Everyone has felt pain, and many experience it daily. But for such a universal sensation, it is still a mysterious one. We are only starting to understand the molecules that produce a painful sensation. Nature, however, is well ahead of us. Many animals are armed with chemicals that hijack the nervous systems of their targets, producing feelings of intense pain. They are unknowing neuroscientists, and by studying their weapons, we can better understand how pain manifests in our bodies.
Take the Texas coral snake. This brightly coloured serpent, clad in warning hues of red, black and yellow, usually shies away from confrontation. When it’s threatened, it defends itself with venom that can cause excruciating and unremitting pain.
It’s a dinosaur tooth, and clearly one that belonged to a predator – sharp and backwards-pointing. But this particularly tooth, belonging to a small raptor called Sinornithosaurus, has a special feature that’s courting a lot controversy. It has a thin groove running down its length, from the root to the very tip. According to a new paper from Enpu Gong of the Chinese Academy of Sciences, it was a channel for venom.
Thanks to a certain film that shall remain nameless, a lot of people probably think that we already know that some dinosaurs are venomous. But the idea that Dilophosaurus was armed with poison, much less spat its toxins at its prey, is non-existent. Some scientists had speculated that they were venomous based on their bizarrely notched and allegedly weak jaws. But these notches have since been found in many other species and no one has ever actually measured the strength of Dilophosaurus‘s jaws.
The best sign that a dinosaur was venomous would be the presence of grooved or hollow teeth. With some notable exceptions, most animals with poison bites use grooves like these to channel their toxins from glands in their mouth to whatever they bite. And grooves are exactly what Gong and his colleagues found in Sinornithosaurus‘s well-preserved skull. Bryan Fry, who discovered venom glands in Komodo dragons earlier this year, says, “It is an absolutely fantastic piece of work. I actually got goose-bumps reading it! Other studies have suggested dinosaurs may be venomous but this is the most solid piece of evidence.”
Sinornithosaurus (meaning “Chinese bird-lizard”) is a small feathered dromaeosaurid (or, more commonly, ‘raptor’) and an early distant cousin of the birds. Its teeth are unusually large and Gong says that those in the upper jaw are “so long and fang-like that the animal appears to be saber-toothed”. They’re very similar to the fangs of back-fanged snakes like boomslangs and vine snakes.
Gong says that other aspects of the skull in support of his venom hypothesis. His team noticed that Sinornithosaurus has a small hollow on the side of its jawbone that could have housed a venom gland. They also found a thin groove running along the animal’s jaw, with small pits at the top of each tooth. They interpret this canal as a “collecting duct” that channelled venom from the gland to the teeth, and each pit could have acted as small, local venom reservoirs. David Burnham, a co-author on the paper, says, “Other fossil animals (dinosaurs, lizards, mammals) have been suggested to be venomous simply on the presence grooved teeth but out work found multiple lines of evidence.”
The Northern short-tailed shrew is a small, energetic mammal that lives in central and eastern North America. The Mexican beaded lizard is a much larger reptile found in Mexico and Guatemala. These species are separated by a lot of a land and several million years of evolution, yet they share astonishing similarities. Not only are they both venomous, but the toxic proteins in their saliva have evolved in very similar ways from a common ancestor, converging on parallel lethal structures independently of one other.
This discovery, from Yael Aminetzach at Harvard University, shows that adaptations are sometimes very predictable. Despite the many changes that could have shaped the course of venom proteins in lizards and shrews, they seem to have gone down a consistent and similar route.
The northern short-tailed shrew is one of the few venomous mammals, but its poisonous bite is painful to humans and can kill smaller animals. The key to its venom is a protein called BLTX, whose job is to cut another protein in two. This chemical reaction frees a molecule called bradykinin, which widens blood vessels and lowers blood pressure. It’s a necessary job, but BLTX is so active that if floods the body with bradykinin – an overdose that leads to paralysis and death.
BLTX is a dark, hyperactive descendant of an ancestral protein called kallikrein-1, which does the same thing but in a much more restrained way. Aminetzach found that BLTX is a longer version of kallikrein and the extra amino acids it has gained have changed the structure of the protein’s ‘active site’.
The active site is the protein’s business end – it allows BLTX to latch onto the right targets and catalyse the relevant chemical reactions. It’s also the part of the protein that has changed the most from the harmless kallikrein model; amino acids around BLTX’s active site have changed about twice as much as the rest of the protein. As a result, the site is larger, more flexible and better at drawing in its target, and the protein as a whole has become hyperactive.
And amazingly, the Mexican beaded lizard has gone through similar changes. Its venom relies on a protein called GTX, which is also descended from kallikrein. Like BLTX, it too is a longer version of its ancestor, and while its extra amino acids have been shoved into different places, the results are the same. The changes have altered the protein’s active site so that it’s larger, more flexible and better at drawing in its target.
These changes are very specific to these toxic proteins. By studying 24 relatives of kallikrein, Aminetzach found that none of the non-toxic members of the family have any of the changes that BLTX and GTX share.
This study demonstrates that evolution doesn’t work with infinite possibilities. Often, there are only a few roads leading to the same destination. Through different amino acid changes, both BLTX and GTX have evolved similar structures and have turned into weapons. This predictability of venom evolution may be useful to us – for example, Aminetzach suggests that it could allow scientists to more easily identify toxins from others species, even distantly related ones.
Reference: Current Biology 10.1016/j.cub.2009.09.022
More on venom:
For the longest time, people believed that the world’s largest lizard, the Komodo dragon, killed its prey with a dirty mouth. Strands of rotting flesh trapped in its teeth harbour thriving colonies of bacteria and when the dragon bites an animal, these microbes flood into the wound and eventually cause blood poisoning.
But that theory was contested in 2005 when Bryan Fry from the University of Melbourne discovered that a close relative, the lace monitor, has venom glands in its mouth. The discovery made Fry suspect that Komodo dragons also poison their prey and he has just confirmed that in a whirlwind of a paper, which details the dragon’s “sophisticated combined-arsenal killing apparatus”.
By putting a virtual dragon skull through a digital crash-test, Fry showed that its bite is relatively weak for a predator of its size – instead it’s adapted to resist strong pulling forces. This is a hunter built to inflict massive wounds through a “grip and rip” style that involves biting lightly but tearing ferociously.
The wounds provide a large open area for the dragon to inject its venom and Fry unquestionably showed that the dragons poison their prey. By placing the head of a terminally ill dragon in an MRI scanner, he managed to isolate the venom glands, which turn out to be more structurally complex than those of any other snake or lizard. He even managed to analyse a sample of venom, which is loaded with toxins that prevent blood from clotting and induce shock.
And as the icing on the cake, Fry concluded that Varanus prisca, a extinct close relative of the Komodo dragon probably also had venom glands. Also known as Megalania, V.prisca was three times the size of the Komodo dragon, making it (to our knowledge) the largest venomous animal to have ever lived.