Adaptations of Desert Amphibians & Reptiles
Thomas R. Van Devender
Amphibians and reptiles have many different adaptations that allow them to live in deserts, avoiding extremes in aridity, heat, or cold. The animals may be active only in certain seasons and at favorable times of the day. Many use the environment to actively regulate their body temperatures, preventing lethal extremes. And some are well adapted to the surfaces they live on — with modified appendages for burrowing or the capacity to run on, dive into, swim in or sidewind across loose sand.
Before vertebrate animals adapted to specific terrestrial habitats, such as deserts, they first had to adapt to living on land. The primary adaptations to life on land occurred in the Paleozoic 400 to 360 mya (million years ago) with the evolution of amphibians. Amphibians, a name derived from the Greek word amphibios (a being with a double life), live in fresh water as larvae and can move onto land as adults. In the amphibian’s metamorphosis from larva to adult, one can read the story of its evolution from lung fish: the larva uses gills to breathe and openings along its lateral line to sense its environment; in the adult these are lost, and lungs, limbs and digits develop. Aquatic larvae and thin permeable skin vulnerable to water loss and sunlight prevent amphibians from entirely living on land and limit their radiation into arid habitats. Although early amphibians had lumbered ashore in search of insects, vertebrates didn’t finally leave the water until later in the Paleozoic when the first reptiles evolved waterproof skin and an egg with membranes (amnion, chorion) to protect embryos from desiccation.
The evolutionary radiations of modern amphibians and reptiles, as well as of modern mammals and birds, began as the dinosaurs declined in the late Cretaceous (98-65 mya). Most general adaptations to aridity evolved in the dry seasons of tropical deciduous forests from the Eocene (about 45 mya) through the middle Miocene (15 mya), long before the deserts of North America came into being. The adaptations of Sonoran Desert endemics likely evolved in tropical deciduous forests or thornscrub. The uplift of the Sierra Madre Occidental by 15 mya changed weather patterns. Preadapted reptiles thrived as increasing aridity formed the Sonoran Desert by the late Miocene (8 mya)
Desert environments present great difficulties to amphibians. Tiger salamanders and lowland leopard frogs enter the desert only near permanent ponds, streams or springs. Tiger salamanders often become neotenic (retaining their larval forms) even reproducing as larvae, and only rarely metamorphosing into terrestrial adults.
The Sonoran Desert toad, desert spadefoot, northern casque-headed treefrog and others survive in the desert because of their abilities to excavate burrows as much as three feet deep where they spend nine or ten months at a time. Spadefoots and the northern casque-headed treefrogs have hardened areas, called spades, on their hind feet with which to dig. To prevent water loss in the burrows, spadefoots secrete a semipermeable membrane that thickens their skins, while the casque-headed treefrog forms a cellophane-like cocoon by shedding outer layers of skin. Spadefoots have a high tolerance for their own urea, since they do not excrete while in their burrows.
The ultimate challenge for desert amphibians is to reproduce in the temporary pools produced by highly sporadic and localized summer thunderstorms. Most breeding occurs at night with females attracted to calling males. The desert spadefoots evolved an accelerated development rate—from egg to toadlet in less than two weeks! In southeastern California, where summer rainfall is less dependable, spadefoots emerge during the first storm, travel to ponds, call and breed, and gorge on lipid-rich, swarming termites, often in single night. The adults may have only enough fat reserves to survive for a year without feeding.
Primitive reptiles were able to radiate into drier habitats than amphibians because of the amniote egg with a leathery or hard shell, and because of their relatively impermeable skin with scales. Populations no longer were concentrated near water sources and embryos developed directly into small adults at hatching.
Since reptiles have thin skin with little insulation and most do not produce heat internally to fuel their metabolisms, adaptations to regulate body temperature (thermoregulation) are very important. Thermoregulation is possible because of complex relationships between body temperature, physiological processes (chemical reactions, hormone production, etc.) and behavior. Activity patterns change with the seasons, from midday in spring and fall to early morning and late afternoon in summer. Nocturnal reptiles such as the banded gecko and most snakes passively exchange heat with the air and soil. In contrast, diurnal lizards absorb heat by basking in the sun. Relatively uniform body temperatures are maintained in a number of ways: through the timing of daily activities, by shuttling in and out of shade and changing body orientation to the sun (insolation), by adjusting contact with the surface to regulate heat transfer (conduction), by changing color (dark skin absorbs energy faster), and so on. Additionally, some desert reptiles can tolerate quite high body temperatures; the desert iguana’s active range, for example, is 100 to 108°F (38-42°C).
During times of environmental stress, desert reptiles spend long periods of inactivity in burrows, often borrowed from those dug by rodents or other mammals. During hibernation in winter and estivation in summer, animals in burrows have greatly reduced metabolic processes. They live on water and nutrients stored in their bodies, while wastes accumulate to potentially-toxic levels. Desert tortoises, for example, have a large urinary bladder that can store over 40 percent of the tortoise’s body weight in water, urea, uric acid and nitrogenous wastes for months until they are able to drink. Urates are separated from water and can be eliminated in solid form, freeing water and ions to be reabsorbed. During extended droughts while the tortoises are inactive, they can reabsorb minerals from their shells to use in their metabolic processes. The giant Isla San Esteban and spiny chuckwallas on islands in the Gulf of California have a pair of lateral lymph sacs in the sides of their bodies that allow them to store extracellular fluid. Chuckwallas and Gila monsters, as well as the barefoot and western banded geckos store water in fatty tissue in their tails.
Species in the Lower Colorado River Valley of Arizona and California and the Gran Desierto of northwestern Sonora have a number of specializations for living in loose windblown sand. Sidewinders have evolved with an unusual form of locomotion where the body contacts the surface at only two points as it lurches along. The flat-tailed horned lizard and the Baja California legless lizard (a snakelike burrowing lizard about the size of a lead pencil, restricted to a small area on the western coast of Baja California) have lost the sand-collecting external ear openings present in most lizards. Several species, including the legless lizard, banded sand snake, and shovel-nosed snake, have small eyes, narrow heads, counter-sunk lower jaws, and very smooth scales—adaptations to swimming and breathing in loose sand. The fringe-toed lizard has pointed, fringe-like scales on the elongated toes of its hind feet to give it traction as it runs across dune surfaces. The wedge-shaped head, nasal valves, ringed eyelids, scaly ear flaps, and fine body scales allow this lizard to escape predators by diving and burrowing into sand.
Thus, amphibians and reptiles use a variety of mechanisms not merely to survive extreme heat and aridity but actually to thrive in hot, dry deserts. Virtually all of these adaptations were inherited from tropical ancestors before the late Miocene formation of the Sonoran Desert.