Juvenile Desert Tortoises Research Articles

Predation on desert tortoises (Gopherus agassizii) by desert canids

Desert tortoises (Gopherus agassizii) are a long-lived reptile vulnerable to predation by many predators, including desert kit foxes (Vulpes macrotis arsipus) and coyotes (Canis latrans). From 2009 to 2014, we assessed canid food habits at a study site in the Mojave Desert in California, USA, by collecting and analyzing desert kit fox and coyote scats. These canids primarily consumed small mammals and invertebrates. Desert tortoise remains occurred at an overall low frequency (<3%), indicating the opportunistic use by these canids of desert tortoise as a prey or scavenged food item. When we analyzed the desert tortoise remains further, there was a significant difference between predator type and desert tortoise age class. Coyotes consumed more than twice as many adult than juvenile desert tortoises, while desert kit foxes only consumed juveniles. Desert kit foxes are a small canid, thus limited to smaller prey items (i.e., juvenile desert tortoises), while coyotes are larger and can optimize a wider size range.

Crowding Affects Health, Growth, and Behavior in Headstart Pens for Agassiz's Desert Tortoise

Abstract Worldwide, scientists have headstarted threatened and endangered reptiles to augment depleted populations. Not all efforts have been successful. For the threatened Agassiz's desert tortoise (Gopherus agassizii), one challenge to recovery is poor recruitment of juveniles into adult populations, and this is being addressed through headstart programs. We evaluated 8 cohorts of juvenile desert tortoises from 1 to 8 yrs old in a headstart program at Edwards Air Force Base, California, for health, behavior, and growth. We also examined capacities of the headstart pens. Of 148 juveniles evaluated for health, 99.3% were below a prime condition index; 14.9% were lethargic and unresponsive; 59.5% had protruding spinal columns and associated concave scutes; 29.1% had evidence of ant bites; and 14.2% had moderate to severe injuries to limbs or shell. Lifetime growth rates for juveniles 1–8 yrs of age were approximately two times less than growth rates reported for wild populations. Tortoises in older cohorts...

Habitat drives dispersal and survival of translocated juvenile desert tortoises

Summary In spite of growing reliance on translocations in wildlife conservation, translocation efficacy remains inconsistent. One factor that can contribute to failed translocations is releasing animals into poor‐quality or otherwise inadequate habitat. Here, we used a targeted approach to test the relationship of habitat features to post‐translocation dispersal and survival of juvenile Mojave desert tortoises Gopherus agassizii. We selected three habitat characteristics – rodent burrows, substrate texture (prevalence and size of rocks) and washes (ephemeral river beds) – that are tied to desert tortoise ecology. At the point of release, we documented rodent burrow abundance, substrate texture and wash presence and analysed their relationship to the maximum dispersal. We also documented the relative use by each individual for each habitat characteristic and analysed their relationships with survival and fatal encounters with a predator in the first year after release. In general, the presence of refugia or other areas that enabled animals to avoid detection, such as burrows and substrate, decreased the overall mortality as well as predator‐mediated mortality. The presence of washes and substrate that enhanced the tortoises’ ability to avoid detection also associated with the reduced dispersal away from the release site. These results indicate an important role for all three measured habitat characteristics in driving dispersal, survival or fatal encounters with a predator in the first year after translocation. Synthesis and applications. Resource managers using translocations as a conservation tool should prioritize acquiring data linking habitat to fitness. In particular, for species that depend on avoiding detection, refuges such as burrows and habitat that improved concealment had notable ability to improve the survival and dispersal. Our study on juvenile Mojave desert tortoises showed that refuge availability or the distributions of habitat appropriate for concealment are important considerations for identifying translocation sites for species highly dependent on crypsis, camouflage or other forms of habitat matching.

