Why Do Most Mammals’ Testes Hang Outside of Their Bodies?

Why do most mammals need a scrotum when birds, which are also warm-blooded, do not? originally appeared on Quora, the best answer to any question. Ask a question, get a great answer. Learn from experts and access insider knowledge. You can follow Quora on Twitter, Facebook, and Google Plus.

Answer by Adriana Heguy, professor of pathology:

The reason why many mammals evolved to require a cooler temperature to produce viable sperm is not precisely known, and it has been a subject to debate for decades, mainly because having the testes exposed creates a vulnerability that seems to be the opposite of an adaptation. And interestingly, not all mammals have dangling testes and scrota, exceptions being not just pinnipeds (seals and sea lions) and cetaceans (whales and dolphins) but also elephants, which keep their testes inside the body.

There are many theories out there as to why sperm needs to be kept cooler and why the testes of most mammals hang outside their bodies, inside the scrotal sac. In my opinion, the least likely is the “handicap principle.” The easiest way to illustrate this principle is the peacock’s flashy feathers: The animal displays his genetic fitness by being able to survive in spite of those cumbersome feathers that attract the predator’s attention. Male mammals with dangly testes signal to the female mammals that they have good genes if they could keep their hanging, exposed, sensitive gonads safe. The reason why this hypothesis is unlikely is because it has not led to increasingly ornate, highly visible scrota, with a few exceptions of mammals with brightly colored scrota (for example, the vervet monkey).

Evolutionary psychologist G.G. Gallup wrote a very detailed paper a few years back, reviewing the current hypotheses and proposing his own, known as “<a target="_blank" —> Read More