Behavior

Iguanids are frequently seen stretched out on a rock or otherwise basking in the morning sun. This behavior falls under the heading of thermoregulation. Because lizards are poi-kilothermic, or "cold blooded," they engage in basking to raise their body temperature to prepare for the day's activities of feeding, perhaps breeding, and evading ever-present predators. Many iguanids spend the night in burrows and emerge each morning to find a sunny spot and raise their internal temperature. As the day gets hotter, ground-dwellers will move into shadier spots so they do not overheat. The more arboreal species may move from a basking spot on an outer tree limb or the sunward wall of a house to the inner branches of a tree or shrub, or closer to the ground where the temperature is lower.

The typical iguanid spends the day in a high state of alert, as predators abound, particularly for the small- and medium-

The Barrington iguana (Conolophus pallidus) is endemic to Barrington Island, Galápagos. (Photo by R. A. Mittermeier. Bruce Coleman, Inc. Reproduced by permission.)
A green basilisk (Basiliscus plumifrons) runs on water aided by the flaps of skin on the underside of its hindfoot. (Illustration by Emily Damstra)

sized species. Many iguanids are cryptically colored and patterned, and are best served by remaining still when a predator passes. Others are quick runners and dash off almost immediately after they spot an animal that is even remotely threatening. Favorite hiding places for these runners are crevices between or under rocks, tangles of vegetation, or anything else that provides adequate cover. Some, like the zebra-tailed lizard (Callisaurus draconoides), start and stop while running or abruptly change direction when momentarily out of view to confound their pursuer. Various lizards, including Uma species, conceal themselves by squirming under the sand. A few inflate their bodies to avert capture. Common chuck-wallas (Sauromalus obesus) are well known for this practice, and inflate their bodies when hiding in a narrow crevice. The enlarged size makes them nearly impossible to extract. The horned lizards also puff up their bodies, but the result is an erection of their numerous spines, an excellent deterrent against biting predators. Several species of these small lizards will go a step further if the attacker is persistent, and squirt up to one-third of their blood supply out of their eye pores. Foxes and coyotes, who are the most common recipients of this behavior, seem to find the blood distasteful and release the lizards.

Another unusual predator-avoidance strategy is employed by the so-called Jesus Christ lizards (Basiliscus sp.) that appear to "walk on water" while making their escape. Normally terrestrial, the basilisks drop into the water when threatened and race on their hind legs across the water surface. Studies have shown that fringes on the hind toes trap a bubble of air beneath their feet and keep them from sinking if they are running quickly enough. Basilisks have been known to race across water as far as 100 ft (30.5 m). Numerous other iguanids engage in bipedal running, and some are very quick sprinters, but none run across the water like the basilisks.

Behaviors between members of the same species also vary among the iguanids. Territoriality is prevalent, and many species exhibit stereotyped behaviors, such as doing repeated "push-ups" or marking their turf with secretions from femoral pores, to defend specific areas for feeding or breeding. In many species, territoriality becomes more pronounced when food resources are low. In some cases, dominance hierarchies may develop. Research on green iguanas (Iguana iguana) has shown that dominant males typically have higher levels of testosterone, as well as particularly strong jaws and enlarged femoral pores. These males were able to attain and defend prime territories against other males, and also to attract more females.

Among many species, including Cyclura carinata and Sauro-malus obesus, territoriality becomes more pronounced during the breeding season. The males of some species captivate potential mates by certain actions. One of the most noticeable mating displays is the flaring of the throat fan, or dewlap, which is often seen in anoles (Anolis sp.). Anoles also flare the dewlap during territorial displays, which may include head bobbing and posturing.

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