What Eats a Desert Tortoise? Predators and Threats

Common ravens, coyotes, Gila monsters, kit foxes, and feral or free-roaming dogs all eat desert tortoises or their eggs, but the threat each poses depends heavily on the tortoise’s age and size. Hatchlings and juveniles with soft shells face by far the greatest predation pressure, while healthy adults are largely predator-proof. The more surprising side of the story is that many of these predators have grown more numerous and more dangerous because of human activity, and predation is only one entry on a long list of threats pushing Agassiz’s desert tortoise toward decline.

Why Age Is Everything

A full-grown desert tortoise, with a shell that can exceed 35 centimeters in length, has very few natural enemies. The shell is too hard and too large for most predators to crack. The real vulnerability lies at the other end of the size spectrum. Hatchlings emerge from the nest at roughly the size of a silver dollar, with shells so soft they can be punctured by a beak or a tooth. It takes years for those shells to harden enough to offer real protection. Research on captive-reared tortoises found that juveniles did not reach full adult shell hardness until roughly ten years of age, and wild juveniles took even longer.

This long window of vulnerability means that the predators most dangerous to the species as a whole are not the ones capable of killing adults, but the ones that specialize in eggs, hatchlings, and small juveniles. A population can absorb the occasional adult death far more easily than it can absorb years of poor juvenile recruitment, because desert tortoises are extremely long-lived and slow to reproduce.

Egg Predators

Desert tortoise nests, typically shallow scrapes in sandy soil containing a handful of eggs, attract a range of opportunistic predators. One well-documented egg predator is the Gila monster, a large venomous lizard native to the same Sonoran and Mojave desert habitats. Researchers observed that tortoise eggs serve as an important food source for Gila monsters and that female tortoises actively and vigorously defend their nests against them, though they are not always successful.¹ Ground squirrels, kit foxes, and various snake species also raid nests when they find them. Because a female tortoise may lay only one or two clutches a year with a modest number of eggs per clutch, even occasional nest predation chips away at an already low reproductive output.

Ravens and the Juvenile Tortoise Problem

If there is a single predator that dominates conversations about desert tortoise conservation, it is the common raven. Ravens are intelligent, adaptable, and increasingly abundant across the Mojave Desert, and they have learned to exploit juvenile tortoises as a food source. A raven can pick up a small tortoise, carry it to a perch, and hammer through the soft shell with its heavy beak. Researchers have found piles of juvenile tortoise shells beneath raven perching and nesting sites, sometimes dozens at a single location.

The concern is not just that ravens eat young tortoises; it is that raven populations in the Mojave have grown sharply. Human communities provide ravens with reliable food from landfills, dumpsters, and agricultural waste, along with water from sewage ponds and irrigation, and elevated nesting sites on power-line towers, billboards, and buildings. A study at a military base in the central Mojave found that raven density tracked human activity closely, with ravens congregating in far greater numbers near the landfill and sewage ponds than at undisturbed sites, and their numbers fluctuating in sync with the base’s troop training cycle.² Because these human-provided resources are more stable and predictable than anything the natural desert offers, ravens have been able to increase their populations far beyond what the landscape would otherwise support.³

The timing and geography of raven predation add further complexity. Field experiments using replica juvenile tortoises found that the risk of predation by ravens was significantly greater in spring than in fall. Interestingly, neither the size of the replica nor its distance from a road or power line significantly influenced the risk, suggesting that raven predation pressure can extend well beyond the immediate vicinity of human infrastructure, persisting for more than a kilometer from raven nesting structures.⁴ That finding is sobering: even tortoises in seemingly remote habitat are not safe from predation if a raven nest is anywhere in the area.

Coyotes, Kit Foxes, and Other Mammals

Coyotes are the most commonly reported mammalian predator of desert tortoises. Unlike ravens, coyotes are capable of killing sub-adults and even occasionally adults, using their powerful jaws to crush or gnaw through the shell. Coyote attacks on tortoises are well documented, and the characteristic tooth-scored shells are a familiar sight to field biologists working in tortoise habitat. Kit foxes, badgers, and spotted skunks can also prey on eggs and very small hatchlings, though they pose less of a structural threat to the population than coyotes do.

Coyotes, like ravens, benefit from human presence. They scavenge from garbage, hunt rodents attracted to irrigated landscapes, and drink from artificial water sources. While the subsidized-predator research has focused most heavily on ravens, the same underlying dynamic applies: human development in and around desert tortoise habitat inflates predator numbers beyond what the natural ecosystem would sustain.

