Friday, June 15, 2012

LIZARDS ARE COOLER WHERE IT'S HOT


We have lizards in the Pacific Northwest, but like all other reptile groups, they become more common and diverse as you travel lower in latitude. In the southwestern deserts, lizards often dominate the landscape, if you exclude the ever-present and noisy birds.

Lizards seem among the most heat tolerant of any vertebrates. You walk around in the desert when it's so hot you really shouldn't be out there, and you find lizards running ahead of you across the burning sand. Most of them are long-legged, and they hold their body above the substrate as they move. The really fast Zebra-tailed Lizard (Callisaurus draconoides) can run bipedally, using only its hind legs for a slightly greater speed. They have been clocked at four meters per second, and they seem to blur as they run. As your eyes follow them, you may not see that they stop.

The long legs and toes, especially the hind ones, are what furnish the combination of speed and sand traction that these lizards need to move about. All of their needs are met by high speed: predator escape, prey capture, and quick moves from shade to shade. Try chasing one down. None of the lizards shown here does much climbing except the Lesser Earless Lizard (Holbrookia maculata), which moves around on rock faces or up in low shrubs.

One adaptation shared by all these lizards to avoid the hot desert sand is to contact it as little as possible. One way of doing this is to rest on their heels and flex their toes upward. Perhaps their toes are especially heat-sensitive. And if you keep watch, you'll see that these lizards usually head for shade in the middle of the day. They're not that heat tolerant!

Earless lizards are unique in lacking external ear openings, perhaps an adaptation to keep sand out of the ear. Oddly, the closely related Zebra-tailed Lizard and fringe-toed lizards (Uma) have ear openings, yet they spend even more time on the sand. Maybe earless lizards just like silence!

The heads of the Zebra-tailed and Lesser Earless lizards are somewhat scoop-shaped, widest at the mouth. They push their head into the sand and, by digging with alternate strokes of the legs, bury themselves very quickly to avoid predators or spend the night.

Most of the lizards of the desert eat arthropods, with two rather different modes of foraging for them. The common mode is as a sit-and-wait predator. The lizard remains in one spot and watches around it. If it sees an insect or spider, it runs and grabs it. The Leopard Lizard (Gambelia wislizenii) is large enough to take other lizards as prey.

The other mode is as an active searcher. This characterizes the whiptail lizards of the genus Aspidoscelis (formerly Cnemidophorus). They move slowly along, digging in the soil with alternate front legs, and scare up or unearth their prey. They are just as fast on the go as the other types, though. The striped pattern helps them disappear when they move into dense cover. As in striped snakes, the animal doesn't seem to move when you see only part of it.

One of the lizards shown here, the Desert Iguana (Dipsosaurus dorsalis), is a herbivore. Typically it takes leaves, flowers and fruits of the low shrubs that are common in its arid habitat. Note its blunt head, not so different from the big green iguanas of the tropics.

Check out the southwestern desert in summer, the lizard season, but be sure to bring water!

Dennis Paulson


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