“… imps of darkness … a hideous looking creature.” — Charles Darwin’s description of Galapagos Island marine iguanas

I cringe every time I read this quote and try to console myself knowing that Darwin still viewed himself as a geologist and a future Anglican pastor when he and the Beagle visited the Galapagos Islands. Also knowing he was creating literary images for readers back in England where lizards are rarely seen, and ultimately aiming at the future sales of his first book, “The Voyage of the Beagle.”

Nevertheless, it reminds me that all too often we humans have had little empathy for reptiles, with the only exception being turtles and tortoises, which were given credit for wisdom or at least perseverance by Aesop in his fable of the tortoise and the hare. Otherwise, fear and loathing of reptiles have been predominant literary themes through history, from Harry Potter to the tale of a snake who convinced Eve to sample forbidden fruits from the tree of knowledge, in that one phrase blaming both reptiles and women for “man’s fall from innocence and grace.” Or, perhaps giving credit to reptiles and women for our species’ never-ending pursuit of knowledge and truth. I prefer that explanation.

Understanding the surprisingly high biodiversity of deserts world-wide, including California’s deserts, requires an appreciation for how reptiles have been able to radiate into so many species despite these seemingly harsh environments. Eric Pianka was one of the first scientists to appreciate the lessons lizards could tell us about how life has diversified on this planet. The 1960s and 70s were a time of great theoretical enlightenment in ecology. One of those ideas was the “ecological niche,” sometimes described as the sum of unique adaptations and environmental constraints that define where a species can live. Pianka wanted to explore this niche concept and did so focusing on where lizards were most abundant — in deserts. He compared the number of species that could occupy the sandy deserts of North America, southern Africa, and Australia. He found the relatively young North American deserts had the fewest species whereas the much older African and Australian deserts held many more species. More time has allowed lizards to divide up the desert resource “pie” into ever smaller slices, allowing more species to coexist.

Compared to the rest of North America’s ecoregions, our deserts, and especially our hot deserts, have by far the highest levels of lizard species richness on the continent. Why do lizards thrive in deserts? Part of the explanation is that their scaly skin reduces water loss, and so their water needs are less. They also void metabolic wastes using semi-solid uric acid rather than the water-wasting urine excreted by many mammals. Part of the explanation is that reptiles are so-called cold blooded, which means they are not using sparse resources to keep themselves warm with metabolically generated heat.  In actuality, at least during the day, lizards are exceptionally good at behavioral thermoregulation, moving from sun to shade and back to keep their body temperature close to constant and optimum for locomotion and digestion while the sun is up. For that active period their blood is as warm as that for most birds and mammals. By conserving resources and limiting activity, larger lizards can survive more than a year without food during extended droughts. And, part of the explanation is that other animals, mammals, birds, and amphibians, are not as good at reducing their water needs and so are not as abundant in deserts. That leaves fewer competitors with lizards for limited resources, and fewer large predators that would find lizards an easy meal.

The niche concept is good for helping understand the ways in which lizards divide up resources in the desert. There is a vegetarian niche occupied by desert iguanas and closely related chuckwallas. The desert iguanas mostly occupy the sandy flats while the chuckwallas are restricted to rock outcrops and so they avoid competing by occupying different habitats. There is an ant-eating niche mostly occupied by horned lizards. Flat-tailed horned lizards live only on the fine-grained sandy flats, desert horned lizards live on coarse-grained sands, gravel, and rocks, and then Blainville’s horned lizards occupy the higher elevations (and coastal areas). They all eat the same thing but live in different habitats, so competition is avoided. There is a sandy flat — food generalist — fast runner niche where the fine aeolian sands are occupied by fringe-toed lizards and the coarser sandy flats and washes are occupied by zebra-tailed lizards. Same food, different habitats, no (or very little) competition.

The diurnal rocky outcrop — insectivore niche appears to be populated by several lizard species, which could mean that there is abundant food to be had there, or that the mechanisms for avoiding competition are more subtle. Part of the mechanisms may be size. In the Mojave Desert there are smaller side-blotched lizards, middle-sized fence lizards, and larger desert spiny lizards occupying this rocky outcrop-insectivore niche. These three differ somewhat in elevation with fence lizards at higher elevations, desert spiny lizards in the middle elevation range, and side-blotched lizards at lower elevations. However, there is considerable overlap in elevations and lizard sizes. And, with climate change the side-blotched lizard populations are occupying incrementally higher elevations every year. These overlaps become even more fuzzy in the Santa Rosa and San Jacinto Mountains (and further south through the Peninsular Mountain Range) where the three rock-outcrop insectivores are joined by granite spiny lizards, banded rock lizards, and southern sagebrush lizards. So, six diurnal rock-outcrop insectivores all overlapping to some extent in size and elevation. Sometimes as many as three species might be sitting on the same rock. Just when you might think you have them figured out, the lizards let you know that there is still much to be learned.

The top carnivore niche is also muddied with elevation and size overlaps. In the Peninsular Mountain Range there are the Baja California collared, leopard, and western whiptail lizards. The collared and leopard lizards prefer rocks, whereas the whiptail prefers sandy habitats. In the Transverse Mountain Range and throughout the Mojave and southern Great Basin Deserts, the collared lizard is a different species, the Great Basin (or Desert) collared lizard, but the leopard lizard and whiptail are also present. They all eat smaller lizards, especially side-blotched lizards, but also all eat larger arthropods like grasshoppers and caterpillars.

Their relatively low resource needs (being “cold-blooded”) coupled with adaptations to reduce water loss (scaled water holding skin, semi-solid urine) make lizards the kings and queens of warm and hot deserts worldwide, reaching densities and species richness levels that surpass all other vertebrates within the desert biomes. An unanswered question is whether these adaptations might make lizards resilient to a changing climate. Some species are clearly shifting their abundances to higher elevations as warmer temperatures and drier conditions become the dominant climate pattern. Others, like fringe-toed lizards and flat-tailed horned lizards, are tied to low elevation aeolian sand fields, and so moving to higher elevation is not an option. How or if these and other lizards might survive a still warmer and drier desert environment is far from clear. Lizards still have much to teach us.

Nullius in verba

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