An eastern diamond rattlesnake recently gave successful birth five years after mating, according to a new paper that describes this longest known instance of sperm storage, outside of insects, in the animal kingdom.
The study, published in the Biological Journal of the Linnean Society, also presents the first documented virgin birth by a copperhead snake. In this case, the female never mated, proving that snakes and certain other animals can either give true virgin -- dadless -- birth, or may store sperm for long periods.
Actual mate-less virgin birthing, known as parthenogenesis, "has now been observed to occur naturally within all lineages of jawed vertebrates, with the exception of mammals," co-author Warren Booth told Discovery News. "We have recently seen genetic confirmation in species such as boa constrictors, rainbow boas, various shark species, Komodo dragons, and domestic turkeys, to name a few.
Booth, an integrative molecular ecologist at North Carolina State University, analyzed DNA from the female copperhead that had been on exhibit -- without a mate -- for years at the North Carolina Aquarium at Fort Fisher. Molecular DNA fingerprinting excluded the contribution of a male in her giving birth, which produced a litter of four normal-looking offspring.
The eastern diamond-backed rattlesnake's birthing moment was even more dramatic, as she suddenly produced 19 very healthy offspring consisting of 10 females and nine males. DNA analysis confirmed that the 19 babies have a dad.
"This snake was caught when it was around one year old, and therefore would be considered sexually immature," said Booth, who co-authored the paper with Gordon Schuett of Georgia State University. "It was housed in isolation from males up to the time that it gave birth. Therefore, this snake was mated in the wild as a sexually immature juvenile."
He and Schuett said internal sperm storage tubules or an ability to twist a portion of the uterus might explain how the rattlesnake stored sperm for five years. To manage the second trick, Booth said "a region of the uterus becomes convoluted and contracted, which may act as a plug sequestering the sperm until ovulation."
Fish, birds, amphibians, insects and other reptiles can also store sperm for long periods.
Mammals are less successful, but a recent study on the Greater Asiatic Yellow House Bat found that females of this species could store sperm for several months. In contrast, women can store it for just hours or days.
Women are incapable of giving true virgin birth since certain genes must come from the man as well as the woman. Schuett explained that, in laboratory settings, scientists have gotten around this requirement for mammals by creating parthenogenic mice.
Both types of unusual birthing, true virginal and long-term sperm storage, have drawbacks and benefits. Storing sperm allows females to overcome climate challenges and other obstacles. Giving virgin birth may hurt genetic diversity and yield only all-female or all-male progeny. On the other hand, it could also weed out mutations that can make individuals less fit.
William Holt, a professor of reproductive biology at the Zoological Society of London's Institute of Zoology, told Discovery News that "this analysis is highly significant because it shows that some previous reports on long term sperm storage in snakes (interpreted as such in good faith by the original authors) were, in fact, caused by parthenogenesis."
Holt believes the implications are that "some species have developed reproductive mechanisms that appear to promote inbreeding, much like cloning technology, and are therefore counter-intuitive in the context of maintaining genetic diversity as a way of promoting fitness."
Paolo Prodohl, a professor of population and evolutionary genetics at Queen's University Belfast, said the next step in this line of research is "robust genetic analysis" to tease apart true virgin births from those due to long-term sperm storage. Schuett added that such studies could directly benefit humans, through possible health breakthroughs and improving our ability to store sperm.