|Source: badgerbadgerbadger.com, obviously|
Everyone thinks you're ugly, frightening or just plain mean. You have no legs. You have to keep yourself warm. Nobody will talk to you at parties.
But perhaps the most difficult thing about being a snake is finding food. Without limbs you can’t catch, trap or use tools. You’re a funny shape, limiting the places where you can hunt. You’re also surprisingly defenceless – no body armour, save scales; and no appendages to use to fight back against reluctant prey while you’ve got your teeth stuck in.
Given such disadvantages, snakes have had to acquire some of the most efficient hunting techniques in nature. In non-venomous snakes, mechanical constriction and jaw-holding are used to great effect, whereas some venomous snakes are eye-wateringly lethal beasties. Sea snakes are some of the worst. I once had a sea snake thrown at me, an experience I am not keen to repeat.
Two tactics are employed by venomous snakes to ensure that lunch is served. First, there are the strike-and-hold behaviours, like that employed by the cobras, clinging on to the prey while the venom takes effect, but this has inherent risks if the prey itself has sharp claws and a propensity to go down with a fight. Then there are the strike-and-release behaviours of the rattlesnakes and vipers, when prey are left to wander off and suffer their final moments alone. This is safer for the snake, but adds a complicating factor: once the venom has done its job, how does the snake find its prey once more, particularly when there may be trails of other suitable, but very much still alive and feisty, prey nearby?
A new paper by Stephen Mackessy and colleagues has just been published in BMC Biology outlining one potential mechanism: a GPS for snakes. A GPSsssssss, if you will.
The team used the western diamondback rattlesnake (Crotalus atrox), looking to see if the snake displayed a preference between different mouse carcasses, some injected with venom, some with saline, and some with specific components, or fractions, of the venom. Presented with the option of envenomed and normal, non-envenomed mice, the snakes flicked their tongues (from which they smell) dramatically more often upon finding prey filled with venom. Among the venom fractions tested, one in particular elicited a particularly excited response from the rattlesnakes, but what could it contain?
The authors predicted that this fraction might be a cocktail of common venom enzymes that are used to break down tissue, and that these might be the markers snakes search for when hunting out their prior victims, but this was not so. In fact, the fraction contained crotatroxin 1 and crotatroxin 2, two proteins belonging to the disintegrin family. These aren’t enzymes at all, but are the by-product of dismantled enzymes, and their roles include the inhibition of platelet aggregation, the phenomenon that usually occurs after injury to halt bleeding. It is also possible that the crotatroxins instigate a process in the prey that causes it to make further potent cues for the snake to follow, although this could not be determined from a mouse carcass. Disintegrins make up only about 2% of the total venom content in Crotalus atrox, but this low amount is more than compensated for by the excessive amount of venom injected in each attack.
So, much like a GPS tag applied to a suspect’s car in a spy film, these snakes strike their prey, leaving behind a protein tag that they can find later, once the venom has kicked in. But not all snake venoms contain disintegrins. So is this response specific to the western diamondback rattlesnake? And what is it about the disintegrins that the snakes find so enrapturing: their smell, or the smell of something else triggered after their administration? Such questions remain and are currently being investigated.
And now I must stop Googling pictures of venomous snakes, BECAUSE THEY TERRIFY ME AND I FEEL A BIT FAINT.
Saviola, J. et al. Molecular basis for prey relocation in viperid snakes. BMC Biology 11, 20 (2013) doi:10.1186/1741-7007-11-20