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 "activation hypothesis." According to his hypothesis, when the sperm cells are ejaculated into the vagina, they experience a sudden rise in temperature, as the female's reproductive tract is at body temperature (37 degrees Celsius). This elevation in temperature activates sperm, temporarily enabling them to increase their motility to go through the cervix and reach the fallopian tubes. The lower temperature in the descended testicles serve to prevent the premature activation of sperm by keeping testicular temperatures below body temperature. Indeed, human sperm that are placed at body temperature become more motile for about one hour, which is the time it would take them to reach the oocyte, and then they slow down. Gallup also speculates that the reason for the extreme sensitivity of the testes and the extreme pain that ensues if they are hit are a result of the activation adaptation. Pain compels male mammals to protect their testes from possible damage. The cremaster muscle, which pulls the testes closer to the body, contracts not only when the outside temperature is too cold, when having an erection (possibly warming the sperm cells a bit, in preparation for the big swim of their lives), and also when there is a hint of possible damage to the testes. All these adaptations make sense in light of the activation hypothesis. This could explain why marine mammals do not need to keep the testes in the scrotum, because they are inside the body but close to the skin and the water temperature may keep them cold enough. But it does not explain the elephant case.
I also came across a recent article that proposes another hypothesis that I like the most, perhaps because it highlights one aspect of evolution: that something that evolved a long time ago, as an adaptation to an environment or physiological conditions that no longer exist, may have become "fixed," and that's why they are still around. It's called "endothermic pulses hypothesis," and it's described in a heavy-duty evolutionary biology paper, authored by the aptly named South African evolutionary biologist B.G. Lovegrove.
This hypothesis argues that the evolution of the scrotum was driven by increases in physiological body temperature (endothermic pulses) that occurred in Boreoeutheria (a clade of mammals, supported by genomic data, that comprises the rodents, primates, lagomorphs such as hares and rabbits, carnivores, bats, and ungulates) during the Cenozoic. These pulses occurred as an adaptation to climate changes, and as an adaptation to cursoriality (the ability to run, which increases body temperature). The model proposes that selection maintained an optimum temperature for spermatogenesis and sperm storage throughout the Cenozoic, at the lower levels of body temperature that prevailed in ancestral mammals for at least 163 million years. The lower temperatures also favor a reduced rate of mutation during spermatogenesis, and evolutionary processes ended up stabilizing these lower temperatures for sperm production and storage. Evolutionary stasis may have been driven by reduced rates of germ-cell mutations at lower body temperatures. The fitness advantages of an optimum temperature of spermatogenesis ultimately outweighed the costs of testes externalization and resulted in the evolution of the scrotum. This hypothesis would explain why elephants do not have external testes (they are not Boreoeutheria, but belong to another clade, Afrotheria, comprising tenrecs, hyraxes, and elephants.
We may never know why exactly evolution favored cooler sperm and dangling testes in a scrotum, but it is fun and useful to hypothesize about the reasons for this adaptation.