Heat sensitivity of sperm in the lizard Anolis sagrei

The heat sensitivity of reproduction is a critical determinant of population persistence under climate change. However, the heat sensitivity of gametes has been much less studied relative to that of adults. We developed a method to measure the heat tolerance limits of lizard sperm cells, and used the method to test several aspects of sperm cell thermal biology in the brown anole lizard (Anolis sagrei). We estimated the repeatability of sperm traits by measuring heat tolerance and baseline motility of ejaculated sperm from the same individuals multiple times over 21 days. To investigate co-adaptation of sperm and adult thermal traits, we tested for a correlation between sperm heat tolerance and the heat tolerance of the adults that produced them. Furthermore, we tested for effects of episodic heat stress experienced by males on sperm performance. Sperm heat tolerance and motility were both repeatable, consistent with evolutionary potential, though there was clear evidence for environmental effects on these traits as well. Contrary to the expectation of thermal co-adaptation, we found no correlation between sperm and adult heat tolerance. A single, episodic extreme heat event experienced by adult males immediately impaired sperm motility, consistent with detrimental effects of adult heat stress on sperm stored within males. Our study adds to the mounting evidence that sperm are heat-sensitive and represent a vulnerability to global warming, but also suggest evolutionary potential for thermal adaptation at the gamete level.