Isla San Esteban Chuckwalla, Sauromalus varius

Isla San Esteban Chuckwalla, Sauromalus varius. Photographed within the confines of the Arizona-Sonora Desert Museum, Tucson, Arizona, February, 2022.

The Isla San Esteban Chuckwalla, Sauromalus varius, is a member of the Iguanidae family of lizards. The family consists of seven hundred members placed in forty genera. There are five species in the Sauromalus genus.  The Isla San Esteban Chuckwalla is also known as the Piebald Chucwalla and Pinto Chuckwalls and in Mexico as del Cachorón de Roca or Chuckwalla Espinoso.

Isla San Esteban Chuckwallas are the largest species of Chuckwalla, reaching 61 cm (2 feet 0 inches) in snout-to-vent length, 76 cm (2 feet 6 inches) in total length, and 1.4 kg (3 lbs 1 oz). They are considered to be a textbook example of island gigantism, as they are 3 to 4 times the size as their mainland counterparts. Fewer predators and an increase in the availability of food has, over time, allowed Isla San Esteban Chuckwallas to increase greatly in size. Their head is relatively small compared to the rest of their body with irregular, black coloring around their eyes and snout. This gives way to splotches of tan and black coloring throughout the rest of their body. Fine scales cover their body, creating the impression of fine sandpaper. Their skin is loose, particularly around their neck and shoulders where it hangs in folds. Their eyes lie back from their snout on the sides of their head. Ear openings are located directly behind their eyes and lack any external structures, possessing only a flap of scale to protect their eardrum. Their torso is dorsally flattened and their sides roll gently outwards. Their legs are short and stout with five long, thin digits that end in talon-like claws. The digits on their hindlimbs are significantly larger than those on their front limbs. Their tail is long and stout with a blunt tip. The entire surface of their tail is fissured with prominent scales, giving it a rough, coarse, and segmented appearance. Sexual dimorphism is displayed with females growing smaller than the males and being duller in appearance with fewer patches. Except for size, juveniles are identical in appearance to the adults. 

Isla San Esteban Chuckwallas increase in activity when temperatures begin to rise early in the year, and sexual activity peaks in mid-spring. During courting, their coloring begins to intensify. Males attempt to gain the attention of females and threaten rivals by performing head bobbing, mouth gaping, and push-up displays. The dominant male clearly exerts his influence over his territory, and he and the other males tolerate one another on the basis of size and hierarchy. Females intrude into male territories to seek a desirable mate. After mating, approximately 5 to 16 eggs are laid in an underground cavity. Incubation lasts approximately 90 days and hatchlings emerge in late September. Outside of choosing a suitable nesting site, no parental care has been discerned. Females conceive every two to three years, depending upon the availability of nutritious food and the build up of body reserves for egg formation. 

Isla San Esteban Chuckwallas must maintain a high level of body heat in order to remain active, so basking on rocks or warm sand occupies much of their morning. During this time they cannot move with agility and are vulnerable. With a rise in body temperature, they then begin to move about in search of food and mates. The rest of their day is spent moving between sun and shade in order to achieve proper thermoregulation. They often seek shelter in rocky crevices to avoid extreme temperatures and predation. If temperatures rise above 40°C (104°F) and food sources become limited, they will go into aestivation. In the winter when temperatures drop, they go into torpor. When threatened, they wedge themselves into a tight rock crevice, gulp air, and inflate themselves so they cannot be removed. Their tail and claws aid in their wedging behavior, and help them to climb over boulders and cling to cliff faces. They also have the ability to break off a portion of their tail to distract predators, allowing the rest of them to escape. While it will grow back, the replacement tail is not as long or of the same color as the original. Isla San Esteban Chuckwallas are omnivores and consume a variety of leaves, fruits, buds, and seeds. Occasionally, they will consume small invertebrates. They gain all of their water requirements from the plants they eat and their waste is eliminated as salty crystals. The importance of territoriality increases when food is abundant and males mark their territory with scent from their femoral pores, located on their hind legs. 

From a conservation perspective, the Isla San Esteban Chuckwalla is considered to be Vulnerable due to their large fluctuations in population size depending on drought and unknown current population size. While this species is adapted to dry conditions, increased severity and/or frequency of drought can affect small populations at a greater degree and result in long-term overall decline. In addition to drought, they are threatened by invasive rats and feral dogs and cats that are sometimes brought to the island. While they are protected by CITIES and Mexican law, the Isla San Esteban Chuckwalla is also threatened by illegal collection for the international pet trade. The Isla San Esteban Chuckwalla was traditionally consumed by the Comcáac Indians of Baja California, but not presently.