Effect of heat stress on the fertility of male mice in vivo and in vitro.

Plain-language summary

This experiment asked whether heat makes sperm worse at their actual job, or simply leaves fewer of them. Male mice were held at 36 degrees Celsius (96.8 degrees Fahrenheit) for two 12-hour periods on consecutive days, then mated with unheated females, and their sperm were separately mixed with eggs in the laboratory. Heated males fathered fewer pregnancies and smaller litters, and they also had lighter testes and fewer sperm, so the two explanations were tangled. To separate them the researchers took equal numbers of moving sperm from heated and unheated males and put them with eggs. Similar numbers of sperm reached the outer coat of the egg, but fewer of the eggs were fertilized and fewer sperm got through. The effects showed up seven days after the heat and were clearer at 10 and 14 days.

Key findings

• Heated males fathered fewer pregnancies, and the litters that resulted were smaller.
• Heated males had lighter testes and fewer sperm.
• With equal numbers of moving sperm from each group, sperm from heated males still fertilized fewer eggs and fewer of them penetrated the egg.
• Similar numbers of sperm from each group reached the outer coat of the egg, so the difference was in getting through rather than in arriving.
• The effects appeared seven days after heating and were clearer at 10 and 14 days.