About Author: Daniel Essary

Posts by Daniel Essary


Rain in the Mojave Desert

A desert tortoise prepares to snack on a desert mallow.

When most people think of the desert, they don’t think much about rain. Well, on August 22, the Mojave Desert experienced record-breaking rainfall, with some areas receiving well over 2.5 inches (6 centimeters) of rain within a 24-hour period, which caused major damage to the area. Most damage was due to washed-out roads and to low-lying property. But altogether, the desert had a much-needed drink for such a hot and dry summer.

Desert plantains have sprouted after record rainfall in the Mojave.

The aftermath of so much rain caused an explosion of plant life to appear throughout the desert. Some plants had not been seen in certain areas for many years. Plants such as the desert plantain Plantago ovate, desert mallow Spaerlcea ambigua, and golden bush from the genus Ericameria, just to name a few, started growing all over the desert. These plants are some of the desert tortoises’ favorite foods, which will help them have a full stomach before they go down for hibernation in the winter.

Daniel Essary is a research associate at the San Diego Zoo’s Desert Tortoise Conservation Center. Read his previous post, Rabbits, Rodents, and Tortoises.


Rabbits, Rodents, and Tortoises

A new perimeter fence for the DTCC.

Hello again to all desert tortoise enthusiasts! It’s about that time of year for desert tortoises to begin their winter hibernation. For anyone who is new to the desert tortoise Gopherus aggaszzi, this is the time of the year when temperatures start to drop and food becomes scarce. With this seasonal change, desert tortoises begin to slow their metabolism and physical performance and search for a dark, comfortable burrow to “sleep in” through the winter until temperatures warm and food becomes more available.

Finding a secure, empty burrow in the Mojave Desert can be more difficult than just finding a hole in the ground. Desert tortoises search or create their own shelters: they may dig burrows, take another animal’s burrow, or find a small cave that protects them from the cool winters of the Mojave Desert. There are many other animals here that also are looking to hibernate or find shelter from the cold; they may cohabit a burrow with the desert tortoise through the winter season. Animals that have been known to share burrows with desert tortoises are black-tailed jackrabbits and desert cottontail rabbits as well as predators like kit foxes, coyotes, and even badgers.

Recently at the San Diego Zoo Desert Tortoise Conservation Center (DTCC), a new, reinforced fence was built to help control the movement of predacious animals onto the property. Prior to the new fence, coyotes, kit foxes, and even badgers would dig below the fence line to enter the property. This situation became a problem due to attacks on the desert tortoises and the destruction of the grounds at the DTCC. Once the new fence was built, there were no more issues with predacious animals entering, killing, or harming the tortoises.

Unfortunately, the DTCC now has a much larger problem with the local fauna. Since larger, predacious animals are not entering the property, nothing is controlling the population of smaller mammals such as rabbits and antelope ground squirrels. These animals do not directly harm the tortoises at the DTCC but harm them indirectly by scavenging the tortoises’ chow, grasses, and water we put out for the tortoises. It only shows that changing an animal’s ecosystem is not always the best answer to conserving that ecosystem.

Daniel Essary is a research associate at the San Diego Zoo’s Desert Tortoise Conservation Center. Read his previous post, Desert Tortoise: Hot, Hot, Hot.


Desert Tortoise: Hot! Hot! Hot!

A desert tortoise seeks shade in a man-made burrow.

Summer months are the best times for people to get out and enjoy the great outdoors, to go swimming, or to just to relax, but for the desert tortoise, it is time to get some much-needed rest. For most animals the summer season is the time to be productive in life by gathering food, finding a mate, or even establishing a home territory. But the Mojave Desert’s summers are harsh, making it difficult to be active with temperatures reaching well above 105 degrees Fahrenheit (41 degrees Celsius) and with very little (if any) rain. The desert tortoise is well adapted to deal with such extreme weather by going into estivation during the extreme heat of the summer.

Estivation (from the Latin word aestas, meaning summer) is a state of summer dormancy similar to winter hibernation, but in summer estivation, tortoises don’t sleep all the time. In most instances, the tortoises are active for a few hours during the morning and retreat back to a favorite burrow to sleep through the day. As dusk approaches, the tortoise  leaves the burrow for a few hours to eat or drink before night falls. In some cases if there are cooler days or even monsoonal rains, the tortoises come out of their burrows to take advantage of the rain and cooler temperatures. But during the months of June through the end of September, desert tortoises mostly remain inside their favorite burrows for summer sleep so as not to use up energy unnecessarily.