Habitat selection by juvenile Mojave Desert tortoises

ABSTRACTGrowing pressure to develop public lands for renewable energy production places several protected species at increased risk of habitat loss. One example is the Mojave desert tortoise (Gopherus agassizii), a species often at the center of conflicts over public land development. For this species and others on public lands, a better understanding of their habitat needs can help minimize negative impacts and facilitate protection or restoration of habitat. We used radio‐telemetry to track 46 neonate and juvenile tortoises in the Eastern Mojave Desert, California, USA, to quantify habitat at tortoise locations and paired random points to assess habitat selection. Tortoise locations near burrows were more likely to be under canopy cover and had greater coverage of perennial plants (especially creosote [Larrea tridentata]), more coverage by washes, a greater number of small‐mammal burrows, and fewer white bursage (Ambrosia dumosa) than random points. Active tortoise locations away from burrows were closer to washes and perennial plants than were random points. Our results can help planners locate juvenile tortoises and avoid impacts to habitat critical for this life stage. Additionally, our results provide targets for habitat protection and restoration and suggest that diverse and abundant small‐mammal populations and the availability of creosote bush are vital for juvenile desert tortoises in the Eastern Mojave Desert. © 2016 The Wildlife Society.

Hiding in plain sight: a study on camouflage and habitat selection in a slow-moving desert herbivore

© The Author 2015. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. Camouflage via animal coloration and patterning is a broadly important antipredator strategy. Behavioral decision making is an influential facet of many camouflage strategies; fitness benefits often are not realized unless an organism selects suitable backgrounds. Controlled experimental studies of behavioral strategies in selection of backgrounds conferring camouflage, however, are rarely paired with observations of wild populations. In order to investigate how substrate composition influenced habitat preference and selection by juvenile desert tortoises (Gopherus agassizii), we completed a manipulative experiment in captivity and an observational study in the wild. In our captive experiment, we found that tortoises spent a greater portion of their time near rocks. We similarly found that wild tortoises preferentially placed themselves in areas with equivalent or larger-sized rocks. Additionally, juvenile tortoises were found to be less detectable on rock substrate by observers than they were on substrate-lacking rocks. We hypothesize that rocks improve juvenile tortoise camouflage and thus that tortoises select for habitat containing rock substrate, in part, due to a survival advantage conferred by such use. The desert tortoise is a threatened species, and the present study provides a model for examining the intersection between behavior and conservation, with implications for how suitable habitat is defined and measured in species conservation programs.

Nutritional Quality of Natural Foods of Juvenile and Adult Desert Tortoises (Gopherus agassizii): Calcium, Phosphorus, and Magnesium Digestibility

Abstract Exotic plants can make up a major component of the diet for some Desert Tortoises (Gopherus agassizii) in the Mojave Desert. If introduced plants differ nutritionally from native plants, they may influence the growth and welfare of young tortoises. Minerals available from a native grass (Achnatherum hymenoides), an introduced grass (Schismus barbatus), a native forb (Malacothrix glabrata), and an introduced forb (Erodium cicutarium) were measured for juvenile Desert Tortoises voluntarily eating single-species diets. We offered tortoises weighed amounts of chopped foods daily for ∼130 days (dry grasses; summer diet) or ∼90 days (green forbs; spring diet). Orts and feces were collected daily and dried to constant mass, and calcium, phosphorus, and magnesium contents of food and feces were measured and used to calculate nutrient digestibilities. Overall, tortoises gained more minerals from forbs than from grasses. Tortoises lost small but significant amounts of phosphorus when eating grasses, which ...

Nutritional Quality of Natural Foods of Juvenile Desert Tortoises (Gopherus agassizii): Energy, Nitrogen, and Fiber Digestibility

Wild Desert Tortoises, Gopherus agassizii, are eating different foods now than they were decades ago, because exotic plant species have invaded and flourished in the Mojave Desert over the last century. Reservations about the nutritional quality of exotic vegetation compared to native vegetation led us to conduct feeding experiments with growing, juvenile Desert Tortoises. We determined the digestibility of dry matter, energy, fiber, and nitrogen in four foods: Achnatherum hymenoides (a native grass), Schismus barbatus (an exotic grass), Malacothrix glabrata (a native forb), and Erodium cicutarium (an exotic forb). The largest nutritional differences among diets were between food types (fresh forbs and dry grasses) rather than between native and exotic species. The two grass diets were higher in fiber content and they contained less digestible energy than the two forb diets. The grasses contained little protein, and tortoises actually lost mass and body nitrogen while eating them. The exotic forb yielded more energy and nitrogen per unit dry mass than did the native forb, but this may be related to differences in phenological stages and associated fiber contents of these foods when they were collected. Juvenile tortoises gained weight rapidly when eating forbs and showed no evidence of having a lower digestive capability than did adults, despite their small size and immaturity. Estimates of nitrogen requirements compared to annual nitrogen intake on these diets suggested that growth of juveniles may be limited in part by dietary nitrogen.

