“Size matters.” — anonymous

Which is the largest lizard living in the California deserts? It depends on how you measure them.

Desert iguanas and collared lizards (both Baja and Great Basin species occur here) are the longest. Desert iguanas’ total length can be 16-24 inches long. However, if weight or girth is your measure, then chuckwallas win the prize. Adult male chuckwallas can be about 15 inches from the tip of their nose to the tip of their tail but can weigh up to one pound whereas the iguanas weigh in at a mere 0.15-0.2 pounds. Both chuckwallas and desert iguanas are in the same family (Iguanidae) and so share a not too distant common relative, both are primarily vegetarians, both live in the same hot deserts with broadly overlapping ranges, so why be different? The answer, like much in nature, is complicated.

Let’s start with chuckwallas. There are currently five recognized species of chuckwallas, all occurring in the Mojave, western Sonoran, Colorado, and Baja California Deserts, with Baja California looking like the center of their origin and subsequent species radiation. Our widespread species, Sauromalus ater, (Sauro – lizard; omalus - flat), used to be S. obesus (“flat-fat lizard”) but in recent years a researcher uncovered an earlier name S. ater, (“flat-black lizard”) and in science the earliest name always wins, even if, as in this case, obesus is more apropos. Chuckwalla is an anglicized version of the Cahuilla name for this species (čaxwal), which is appropriate since the Cahuilla knew this lizard long before modern science ever described it. The other four chuckwalla species all occur on islands off the eastern coast of Baja California in the Gulf of California. Two of those are “island giants.” The Isla Angel de La Guardia and Isla San Esteban chuckwallas are at least three times the size of each of the rest of the species. These two chuckwallas are almost the size of small dogs, weighing in at over three pounds and are at least 10 inches longer. Again, this begs the question of why are they different?

Islands and the differences between island species and mainland species have always presented both a challenge and an opportunity for evolutionary biologists to explain. Island species are often either much larger or much smaller than their mainland counterparts. Oddly, big species are often smaller on islands and small species are often much larger. For example, on Santa Cruz Island, one of the Channel Islands off the California coast opposite Santa Barbara, there were once “pygmy” mammoths, elephant-like mammals that were no taller than a human. On the same island there were “giant” deer mice (the size of large rats) and a plant in the sunflower family, giant coreopsis, that often exceeds three feet in height and is a perennial, while their closest mainland relatives are comparatively diminutive, reaching just few inches tall and are annuals. So, of the four island species of chuckwallas, two are giants and two are no different in size from the mainland S. ater. At least one of the explanations is that the two giants occur on islands that were separated from the mainland and each other by deep ocean channels throughout the Pleistocene despite changing sea levels between repeated glacial maxima (low sea levels) and glacial minima (high sea levels). The two island chuckwallas that are the same size as mainland chucks are on islands that were connected to the mainland during each glacial maxima; they haven’t had sufficient time nor isolation to evolve different sizes.

Still, why in this case was it beneficial for the two giants to become giants? Part of the reason is food supply. Arid islands have limited food; a giant mammoth would have eaten all of their food supply in a short time, whereas smaller mammoths might be able to sustain their populations much longer. Perhaps counter intuitively, for reptiles, food shortages can be a reason for getting bigger. During a drought-catalyzed famine reptiles can crawl into a crevice or a burrow and shut down metabolic activity to minimum levels. Using that strategy, a lizard, snake, or tortoise might be able to survive a year or even two without food or water. However, in contrast to a mammal or bird, which need constant food to sustain a warm body, a bigger reptile draws upon fat and muscle storage during a famine, and of course has no need to produce heat, so they can live much longer on much less. Eventually, when rains return and vegetation recovers, a larger reptile can quickly replenish their spent body storage capacity, preparing themselves for the next famine.

Another reason to get big is to avoid getting eaten. On the Galapagos Islands marine iguanas, cousins to our chuckwallas and desert iguanas, grow big fast, to avoid becoming prey for Galapagos hawks, red-tailed sized birds of prey that specialize on eating lizards. Lava lizards and hatchling-juvenile marine iguanas are their prey. Once the marine iguanas get large (they can exceed 20 pounds in weight), they become too big to be hawk food. The giant chuckwallas on Isla Angel de La Guardia and Isla San Esteban probably suffer predation pressure from red-tailed hawks, prairie falcons, and ravens. Bigger lizards are more immune from such a fate. 

Famine is also a factor in the marine iguanas’ race to get big. Like our iguanas and chuckwallas, these lizards are also vegetarians; they eat marine algae, with larger males diving deep to reach algal sources while females and juveniles stick to the shallower pools. However, during El Niño events warmer water diminishes the algal supply. During those periods of famine, the smaller juveniles are the first to succumb. However, there is one of the Galapagos islands, Isla Genovesa, where all the marine iguanas are small, not much bigger than our desert iguanas. The seas surrounding that island are warmer so the algal supply may be less, but perhaps less impacted by the periodic El Niño shifts, and the Galapagos Hawks do not occur there. No predators and warmer water with probably less abundant but less fluctuating food has resulted in no giants.

Our mainland chuckwallas and desert iguanas are big, but not giants. One reason is that rather than just avoiding being eaten by birds of prey, our lizards are eaten by coyotes and kit foxes, as well as ravens, roadrunners, hawks, and falcons. Being bigger does not make them immune to coyotes, badgers, and foxes, predators absent on those Baja islands where chuckwalla giants are found. The iguanas need to run fast, and the chuckwallas need to quickly slip into rock crevices. Being bigger would not help them avoid being invited to lunch by the suite of predators they face. But being big does help survival in a desert where droughts are always being countered by wet years of food plenty. With extreme aridity becoming more common and less the exception, it appears that many reptile populations are shifting abundances to higher elevations where the aridity is not quite so severe, and where their strategies for surviving famine still work. Size is part of that complicated balancing act between feast and famine and predation.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe