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Desert Tortoise (Gopherus agassizii) Fact Sheet: Behavior & Ecology

Desert Tortoise (Gopherus agassizii)

Activity Cycle

  • Rainfall and temperature control many activities (Ernst & Lovich 2009)
  • Most active spring and early summer; also in fall before cold weather; after rain in summer.
    • Hibernate in winter den; annual cycle of shifts from months of inactivity to utilizing summer burrows, with daily active periods.
    • Nest in June or early July to Aug. in Sonoran Desert; May and June in Mojave
  • Much behavior determined by fact that these tortoises live in hottest habitat of any tortoises on earth (Stanford 2010)
    • During heat of day, between May-October temperatures may exceed 50 degrees C (122 degrees F)
  • Seasonal activity patterns differ for Mojave and Sonoran populations
    • Two seasons of rain in Sonoran Desert - summer and winter; tortoises most active July to September (Averill-Murray 2002)
    • One main season of rain in Mojave Desert - winter;
      • Western Mojave is winter-rainfall desert; usually has more rainfall than eastern
      • Easter Mojave has two seasons - main winter and unpredictable summer rainfall
  • In a 1974-1975 study of tortoises in southern Nevada, active season is spring, summer, early fall; inactive winter season (Burge 1977)
    • Change from active to inactive begins late October
    • Change from inactive to active in March-April
    • Activity pattern controlled by temperature of environment and precipitation.
  • After active foraging, often assume a limp, spread-eagle posture with limbs and neck extended. (Luckenbach 1982)
    • Perhaps is technique for increasing body temperature and aiding digestion

Use of Dens

(Woodbury & Hardy 1948)

  •  Occupy two types of underground shelters according to study of wild population in SW Utah.
    • Winter dens dug horizontally 2.4 to 9.1 m (7.9 to 30 ft) into banks of dry washes
    • Summer burrows dug down 0.9 to 1.2 m (3 to 3.9 ft) from level ground
  • Winter dens re-used year after year, but same den not necessarily used each year
  • Burrows less permanent and multiple holes may be dug
    • May occupy temporary shelters several minutes or several weeks before moving to another shelter (Riedle et al 2008)
  • Several tortoises may occupy a winter den at the same time
    • One record of 17 tortoises in a single large den
    • 3.1 is the average number/den
  • Burrows used normally by single individual, or a male and female together
  • Hibernating tortoises show much variation in the timing of their fall entrance and spring exits from dens (Haines et al 1999)
    • Environmental cues did not seem to play a large role in affecting the timing

Home Range

(Berry 1986)

  • In general, yearly home range size estimates vary from 1 to 89 ha (2.5 to 220 acres) with high variability. (Duda et al 1999)
  • Each tortoise occupied a home range: 11-53 ha (27-131 acres) in 5 study sites in California, Nevada, Arizona
    • Range overlaps range of other tortoises
    • May be a very conservative estimate of the actual home range
    • May not account for excursions for denning, mating, mineral licks, water
    • Tortoises traveled distances of 1.4-7.3 km (0.9-4.5 mi) over periods ranging from 16 days to 5 years
  • Territory not defended by individual tortoises
  • Density of tortoises: 300 individuals/1200 acres for a study site in SW Utah. (Woodbury & Hardy 1948)
  • Choice of sites for relocation of tortoises needs to be based on
    • Awareness of complex tortoise social systems
    • Disease status of resident and imported tortoises
    • Homing capacity of tortoises
    • Whether new habitat is suitable
      • Edible plants
      • Shelter sites
      • Water
      • Mineral licks
      • Low enough density of resident tortoises,
      • Lack of ORV and road traffic, predators and toxins

Communication

(Ruby & Niblick 1994)

Displays and Olfactory Signals

  • Head bobs may be one factor allowing species recognition in Gopherus.
  • Postures (and odors from trails, fecal pellets, urine) help maintain dominance hierarchies (Berry 1986)
  • Secretions of chin glands used for sex and species identification (Alberts et al 1994) (Miller & Dinkelacker 2008)
    • Glands not active until sexual maturity
    • Males' glands are larger than females'
    • Dominant males' chin glands larger and contain more testosterone; size varies seasonally (Alberts et al 1994)
    • Enlarged chin glands serve as chemical but also a visual signal

Vocalizations

  • Sound production common in the testudinids (Auffenberg 1977), but not well studied. (Berry 1986)
  • Hiss, grunt, moan. (Ruby & NIblick 1994)
  • Male vocalizes during mating (Auffenburg 1977)

Agonistic Behavior and Defense

(Ruby & Niblick 1994) (Berry 1986)

  • Male/male aggression common during breeding season
    • Opponents attempt to overturn each other, may bite and ram opponents, face to face
    • Male/female aggression not known
  • When surprised by intrusion may lie down quickly (all legs withdraw and shell hits the ground)
  • When handled by humans or excessively stressed, may void waterstored in the bladder.
    • Can be life-threatening to tortoise in times of drought
  • Larger tortoises are usually dominant; males dominant over females but females decide mate choice.
  • Female tortoises observed defending nest from Gila monsters (Heloderma suspectum)
  • Defensive behavior (biting, nipping, hopping forward, hissing) by hatchlings in wild

Locomotion

(Woodbury & Hardy 1948)

  • Have normal quadruped gait (forelimb, opposite hindlimb, other forelimb, other opposite hindlimb) (Williams 1981)
  • Walking speed on land: rate of .5 to 12 hours per km (.8 to 19.3 hours per mile),
    • Can't keep up such a pace due to danger of overheating
  • Can climb inclines, sometimes at angles of 45 degrees to reach dens
  • Poorly adapted for swimming; has difficulty placing nostrils up high enough to breath above surface
  • Digging achieved with forelimbs; hindlimbs push soil out of burrow
  • Females dig nest holes with hind feet

Interspecies Interactions

(Woodbury & Hardy 1948)

  •  Many animals make use of desert tortoise dens and holes:
    • Pack rats (most common associated species); tortoises often found covered with pack rat 'rubbish'.
    • Banded geckos
    • Desert scaly lizards
    • Spotted night snakes
    • Sidewinder rattlesnakes
    • Great Basin rattlesnakes
    • Gopher snakes
    • Burrowing owls
    • Cactus wrens
    • Poorwills
    • Horned larks
    • Roadrunners
    • Arizona cottontail rabbits
    • Desert jackrabbits
    • Kangaroo rats
    • Mojave pocket mice
    • Spotted skunks
    • Black widow spiders

Face-Off

two desert tortoises square off

Larger tortoises are usually dominant over smaller ones.

Aggression is observed between male tortoises during the breeding season. Opponents attempt to overturn each other, may bite, or ram opponents.

Image credit: © San Diego Zoo Global. All rights reserved.

Page Citations

Berry (1986)
Burge (1977)
Ernst & Lovich (2009)
Luckenbach (1982)
(Woodbury & Hardy 1948) (Riedle et al 2008) (Ruby & NIblick 1994)

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