Many would shriek in horror if a venomous viper slithered across our path. We would surely be even more alarmed to come across a pair of snakes having a fight, their undulating bodies twisting and turning around each other as they fought for supremacy. Even the bravest might think twice before getting too close.

But not Dawn Kelly. When the Arkansas confectioner noticed two metre-long snakes wrestling in the grass, she pulled out her smartphone and recorded a video.

It is a lucky thing she did. What she filmed stunned herpetologist David Steen of Auburn University in Alabama when he saw it: a copperhead grappling with a cottonmouth.

A pair of snakes wrestling is not surprising in itself. Males engage in their ritualistic "combat dances" all the time.

What is surprising is that the two are members of different species. That has never been seen before, and it should not have happened.

When two snakes fight like this, their primary goal is to knock the other to the ground. The main exceptions are rat snakes and king snakes, which tend to stay on the ground while in combat.

The purpose of combat is not to fight to the death

Depending on the species, the highly-choreographed routine can involve anything from tail whipping and head bobbing to a "lateral punch," in which one snake slams a part of its body against a second snake in an effort to loosen the second snake's coils.

The snakes wiggle and writhe, slamming their bodies into each other and the ground, all in an effort to prove their mettle. The winner gains access to a territory and to whatever females might be within it, while the loser slithers off to regroup. Eventually, he might challenge the dominant individual again.

It may seem odd that two such venomous creatures spar without employing the piercing venom-delivery devices attached to their faces, but like similar tussles in mammals, the purpose of combat is not to fight to the death.

It is possible that the cottonmouth and copperhead, similarly, were fighting over a nearby female. This is pointless, of course, since only one of them could mate with her anyway, depending which species she was.

Alternatively, Steen says that there could even have been two females nearby, one cottonmouth and one copperhead. This would make the fight even more of a waste, since each snake could ostensibly pair off with its respective partner.

It's never been observed before, to my knowledge, between two different species

We cannot tell, because the video does not show a female.

There might not have been a female present at all, says Phil Senter of Fayetteville State University in North Carolina, who has studied the evolution of snake combat dances.

"It's a territorial thing, which means potential females, not necessarily an actual one," says Senter. "It could have just been a border dispute."

Still, that does not explain why males of two different species, which should not be fighting over the same pool of potential mates, would be locked in a contest for dominance.

"It's never been observed before, to my knowledge, between two different species," says Steen.

"I've never seen that before, two different species fighting each other as if they're the same species," agrees Senter.

The simplest possibility is that the fight was a mistake.

Breeders have been known to artificially hybridise the two species, resulting in "cottonheads" or "coppermouths"

Copperheads and cottonmouths are members of the same genus, Agkistrodon. What's more, according to an analysis of combat dance manoeuvres that Senter published in 2014, both species rely exclusively on the same three ritualised actions: head-raising, downward-pushing, and swaying. This means mistaken identity is at least a possibility.

"Snakes are dumb," says Senter. "Their brains are very, very small. At least, the thinking part of it is."

During these kinds of social interactions, snakes generally respond to each other in a predictable, scripted way. One snake provides a stimulus, and the second snake responds, then the first snake responds to the second snake.

"They get the back-and-forth going, and then it escalates," says Senter. "There's no stopping a combat coming next."

However, there is another, more intriguing possibility. It is conceivable that the two snakes really were fighting over a female – and that it was in their interests to do so.

The reptile trade is a lucrative one, and breeders have been known to artificially hybridise the two species, resulting in "cottonheads" or "coppermouths". This tells us that it is at least biologically possible for the two species to mate and produce offspring.

Steen says he can only recall hearing of a wild cottonmouth-copperhead hybrid on, perhaps, one occasion. This would imply that, even if the two species can interbreed and produce offspring without human help, such hybrids are probably exceedingly rare.

Snakes are dumb. Their brains are very, very small. At least, the thinking part of it is

Still, it is conceivable that both snakes had a good reason to fight over a female.

Regardless, the incident is a reminder that biologists alone cannot see everything worth seeing.

"There are only so many formally-trained scientists, and many of them have to spend more time in front of the computer than you may expect," says Steen. That is why citizen scientists like Dawn Kelly are so important.

Steen is planning on working with Kelly to publish her observation in a scientific journal. "Citizen scientists really can perform an important service by reporting observations," he adds. "Doing so can contribute to a greater understanding of wildlife."

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