Dogs Near Human Settlements

One predator that often gets overlooked in wildlife management discussions is the domestic dog. Free-roaming and feral dogs can inflict devastating injuries on tortoises, and their impact grows dramatically near human communities. A retrospective analysis of nearly 6,700 wild desert tortoises examined across the Mojave and western Sonoran deserts over three decades found that the risk of severe trauma to shell and limbs quadrupled between the 1970s and the 2000s, and the risk of damage specifically to the gular horns, the bony projections at the front of the shell used in combat between males, increased more than 16 times.⁵ The increase in severe trauma grew exponentially the closer a tortoise site was to a settlement, with the steepest rise beginning at roughly 12 kilometers from the nearest community.

Dogs attack tortoises in ways that other predators generally do not. A coyote kills to eat; a dog may chew on a tortoise out of curiosity or play, inflicting massive shell damage without consuming the animal. The resulting injuries, including cracked and missing scutes, exposed bone, and amputated limbs, often prove fatal or leave the tortoise vulnerable to infection and dehydration. Because dog-related trauma concentrates near settlements, it is a largely preventable threat, but one that has been growing as desert-edge development expands.

Upper Respiratory Tract Disease

Not everything that threatens a desert tortoise has teeth or a beak. One of the most significant non-predator threats is upper respiratory tract disease, caused by the bacterium Mycoplasma agassizii. The disease produces a chronic infection that eventually clogs the nasal passages with thick discharge and destroys the respiratory lining, making it progressively harder for the tortoise to breathe and forage.⁶ Sick tortoises lose weight, become lethargic, and are more susceptible to other stresses. In populations already weakened by drought or habitat degradation, the disease can accelerate decline.

The picture is not entirely straightforward, though. Researchers have challenged the idea that Mycoplasma agassizii causes consistent mortality everywhere it appears, arguing instead that the disease behaves in a context-dependent way. In some populations, exposure to the bacterium causes serious illness and death; in others, tortoises carry the organism with few obvious effects. Environmental stressors like drought, poor nutrition, and crowding appear to influence how severely the disease manifests.⁷ The release of former pet tortoises into the wild has been identified as a major route of disease transmission, because captive animals often carry the pathogen and introduce it to naïve wild populations.

Roads and Vehicle Strikes

Highways and paved roads cut through desert tortoise habitat across the Mojave and Sonoran deserts, and they kill tortoises in two ways: directly, through vehicle strikes, and indirectly, by degrading the habitat around them. Tortoises are slow, low to the ground, and tend to freeze or retract into their shells when startled, all behaviors that are exactly wrong for surviving on a road. Research on a highway in the Mojave found that tortoise density was suppressed near the road and did not recover to background levels until somewhere between 400 and 800 meters away, a zone the authors described as the road’s “road-effect zone.”⁸ Within that zone, tortoises are not simply getting hit; they are being removed from the population year after year, creating a demographic sink that can extend for hundreds of meters on either side of a highway.

Roads also fragment habitat in ways that compound other threats. A tortoise population split by a highway becomes two smaller populations, each more vulnerable to local extinction from drought, disease, or predation. Fencing along highways can prevent road mortality but also prevents natural movement between populations, creating its own long-term genetic and demographic problems.

Wildfire and Invasive Grasses

The Mojave Desert is not supposed to burn. Native desert scrub communities historically had too little continuous fuel to carry fire across the landscape. That has changed with the spread of non-native annual grasses, especially red brome and Mediterranean grass, which fill the gaps between shrubs and create a continuous carpet of fine fuel during wet years. The proliferation of these grasses has increased wildfire frequency and extent across the Mojave in recent decades, and the trend continues to accelerate the conversion of tortoise habitat.⁹

Fire kills tortoises directly when they cannot retreat underground quickly enough, but the bigger problem is habitat transformation. After a burn, native perennial shrubs that tortoises depend on for food and shade may take decades to recover, while the invasive grasses bounce back within a season. The result is a feedback loop: fire promotes grasses, grasses promote more fire, and each cycle pushes the landscape further from the shrub-dominated community tortoises need. Burned areas often support lower tortoise densities for years afterward.

Drought and Climate Stress

Desert tortoises are built for drought. They can store water in their bladders, drastically slow their metabolism, and spend most of the year inactive in burrows. Even by reptile standards, they have remarkably low energy demands, which allows them to tolerate long periods of chronic energy shortage during dry years.¹⁰ But there are limits. Multi-year droughts push tortoises past the point where stored water and fat reserves can sustain them.

