NATIONAL
GEOGRAPHIC (Washington , DC ) 5/4/13 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.”
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