As winter approaches, hibernating animals will finalize their preparations for taking a ‘long winter’s nap’. These remarkable animals reduce their metabolism to help conserve energy at a time when environmental resources are scarce and considerable energy would otherwise need to be spent on just staying warm. For many species of hibernators, periodically entering this state of torpor allows them to reduce their energy needs by 90% compared to intermittent states of arousal. When these animals arouse from hibernation in the spring, they rapidly switch from a state of energy conservation to a state of reproduction. Some researchers have speculated that male hibernators either stop producing sperm during hibernation or limit sperm production to periods of arousal. A concern is that states of arousal from torpor are associated with oxidative stress – which may damage the very cells responsible for producing healthy sperm. Similarly, oxidative stress is responsible for some cases of infertility in humans.
A new study published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology set out to examine how hibernation impacts male reproductive physiology in male captive eastern chipmunks, Tamias striatus. The researchers divided the animals into two groups, shallow and deep hibernators, based on time spent in torpor and the length and depth of each torpor bout. This information was collected with the help of collars that were equipped with sensors that recorded body temperature.
Not surprisingly, the researchers found that sperm production was inhibited during late hibernation in the deep hibernators, but these animals were able to resume normal sperm production in the spring. In fact, they found springtime sperm production did not differ between the shallow or deep hibernators. Interestingly, they also found that shallow hibernators had more DNA damage to the cells that produce sperm near the end of hibernation whereas deep hibernators had more DNA damage in the spring. It is thought that arousal from torpor and resumption of normal metabolism is what causes cell damage as the mitochondria in the cells ramp up metabolism but produce free radicals in the process.
MF Gagnon, C Lafleur, M Landry-Cuerrier, MM Humphries, S Kimmins. Torpor expression is associated with differential spermatogenesis in hibernating eastern chipmunks. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 319(4): R455-465, 2020. https://doi.org/10.1152/ajpregu.00328.2019