Desert tortoise hatchlings at the entrance to several burrows

We are currently conducting an experiment at the Desert Tortoise Conservation Center that may benefit the well being of the desert tortoises on site. We are doing a burrow temperature study that will help determine if the artificial burrows we dig for tortoises maintain the same temperatures as burrows that tortoises dig themselves. Since tortoises spend 95 percent of their lives in burrows, this is very important information for us to know! We placed temperature data loggers in both natural and artificial burrows and set them to record temperature throughout the day.

By analyzing the temperatures in both artificial and natural burrows, we will find out if we need to change the way we dig the artificial burrows so that tortoises can comfortably estivate in summer and hibernate in winter. If temperatures are too high inside the burrow, the tortoise living inside it can get very sick, or even die, so we want to make sure that every burrow we dig provides them with all the protection they need from the harsh heat of the Mojave summer.

Daniel Essary is a research associate at the San Diego Zoo’s Desert Tortoise Conservation Center. Read his previous post, A Long Winter’s Sleep.


A Long Winter’s Sleep

A desert tortoise hatchling heads for a burrow.

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.

Desert tortoise hatchlings have their choice of burrows at the DTCC.

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!

A juvenile desert tortoise enjoys time in the sun.

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.


Desert Invasions

A desert fire survivor.

This spring was more beautiful than ever. Due to higher-than-average rainfall this winter, many plants and animals are thriving in the Mojave Desert, a beautiful sight to see for desert rats like staff at the San Diego Zoo Desert Tortoise Conservation Center (DTCC) in Las Vegas! Desert critters are feasting on a buffet of flowers such as the desert globemallow, which happens to be a desert tortoise favorite. We recently released healthy tortoise back into the wild, something no other organization in the state of Nevada is permitted by U.S. Fish and Wildlife Service to do, and we are confident that this release will be a huge success because of the ample forage available.

While we love to see all the native plants, this abundant bloom also presents a potential setback for the desert. Invasive plants such as brome and cheat grass are also thriving from the winter rains, and they pose a potential problem to the desert tortoise and other desert life. As summer sets in, most desert plants dry up while we are seeing invasive plants persisting all over the desert. The invasive plants can potentially catch fire from lightning strikes during late summer, causing devastation to the animals and plants of the Mojave Desert. Without these invasive plants, a lightning strike would set only one or a few native plants ablaze, but the invasive plants create a ground cover of highly flammable material that has been responsible in the recent past for fueling fires over thousands of acres of desert.

The desert after a fire.

When fire occurs in the desert, many animals can flee from the fire if they are fast enough; for the desert tortoise, however, fast is not much of an option. During a fire many desert tortoises will survive if they are in a burrow, but that doesn’t mean they are safe. After the fire is over, a tortoise comes out of a burrow to find that there is nothing to eat, which means that it must travel long distances in search of food and basically start over in finding or building a new burrow; this can cause stress on the tortoise during the extremely hot summers. The picture above is of a tortoise that survived a fire north of Las Vegas. He was tracked since the time of the fire, and I am happy to say that four and half years later he is thriving and living a fruitful life in the Mojave Desert. He was a lucky one.

Please consider xeriscaping your lawn and garden if you live in or near the desert. Strong winds blow the seeds from neighborhoods in the Las Vegas Valley out to the nearby desert, so by xeriscaping your yard, you not only have a beautiful native landscape to enjoy, but you help us protect the desert tortoise’s habitat!

Daniel Essary is a research associate at the San Diego Zoo Desert Tortoise Conservation Center. Read his previous post, Watch Where You Step.


Watch Where You Step!

A nickle placed on cryptobiotic soil shows how small the lichen is.

The desert soil is alive! Well, the soil itself isn’t really living, but life occurs throughout the soil of the Mojave Desert, so it’s important to always stay on designated trails and roads when you are in the desert.

Small microorganisms called cyanobacteria, which are from the same family as blue-green algae, actually live on the surface of bare soil in the desert. For most of the Mojave Desert, the soil is usually characterized by rough dark patches as shown in the photo, but these cyanobacteria, with the aid of different types of lichens, mosses, and other colonies of microorganisms, can sometimes produce colorful soil crusts. In both cases, the soils are called cryptobiotic crust.

