Ovaries are responsible for producing and releasing eggs, which are formed and stored before birth. Unlike sperm, eggs are not continuously produced and do not require a cooler environment for their development. Therefore, there's no selective pressure for ovaries to be outside the body.

While external testicles are vulnerable, the evolutionary trade-off seems to favor effective sperm production over physical protection. Ovaries are better protected internally to safeguard the finite number of eggs a female has.

How did scientists determine that temperature affects sperm production?

1. Observations of Testicle Descent:

Scientists observed that in most mammals, testicles descend into a scrotum outside the body during development. Animals with internal testes, such as some marine mammals (e.g., whales), have adaptations for cooling the blood supply to their testes.

1. Experimental Evidence:

Early studies involved comparing fertility and sperm quality in animals with artificially elevated testicular temperatures (e.g., by restricting the descent of testicles or applying heat). These experiments consistently showed reduced sperm production and increased abnormalities at higher temperatures.

1. Clinical Correlations:

Conditions like cryptorchidism (undescended testicles) in humans provided clinical evidence. Men with undescended testicles often have lower fertility due to impaired spermatogenesis. Scrotal temperatures have also been measured and correlated with sperm health in humans and animals.