Today, we’re staying in snakes, but we’re going to look at the evolution of a really hot-sauce characteristic in the group: venom. Venom is probably the reason that people have such an irrational fear of snakes, and is also one of the most intriguing of the features found in snakes. Of course, there are other venomous reptiles — the beaded lizards and gila monsters are the best-known venomous lizards, and venom has been implicated as one of the reasons, in addition to massive bacterial load, that the bite of komodo dragons is so devastating (Incidentally, if anyone wants to buy me a pet gila monster, I wouldn’t say no.)
Snakes use their venom for both prey capture and defense — it’s utility in each is fairly obvious: as a defensive mechanism, it is enormously effective in deterring a would-be attacker by killing them or causing fairly excruciating pain, while in prey capture, venom subdues the prey and begins the digestion process in the snake. Both uses are very well-documented in venomous snakes, but the question remains: which came first, the chicken or the egg? Or, in this case, what is the primary purpose of venom? What did it evolve for? And if one of those functions disappears, will there still be selection pressure to produce venom?
To answer this question, we’re turning to a fascinating group of snakes: the sea snakes. Snakes make up about eighty percent of marine reptile life (the other twenty percent consists of sea turtles and the saltwater crocodile). The true sea snakes, or Hydrophines, make up the majority of that salt-water snake diversity. These animals are highly specialized, with salt-excreting glands, paddle tails, and, most importantly for our purposes, an incredibly strong neurotoxic venom. The vast majority of these snakes are fish hunters, which accounts for the potency of their venom — in order to subdue fish prey, and to keep it from being stolen or swimming away, their venom has to be very fast-acting indeed. So, while these snakes are, for the most part docile, they are far from harmless and can easily kill an adult human.
The olive sea snake, shown above, is a pretty typical Hydrophine snake. So, why do sea snakes make my remarkable reptiles list? And what do they have to do with venom evolution? To find out the answer to that question, read on.
The answer is that no all sea snakes are fish eaters. There are two species, in particular that have been well-documented as fish-egg eaters: Emydocephalus annulatus, the turtle-headed sea-snake and Aipysurus eydouxii, the spine-tailed sea snake. This is remarkable because it differs pretty enormously from the standard diet of snakes in which prey is relatively huge compared to the size of the snake (think of rock pythons swallowing gazelle whole), and there are an enormous number of specialized adaptations in the respiratory system, skull and physiology of snakes that reflect this specialized mode of life. By contrast, roe-eating snakes eat items which are a tiny fraction of their body weight, and are pretty easily consumed and digested. They don’t have to subdue the prey item, or expend a lot of effort chasing it down — what they have to do is eat a lot. And, so, what evolves is a snake with dietary behaviors that more closely resemble those of a browsing herbivore, moving between patches of vegetation than a typical snake, which chases down a large prey item and spends a good deal of time and energy to capture one animal.
And, as you can imagine, these snakes don’t need venom to subdue their prey. So, while Aipysurus and Emydocephalus are descended from and closely related to fish-eating snakes that used venom to hunt down prey, they don’t need to do so anymore. Thy are, therefore, perfectly poised to answer fundamental questions about the whys and wherefores of snake venom evolution.
And the results turn out to be surprising, to say the least. The more remarkable of the two snakes, in my opinion, is Aipysurus. This animal still possesses the venom-encoding genes typical of other sea snakes (and, indeed, of all other venomous reptiles: a three-finger toxin), but the genes are plagued with mutations that reduce the efficacy of the toxin. In essence, without prey-capture at stake, the venom encoding gene is degenerating into a much less functional toxin, and, with the selection pressure gone, evolving far more slowly than in its venomous relatives. Beyond this, the snake has had an interesting loss of skull characters: unlike its closest relatives, it lacks the fixed front fangs used to deliver venom. This snake and the change in the evolution of its venom since its shift in diet provides fairly good evidence that the primary purpose of venom is for prey capture, not self-defense, and that in the absence of this primary prey-capture utility, the venom loses much of its function.
And what of this lovely fellow, the turtle-headed sea snake? Well, he’s just as fascinating as his counterpart. These snakes still have a weak, if functioning venom gland, but have lost their fangs as well, and have even the teeth on their lower jaw. They are sexually dimorphic: only males have that sharp protuberance you see on the tip of the animal’s snout, and they may even exhibit social behavior.
So, there you have it. Venom evolved for prey capture, not self-defense, and in the absence of a prey-capture utility, it is lost. That’s a pretty cool story for such a silly-looking snake to tell, don’t you think?
But, if you still aren’t convinced that the diversity of reptiles is wild, wonderful and bizarre, just wait until tomorrow …
A novel foraging mode in snakes: browsing by the sea snake Emydocephalus annulatus (Serpentes, Hydrophiidae
R. Shine et al., Functional Ecology, February 2004
Eggs-Only Diet: Its Implications for the Toxin Profile Changes and Ecology of the Marbled Sea Snake ( Aipysurus eydouxii)
Min Li et al., Journal of Molecular Evolution, February 2005
Putting the brakes on snake venom evolution: the unique molecular evolutionary patterns of Aipysurus eydouxii (Marbled sea snake) phospholipase A2 toxins.
Min Li et al., Molecular Biology and Evolution, April 2005
Synchrony in capture dates suggests cryptic social organization in sea snakes (Emydocephalus annulatus, Hydrophiidae)
Richard Shine et al., Austral Ecology, 2005.
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