From Scientific American:
The male lion’s magnificent mane sets him apart from other cats–and it’s a great charmer for the ladies–so why would he do without it? That question has puzzled scientists since 1833, when the first reports of “maneless” lions trickled in from around the world. Now, a research team reports that lions from the Tsavo region of Kenya deliberately delay mane growth to cope with the region’s harsh temperatures.
Some researchers suggested that lions lost their manes because they were snagged too many times in Tsavo’s ubiquitous thorn scrub. But zoologist Thomas Gnoske at the Field Museum in Chicago, Illinois, considered something these speculators didn’t: lions shipped to zoos in cooler climates grow longer manes. This made him wonder whether hot temperatures account for Tsavo’s thinning tomcats. To find out, Gnoske and colleagues studied museum specimens and conducted 10 years of fieldwork in Tsavo and in the Serengeti, which is about 10 degrees cooler. In an article 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 breed–but that Tsavo’s lions don’t have much of mane until age 8, well past their reproductive prime.
Gnoske thinks smaller manes improve a young, vigorous 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 turn off that program, now that they’re in a place like Tsavo. “They’re hard-wired to grow a mane, period, and they’ll develop as large of a mane as they possibly can.”
More here.