Leaping Lizards and Male Impersonators: Are There Hidden Messages? by Imitating Male Mating Behavior, All-Female Lizard Species Apparently Enhance Their Own Ability to Reproduce
Edwards, Diane D., Science News
Leaping Lizards and Male Impersonators: Are There Hidden Messages?
What do dandelions and certainspecies of fish have in common? There's not a male among them --yet they do very well, thank you. As one of several procreation options available in nature, all-female species are intriguing. But can this extreme form of asexual independence teach anything about the evolution of sexual behavior in higher animals, including Homo sapiens? Maybe, maybe not, say scientists who study lizards lacking the true male touch.
For most, life without the male of thespecies would lack a certain joi de vivre. The same might be said of a female-free world, with an added technicality: Life itself would be in shorter supply. While females and their eggs are crucial in species with sex differentiation, males can be superfluous in those that rely at least in part on the process called parthenogenesis, in which egg cells develop into individuals without fertilization.
Some species can alternate betweenparthenogenesis and sexual reproduction, depending on environmental conditions. Others stay true to parthenogenesis and its production of identical daughters--which, biologists point out, is a far more efficient way to reproduce than by sexual means, in which two cells are required for reproduction instead of one.
Giving a twist to the tale ofparthenogenesis, David Crews in 1979 reported a type of male impersonation among parthenogenetic whiptail lizards of the genus Cnemidophorus (SN: 12/22&29/79, p.423). While at Harvard University, he found that captive members of the all-female C. uniparens imitated mounting and mating postures of the male C. tigris, a Cnemidophorus species containing both males and females that reproduce sexually (see photos). Crews, now at the University of Texas in Austin, and his co-workers have since described this pseudosexual behavior in captive lizards from four other parthenogenetic whiptail species collected in the southwestern United States.
The lizards could change some oldtheories about sex, says Crews. He suggests that sexual behavior in animals may have evolved before the two sexes evolved. This may represent "a reversal of the old argument of first there was sex, then there was sexual behavior,' he says.
Although all are females, individualC. uniparens lizards are in a sense bisexual in their behavior, alternating their roles, says Crews. Which one plays the male in the aggressive pseudocopulatory event appears to be related to ovulation cycles, rather than size and age. The "male' of the pair either is past the point of ovulation (egg release) in its reproductive cycle, or its reproductive system is inactive. The preovulatory "female,' however, has large ovarian follicles and lays eggs about a month after the spurious mating dance.
Crews began searching for an explanationof the male-like behavior, which seems unnecessary in an all-female species capable of parthenogenesis--unless it actually serves some biological function. From observations made during a series of subsequent laboratory experiments, Crews concludes that pseudosexual behavior in C. uniparens does actually enhance the reproductive capability of the parthenogenetic lizard--a conclusion that has been met with some skepticism as well as interest.
Crews examined how productivity in C.uniparens is affected by such variables as male homrones and the presence or absence of different female cagemates. Similar studies in a parthenogenetic strain of fly were used for comparisons.
Data collected by the Texas group showthat, if C. uniparens are placed in isolation or with cagemates that have had their ovaries removed, the average number of egg batches (clutches) laid each breeding season drops to about 1, compared to an average of 1.5 clutches born to individuals housed with a cagemate having ovaries. But if a cagemate without ovaries is treated with the male hormone androgen --which causes male-like behavior in the lizards--the average number of clutches is comparable to the 2 to 3 clutches laid each season in a natural environment. …