illinois，considered something these researchers didn't：lions shipped to zoos in cooler climates grow longer manes. This made him wonder whether hot temperatures account for Tsavo's thinning lions. To find out，Gnoske and his colleagues studied museum records and conducted 10 years of fieldwork in Tsavo and in the Serengeti，which is about 10 degrees cooler. In an artick published online this month in the Journal of Zoology，the team reports that lions in the Serengeti grow a full mane in 5 years—by the time they're ready to be parents—but that Tsavo's lions don't have much of mane until age 8，well past their reproductive period.

　　Gnoske thinks smaller manes improve a young，energetic lion's ability to keep cool. Bushy manes probably evolved to attract females in cooler climates where heat stress was not an issue，Gnoske says，and lions can't just

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