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Desert Tortoises (Gopherus agassizii) Fact Sheet: Diet & Feeding

Desert Tortoise (Gopherus agassizii)

Diet

  • Mostly grasses, leafy plants, flowers. (Bonin et al 2006).
  • A study of foods of desert tortoises in California: (Bury 1982)
    • Desert annuals are preferred food (flowers and vegetative portions)
    • Can consume enough annuals at peak blooming time to survive summer and winter dormancy periods.
  • In years of high rainfall, tortoises can afford to be more selective in food choices (Oftedal 2002)
    • Observed choosing plant species that aren't common
    • May choose many high-moisture plant
  • Major plant food consumed in Mojave and Sonoran Deserts: (Oftedal 2002)
    • 43 plant species are important
    • Only 8 species were major food item in both deserts
  • Diversity of annual plants available varies markedly from year to year; an "average" tortoise diet hard to characterize. (Oftedal 2002)
  • Non-native plants may pose a threat to tortoise's health (Boarman 2002)
    • Split grass (Schismus barbatus) - may deplete nitrogen, phosphorus, and water, causing weight loss
    • Russian thistle (Salsola tragus)
    • Red brome (Bromus madritensis rubens) - sharp awns cause mechanical injury to tortoise nostrils
    • Cheat grass (Bromus tectorum)
    • Many introduced weed species are high in potassium, (in high concentrations, toxic to tortoises)
  • Dry grasses often consumed in summer may be the least toxic food available, but these plants also dehydrate the tortoise. (Nagy et al 1998)

Feeding

Adaptations to drought

  • Stay in burrows in drought years; rarely feed; emerge to drink and feed after rains. (Oftedal 2002)
  • Winter rainfall of at least 26 mm (1.0 in) necessary for annual plants to germinate in Mojave and feed tortoises
  • Seem to sense coming summer rains; move to low areas to secure drinking water. (Averill-Murray 2002)

Nutritional aspects

  • Wild tortoise diet is a delicate balance between their need for protein and water and need to excrete excess toxic potassium. (Oftedal 2002, 2003)
    • Rains allow tortoises to drink and void potassium, then switch to a diet of dry grasses to gain weight. (Nagy & Medica 1986)
    • Without rainwater to drink, tortoises need to get water and protein from plants with a high potassium excretion potential (PEP), to avoid buildup of too much potassium
    • Oftedal suggests shortage of high PEP plants led to tortoise population crashes
  • Digestive physiology similar to that of rabbit or horse: plant fiber is a crucial nutrient for all these hindgut fermeters
  • Known to seek out sources of calcium, in soils, bone (Walde et al 2007)
    • Hansen et al (1976) found bird feathers, mammal hairs, snake and lizard skins, and arthropods in tortoise fecal pellets
    • Walde et al (2007) describe tortoises deliberately consuming bone from tortoise carcasses and skeletons
    • Murray (1997) reports desert tortoise ingesting a vulture pellet; clumps of hair from the pellet noted on tortoise's beak
    • Known to ingest calcium salts from desert soils by digging down to depths of 2-6 cm (0.8-2.4 in) where salts occur. (Marlow & Tollestrup 1982) but other researchers caution that the tortoises might have been seeking associated minerals (Oftedal 2002)
  • Walde et al (2006b) observed consumption of rabbit feces by desert tortoises and notes other records of eating scat of woodrats, lizards, peccaries and other tortoises; such behavior may add needed nutrients or beneficial gut microbes to the tortoise diet

What Does Pink Taste Like?

Desert tortoise eating a cactus flower

When available, desert tortoises selectively feed on high-moisture plants.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Page Citations

Averill-Murray (2002)
Boarman (2002)
Bonin et al. (2006)
Bury (1982)
Hansen et al. (1976)
Jarchow et al. (2002)
Marlow & Tollestrup (1982)
Murray (1997)
Nagy et al. (1998)
Nagy & Medica (1986)
Oftedal (2002, 2003)
Walde et al. (2006)

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