The lion’s mane has long been an iconic symbol, yet there has been no clear answer as to why lions have manes, or what function they serve. Charles Darwin was the first to suggest that the mane may be a result of sexual selection, meaning that the mane increases reproductive success. The mane may protect a male’s neck during a fight with another male, or it might signal physical condition, allowing males to assess each other’s fighting ability and females to choose superior mates.
If the mane primarily serves as a shield, the neck should be a special target of attacks or wounds to the neck should be particularly dangerous. However, wounds in adult males and in females and sub-adult males (whose neck areas are bare) were no more common on the neck than on other parts of the body nor were wounds to the neck more likely to be fatal.
So what information is conveyed by the length and coloration of the lion’s mane? And if longer, darker manes are more intimidating/attractive, why don’t all male lions have extravagant manes?
Our longterm records show that males with shorter manes had often been recently injured or sick, suggesting that mane length indicates current fighting ability. Males with darker manes had higher testosterone levels, were more likely to recover from injury, spent more time resident with prides, and had higher offspring survival. Thus, mane color appears to convey information about male aggressiveness and potential reproductive success.
Variation in a sexually selected trait generally results from the inherent costs of expressing such an elaborate physical characteristic. For example, the peacock’s tail makes the male more vulnerable to predation, and only the highest-quality males can evade predators while carrying such exaggerated tails. To determine the costs of growing a large mane, we used an infrared camera to measure the surface temperatures of male and female lions. Maned males, but not those with abbreviated manes, were hotter than females, suggesting that the mane imposes a general increase in heat sensitivity. Furthermore, males with darker manes were hotter than males with blond manes. Referring again to our longterm records, we determined that males with darker manes had higher proportions of defective sperm, and ate relatively small meals in hotter weather (increasing the ratio of foraging time to food consumption). These costs are most likely heat-related and they are burdens that only superior males can bear; for inferior males, a dark mane would inflict physiological costs that outweighed the reproductive benefits.
Thus the lion’s mane appears to be a sexually selected signal by which a male advertises his quality to other lions. Our study also highlights the importance of temperature to lion ecology and behavior—which will become increasingly relevant in the face of global climate change: the Serengeti lions are already developing lighter and shorter manes than in the past. These findings were presented by Science magazine in a paper entitled “Sexual Selection, Temperature and the Lion’s Mane” by Peyton West and Craig Packer.