Cnemidophorus velox, a parthenogenic lizard. From Colorado Division of Wildlife.

What Kind of an Animal is It?

Lizards are vertebrate animals (with backbones). They belong to the class of Reptiles, animals with a number of advanced features:

• A scaly waterproof skin
• 4 legs (they are tetrapods)
• Well-developed lungs
• Internal fertilization
• Most lay special amniotic eggs which conserve water; an adaption for life on land

The whiptail lizards are in the family Cnemidophorus, and are noted for a long slender tail. There are 45 different species within this family.

What is its Lifestyle?

Several of these features help reptiles adapt to dry environments with little water. The whiptails live in the deserts of the southwestern US and Mexico. They run around under mesquite and yucca bushes and catch spiders, insects and scorpions for food.

Some species have a very unusual mode of reproduction (see next section).

Whiptail Lizards and Parthenogenesis (Virgin Birth)

In 15 of the Cnemidophorus species there are no males. They reproduce without fertilization, a process known as parthenogenesis of "virgin birth". Parthenogenesis is well known in lower animals, such as aphids,bees, and Daphnia but is rare in vertebrates. The offspring of parthenogenic lizards are clones, identical to the mother.

There are some advantages to a parthenogenic lifestyle:

• All members of your species can lay eggs and reproduction is more efficient
• Good mutations are passed on more efficiently in clones than in sexual species
• You don't waste a lot of time and energy searching for a mate (this might not be true in this species, since the females do pair up)

One of the surprising things about unisexual whiptail reproduction is that a courtship ritual is still required even though there is only one sex. Unisexual whiptails pair up. In the courtship ritual one female takes the part of a male, while the other takes the role of a female. Later the 2 lizards switch roles. The switch is caused by hormones: estrogen promotes female behavior; progesterone stimulates male behavior. The mating ritual is required for survival of the species: without it few eggs are released (ovulation).

Some Things to Think About

• Are there any advantages to having 2 sexes? Will males in our species someday also disappear like those of the whiptail?
• Why is nature still experimenting with so many types of reproductive strategy?
• How are the hormonal rhythms regulated so that one of each pair has high progesterone (male role) at the same time that the other has high estrogen (female role)?

More Information

Whiptail lizards without males have been described in 2 Scientific American articles:

• Charles J. Cole. Unisexual lizards. Scientific American. January, 1984, p. 94-100.
• David Crews. Courtship in unisexual lizards: a model for brain evolution. Scientific American. December, 1987, p. 116-121.

Click to see a picture of another parthenogenic whiptail, Cnemidophorus neomexicanus.