Sonoran Desert Tortoise

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Sonoran Desert Tortoise


Sonoran Desert Tortoise,

Gopherus morafkai Murphy et al., 2011


The plastron is yellow and has no hinge. The hind limbs are very stocky and cylindrical; forelimbs are flattened and covered with large conical scales. Males have elongated gular shields, chin glands on each side of the lower jaw, and a concave depression on the plastron. This tortoise has a narrower shell and shorter gular projections than the Mojave Desert Tortoise, but the two species are best distinguished by geography. Adults reach 381 mm. The carapace is high-domed and usually brown, with prominent growth lines.

Distribution and Habitat. The Sonora Gopher Tortoise ranges east and south of the Colorado River in Arizona. In Mexico,

it is present in Sonora, including Tiburon Island. Introduced individuals or populations are present in North America and possibly elsewhere, but these are likely hybrids between Gopherus morafkai x agassizii. Sullivan et al. (2016) noted Sonoran Desert Tortoises differentially use incised washes and rocky slopes and avoid open flats and intermountain valleys, except during emigrations.

Natural History. Native grasses, forbs, and flowers form the diet of this tortoise. Today, many tortoise populations are eating different plants than those the species consumed 50 years ago. This is the result of the invasive Buffelgrass (Pennisetum ciliare), a perennial, C4 bunchgrass native to Africa and Asia. Buffelgrass was introduced to the southwestern United States in the 1930s and Mexico in the 1950s for erosion control and as cattle forage. The invasive grass creates a dense layer in areas where ground cover was historically sparse. The increase in biomass increases the frequency of fire in areas where fires have been rare. Additionally, the dense buffelgrass excludes native plants, reduces the abundance and distribution of species that provide food for tortoises. Sonoran Desert Tortoises reach their maximum densities on the steep, rocky slopes the same habitat that Buffelgrass thrive in (Gray and Steidl 2015).

Sonoran Desert Tortoise reproduction was studied by Averill-Murray (1993, 2018). Females laid a single clutch of eggs near the onset of the summer rainy season, but not all females reproduced every year. Both winter and spring rainfall influenced clutch frequency. The smallest female to lay eggs was 220 mm midline carapace length, but minimum reproductive size was negatively correlated with winter rainfall. The mean clutch size ranged from 3.8 to 5.7 eggs and was not related to female body size or rainfall. Mean egg width was not related to year, rainfall, or clutch size, but large females laid larger eggs than did small females.

Nest predation appeared to be high; some hatchlings emerged from nests during late summer, but hatchlings from clutches laid late in the year may overwinter in the nest.

Substantial follicle growth occurred during the spring after emergence from hibernation. The average date for females to emerge from hibernation was 8 March.  Vitellogenesis and egg production varied considerably among individuals. No female produced more than one clutch per year and annual clutch frequency varied from 0.35 to 1.00. Compared to small females, large females were more likely to reproduce each year and produced larger eggs, but body size did not affect clutch size (Averill-Murray et al. 2018).

These reproductive traits contribute to a life history that resembles an income breeder compared to the more capital-breeding strategy of the closely related Mojave Desert Tortoise (Gopherus agassizii). The differences in the life history traits may convey different reproductive and population consequences of climate change.

Captive tortoises subjected to olfactory and visual cues of coyotes were then measured for their resulting foraging behavior, burrow use, and other antipredator behaviors. This was an effort to see if tortoises’ behavioral decision-making would minimize their risk of predation. Nafus et al. (2017) also measured risk of coyote predation in wild tortoises based on burrow use. Willingness to feed and time spent feeding were unaffected by the presence of coyote urine, but higher air temperatures at the start of feeding caused a decrease in time spent at their food dish. Captive tortoises chronically exposed to coyote urine did, however, spend more time in their burrows than when exposed to rabbit urine. When exposed to a coyote decoy, captive tortoises showed more general antipredator behaviors than those exposed to a control stimulus (deer decoy). Wild tortoises were less likely to be preyed upon by coyotes if they were encountered in burrows more frequently. Human-subsidized predator populations may negatively affect declining species, including reptiles, because of behavioral modifications in response to predator cues (Nafus et al. 2017).

Little is known about how tortoise use space and even less about use of space is differs between males and females. Sullivan et al. (2016) observed activity of adult and juvenile via radiotelemetry, and hatchling activity observations were made incidentally, over a three-year period in central Arizona. Tortoises were most active in the fall (August–October) but exhibited a second peak of activity in the spring (April). On average, males moved longer distances than females in every month of the year when tortoises were active. Distance moved by females in the fall was significantly greater than all other months except April. Males showed a similar pattern with greater movement in the fall but it was not statistically significant. Adult activity was detected in every month of the year except January; at least one hatchling was observed active in every month of the year. The authors conclude that adult home ranges are consistent in size and placement across multiple years and, females may include a “migratory” pattern to north slopes following summer rains, where they encounter a higher diversity and abundance of food plants. Female home ranges greatly overlap but males show less overlap. Also, home ranges contain a few refuges in relatively low elevation washes that are used consistently, especially during the hot, dry summer months of May and June. In these washes tortoises consume caliche.

