Natural History

Mary Copperhead with one her baby Jesuses.

Parthenogenesis is a form of asexual reproduction that is found in certain multicellular species, including vertebrates.

Certain species of whiptail lizard exist only as females that reproduce through “lesbian sex” (though it’s not technically “sex”). This mating behavior is thought to stimulate ovulation, and the female lizard lays eggs that consist only of other females. There is no male contribution to the offspring.

These lizards engage in obligate parthenogenesis. but another form of parthenogenesis also exists– facultative parthenogenesis.

In this form of parthenogenesis, the females switch between sexual and asexual forms of reproduction. This particular form has been observed in many species, including several species of shark and the Komodo dragon.

In squamates (snakes, lizards, and “worm lizards”), it was thought that facultative parthenogenesis happened only in captive females.

But a recent study of wild caught copperheads and water moccasins shows that even wild female squamates occasionally in parthenogenetic reproduction.  This was the first time facultative parthenogenesis had been observed in wild vertebrates.

Female snakes apparently produce only male offspring through parthenogenesis.

It was thought that isolation from males of their species would stimulate females to reproduce in this way, but this study shows that even females that have lived around males of their species will occasionally produce parthenogenetic offspring.

The researchers looked at microsatellites in the DNA of offspring born to 22 wild-caught female copperheads and 37 female water moccasins.

They found that one female in each species sample had given birth without having mated with a male, which means that as much as 5% of all litters produced by these snakes are produced through parthenogenesis.

Of course, another study with a much larger sample size will be needed to determine the full extent of this form of reproduction in these two snakes, but the fact that it was found it two distinct species suggests that it is relatively widespread in both of them. If it had been found only copperheads, we could say that the single parthenogenetic litter was just a fluke. It would have been the first time it had been confirmed in the wild, but it wouldn’t have been so interesting.

But finding it in both species raises important questions about exactly how widespread it is in squamates.

Copperheads and water moccasins (also called “cottonmouths”) are two close relatives.  The copperhead is found throughout the Eastern and Central US, while the water moccasin is found throughout the subtropical South and along the Mississippi drainage as far north as extreme southern Illinois and extreme southwestern Indiana.

They classified in the genus Agkistrodon.  The copperhead is A.  contortrix, and the water moccasin is A. piscivorus.  I don’t think the two species can interbreed, but I don’t know if any genetic studies have looked at when the two species last shared a common ancestor.

West Virginia has only copperheads, which are the less venomous of the two species. I have had a lot of experience with copperheads. When I was a kid, I was told never to go walking at night, because I’d likely be bitten by a snake. I did encounter several copperheads on my walks, but they always slithered away. They are very reluctant to bite.

However, we had more than a few dogs bitten by them. I remember one Jack Russell that my grandpa had custody over that never seemed to learn about copperheads. One time she bayed something under his building, and when she came out, she seemed fine. But about an hour later, the flesh on her lower jaw began to swell up, and it wasn’t long before she looked like a pelican. An emergency trip to the vet was in order, where she got an antivenin shot and antibiotics.

And that was only one encounter.

For whatever reason, golden retrievers tend to leave the snakes alone. Perhaps they have too much sense to waste time on snakes, but they just never seemed to cause problems with them. (I know that’s not a universal concept, but I’ve never seen a golden retriever go out looking for snakes in the way I’ve seen other dogs do it.)

The West Virginia tradition was always to kill every copperhead one encountered, but I’ve often wondered about the wisdom of such a custom. One can teach a dog to leave copperheads alone, and as venomous snakes go, copperheads have fairly mild venom. They are also very unlikely to bite.

When I was a very young child, my dad killed one that was “pregnant.”   These snakes are ovoviviparous, which means that the embryos form in eggs inside the mother’s body. When they hatch, they are born. Unlike mammals, they are not fed by a placenta. It is an actual egg inside the mother’s body. Dad hacked open the snake’s body to reveal the developing embryos, and I distinctly remember that being one of the coolest things ever.

Maybe that female copperhead had been mated by male.

Maybe not.

But these snakes certainly deserve a little more respect than they have received.

They are not cute and cuddly.

They are not social animals.

They are not wolves, which remind of dogs and “noble savages.”

They are not panda, which remind of teddy bears.

They are the evil snake of Christian legend. Satan incarnate in the Garden of Eden.

The great irony is that humans can never have a virgin birth.

But the serpents can.