It’s that time of year again: the time for desert tortoises to sleep for the winter. As some animals head south for the winter in search of warmer weather, desert tortoises stay in their favorite burrows right here in the Mojave Desert to escape the winter chill. Every year around October, desert tortoises begin to slow down and find a perfect burrow to hibernate in for at least the next few months of winter. At the San Diego Zoo Desert Tortoise Conservation Center (DTCC), we dig artificial burrows for many of the tortoises to make sure they are well protected over winter, and we berm them in by building up a mound of dirt in front of the burrow to prevent cold air and winter monsoon rains from getting in.
It is not uncommon for the Mojave Desert to receive some rainfall during winter months, and occasionally desert tortoises come out to have a drink. That was certainly the case a few weeks ago when it rained for seven days straight, and we were completely flooded! In most cases, tortoises come out of their winter sleep because of the smell of the rain and creosote in the air, and they either find low-level ground where water has collected, or, in some instances, they actually dig a small impression in the ground so water can pool up for drinking. After their drink, they return to their burrows to sleep for the rest of their hibernation period.
Things are a bit different around here for little desert tortoises, though. As winter approaches, young tortoises are also starting to go to sleep for the winter, but before they can hibernate, we must do a health assessment on them to make sure they are healthy. Neonates (hatchlings from this year) and very young desert tortoises that weigh less than 3.5 ounces (100 grams) and are less than 3.1 inches (80 millimeters) long are kept in special predator-proof enclosures until they are large enough to be moved to an unprotected pen. Every one of the tortoises that is held in these pens must undergo a pre-hibernation health assessment, and this keeps us really busy!
This winter there are over 400 young tortoises in the predator-proof pens, so it took us two full weeks to assess them all. Luckily, almost all of them were in great condition and were ready for hibernation, but the few that weren’t feeling so well were taken to the medical center for special care over the winter (see post Desert Tortoise Hibernation: Not for All). The best thing for a young desert tortoise to do over the winter is to hibernate; we have seen that young tortoises that are allowed to hibernate outdoors, especially in their first year of life, grow up healthier than those that don’t! But if a little one is not healthy enough to hibernate, then it’s best to allow it to stay inside and awake over winter so he or she can get the care needed before spring.
Daniel Essary is a research associate at the San Diego Zoo Desert Tortoise Conservation Center. Read his previous post, Desert Invasions.