Lichen covers cryptobiotic soil in the Mojave Desert.

Cryptobiotic crust is very important to the health of the desert—a great sign that barren land is actually growing and thriving. In fact, cryptobiotic crust helps produce nutrients and organic material that are recycled back into the soil, and this supports vegetation in the desert. This is great news for all the desert animals, like desert tortoises, that feast on plants as their main source of nutrition. The organic structure of cryptobiotic soil can also help native seeds to germinate (sprout), again an important feature for plant eaters like desert tortoises.

It takes a very long time for cryptobiotic soil to form, and it is also very sensitive to changes in its environment, so when it is disturbed, it does not have an easy time recovering. Some estimates indicate that it takes 250 years for damaged desert habitat to recover! When people use the desert for recreation, they have the opportunity to see and experience some of the most amazing scenery in the world. But if they are not careful, or they purposefully hike or drive off designated trails, cryptobiotic soil can be devastated.

When you step on cryptobiotic soil or drive over it, you kill millions of organisms that support the plant life that desert tortoises eat. If the soil is destroyed, then plants cannot grow, and tortoises will have nothing to eat. So if you know anyone who drives or hikes off trail and they tell you it’s okay because they are always careful not to run over tortoises or their burrows, you can now tell them it’s not okay because they are destroying cryptobiotic soils that allow plants to grow to feed the tortoises that they are being so careful to avoid!

As you can see, cryptobiotic soil is very important to the Mojave Desert ecosystem, and we should make every effort to avoid walking on or touching the soil. The next time you are out on a desert hike or driving down an old desert road, please stay on the designated routes to avoid harming the living soil below you.

Daniel Essary is a research associate at the San Diego Zoo’s Desert Tortoise Conservation Center in Las Vegas. Read his previous post, A Desert Tortoise Isn’t Just Any Old Tortoise.


A Desert Tortoise Isn’t Just Any Old Tortoise

Desert tortoise

Desert tortoise

The southwestern United States is lucky to have a wide variety of animals throughout its desert region. One special and ecologically important animal that is found here is the desert tortoise. This land-loving prehistoric critter has worked its way into the hearts of the public and has gained a lot of attention due to its declining populations resulting from encroachment on its habitat, droughts, and exposure to bacteria that cause upper respiratory tract disease.

The desert tortoise is one of four North American tortoise species that are uniquely adapted to the different habitats in which they thrive. These four tortoise species are all members of the genus Gopherus, and it can be tricky to tell them apart. We have seen all four species here at the Desert Tortoise Conservation Center (DTCC)! Following are some characteristics and life history traits of each species.

Gopher tortoise Gopherus polyphemus:

The gopher tortoise is a master burrower found in the southeastern U.S. from southern Louisiana all the way up to the southern tip of South Carolina, as well as throughout Florida (except in the range of the Everglades). The gopher tortoise is found in shrubby, prairie-like areas with sandy soil, as well as in longleaf pine/scrub oak habitat. Gopher tortoises are known for their long deep burrows, with some burrows reaching up to 40 feet (12 meters) long. These amazing burrows serve to protect gopher tortoises and over 300 other animals such as snakes, skunks, lizards, and armadillos from predators and harsh weather conditions.

Texas tortoise Gopherus berlandieri:
The smallest of the four Gopherus species, the Texas tortoise is found in southern Texas and northeast Mexico. It can be found in a variety of habitats but is mostly found in dry scrub and grassland areas. Unlike its cousin the gopher tortoise, the Texas tortoise doesn’t dig its own long burrows but instead uses a burrow dug by another animal or digs a shallow burrow under small shrubby plants. The Texas tortoise population has declined because of the pet trade in recent years, but new laws in Texas will be giving the tortoise a chance to recover.

Bolson tortoise

Bolson tortoise

Bolson tortoise Gopherus flavomarginatus:
The Bolson tortoise, also called the Mexican tortoise, is the largest of the four Gopherus species and is believed to be the rarest as well. It is found in a small region in north-central Mexico and only at an altitude of 3,300 to 4,300 feet in arid, sandy desert habitat characterized by the unique flora and fauna of that region. The significant declines in the populations of this species are attributed to cattle ranching, road construction, the pet trade, and even collection by locals as food. Recently, a biosphere reserve was established for the conservation of the Bolson tortoise and its habitat.

