| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
|
|
|
|
|
wild lives Alligator Lizards Get A Grip
My first encounter with an alligator lizard was on a natural history reserve in northern California. I greeted the foot-long, rough-scaled creature with a flat palm, trapped it against the matted meadow grass, and lifted it off the ground. It greeted me by performing a curious "crocodile roll"-the spiraling motion a crocodile does to dismember its captured prey (and that alligator lizards sometimes do after a successful catch)-and aggressively smearing excreta across my palm and wrist. I reached toward it with my free hand and wham! it clamped its powerful jaws between the first and second knuckle of my index finger and started shaking its head like a killer whale thrashing a sea lion. I shook my hand, and the little terror, with a mouthful of my skin, fell to the ground and half-slithered, half-scurried with its awkwardly skinny limbs to the safety of the leaf litter. At first glance, they bear an uncanny resemblance to their much larger and more fearsome namesakes. An armor of thick, bone-reinforced scales coats them like alligator hide, covering all but a fold of skin along each side. These "lateral folds" allow their rigid bodies to expand after a large meal or while gravid, and aid breathing. When under attack, the lizard may aggressively strike back, lunging and biting at the assailant's head or neck. Peer closer and they start looking like snakes. They have pointed, chemoreceptive tongues and shed their skin in one piece like a snake. When they run, disproportionately small and widely spaced legs carry their undulating bodies. It is easy to imagine their descendents, one day legless, slithering and flicking forked tongues. Indeed, alligator lizards often break into near-slithering, limbs held at their sides like the Grinch when he stole Christmas. They belong to the family Anguidae along with legless lizards and other genera that look more like worms and snakes than lizards. But alligator lizards are really their own, weird-and impressive-animal. For self-defense, it strategically positions its tail between its head and the predator, increasing the chance that the appendage will be first to be injured. If clearly overwhelmed, it drops its injured tail and flees, leaving the writhing decoy for the seemingly victorious predator. This special tail, which regenerates within a few months, is prehensile; if the lizard is threatened by a hungry raptor, it may wrap its tail around a convenient branch and hold the tip in its mouth to prevent being carried off. A predator that seizes an alligator lizard will need to endure nauseating, repulsive excreta to get a meal. To the untrained eye, the two species of alligator lizards found in the western United States seem identical. Individuals of both species vary in color from gray green to brown and in the degree of dark banding which traverses their backs. They typically grow to about 14 inches long, including their tails. Both species mate from April to June; the young appear in August and September. Prior to mating, the male stalks the female like a predator. Once within striking distance, he lunges and clamps his vise-like jaws around her, eventually sliding his grip toward her neck. The two lizards maintain this mating position, with the female's neck clamped firmly between the male's jaws, for up to 24 hours. However, southern alligator lizards, Elgaria (previously Gerrhonotus) multicarinata lack pigment in their eyes, which appear light yellow, and their belly-side scales have longitudinal stripes running through them. Northern alligator lizards (E. coerulea) have brown or black eyes and stripes that run between the scales. But their biggest distinction by far is in the way they are born. Northerns are viviparous, or live bearing, whereas Southerns lay eggs like most lizards-a difference that makes them an important model for scientists studying reproductive evolution. The alligator lizard that escaped my amateur grip is known as Number Nine, a captive of University of California at Berkeley graduate student Judy Sheen. She is based at Hastings Reserve in Carmel Valley, spending her fourth year assessing the "costs" and "benefits" of each reproductive mode. For example, a live-bearer that retains her eggs through development may avoid nest predation, but she might run more slowly and make herself more vulnerable to predators. Viviparity has evolved independently over 100 times in reptiles alone-between species of the same genus (as alligator lizards), and even between populations within some lizard species. About 20 percent of all lizards, snakes, and amphibians produce live young. But to this day, there is no solid theory as to how this common strategy arises or why. There must be a selective advantage, but what is it? Retired Kansas University herpetologist Henry Fitch suggested in his 1930 college senior thesis that viviparity in E. coerulea "may have been in response to a high altitude environment," where conditions are generally cooler. Cold temperatures generally retard egg development. So a female that retained her eggs, he reasoned, could ensure high incubation temperatures and proper egg development by basking, giving her a selective advantage. Seventy years later, Fitch's thesis remains the only comprehensive published description of alligator lizard natural history, and his idea, now called the "cold climate hypothesis," still offers a good starting point for explaining viviparity in lizards. Both alligator lizard species occupy a wide variety of habitats in California-from sandy coastal scrub (their ranges overlap from Sonoma to Monterey counties), to grassy foothills, to chaparral in the Sierra Nevada (their ranges overlap again south of Lake Tahoe). But live-bearers do tend to inhabit higher latitudes or altitudes than their egg-laying cousins. And the bulk of the Northerns' range is in Can-ada, usually on the edge of moist conifer forests and up to 10,000 feet above sea level. Southerns generally live near dry oak woodlands of lower elevations and occupy the western half of California into Baja, excluding the San Joaquin Valley. But the cold climate hypothesis is based on today's population distributions and, though the correlation is strong, it alone cannot explain the origin of viviparity. So researchers like Sheen continue to collect more details. Herpetologists have other information about reptiles to work with. Incubation temperatures of developing young can affect a reptile's appearance and its behavior; in crocodiles, temperature can even determine the baby's sex. Embryos developing within a viviparous female are kept warmer and develop more quickly than eggs do in soil. Such early-bird babies do have advantages over egg-born cousins; they have more time to establish territories and prepare for the winter months. Sheen aims to determine how such factors may help or hinder alligator lizard survival. To do this, she's comparing the body temperatures of both species. A consistently high body temperature may speed up embryonic development, indicating more frequent basking; however, more basking increases the risk of predation. Sheen is also calculating their accelerations and observing how they move. Alligator lizards bolt for cover when threatened, and the way they curve their bodies to increase stride and accelerate is key to their survival. How does carrying a load of eggs affect a Southern's acceleration? And pregnancy, a Northern live-bearer's? The answers to these questions may help illuminate the rise of viviparity. Sheen is quick to point out that viviparity is not, necessarily, a more advanced condition, that a successful reproductive strategy depends on what works best where. "Just because we are viviparous, doesn't mean viviparity is an evolutionary goal," she says. Whatever selective forces are involved, surely, alligator lizards will help scientists get a grip on a working theory. But before reaching for one, I recommend, at the very least, wearing a glove. Dave Brian Butvill is Assistant Editor of California Wild. |
