Virgin births among captive animals have been described as a desperate attempt by females with no access to males to procreate, but a new study documents several cases of birth by wild female snakes that had no help from the males.
The study, published in the latest Royal Society Biology Letters, represents the first time that virgin births have been detected in the wild. It also suggests that absence of males does not always instigate the phenomenon, which occurs among chickens, turkeys, lizards, sharks, insects and many other animals.
For this latest study, the researchers focused on two closely related species of North American pit viper snakes: the copperhead and the cottonmouth.
"In these populations, males are relatively common, hence females were not restricted from access to males, and therefore isolation from males is not a driving factor for parthenogenic reproduction (virgin births) here," lead author Warren Booth, an assistant professor of molecular ecology at the University of Tulsa's Department of Biological Sciences, told Discovery News.
Booth studied field-collected pregnant snakes and worked on the research with colleagues Charles Smith, Pamela Eskridge, Shannon Hoss, Joseph Mendelson III, and Gordon Schuett.
Out of a total of 59 litters from the snakes, the scientists selected two for DNA analysis. These two already showed signs of virgin birthing, since the eggs had multiple yolks and the litters included just a single male offspring.
The genetic analysis supported the suspected lack of paternal DNA contribution. Booth explained that, in each case, the female's egg cell "fused to a part of itself, and her chromosomes doubled." The offspring wound up having two copies of her set of chromosomes, and therefore half the genetic materials.
"This means she has very reduced diversity across her genome," he said. "This is essentially an extreme form of inbreeding."
Loss of genetic diversity can be a problem, leading to deleterious genes in the population. On the other hand, the process for certain species can sometimes purge out bad genes.
For example, Booth said, "We see extreme inbreeding in many insect species, such as bed bugs and cockroaches, and they thrive. So while inbreeding is never ideal, it is not necessarily bad in all cases."
So far, the offspring of the studied snakes "are outwardly healthy," he shared, "and on their way to sexual maturity."
Demian Chapman, an assistant professor at Stony Brook University's School of Marine and Atmospheric Sciences, is a leading shark expert. He has documented virgin births in sharks, such as in a female blacktip that was once housed at the Virginia Aquarium.
At the time, Chapman told Discovery News that such a male-less birth "may just be an occasional mistake that sometimes occurs when eggs are left unfertilized."
But the latest findings suggest that this form of birth may be far more common among some animals than previously realized. Booth said that, based on his own past research on boa constrictors and cottonmouths, virgin births produce about 2.5 to 5 percent of litters. While those numbers aren't huge, they indicate that dad-less snakes aren't just an every-so-often novelty.
It remains a mystery now as to why these births happen, and what triggers them. The copperhead that gave virgin birth was small, Booth said. "If she had never mated, it is possible that she was overlooked by males in favor of larger, more fecund females."
Another theory is that females produce a single male offspring so that they can later establish a population with their son via inbreeding. Yet another is that bacteria or disease may trigger virgin births.
One thing is certain, at least so far: Mammals, including humans, cannot achieve the feat without significant intervention from scientists. Booth explained that the process seems to require a lack of genomic imprinting (whereby genes of different parental origin must interact). Reproduction among all mammals, save for the platypus and echidna, involves genomic imprinting.