tortoise burrows


Students Help Desert Tortoises

Part of the West Tech Team


The team measures a desert tortoise burrow.




-Michael Vogel




Desert Tortoises Step Closer to the Wild

Can you find the desert tortoise in this burrow?

We recently completed our inventory of all the tortoises on site at the San Diego Zoo Desert Tortoise Conservation Center (DTCC). It took us two full active tortoise seasons, but we did it! (See Pam’s previous post, Counting Tortoises). Although this task has occupied nearly all of our time, we have somehow also been able to move healthy tortoises into our newly designated 20-acre translocation enclosure. Healthy, ELISA-negative tortoises (see post Tortoise Science: Cooler than You Think about ELISA testing) that have remained free of upper respiratory tract disease (URTD) symptoms for at least 30 days will be moved to the translocation enclosure (we call it a “pen”), the final step before being relocated into the wild next spring!

We had to make sure that the translocation pen could accommodate the number of tortoises we wanted to put in it, so our four seasonal research assistants had the task of adding burrows to provide the tortoises with protection from the heat and cold and installing a water delivery system to this pen. Each burrow takes an hour or more for one person to dig, but we have successfully added 50 burrows to this pen over the course of 2 months while still conducting inventory!

We now have over 100 healthy tortoises waiting to be relocated to a U.S. Fish and Wildlife Service-approved translocation site here in southern Nevada! We plan to put radio transmitters and GPS data loggers on the healthy tortoises prior to the long-anticipated release in the spring of 2011. A team of experienced telemetry technicians will follow the signal from these transmitters, allowing us to track and study the movements and habit use of these animals.

Why is translocation into the wild so important? Wild populations of Mojave desert tortoises have reportedly declined by approximately 90 percent in the past 30 years; it is estimated that there are only about 150,000 wild Mojave desert tortoises remaining in critical habitat. The desert tortoise is a keystone species, meaning that it plays a critical role in its environment. How? Desert tortoise burrows serve to protect other desert species from predators and harsh weather conditions, and they disperse seeds from the native plants that they eat, repopulating the desert ecosystem with them.

Our mission is to play a significant role in the conservation of the Mojave Desert, including the recovery of the desert tortoise and its habitat. Through all of our work, including translocation, we are well on our way to fulfilling our mission!



Desert Tortoises: Unexpected Nests!

desert_tortoise_nestOne 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.

desert_tortoise_eggsWhile 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?