The Parkes Lecture. Heat and the testis.

Plain-language summary

This lecture is the field's own summary of why the testicles sit outside the body and what happens when they get warm. It sets out the evidence that the testes of many mammals run cooler than the core, discusses why they descended into a scrotum at all, and describes the methods used to study heat, from warming the whole animal to warming the testicles alone by insulating them or immersing them in a water bath. It then works through what heat does: to the testicle itself, including its weight, its structure and the hormones it produces, to the number and movement of sperm, to whether those sperm can fertilize an egg, and to how the resulting embryos develop. Its author is also a co-author on the three mouse heat studies submitted alongside this one, all of which were published after this lecture.

Key findings

• The testes of many mammals run cooler than the body's core, and the scrotum appears to be what keeps them there.
• Heat has been studied by warming the whole animal, by warming the testicles alone, and by warming tissue preparations outside the body.
• Heating the testicle affects its weight, its structure and the hormones it makes.
• Heat reduces the number and movement of sperm and their ability to fertilize an egg.
• Heat also affects how embryos develop when the sperm that fertilized them came from a heated testicle.