Reptiles
Western Fence Lizard
Sceloporus occidentalis · also called blue-belly, blue-bellied lizard
The lizard doing push-ups on a sunlit fence post almost anywhere in California is the western fence lizard, the blue-belly. It is one of the most familiar reptiles in the West, and it also quietly reduces the amount of Lyme disease in the landscape.
Overview
Sceloporus occidentalis ranges across western North America from Washington to Baja California, from the coast into the mountains, on rocks, logs, chaparral, and fences. It is a day-active insect and spider hunter that basks in the open and dashes for prey.
Push-ups and blue
Adult males have vivid blue throat and belly patches, flashed during rapid push-up and head-bob displays from a prominent perch to warn off rivals and court females. The lizard can also darken or lighten its skin to manage its temperature.
The Lyme disease effect
When a young western black-legged tick feeds on this lizard, a protein in the lizard’s blood kills the Lyme bacterium in the tick’s gut. Because so many juvenile ticks in the West feed on fence lizards, this is thought to help keep Lyme disease rarer there than in the Northeast.
Status
Least Concern and very common, tolerant of gardens and farmland, though feral cats thin local populations.
Western Fence Lizard: common questions
How does the western fence lizard lower the risk of Lyme disease?
Lyme disease is caused by a spiral bacterium that ticks pick up when they feed on an infected animal, usually a small mammal or bird, and then pass on at their next meal. In much of the western United States the young stages of the western black-legged tick feed heavily on western fence lizards. The lizard's blood contains a protein that kills the Lyme bacterium, so a tick that feeds on a fence lizard has the infection cleared from its gut and cannot transmit it afterwards. Because so many juvenile ticks in the West take their blood meal from these lizards, this effect is thought to be one reason Lyme disease is less common in California and neighbouring states than in the northeastern United States, where ticks feed instead on mice that amplify the bacterium.
Why do fence lizards do push-ups?
It is territorial and sexual signalling. A male western fence lizard defends a patch of ground with good basking spots and does its push-up and head-bob display from a raised, visible perch. The movement, combined with flattening the body sideways and flashing the blue throat and belly, tells rival males that the territory is occupied and advertises the displaying male's size and condition to females. Rivals often answer with their own push-ups, and disputes can escalate to chasing and biting. Females and juveniles push up too, in a less emphatic way, usually as a signal of awareness or mild threat.
Is the blue-bellied lizard venomous or harmful?
No. The western fence lizard has no venom and is harmless to people. If cornered it may gape, and a large one can give a nip that barely registers, but it would far rather flee, and if seized it will often drop its tail and escape. It is beneficial in a garden, eating large numbers of insects and spiders, and its role in clearing the Lyme bacterium from ticks makes it useful to have around. Handling should be brief and gentle, supporting the body and never holding the lizard by the tail.
What is the difference between a western fence lizard and a sagebrush lizard?
They are close relatives and look similar, and their ranges overlap in parts of the Great Basin and the mountains. The western fence lizard is generally larger and chunkier, with coarser, spinier scales, blue belly patches that usually have a black border in males, and often yellow or orange on the rear surfaces of the limbs. The sagebrush lizard is smaller and slimmer with finer scales, the male's blue belly patches are usually smaller and lack a strong black edge, and it tends to occupy higher, cooler, more sagebrush-dominated habitat. In the field the two can be genuinely hard to separate without a close look.