These are the famous Tsavo Man-Eaters. In 1898, they killed over 135 people, and stopped work on a railroad before they were finally shot. Like many male lions in the Tsavo region, they have no manes. A 10-year study shows us part of the reason why. The Tsavo lions have been sitting in the Field Museum since Lt. Col. John Henry Patterson, the man who shot them, sold their skins and skulls to the museum in 1925. I guess it’s not possible to spend a career staring at a weird-looking lion and not try to find out about it, because in 2006 Bruce Patterson, the curator of mammals at the Field Museum, announced that he knew at least part of why these male lions don’t have their manes. The area that is now Tsavo East National Park in Kenya is blisteringly hot. A full mane is also hot.
It makes sense that, over generations, Tsavo males would lose their manes, but that’s not quite all of what happens. Patterson took a look at lions in zoos all over North America, measuring their mane length and comparing that with the climate they resided in. The paper he wrote describes what he found:

“Mane length and density were inversely correlated with temperature; color variation was unrelated. Mane development was correlated with January but not July conditions, suggesting a stronger response to cold than to heat. Climate-induced variation in manes of captives accounted for up to 50% of variation seen.”

Read this article. about the Tsavo lions and using the words "adaptation" and "natural selection", explain why these lions do not have manes.