Predators have a remarkable evolutionary effect on the morphology and behaviour of their prey. In their attempt to reduce the likelihood of being killed, prey species have evolved a variety of counterstrategies, including the ability to smell like their predators to discourage pursuit or attack.
It seems like an uneven match. In one corner, the unassuming California ground squirrel (Spermophilus beechyi) or rock squirrel (Spermophilus variegatus), 30cm in length. In the other, the rattlesnake (Crotalus spp.), more than twice the length of the squirrel, and armed with hinged fangs that pack a lethal venom. But thanks to a cunning adaptation, the squirrel often gets an unexpected upper hand in this bout.
Chemical substances unique to one species are occasionally found on the body of a different species. Animals often ingest such foreign substances and sequester them into their integument, but here is a case of direct application of heterospecific substances to the body. Female California ground squirrels and rock squirrels chew on skins shed by rattlesnakes then lick themselves and their pups. This "snake scent application" (SSA) to their fur apparently anoints the squirrels with the odour of their enemy, most likely premeditated for antipredator shield.
To find out the function of this olfactory disguise, Barbara Clucas and company of the University of California in Davis captured 30 California ground squirrels (14 adult females, 9 adult males, and 7 juveniles) and 41 rock squirrels (16 adult females, 14 adult males, and 11 juveniles) and offered them frozen shed skins from different rattlesnakes. Their results ruled out ectoparasite defence and conspecific deterrence, and indicated that SSA most likely serves an antipredator function for ground squirrels.
This intriguing form of adaptive behaviour is a wonderful illustration of the many evolutionary arms races between predator and prey. Wild, snake-scented ground squirrels may evade attack because they no longer smell tasty. Fascinatingly, the weaker adult females and juveniles are known to spend more time applying scent than did adult males. Adult ground squirrels have essentially developed a certain degree of immunity to snake venom and their agility helps them avoid strikes. But their pups are still vulnerable and due to maternal instinct, adults disguise their scents by chewing on the discarded skins of rattlers and licking them.
The abovementioned study has established one of a remarkable package of defences that squirrels use against rattlesnakes and could very well be an onset to prove SSA as a novel form of defence behaviour in vertebrates!
Reference:
Clucas B., M. P. Rowe & D. H. Owings, 2008. Snake scent application in ground squirrels, Spermophilus spp.: a novel antipredator behaviour? Animal Behaviour, 75(1): 299-307.
Image of California ground squirrel taken from here;
Video clip of California ground squirrel chewing on shed rattlesnake skin taken from here;
Image of Southern Pacific rattlesnake (Crotalus oreganus helleri) taken from here. Photo credit: Russ Smith.
Related stories on ground squirrels and SSA:
Clucas, B., D. H. Owings & M. P. Rowe, 2008. Donning your enemy’s cloak: ground squirrels exploit rattlesnake scent to reduce predation risk. Proceedings of the Royal Society: Biological Sciences, 275(1636): 847-852.
“Smelly squirrels fool hungry snake.” 2006. The New Scientist, 191(2565): 17.
“Squirrels use snake scent.” UC Davis News Services, 19 Dec 2007.
More about ground squirrels and antipredator tactics:
Rundus, A. S., D. H. Owings, S. S. Joshi, E. Chinn & N. Giannini, 2007. Ground squirrels use an infrared signal to deter rattlesnake predation. Proceedings of the National Academy of Sciences of the United States of America, 104: 14372-14376.
Swaisgood R. R., D. H. Owings & M. P. Rowe, 1999. Conflict and assessment in a predator-prey system: ground squirrels versus rattlesnakes. Animal Behaviour, 57(5): 1033-1044.
Swaisgood, R. R., M. P. Rowe & D. H. Owings, 1999. Assessment of rattlesnake dangerousness by California ground squirrels: exploitation of cues from rattling sounds. Animal Behaviour, 57(6): 1301-1310.
More awesome animal defences:
Brodie E. D., 1977. Hedgehogs use toad venom in their own defence. Nature, 268: 627-628.
“The snake that eat toads to steal their poison,” by Ed Yong. 20 Feb 2007.
“Moths mimic each others’ sounds to fool hungry bats,” by Ed Yong. 20 Jun 2007.