Unlike many people who study reptiles, I didn’t grow up catching snakes for fun. I’m still a little nervous, therefore, about encountering venomous snakes. In the last few years, two snakes in particular have loomed over my time outdoors. In India, it’s the saw-scaled viper, a snake that’s widespread in dry, rocky habitats but active only at and after sunset. In the US, it’s also a viper, the cottonmouth, which is active during the day but usually remains in or near water.
Being bitten by either of these snakes would be far from pleasant, and as one of the four most widespread venomous snakes in India, the danger posed by the saw-scaled viper can’t be dismissed. But to prevent the thought of being bitten by the snakes from spiralling into incapacitating nervousness, I remind myself that these snakes can’t eat me and, therefore, have little interest in wasting their venom on me. In fact, each of these animals has, over evolutionary time, finely tuned aspects of its feeding biology exactly to the prey available to it.
Though most vipers, including several species of saw-scaled vipers, feed primarily on vertebrates, such as lizards, frogs, birds and mice, other species of saw-scaled vipers have shifted to a diet that contains lots of invertebrate arthropods — insects, spiders, scorpions and their ilk. Of course, mice and scorpions are very different beasts, but I would have imagined that viper venom would be a one-size-fits-all poison, potent enough to kill anything. But like many of our instincts and much of our common wisdom about venomous snakes, my imagination was off the mark. In fact, the venom of each species of saw-scaled viper is carefully calibrated to the composition of its prey — species that eat mostly arthropods have venom that is far deadlier to arthropods than the venom of species that eat mostly vertebrates.
This precise match between prey type and the venom’s deadliness has been maintained across at least three shifts in diet in the saw-scaled vipers, which makes perfect sense if we remind ourselves that making venom is energy-wise costly. Any snake would therefore benefit from needing to inject as little venom as possible every time it tries to kill something. A scorpion-eating saw-scaled viper could achieve its desired goal — a dead scorpion — with only a small quantity of venom that is maximally potent towards scorpions. I’ll try to remind myself of their venom’s precision if I am ever so careless or unlucky as to get bitten by and injected with the venom of a saw-scaled viper.
Cottonmouths are found across the south-eastern United States, and in most of their range, cottonmouths aren’t too picky about what they eat. I’ve even seen a cottonmouth scavenging on decomposed animal matter, which is unusual for a snake. But there are islands off the coast of Florida with thriving populations of cottonmouths that have a peculiar fondness for fish that fall from trees.
Given their unfussy appetite for almost any type of prey, I would have imagined that, finding themselves in the vicinity of bird nests, cottonmouths would show no hesitation in feeding upon birds’ eggs and small chicks. In most habitats, the cottonmouths would indeed gobble down this easy prey. But on the island of Seahorse Key, cottonmouths and nesting water-birds seem to have reached an accord — the cottonmouths ignore the birds’ eggs and chicks, and instead feed almost exclusively on dead or regurgitated fish that fall to the ground as the adult birds feed their young. To facilitate their search for this peculiar food source, these island-dwelling cottonmouths have developed a sense of smell that is particularly tuned to detect and track the sources of the distinct odours of dead fish. But this single-minded dedication to tracking down dead fish smells may have gotten a bit out of hand — cottonmouths on Seahorse Key have been observed ingesting large volumes of algae that smell a bit fishy, eating clumps of mud soaked in the vomit of a brown pelican, and attempting to swallow a large stick covered in water-bird excrement.
The cottonmouths in the patch of forest in Florida where I work are likely not eating any dead fish and, therefore, probably don’t find the smell of dead fish too compelling. This throws a wrench into what I initially thought would be a foolproof cottonmouth-avoidance plan of leaving decoy fish to attract the cottonmouths away from where I work. Nevertheless, since the cottonmouth’s expansive diet can’t include a human-sized organism, I have little to worry about. All I need to do is stay out of the way when the cottonmouths in our forest patch are making a beeline for whatever it is they have decided to eat, hot on the trail of whatever scent they have decided to follow.
(Ambika Kamath studies organismic and evolutionary biology at Harvard University)