San Esteban Island chuckwalla  (Sauromalus varius)

This species is yet to be classified by the IUCN, but is listed on Appendix I of CITES (3).

This plump lizard can grow up to 60 centimetres, making it by far the largest of the chuckwallas (Sauromalus), and similar in size to its large iguana relatives (4) (5). This is a prime example of ‘insular gigantism’, the tendency of small mainland animals to increase in size once established on an island due to fewer natural predators (2) (4). The chuckwalla’s grainy, sand-coloured skin is splotched with shadowy grey blotches over the entire body, providing almost perfect camouflage against potential predators in its rocky habitat. Sadly, this lizard’s large size, rather small head, loose, gathered folds of skin, and long, thick, blunt tail means that it has sometimes been mistaken for the poisonous gila monster, and persecuted as such (6).

Found only on the small islands of San Esteban, Roca Lobos and Pelicanos in the Gulf of California (4).

Chuckwallas occupy open flats, rocky outcrops, dry canyons and washes, often being found near large rocks and boulders (7) (8).

This diurnal lizard emerges in the morning and basks in the sun to raise its body temperature before foraging for food (8). Chuckwallas are primarily herbivorous, feeding on a variety of fruit, leaves, buds, succulent stems and flowers (8), with some species supplementing their diet with the occasional insect (7). When danger is sensed, chuckwallas display a unique defensive strategy of evading a potential predator. The threatened animal scurries into a rock crevice wherein it firmly lodges itself by inflating its lungs, thereby making removal by a predator almost impossible (5) (8).

Relatively little is known about the reproductive biology of the San Esteban Island chuckwalla. Chuckwallas generally lay a clutch of between 5 and 16 eggs in June to perhaps August, although some females may not lay eggs every year (7).

The precise threats to the San Esteban Island chuckwalla are unknown, although historically it has been widely captured for the burgeoning United States pet trade (6).

Chuckwallas have been maintained in captivity at the Arizona-Sonora Desert Museum (ASDM) since 1977 (9), where there have been ongoing studies of the species’ behaviour and reproductive biology within the large breeding colony held there (10). The San Esteban Island chuckwalla is listed on Appendix I of CITES, banning all international trade in the species (3).

• Diurnal: active during the day.
• Herbivorous: diet comprises only vegetable matter.

1. ITIS (September, 2008)
   http://www.itis.gov/
2. Beacham, W., Castronova, F.V. and Sessine, S. (2000) Beacham’s Guide to the Endangered Species of North America. Vol. 1: Mammals, Birds, Reptiles.Gale, Unknown.
3. CITES (January, 2007)
   http://www.cites.org
4. Rigor Vitae: Life Unyielding: Vanishing Circles (January, 2007)
   http://rigorvitae.blogspot.com/2006/05/vanishing-circles.html
5. Northern Arizona University: Caitilin McCracken’s Master's Thesis (January, 2007)
   http://www.nau.edu/~qsp/students_mccracken_thesis.htm
6. U.S. Fish and Wildlife Service (USFWS): Department of the Interior – News Release, 1979: Status Review of the San Esteban Island Chuckwalla (January, 2007)
   http://www.fws.gov/news/historic/1979/19790306c.pdf
7. Arizona-Sonora Desert Museum (ASDM) (January, 2007)
   http://www.desertmuseum.org/center/edu/docs/3-4_Reptiles_p21-27.pdf
8. Phoenix Rock Gym (January, 2007)
   http://www.phoenixrockgym.com/PRG/chuckwalla.html
9. Lee Rex McAliley, Ph.D. (January, 2007)
   http://www.webpages.ttu.edu/lmcalile/
10. DesertUSA: The Ultimate Desert Resource (January, 2007)
    http://www.desertusa.com/sept96/du_chuckwalla.html