A long-term study at a Mojave site tracked tortoise survival from 1978 to 2012 and found that survival was high for nearly two decades before dropping sharply between 1997 and 2002, a period that coincided with persistent drought. The best predictor of adult survival turned out to be a three-year rolling average of winter precipitation: when several dry winters stacked up, tortoise deaths climbed. When researchers surveyed the site in 2012, the postures and positions of most dead tortoises they found were consistent with death by dehydration and starvation.¹¹ As droughts in the American Southwest become more frequent and severe with climate change, this slow-motion threat may ultimately prove more dangerous to the species than any predator.

Illegal Collection

Agassiz’s desert tortoise has been federally listed as threatened since 1990, and it is illegal to collect, harass, or harm them. That has not stopped illegal collection entirely. Tortoises are taken from the wild as pets, sometimes by people who do not realize the animal is protected and sometimes by people who do not care. Researchers have noted that while road mortality and habitat loss have been well documented, illegal tortoise collection remains insufficiently addressed as a factor in population declines.¹² The challenge is partly one of detection: a tortoise taken from a remote desert site leaves no trace, and enforcement resources in vast, sparsely populated landscapes are thin.

Collection disproportionately removes adults, which are the demographic backbone of the population. Because desert tortoises take 15 to 20 years to reach sexual maturity and may live for 50 years or more, every reproductively active adult removed represents decades of lost reproductive potential. Even low-level collection, sustained over years, can quietly hollow out a population.

Managing Ravens to Protect Tortoises

Because raven predation on juveniles is such a significant obstacle to tortoise recovery, wildlife managers have invested considerable effort in trying to control it. Simply killing ravens is legally complicated and raises public backlash, so researchers have developed less-lethal approaches focused on reducing raven reproductive success. One method involves oiling raven eggs in the nest, which halts embryo development without removing the nest and causing the adults to re-nest elsewhere. Field trials using a specialized tool called the Remote Fluid Application System, which can be mounted on telescoping poles or drones, achieved a 100 percent success rate in halting raven nesting events across 46 nests by spray-oiling eggs on cliffs, in trees, and on other elevated structures.¹³

Early results from multiple sites have been encouraging. At tortoise conservation areas in the Mojave Desert, juvenile tortoise survival improved following raven egg-oiling campaigns.¹⁴ Egg oiling does not eliminate ravens from the landscape; it reduces the number of new ravens entering the population each year. Combined with efforts to reduce the food subsidies that sustain inflated raven numbers, like covering landfills and managing trash at campgrounds and rest stops, the approach aims to bring raven populations closer to levels the desert ecosystem can naturally support.

Head-Starting Programs

Given that soft-shelled juveniles are so vulnerable and that wild recruitment is so low, some conservation programs have turned to head-starting, the practice of raising hatchlings in protected outdoor enclosures until they are large enough to survive in the wild. The logic is simple: skip the most dangerous years. An 11-year study of head-started desert tortoises in the Mojave found that annual survival in the enclosures averaged 96 percent, and seven to ten times as many tortoises survived compared to what would be expected for free-living hatchlings over the same period. Successive improvements in predator exclusion, food and water supplementation, and pen design pushed survival rates higher with each cohort.¹⁵

Head-starting is not a silver bullet. It is expensive, labor-intensive, and only addresses the juvenile recruitment bottleneck. If the threats waiting in the wild, from ravens, drought, disease, and habitat loss, remain uncontrolled, releasing larger juveniles into a hostile landscape simply delays mortality rather than preventing it. Researchers have found that head-started tortoises released into the wild showed variable survival and space-use patterns, and that ongoing threat management at release sites is essential for the investment to pay off.¹⁶ The programs work best as one piece of a broader conservation strategy, not as a stand-alone fix.

How Threats Interact

What makes desert tortoise conservation so difficult is that few of these threats operate in isolation. A prolonged drought weakens tortoises physiologically, making them more susceptible to upper respiratory tract disease. Disease further reduces their ability to forage and maintain body condition, which in turn makes them less capable of surviving the next dry spell. Meanwhile, ravens subsidized by nearby landfills are picking off the juveniles that would otherwise replenish the population, and invasive grasses are converting the remaining habitat into a fire-prone landscape that no longer supports the plant communities tortoises rely on for food. A highway running through the middle of it all fragments the population into smaller, more isolated groups that cannot recolonize after local die-offs.

This layering of stresses is why desert tortoise populations have declined across large portions of their range despite decades of legal protection. The tortoise is not being overwhelmed by a single catastrophic threat. It is being ground down by a combination of predators, diseases, habitat changes, and climate shifts that collectively push mortality higher and recruitment lower than the species’ slow reproductive rate can offset. Each individual threat might be manageable on its own; together, they create a conservation challenge that requires action on multiple fronts simultaneously.