Researchers have discovered a female boa constrictor that can produce offspring without mating — so-called virgin births — a rare phenomenon among vertebrates.

A female boa produced two large litters of female babies with no help from any male. More significant, however, was a finding that the offspring all had a genetic makeup never before recorded naturally in the vertebrate world.

One of the 22 baby boa constrictors produced without the help of a father is pictured here. ((Courtesy Warren Booth))

Normally, female boa constrictors have a Z and a W chromosome, while male boas have two Z chromosomes.

What was so extraordinary in the case of this boa super-mom is that all of her female offspring had two W chromosomes — something that was thought to be impossible. The 22 babies all had the mother's rare colour mutation.

In effect, the babies are all half clones of their mother.

Incidents of virgin births have often been attributed to an absence of males. But the mother's two virgin birth litters were produced while she was being housed with male snakes, and she had previously given birth to litters after mating with a male.

Virgin births common among invertebrates

Lead author Warren Booth, a geneticist at North Carolina State University, said the results may require scientists to take another look at reptile reproduction. He suggested that asexual reproduction in snakes could be more common than previously thought.

Such virgin births are common in the invertebrate world. Many insect species, for instance, can produce offspring without mating. But asexual reproduction in vertebrates is much rarer.

Incidents of virgin births — known in the scientific community as parthenogenesis — have been reported previously among captive female hammerhead sharks and in Komodo dragons, but never before among boa constrictors.

Researchers at North Carolina State University published the results of their study in Biology Letters, a Royal Society journal.

"These findings provide the first evidence of parthenogenesis in the [Boa family], and suggest that WW females may be more common within basal reptilian lineages than previously assumed," the authors write.

It's not clear if the all-female snake babies will eventually mate with a male, or reproduce asexually, or do both as their mother did. But because of their WW chromosomes, any offspring they produce will be female.