Infrared Thermography and the Mane

An infrared camera takes digital pictures, and each pixel in the picture corresponds to a specific temperature. The technique is astonishingly accurate (usually to within 0.1° C) and the cameras are hardy and portable making them ideal for a field study. Unfortunately, the cameras are also extremely expensive, but in 1999 we received a special supplement to our National Science Foundation research grant to support a 3 month rental of an infrared camera. Thanks to special assistance from Flir Systems Inc. (a manufacturer and distributor of infrared technology), we obtained a camera in June, 2000 and again in May, 2001 and brought them to East Africa.

Our hypotheses were that males are generally hotter than females, and that males with darker or longer manes are the hottest of all. We addressed these hypotheses by taking infrared images of males and females in the Serengeti and Ngorongoro. We found that Serengeti/Ngorongoro males were hotter than females but it was impossible to determine whether this difference arose because males are bigger (lowering their surface area: volume ratio) or because of the male's mane. To answer this question we traveled to Tsavo East in Kenya, where the males are largely maneless. In Tsavo, as in the Serengeti, males are bigger than females, but when we measured their surface temperatures we found that they were no hotter than females. This allowed us to infer that in the Serengeti, it is the male's mane which makes him hotter than females.

When we compared the surface temperatures of individual males in Serengeti/Ngorongoro, we found that males with darker manes were hotter than those with lighter manes. This suggested that darker-maned males pay a higher cost in terms of heat stress. Analysis of our long-term data supported this supposition; high temperatures interfere with sperm production, and males with darker manes had more abnormal sperm. Eating large meals is known to increase heat load and males with darker manes at less in hot months; thus it appears that when temperatures rise, males with darker manes must limit their food intake to stay cool.