Amber & Ischial Callosities

Yesterday was the first session of my new zoo class on primates. Mark, a former keeper in the ape house who now teaches primate social behavior at the University of Maryland, is teaching the class. Our first session focused on taxonomy and was full of fascinating details.

We learned about many of the differences between humans and other primates. For example, we have a uniquely angled femur which positions our legs under our body and makes walking upright more graceful. Or, we are the only primate with longer legs than arms. And we have the largest relative brain size, where brain weight is looked at in comparison to body size. But one thing we miss out on as humans is having an ischial callosity. Since other primates, primarily Old World monkeys, spend so much time sitting on their rumps eating leaves and fruits, they have developed soft built-in pillows in their butts. We invented cushioned chairs and couches, but primates seem to have an advantage in always having their cushion with them.

Additionally, the ischial callosities serve in many species as a sexual characteristic. When a lady primate nears ovulation, her ischial callosities become red and swollen to tell the boys around her that she is ready for some lovin'. Not only do they have a built-in butt pillow, but they also have a hot little red dress ready to go. No tempting fate for these ladies and hoping they catch the attention of a randy gentleman. They're advertising in a major way.


Evolution is so smart.