Reptiles
Whiptail Lizard
Aspidoscelis · also called whiptails, racerunners
A whiptail lizard is almost never still: a slim, striped, twitchy lizard that forages the hot open ground at a jerky trot, tongue flicking, and sprints off on its hind legs when startled. And in about a third of whiptail species there are no males at all — every lizard is a female that lays fertile eggs and hatches near-perfect copies of herself.
Overview
Whiptails are the genus Aspidoscelis, over 90 species across the Americas, most diverse in the arid southwestern US and Mexico — deserts, dry grassland, thorn scrub, and dunes. They are active-foraging insectivores, digging termites and beetle larvae out of soil and rotting wood rather than ambushing, and they keep a tight daily schedule, working through the hottest hours and retreating underground before dusk. The name racerunner comes from the flat-out bipedal sprint.
The all-female species
Roughly a third of whiptail species reproduce by parthenogenesis — females lay fertile eggs with no mating, and the daughters are near-clones. Each such species arose from a single hybridisation between two sexual whiptail species; the founding hybrid female could reproduce alone, and her exact genome passed to every descendant. Some carry three chromosome sets from a later backcross.
Going through the motions
Even without males, parthenogenetic whiptails perform a courtship and pseudocopulation ritual, one female mounting another, with the hormonal state deciding who plays the male. Females given this stimulation lay more eggs — a retained behaviour from the sexual ancestors.
Status
Most whiptails are Least Concern and stable, well suited to hot disturbed ground. Several small-range species are threatened, including the Saint Croix ground lizard, driven from its main island by introduced mongooses and rats and recovering on protected islets.
Whiptail Lizard: common questions
How can a lizard species be all female?
Through parthenogenesis, where females produce viable eggs without any genetic contribution from a male. In the all-female whiptails, a female doubles her chromosomes before egg formation and then divides them in a way that restores the full set, so each egg develops into a female that is essentially a clone of her mother, carrying the same combination of genes. Because there are no males, there is no mating in the usual sense and no mixing of genes between individuals. About a third of whiptail species reproduce this way, which makes the genus a major natural laboratory for studying how and why sexual reproduction is usually maintained.
Where did the all-female whiptail species come from?
Each one traces back to a single hybridisation event between two normal, sexually reproducing whiptail species. When those two species interbred, most hybrids were sterile, as hybrids usually are, but in whiptails some hybrid females turned out to be able to reproduce by parthenogenesis. That founding female passed her exact hybrid genome to all her descendants, so a parthenogenetic species such as the New Mexico whiptail is effectively a clone lineage descended from one hybrid individual, sometimes tens of thousands of years old. A few have picked up a third chromosome set from a later backcross with a sexual species.
Why do the females still court and mount each other?
Even without males, parthenogenetic whiptails go through a mating-like ritual, with one female climbing onto another, gripping her with the jaws and hind legs, and curving her body against her partner in a way that mimics copulation in the sexual species. Which lizard takes the male-like role depends on where each is in her reproductive cycle, with a female that has recently laid eggs and has low progesterone tending to act as the male. Experiments show that females given this pseudosexual stimulation lay more eggs than isolated females, so the behaviour, a leftover from the sexual ancestors, has been retained because it boosts reproduction.
Are whiptails hard to catch or keep?
They are notoriously difficult to catch by hand because they are fast, wary, and almost never still, bolting for a burrow at the first disturbance. Some are kept by specialist hobbyists, but they are demanding, needing a large, hot, brightly lit enclosure with deep sand for digging, high basking temperatures, and a steady supply of small live insects, and they do not tolerate handling well, staying flighty. The parthenogenetic species have a curiosity value because a single lizard can, in principle, found a whole colony, but they are not a beginner animal.