Riedle et al. (2010) found annual survivorship was high (89–97%), and it did not differ between sexes, and was comparable to previous studies using mark–recapture methods. At one site survivorship between sexes did differ seasonally, based on differences in seasonal activity patterns and differential exposure to predation by mountain lions. However, in the absence of mammalian predation, seasonal survivorship did not differ between sexes. The next leading cause of mortality was failure to turn over after a fall or after being flipped during mating or a combat.

The remains of 10 radio-tagged tortoises were found at one location and probable cause of mortality was determined for seven individuals (Riedle et al. 2010). One individual was wedged between two boulders, probably the result of a fall. One male and two females were found dead and lying on their carapaces, but causes of their deaths could not be determined. Six tortoises showed signs of predation, with Mountain Lions responsible for five of those based on canine marks on the shell and tracks in the vicinity of the carcass. Eleven tortoises preyed upon by lions had up to two-thirds of the carapace removed by the lions. A sixth carcass was missing its head and limbs but cause of death was uncertain it could have by predation or scavenging. Actual annual mortality of radio tagged individuals ranged from zero to three per year, and any year where mortality was 0.1 included at least one predation event by lions. All predation occurred during the primary activity season for tortoises (spring and late summer), suggesting random encounters between tortoises and lions.

 Curtain et al. (2009) examined the age structure of tortoise populations from the Sonoran and Mojave Deserts to determine whether the difference in resource availability has driven an evolutionary divergence in life history strategies. Age and growth rates strongly reflect the ecological adaptation of the two populations. The oldest Sonoran males reached 54 years, compared to only 43 years in females. The oldest West Mojave males reached 56 years, compared to only 27 years in females. Mojave Desert Tortoises grew faster and females reached sexual maturity at younger ages (~ 17-19 years) than Sonoran Desert Tortoise females (~ 22-26 years). These traits and the higher rate of clutch production in the Mojave Desert Tortoises are likely the evolutionary adaptation for low juvenile survivorship and a significantly shorter life span. Frequent droughts in the Western Mojave Desert and the lowest annual rainfall area within the range of the tortoises cause chronic physiological stress, and Curtain et al. this as a major selection force producing contrasting life-history strategies.

Winter behavior was studied by Sullivan et al. (2014). They observed adult, juvenile, and hatchling G. morafkai active during November, December, January, and February at 3 field sites in upland Sonoran Desert in central Arizona. At one site thirty-six individuals under observation, including males, females, and hatchlings, emerged to drink during the first heavy (> 20 mm) rainfall event in December. At all three sites, females were actively basking and foraging during winter more frequently than males. Food and water are restricted resources, and the tortoises exploit them in arid environments when they are available, even during the winter.

Predicting the impact of climate change on the Sonoran Desert Tortoise can be challenging given the diverse aspects of its life history that may be impacted. One strategy to improve forecasts is to evaluate how the species responded to climatic variation in the past. Zylstra et al. (2013) used 22 years of capture-recapture data for Sonoran Desert Tortoises collected from 15 locations across their geographic range in Arizona to evaluate how environmental factors affected spatial and temporal variation in survival. Although rates of annual survival were generally high, survival of adults decreased with drought severity, especially in portions of their range that were most arid and proximate to cities. In three locations where large numbers of carcasses from marked tortoises were recovered, survival of adults was markedly lower during periods of severe drought (survival = 0.77–0.81) compared to all other periods (survival= 0.93–0.98).

Assuming continued levels of dependency of humans on fossil fuels, survival of adult tortoises is predicted to decrease by an average of 3% during 2035–2060 relative to survival during 1987–2008 in 14 of the 15 populations studied. This decrease could reduce persistence of tortoise populations, especially in arid portions of their range. Temporal and spatial variation in drought conditions are important determinants of survival in adult desert tortoises (Zylstra et al. 2013).

Taxonomy & Systematics. Cooper (1861:118) considered it part of Xerobates agassizii;  Cope (1875) used the combination Testudo agassizii. Stejneger (1893) used the combination Gopherus agassizii; Bramble (1982) used the combination Scaptochelys agassizii; Murphy, Berry, Edwards, Leviton, Lathrop & Riedle (2011) described it as Gopherus morafkai. van Dijk et al. (2014:71) followed and used the combination Gopherus morafkai.