Western Fence Lizard: California’s Blue Belly Is Evolving in Cities
Meet California’s western fence lizard, the blue-bellied reptile found on fences, rocks, and trails across the state. Here’s the science behind this ubiquitous backyard lizard.
Updated
Los Angeles looks like a triumph of concrete over nature, but look closely and there’s wildlife everywhere, from coyotes trotting through neighborhoods to hawks hunting over the freeways. One of the most common city dwellers is the western fence lizard. And new research shows it isn’t just surviving in the city. It’s evolving to fit in.
If you live in Southern California, you’ve probably had the experience of walking out your front door and catching just the slightest flicker of movement in your peripheral vision. Something skittered, something small. You glance over and, sure enough, it’s Sceloporus occidentalis, better known as the western fence lizard.
It’s a regular in the California cast of critters, probably the most common reptile in our yards, on our patios and along our hiking trails.
Western fence lizard’s blue belly.
The males are the flashy ones, with bright blue patches on their bellies and throats, which is why people call them blue-bellies. But both sexes can change shade. When it’s cool, they darken to soak up more heat from the sun, and when it’s hot, they lighten up to reflect it. It’s nothing like a chameleon, but it’s an important tool for thermoregulation.
Western Fence Lizard
Fence lizards belong to the genus Sceloporus , the spiny lizards, which has more than 90 species. Their closest relatives include the Sagebrush Lizard and the Eastern Fence Lizard, which look a lot like them. Their gray-brown, spiny backs are great camouflage. (They really do look like a piece of dry wood or a rock until they move.)
They live almost everywhere, from coastal sage scrub and chaparral to forests and grasslands. You see them everywhere in the San Gabriel Mountains. They hang out on rocks, fence posts and branches, soaking up sun and watching for predators and snacks. They eat crickets, ants, beetles, spiders, grasshoppers and flies, so a few of them in your yard is free pest control.
They may even protect you from Lyme disease. When ticks feed on fence lizards, a protein in the lizards’ blood kills the bacteria that cause Lyme disease, cleaning the ticks of it. Scientists think that’s one reason Lyme disease is less common in California than in the Northeast.
The males sport vibrant patches of blue on their bellies and throats, making them easy to identify. (Erik Olsen)
In mating season, roughly April to July, the males show off by doing push-ups, flashing their blue bellies and throats. It’s a warning to other males and a pitch to females, who seem to go for the most energetic, flashiest guys. It’s not unlike what happens on a typical summer afternoon down at Muscle Beach in Santa Monica.
Females dig shallow nests in moist soil, lay their eggs and cover them up, and a couple of months later the babies hatch. I love it when this happens because very often there are a dozen or more adorable little baby lizards hanging out around the yard.
Fence lizards are also a great example of urban ecology, the study of how living things cope with cities. Studies around the world have found animals genetically changing in response to urban life, and it’s happening here too.
“We usually think of evolution as being this super slow process that takes millions of years to happen and that no human could ever observe,” says San Diego State University herpetologist Kinsey Brock. “But really, things can happen fast when the environment changes quickly, and if they don’t, you don’t persist.”
Researchers have found western fence lizards in cities are evolving traits that help them live alongside millions of people. City lizards are less skittish than country lizards and let people get much closer before running. When you live with constant foot traffic, cars and bikes, bolting every time someone walks by is a waste of energy.
Their bodies are changing too. A recent study on western fence lizards in Los Angeles by researchers at University of California Los Angeles found that urban fence lizards have shorter legs and shorter toes than rural ones, probably because walls, sidewalks and other flat, man-made surfaces call for different moves than rocks and branches.
Western fence lizard (Erik Olsen)
The same research found that city lizards have fewer, bigger scales, likely because of the urban heat island effect. Lizards lose water through the skin between their scales, so fewer, bigger scales mean less exposed skin and less drying out, which helps in a city that runs hotter than the surrounding countryside.
That’s natural selection, happening in the last few decades, in our backyards. Next time a blue-belly does push-ups at you from the fence, give it some respect. It’s evolving right in front of you.
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