Virile Reptile

AFTERNOON SUNLIGHT filters through the pines near Sentinel Peak in the Chiricahua Mountains of Southeastern Arizona. A small swarm of tiny flies rises and falls within a shaft of light. From far off to the south, the sound of thunder drifts faintly in from a distant summertime monsoon storm.

On the exposed gray boulders that sit as if tossed about on the ridge, a dozen or so fat-bellied mountain spiny lizards (Sceloporus jarrovi) lounge about in the sun with their legs splayed out to the side, rounded abdomens pressed close to the sun‑warmed rocks. The lizards look as if they are asleep except for one stalking a black fly that made the mistake of landing a few feet away. The lizard creeps across the boulder like a cat, then rushes forward to snap up the insect. A gulp of its big mouth, and the fly disappears.

Although the rest of the lizards appear to be taking it easy, the larger males have been battling one another at intervals over the last month or so. In the days of late summer, male mountain spinies begin manufacturing testosterone, which transforms laid-back lizards into World Wrestling Entertainment reptiles. They start to wander about looking for trouble.

Confrontations often begin when a male struts into   another’s domain with his back arched and sides flattened, which makes him look as large as possible   while also showing off his bright blue throat patch. From time to time, the intruder performs a series of intimidating push-ups. The opponent responds in kind as the two maneuver, sometimes lining up side by side only a few inches apart but facing in opposite directions. The intruder may then concede to the other male, slinking back to the rock from which it came.

But on occasion, puffed‑up display gives way to no‑holds-barred biting. Sooner or later — generally sooner — the match is over with one male breaking free and departing on the double, often encouraged on his way by a parting nip to the tail. Usually the intruder is the one to give up, but occasionally a male that had been holding out on an attractive boulder is forced to relinquish it.
 

The result of these fights is the subdividing of lizard habitat into a set of mutually exclusive male territories. But territoriality among mountain spiny lizards is only a late-summer and early-fall phenomenon. Life is tranquil in June when females give birth to as many as 15 tiny youngsters. During the summer rainy season in July and August, all the lizards, large and small, male and female, stay busy snapping up insects. Adult females use their meals to produce a new clutch of eggs internally, which mature by late summer and early fall. At this time they become sexually receptive. Not coincidentally, adult males get feisty at the same time under the spell of their own testosterone. They stake out their territories, which enclose the smaller patches where females feed, bask and hide. Thus, although males fight directly for possession of real estate, they are indirectly fighting for females, which occur more often in some places than others. The male who is able to control a female‑rich territory will leave more descendants than a male who is forced off to the side, where females are scarce.

The fact that males only pump themselves up with   testosterone in time for the breeding season suggested to my colleagues, Cathy Marler and Michael Moore, while both were at Arizona State University, that there might be some disadvantages associated with being hormone‑soaked, highly combative male lizards. To check out this proposition, they operated on a batch of mountain spiny lizards in June and July, slipping a slow-release capsule of testosterone under the skin of some of their subjects, while others underwent the same operation but received an inert chemical instead.

They put the two classes of lizards back into the wild at a time when untreated males had little testosterone and little interest in being nasty toward one another. As expected, the testosterone‑supplemented lizard group became aggressive much sooner than they would have naturally, and experienced a much higher rate of mortality than other males, almost certainly because these hyper‑territorial males spent too much time looking for a fight and not enough time feeding. Apparently, for mountain spiny lizards, as well as human athletes, the use of steroids comes with a price. 

John Alcock is a Regents’ Professor of Biology in the Division of Life Sciences at Arizona State University in Tempe. He is the author of several books on the biology of Arizona’s animals and plants, including Sonoran Desert Spring and Sonoran Desert Summer, both published by The University of Arizona Press.