One morning, while cleaning his rattlesnake cages at the University of Colorado at Boulder, David Chiszar was greeted with a surprise.
In the cage of a female timber rattler that had never encountered a male in its adult life, he found a live baby snake and two stillborn young.
What the psychology professor at Colorado discovered may have been one of the first recorded observations in snakes of parthenogenesis, from the Greek for “virgin birth.” Or it wasn’t.
“It could be a dead end, or it could be of incredible importance,” says Dr. Chiszar. “We don’t know yet.”
He acknowledges, in any case, that his discovery was pure luck. “Coming upon it really was a serendipitous matter,” he says.
Scientists aren’t certain what to make of the report, one of five cases of parthenogenesis in snakes that Dr. Chiszar and 13 other researchers detail in two papers in the current issue of the journal Herpetological Natural History.
Some herpetologists and evolutionary biologists view the reports with skepticism, noting that the females in some species of snakes have been known to store sperm for years and thus could be incorrectly described as having given birth parthenogenetically.
Other scientists, though, are persuaded by genetic tests on the offspring, which suggest that the females had no reproductive contact with males. Some experts say they wouldn’t be surprised by snake parthenogenesis, because some lizards are able to accomplish the feat.
Still, all agree that if snakes can, indeed, reproduce parthenogenetically, it would not only force herpetologists to change their views, but might also require biologists to rewrite the already complicated picture of snake evolution.
In the two articles, Dr. Chiszar, Gordon W. Schuett, an assistant professor of integrative biology at Arizona State University West, and 12 colleagues at other institutions document five purported cases of snake parthenogenesis. The cases involve five species, from varied environments.
Each of the snakes, held captive and isolated from males for many years -- in some cases since birth -- produced as many as several offspring, although most died as fetuses. But a few were born alive and have survived.
The researchers ran genetic tests that they say demonstrate that the DNA of the offspring came entirely from the mother. The DNA tests were done to address the most obvious suspicion: that the offspring resulted from contact with male snakes that the researchers did not know about.
Even so, David B. Wake, a professor of integrative biology at the University of California at Berkeley who is a herpetologist and evolutionary biologist, says he needs further evidence that these cases are not merely freak reproductive events.
“Whether it’s a common or a rare phenomenon is a really critical issue,” he says. After all, he points out, a mechanism that occurs so infrequently and produces so few viable offspring “is not a winning proposition in an evolutionary sense.”
If researchers show that snakes can use either parthenogenesis or sexual reproduction from generation to generation -- depending on the availability of mates -- that would be “a winning proposition” for the snakes evolutionarily, he points out. “It would explain why certain organisms can be very rare.”
For example, Dr. Wake says, some tropical snakes and salamanders are so rare that herpetologists cannot figure out how males and females find each other.
“That’s what’s intriguing about this observation. But it really needs to be nailed down.”
Parthenogenesis is known to occur in a variety of animals, including reptiles. Scientists speculate that its evolutionary role may be to allow animals that don’t often encounter mates to reproduce, but they acknowledge that they are not yet close to understanding the phenomenon fully.
Some lizards reproduce that way, as do many invertebrates. Domesticated turkeys occasionally do, too.
Fowl sometimes produce both parthenogenetic and non-parthenogenetic eggs within the same clutch. That suggests, scientists say, that parthenogenesis may sometimes simply be one possible, though unusual, result of the sequence of cellular processes that lead to the development of offspring.
With one exception, though, no parthenogenetic snakes were suspected to exist. The exception was the blind Asian Braminy snake, a species closely related to lizards. But its existence had not led herpetologists to look for other parthenogenetic snakes, because, like parthenogenetic lizards, the species is made up entirely of females. Biologists call it an “obligate parthenogen,” because it is has no choice but to reproduce that way. “Facultative parthenogenesis” -- the kind that can be used only when needed -- had not been seen before in scaled reptiles.
As it is, herpetologists have long known that some varieties of snakes can store sperm, apparently for years, before using it to reproduce. Dr. Schuett and his colleagues now wonder whether many such reported cases were not, in fact, instances of parthenogenesis. They believe they have examples in snakes of the same form of parthenogenesis that has been observed in domesticated turkeys for more than 40 years.
In turkeys, the few parthenogenetic offspring that live -- all are male -- do not reproduce. In the case of snakes, the researchers say in their paper, the reproductive ability of snakes born parthenogenetically hasn’t been determined, because the snakes are not yet sexually mature.
All of the snake offspring in the study are male. Dr. Chiszar says he will rear the offspring of his apparent parthenogenetic timber rattler, to gauge the offspring’s fertility. That will take time, as rattlers do not become sexually mature for four years.
Dr. Schuett and Dr. Chiszar are the first two researchers to report possible parthenogenesis in snakes that normally reproduce sexually. Like Dr. Chiszar, Dr. Schuett chanced upon his finding. He was on the verge of publishing a paper on cases of long-term sperm storage when it occurred to him that parthenogenesis might offer a more likely explanation for otherwise unaccountable births.
Working with Philip J. Fernandez, a professor of biology at Grand Canyon University, he conducted genetic tests on the offspring of a female garter snake that, Dr. Fernandez said, had not been in contact with a male snake for 10 years. The tests strongly suggested that no father had been involved.
A year later, when Dr. Chiszar made his own discovery, he recalled hearing about Dr. Schuett’s findings, and the two began to collaborate.
Dr. Chiszar contacted a geneticist on his campus, Jeffry B. Mitton, to ask him to compare the DNA of the mother rattler with that of its offspring. If the offspring had been produced sexually, it would have genes different from the mother’s.
When Dr. Mitton first heard his colleague’s speculation about parthenogenesis, “I thought it was probably a mistake,” he says. “But the DNA fingerprinting clearly was able to reject any hypothesis that a second individual was involved, any father.”
Soon the researchers began hearing of other cases -- an Aruba Island rattlesnake at the Toledo zoo, a checkered garter snake at the Phoenix zoo. Then a case involving an Australian file snake came to light at the Brookfield Zoo, in Chicago. It became the basis of the second paper in the herpetology journal, written by three zoologists at Brookfield.
“I went from thinking it was probably an inaccurate assessment to now thinking that parthenogenesis is probably rare, but at least moderately widespread among snakes,” says Dr. Mitton.
He speculates that the strategy may be an ancestral trait that is triggered when females have trouble finding mates. He and his colleagues agree that more studies will be needed to determine whether the phenomenon plays a role in evolution. For that to be true, parthenogenesis would have to occur in the wild.
Ironically, says Dr. Schuett, “none of us were actually looking for this phenomenon or even thinking we would be looking for parthenogenesis.”
David Green, an associate professor of biology at McGill University and a herpetologist, notes that in the researchers’ reports of parthenogenetic snakes, however, there was only an almost complete match of the DNA of offspring and mothers. To be unquestionably a case of parthenogenesis, the match would need to be complete, he says.
The researchers’ explanation of their data may be correct, he says, but the lack of an identical match of DNA may prove to be a crucial sticking point. “These snakes can retain sperm for years, and if the male came from some population that is highly inbred, where there is very little differentiation between individuals, then the amount of difference between the offspring and the mother may be very slight, but it would still be some,” he argues. “And they did find some. They chose not to interpret it as a contribution of a male, but you can’t rule that out.”
To address that objection, the Brookfield researchers compared suspected parthenogenetic Australian file snakes with wild snakes of the same species. They said their findings were inconclusive.
No one knows how all of this will turn out, but the reports of virgin snake births are intriguing to Dr. Green and many other herpetologists. “I’d like to stay tuned to this story,” he says.