One very important part of my job is to maintain the outdoor tortoise enclosures here at the San Diego Zoo’s Desert Tortoise Conservation Center (DTCC), and in particular to ensure that the burrows in which tortoises live are well insulated from the harsh desert sun and heat. Tortoises spend over 90 percent of their lives in their burrows, so it is incredibly important that they are comfortable in them. While taking care of the burrows last week, I unexpectedly located several tortoise nests! You may be wondering how it is that I didn’t know there would be nests in the enclosures, but here’s what happens…
Sometimes a male and a female will be placed together in an enclosure, especially if they came from the same house, and of course mating can and will happen, so we can expect there to be a nest in such a case. However, female desert tortoises can store sperm for five years, and maybe even longer, so even if she has no contact with a male for many years, she can still lay viable clutches if she mated with a male at some point earlier in her life! With hundreds of female tortoises on site, there’s no way for us to know which females laid eggs and which eggs will be viable.
Eventually, all the tortoises at the DTCC cycle through our system and move to new pens, or we release them to a translocation site, but the nests they leave behind are deep and well hidden, so there is no way to know for sure if there is a nest in an enclosure until we start digging. After we move tortoises out of a pen, we pull up all the man-made burrows to sterilize the area before placing new tortoises in the enclosure.
While doing this just last week, I came across a nest of eggs where the old burrow had been located, but it was almost four feet (1.2 meters) inside of where the mouth of the old burrow rested. The eggs were 3- to 5-inches (8- to 13-centimeters) deep into the ground in a 5-inch diameter hole. I excavated the nest and collected the eggs, marking the top of each one with an X, to ensure that I did not disturb its position, and carefully placed them in a plastic container filled with dirt from the nest. During that same day, we found 18 eggs all together in 3 nests, and the 15 viable eggs we collected are all in the incubator waiting to hatch.
We hope to some day soon start a “headstarting” program in which we can hatch out baby tortoises and grow them up until they are big enough and strong enough to survive on their own in the desert so we can recover this threatened species in the wild. Stay tuned in another month for reports of hatchlings!
Daniel B. Essary is a research associate at the San Diego Zoo’s Desert Tortoise Conservation Center. Read his previous post, Desert Tortoises: Male or Female?