Puff adders lie in wait for their prey. Their patient approach to sustenance should make them vulnerable to becoming dinner themselves. Except they have appear to have evolved a suite of adaptations that renders them virtually undetectable.
Most obviously, the puff adder’s beautiful pattern make them incredibly hard to see. When confronted by danger, they tend to freeze too, making them still harder to detect.
Yet many would-be predators of puff adders use their noses rather than their eyes to locate their prey. Jackals, wild dogs, hyenas, mongooses, genets and so on should all be able to sniff out puff adders with ease. But they can’t.
Researchers at the University of Witwatersrand in Johannesburg, South Africa have been radiotracking puff adders for several years and the snakes would sometimes slither onto a property policed by dogs. “I’d find my puff adder with the transmitter in it in ambush position in one of the flowerbeds,” explains Graham Alexander in a video posted on YouTube. The dogs would bound over to greet him and run straight over the snake, he says, “oblivious to its presence.” The clip contains remarkable footage of a Cape porcupine trampling right over the top of a puff adder without noticing at all.
With these observations in mind Alexander and colleagues set out to test whether the puff adder Bitis arietans really is as inodourous as it seems. Their findings are published in the current issue of Proceedings of the Royal Society, Series B.
The study involved “asking” dogs and meerkats if they could recognise the scent of various species of snake. The researchers presented the putative predators with an odour lineup of six vials. One of these contained a serpent-scented cloth; the other five were controls. “We predicted that both dogs and meerkats would have difficulty in detecting puff adder scent, despite a demonstrable capacity to recognize the scent of snakes that forage actively,” write the researchers.
This is pretty much what they found. Both dogs and meerkats identified the target odour with impressive accuracy, except when it was a puff adder. Then they were hopeless. Sometimes they lucked out and indicated the puff adder tube, but at about the same low frequency that they nuzzled at controls. “The remarkable and stark difference in detectability between puff adders and other snake species provides strong evidence for our hypothesis of chemical crypsis in puff adders,” they conclude. Unless anyone knows better, “this is the first evidence of the employment of chemical crypsis by a vertebrate organism.”
So how do the puff adders do it? On this question, the authors have some ideas. If the puff adder has a lower metabolic rate and lower body temperature than other snakes, it will give off fewer volatile molecules less frequently. It is also possible that the products of metabolism themselves could be heavier than in other snakes, making them less likely to waft off into the surrounding air. This could be tested easily by studying the odour profile of each species, but the researchers are saving this for another paper.
More generally, Alexander and friends propose that this kind of selection for chemical crypsis could be pretty widespread in the animal kingdom, “especially for predator and prey species that spend extended periods immobile.”
Miller, A.K et al. (2015) An ambusher’s arsenal: chemical crypsis in the puff adder (Bitis arietans). Proceedings of the Royal Society, Series B 10.1098/rspb.2015.2182