Desert tortoise Gopherus agassizii:

The desert tortoise that we know and love right here in the Mojave Desert is naturally found in northwest Mexico, southern California, southern Nevada, southern Utah, and throughout much of Arizona. The desert tortoise’s range spans two different deserts, the Sonoran and the Mojave. Scientists have determined that there are genetic differences between the tortoises found in those two locations. Like the gopher tortoise, the desert tortoise is an excellent burrow digger, and it uses burrows to escape the extreme temperatures of the desert, which range from above 120 degrees Fahrenheit in the summer to below freezing in winter. Burrows also help to protect the desert tortoise from predation by coyotes, ravens, and even mountain lions. Desert tortoises have been known to live up to 100 years in managed care. They can lay up to two clutches of eggs in a single year if resources like food and water are abundant. Unfortunately, desert tortoises are a threatened species, and populations are declining due to upper respiratory tract disease and human encroachment on their habitat.

So how do we tell the difference between these four animals if they are all lined up in front of us?! Well, Texas tortoises and desert tortoises show one small characteristic on their carapace (top shell) that is different from the others and from each other. Texas tortoises are small and have a slightly different head structure from the others, while Bolson tortoises are big and do not show sexual dimorphism (you can’t tell males from females externally). Finally, gopher tortoises sometimes have a darker and slightly different-shaped carapace than the others. Not very definitive, is it?! That’s why we rely on genetic testing to be certain which species we have here at the DTCC!

Daniel Essary is a research associate at the San Diego Zoo’ Desert Tortoise Conservation Center. Read his previous post, Desert Tortoises: Unexpected Nests.


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?


Desert Tortoises: Male or Female?

During the years I have spent working with desert tortoises, the one thing that I am continuously asked is “How do you know if it is a female or a male?” Well, it’s not an easy answer because there’s more than one way to determine the sex of a desert tortoise, and it can take some practice to get it right.

Plastrons of female, left, and male desert tortoises

Plastrons of female, left, and male desert tortoises

But first things first: it takes 15 to 20 years for a desert tortoise to reach sexual maturity, so it can also take that long before it starts showing physical characteristics that are typical of the two sexes, including plastron concavity, gular shape and size, tail size, and the presence or absence of chin glands. So we don’t know the sex of a young tortoise unless we hatched it ourselves at a particular temperature in the incubator (tortoise sex is based on the temperature of the nest, not on genetics).

But once a tortoise reaches adulthood, I find that the best way to identify the sex is to look at the plastron (the lower shell); but please do not flip a tortoise on its back, since that can be very stressful to the animal. If you lift the tortoise just slightly off the ground and get down there with him or her, you will see that a male desert tortoise has a concave plastron, or an indented curve, toward the tail end. Females, on the other hand, have a flat plastron, though on a rare occasion you will see slight curvature of the plastron, causing you to second guess yourself as to whether the tortoise is a male or female.

If you’re not sure about the plastron then check the gular (a long, extruding piece of the plastron under the neck). Males tend to have a large gular, which is used to fight and flip other males. However, a long gular may be damaged in a fight, broken off during an encounter with an off-road vehicle, or may even be bitten off by a predator, so it‘s important to check for other sex-specific characteristics as well.

Female, left, and male

Female, left, and male

Another way to determine the sex of a desert tortoise is by its tail, which is a great way to help determine if a tortoise is male or female without actually picking up the animal. Males tend to have longer tails than females, but the female’s tail is usually no longer than the end of a cotton swab, so the male’s tail is not exactly gigantic, making this another tricky technique that takes time and practice to use.

Tails of female, left, and male

Tails of female, left, and male

So finally, the last method we use to identify the sex in desert tortoises is to look for chin glands. Sometimes, especially during the active mating season, male tortoises will have enlarged glands under the chin, and they may have a gooey liquid extruding from them, while females rarely, if ever, show these glands.

Female, left, and male

Female, left, and male

The moral of the story is that there are many ways to help determine the sex of a desert tortoise, but to avoid sexing your pet desert tortoise incorrectly, you should use all the techniques described. Or better yet, the best thing to do is just look and enjoy. Remember, the desert tortoise is a threatened species, so it is against the law to touch one in the wild, and you should never flip a desert tortoise over, even if it’s your backyard pet.

Daniel B. Essary is a research associate at the San Diego Zoo’s Desert Tortoise Conservation Center.

Read a previous post, Desert Tortoise: Twizzler.