How prevalent, then, is virgin birth? And could it possibly extend to humans?

"In terms of other species, it is evident now that reptiles are a group that appear predisposed to parthenogenesis, whether facultative, as we address here, or obligate, where the primary reproductive mode is parthenogenesis and few or no males are known within the species," Booth said.

Obligate parthenogenesis may have arisen from ancestral interbreeding between species, though scientists aren't sure why some animals seem to randomly give birth without help from the male (the facultative type).

"What is common to those that reproduce facultatively is the lack of genomic imprinting — by that, I mean a process in which a specific set of genes are provided by the mother, and a second set from the father," Booth said. "These genes of different parental origin must interact in a process called genomic imprinting in order for the development of an embryo. This, as far as we are aware, occurs in all mammals with the exception of the monotremes — platypus and echidnas — and therefore explains why we cannot have facultative parthenogenesis in mammalian species without significant intervention by scientists."

Originally, Booth and his colleagues thought such virgin births might happen if potential mates were not present, but over the years, they have seen six captive female boa constrictors give birth via parthenogenesis even when males were around during their breeding cycles. The number of times virgin births have occurred with different females also seem to rule out a freak accident causing it to occur, Booth and colleagues said. They are now investigating other possible causes for these virgin births — "these include genetics, viruses, tumors and bacteria," Booth said.

In the future, the researchers also hope to investigate other species for virgin births, such as water snakes in Oklahoma. In addition, they plan to see how well the offspring of virgin births survive and reproduce. It may be that virgin mothers can establish whole area populations of snakes by themselves. "We will know if this is possible in the next two to three years," Booth said.

The scientists detail their findings online Sept. 12 in the journal Biology Letters.