Rates of Food Passage in the Digestive Tract of Young Desert Tortoises: Effects of Body Size and Diet Quality

Diet quality and body size affected passage time of 2 diets (higher and lower in fiber) in hatchling and juvenile desert tortoises (Gopherus agassizii). Mean retention time of the liquid fraction of each diet was significantly shorter than that of the solid fraction, and hatchlings had shorter passage times than juveniles. Habitat disturbances that limit access of hatchlings to diets lower in fiber may negatively impact the physiological and behavioral ecology of young desert tortoises.

SPATIAL PATTERN OF RISK OF COMMON RAVEN PREDATION ON DESERT TORTOISES

Common Ravens (Corvus corax) in the Mojave Desert of California, USA are subsidized by anthropogenic resources. Large numbers of nonbreeding ravens are attracted to human developments and thus are spatially restricted, whereas breeding ravens are distributed more evenly throughout the area. We investigated whether the spatial distribution of risk of predation by ravens to juveniles of the threatened desert tortoise (Gopherus agassizii) was determined by the spatial distribution of (1) nonbreeding ravens at human developments (leading to “spillover” predation) or (2) breeding individuals throughout developed and undeveloped areas (leading to “hyperpredation”). Predation risk, measured using styrofoam models of juvenile desert tortoises, was high near places attracting large numbers of nonbreeding ravens, near successful nests, and far from successful nests when large numbers of nonbreeding ravens were present. Patterns consistent with both “spillover” predation and “hyperpredation” were thus observed, attributed to the nonbreeding and breeding segments of the population, respectively. Furthermore, because locations of successful nests changed almost annually, consistent low-predation refugia for juvenile desert tortoises were nearly nonexistent. Consequently, anthropogenic resources for ravens could indirectly lead to the suppression, decline, or even extinction of desert tortoise populations.

Managing a subsidized predator population: reducing common raven predation on desert tortoises.

Human communities often are an inadvertent source of food, water, and other resources to native species of wildlife. Because these resources are more stable and predictable than those in a natural environment, animals that subsist on them are able to increase in numbers and expand their range, much to the detriment of their competitors and species they prey upon. In the Mojave Desert, common ravens (Corvus corax) have benefited from human-provided resources to increase in population size precipitously in recent years. This trend has caused concern because ravens prey on juvenile desert tortoises (Gopherus agassizi), a federally threatened species. In this paper, I discuss management strategies to reduce raven predation on desert tortoises. The recommendations fall into three categories: (1) managing raven populations by reducing access to anthropogenic resources; (2) removing offending ravens or other birds in specially targeted tortoise management zones; and (3) continuing research on raven ecology, raven behavior, and methods of reducing raven predation on tortoises. I also recommend approaching the problem within an adaptive management framework: management efforts should first be employed as scientific experiments--with replicates and controls--to yield an unbiased assessment of their effectiveness. Furthermore, these strategies should be implemented in concert with actions that reduce other causes of desert tortoise mortality to aid the long-term recovery of their populations. Overall, the approaches outlined in this paper are widely applicable to the management of subsidized predators, particularly where they present a threat to a declining species of prey.

Water Balance in Neonate and Juvenile Desert Tortoises, Gopherus agassizii

Water Balance in Neonate and Juvenile Desert Tortoises, Gopherus agassizii

Winter Activity of Juvenile Desert Tortoises (Gopherus agassizii) in the Mojave Desert

Winter Activity of Juvenile Desert Tortoises (Gopherus agassizii) in the Mojave Desert

Spring Activities and Habits of Juvenile Desert Tortoises, Gopherus agassizii, in California

Spring Activities and Habits of Juvenile Desert Tortoises, Gopherus agassizii, in California