I have lizards in my backyard. How do I tell if they are a male or female? — Macie M., Dyersburg, TN
Great question, Macie! If you’re seeing multiple lizards in your backyard, then you’re probably seeing both males and females. Most lizards, like many animals, are sexually dimorphic, meaning not only are the genitals different, so are other characteristics such as size, shape, and appearance.
While there are few hard and fast rules in the natural world, here are a some things to look for when trying to determine the sex of a lizard. As with most examples of sexual dimorphism, a lot of them have to do with males competing for both territory and mates.
First, look at the size of the lizard’s head. In many lizard species, males engage in territorial combat and therefore have noticeably bigger, bulkier heads (and bodies) than females. They use these for biting and wrestling each other, and the encounters can actually become pretty bloody and intense. So if you see a lizard with a large head, it’s likely a male.
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Second, look at the lizard’s coloration. Males tend to be bolder in coloration than females, and like male birds they use these colors as a form of communication. The male western fence lizard (Sceloporis occidentalis), the most commonly seen lizard here in the Bay Area and known colloquially as the “blue belly”, does indeed have deep blue markings along the side of its belly. When displaying (either for territorial reasons or to attract a female), male blue bellies will do “push-ups” and extend their abdomens, showing off that color. You’ll even sometimes see them doing this to you if it’s particularly bold! Female blue bellies can also have blue markings, but they are much less intense.
One of the most interesting cases of coloration in male lizards occurs in common side-blotched lizards (Uta stansburiana), which can range into the Bay Area but are more commonly seen in Southern California and throughout the southwest. Males of this species have throats that are predominantly either orange, blue, or yellow, and each color morph has its own mating tactic, creating a “rock-paper-scissors” situation. Orange-throated males mate with many females and aggressively defend their territory. However, males with yellow throats do not defend territory; they instead mimic female lizards and sneak into orange-throated males’ territories to mate with his females. Blue-throated males defend mates instead of territory —they can chase off the weaker yellow-throated males, but not the aggressive orange-throated ones. Pretty amazing! Here’s a page from UC Santa Cruz that describes this phenomenon.
And third, look for any anatomical “decorations” on the lizard, such as spikes, horns, and dewlaps (flaps on the throat). If you see those, then you are most likely looking at a male lizard. One of the most ostentatious examples of this is the green iguana (Iguana iguana), the males of which have extremely large “jowls” at the bottom of their mouths and throat, in addition to longer dorsal spikes. As with the other signifiers mentioned above, all of these “decorations” are used to defend territory (and scare off rivals) in addition to attracting females.
If you’re lucky enough to get your hands on a lizard, check out the back legs and surrounding area. Males lizards often have large “femoral pores,” or little raised bumps, on the bottom side their back legs, which are used to secrete pheromones; females generally either don’t have them or have much smaller ones. Males also store their hemipenes (like many reptiles, male lizards have two pensis) just past their vent, so look for a bulge at the base of the tail.
Phew! I think that about covers it for lizards. As for other reptiles, things can be a bit more difficult. For turtles and tortoises, males are usually smaller than females and will have a concave belly, which allows them to fit on the female during mating; and male pond turtles have longer claws on their front feet, which they use to “tickle” females and fight off mates. Thankfully you didn’t ask about snakes — determining the sex of the snake usually involves probing its cloaca, which should only be done by a trained professional!
Ask The Naturalist is a bimonthly column produced in partnership with California Center for Natural History, a community of experienced naturalists who educate the Bay Area public about the natural world.