Venomous lizards of the desert: studies of Gila monsters and beaded lizards have uncovered an array of surprising characteristics, from odd fighting rituals, to extreme energy efficiency, to a venom useful in treating diabetes

Natural History, July-August, 2004 by Daniel D. Beck

As usual, though, when one tries to lump the monstersaurs into one category or another (daytime animals, for instance), contradictions emerge. Lizards that forage widely, such as the monitors, are generally more active and have higher resting metabolic rates than sit-and-wait predators. Monstersaurs buck the trend: Even though they do forage over wide ranges, their overall activity levels and their resting metabolic rates are remarkably low. Rather than being steady performers, they are like athletes who perform superbly in bursts, then spend the rest of their time on the bench.

Even in the spring and summer, when monstersaurs are most active, they spend more than 90 percent of their time hidden in shelters. And over an entire year they spend far less energy in activity than most sit-and-wait predators do, Such apparent sloth is the result of extreme seasonal fluctuations in the monstersaurs' environment. Food, when available, needs to be gorged on, stored as fat, and used slowly. Compared with monitors, monstersaurs can feed only infrequently, but when they do, they can eat meals that put supersizing to shame. I once watched a Gila monster eat four nestling rabbits in a single "sitting," an Easter dinner with portions equal to a third of the lizard's weight and providing a third of the energy it needs for a year. (For a 140-pound person, that would be the energy equivalent of downing 325 twelve-ounce sirloin steaks or 450 fast-food double cheeseburgers at one sitting.) Of course, monstersaurs don't use up all that energy right away; they store it in their sausagelike tails and within their body cavities. Because the capacity to store energy is directly proportional to body mass, the larger the lizard, the longer it can subsist between meals. Gila monsters and beaded lizards, two of the largest lizard species in the New World, therefore have impressive abilities to store energy.

But if monstersaurs are good savers of energy, they're even better at spending it very, very slowly. At rest, Gila monsters and beaded lizards have among the lowest metabolic rates ever measured in lizards--far lower than the rates in monitor lizards. Furthermore, because monstersaurs spend so much time at rest--much of it in shelters where reduced temperatures confer even greater energy savings--the stored energy is used frugally. Monstersaurs are ectotherms (or, as they are sometimes erroneously known, cold-blooded animals), lacking internal mechanisms to regulate their body temperatures. With each drop in body temperature of eighteen degrees Fahrenheit, their metabolic rates decrease threefold.

The resulting energy conservation makes frequent foraging unnecessary, and enables the monstersaurs to occupy a feeding niche exploited by few other lizards. Indeed, their frugal lifestyle is causing herpetologists to rethink what they know about lizards, some of which turns out to be based on--you guessed it--overinterpretations of experience. Contrary to the received wisdom, predators that search widely can have low rates of overall activity--provided they have the right combination of high storage capacity, large meals, and low metabolic rates.