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http://phenomena.nationalgeographic.com/2013/04/05/pythons-are-still-a-little-venomous/
Pythons are still a little venomous
by Ed Yong
According to popular knowledge, venomous snakes are in the minority. Most kill their prey through other means. The pythons and boas, for example, squeeze their prey to death, constricting them in powerful coils until they can no longer breathe.
But that doesn’t mean they lack venom.
The ‘venom’ glands of these constrictors mostly produce lubricating mucus, which helps the snakes to swallow prey easily. But Bryan Fry from the University of Queensland has found that the glands still produce small amounts of venom proteins. So do the equivalent glands of iguanian lizards—the group that includes iguanas, anoles and chameleons.
These snakes and lizards are unlikely to be using their venom to subdue prey or to defend themselves, but they clearly still make the stuff. Their toxins are the equivalent of a kiwi’s wing or the sightless eyes of blind cavefish—defunct remnants of a functional past.
This is not the first time that Fry has shaken our understanding of animal toxins. In 2009, he showed that the Komodo dragon kills its prey with venom, rather than blood poisoning caused by a filthy bacteria-laden bite. And earlier, in 2006, he showed that venom is a far older and broader reptile invention than anyone had guessed.
Until then, everyone thought that there were only two venomous lizards—the Gila monster and the Mexican beaded lizard—which evolved their toxins independently from the hundreds of venomous snakes. Fry showed otherwise. While capturing monitor lizards in the field, he noticed that they had bulges in their heads at the same place as the Gila monster’s venom glands. “It was a Captain Obvious moment,” he says.
Fry eventually isolated venom proteins from many supposedly non-venomous species of lizard and snake, including all monitors and frequently kept pets like bearded dragons and ratsnakes. He argued that reptile venom evolved only once, in the common ancestor of this reptile group, which he called Toxicofera. It covers all snakes and a significant proportion of all lizards.
The Toxicoferan ancestor had two pairs of venom glands, one in the upper jaw and one in the lower, which secreted an already complicated set of venom proteins. Its descendents duplicated the genes that produced these proteins, and tweaked them to produce even more chemical weapons. They also streamlined their venom glands—some venomous lizards, like the monitors and Gila monster, lost the top pair, while the snakes downplayed the bottom set.
Fry’s new study is a sequel to this classic work. He took a much closer look at the venom glands of several constrictors like pythons and boas, and iguanians like the veiled chameleon and the common iguana. He dissected them, stuck them in medical scanners, catalogued their proteins, and more.
For a start, he doubled the number of known venom glands. He studied the red-tailed pipe snake—a member of one of the most ancient of snake lineages—and found that it secreted venom from four glands at the corners of its mouth called rictal glands. These structures had been completely ignored since the 1920s, but Fry showed that they produce venom.
His also found venom proteins in the constricting pythons and boas, and in iguanians. The levels are too low to be used as a defence or to kill prey (although the more predatory iguanians did have more protein-secreting cells in these glands—maybe a killing role isn’t out of the question). “Nothing in evolution is every really lost,” Fry says. Even if venom glands have been repurposed for making mucus, you’d expect them to still produce traces of venom.
Nicholas Casewell from Liverpool School of Tropical Medicine, who studies venom evolution, says that the study addresses unanswered questions from Fry’s earlier work, which “has been contentious”. For example, the fact that the boas and pythons have tiny amounts of venom fits with the idea that they evolved from venomous ancestors and have since downplayed their toxic heritage.
Casewell adds that the new study helps to answer another baffling question: “Why would a vegetarian iguanid require the secretion of venom toxins?” In the iguanians, the most common of the venom proteins—crotamine and crystatin—originally evolved as defences against microbes.
Fry thinks that reptile venom actually has its origins in killing microbes rather than prey. The common ancestor of the venomous snakes and lizards had glands that churned out proteins that kept bacteria at bay. By tweaking these proteins to kill other animals instead, and ramping up their manufacture, these early reptiles turned their chemical shields into swords. Indeed, some of the iguanian and constrictor venom proteins are still evolving, and rapidly so in some cases. Perhaps they are changing to regain their old protective roles?
This isn’t just for academic interest. Since 1979, Australians have relied on the Commonwealth Serum Laboratories Venom Detection Kit to identify the species responsible for venomous snakebites. Some people have tested positive using this kit despite being bitten by an apparently non-venomous python. Everyone just shrugged and regarded it as a mistake.
But Fry’s work shows that the test is picking up genuine venom proteins, which pythons share with other snakes. “It’s not enough to affect a human or a prey animal, but enough to set off the very sensitive test and give a false-positive,” says Fry. “In which case, the person bitten might be given very expensive anti-venom that they don’t need.”
Reference: Fry, Undheim, Ali, Debono, Scheib, Ruder, Jackson, Morgenstern, Cadwallader, Whitehead, Nabuur, van der Weer, Vidal, Roelants, Hendrikx, Gonzalez, Jones, King, Antunes & Sunagar. 2013. Squeezers and leaf-cutters: differential diversification and degeneration of the venom system in toxicoferan reptiles. Mol Cell Proteomics
