Eau de danger

Natural History, April, 2003 by Stephan Reebs

Do animals smell fear? Mostly the answer is no. Sight, not smell, is what reveals a frightened creature's (or person's) emotional state. But for animals in a watery environment, the smell of fear does indeed act as a strong signal--albeit more as a warning to potential victims than as a giveaway to predators.

It is known that frightened crayfish, crabs, fish, and tadpoles spurt ammonia in their urine and through their gills. For neighboring animals--even unrelated species--the fluids serve as a kind of universal "disturbance cue," causing them to seek cover or become more circumspect, even though they may not be able to sense the predator directly.

Now two biologists at the University of Saskatchewan in Saskatoon have shown that disturbance cues can even lead some fish to identify previously unknown predators. Reehan S. Mirza and Douglas P. Chivers scared young brook charr (also known as brook trout) by striking the water surface of the charr's tank with a fake heron head. Then they collected a sample of the water. They also collected water from tanks containing predatory northern pike. Then they subjected two new groups of charr to a few ounces of water from the pike tanks. One of the new groups (the experimental group) also received a few ounces of the water from the tank that had been disturbed by the fake heron head. The second new group (the control group) received the same amount of water from a tank containing charr that hadn't been disturbed.

When the charr in the experimental group then encountered a pike in a test tank, they gave the unfamiliar predator a wider berth and avoided capture for a longer time than did the charr in the control group. Mirza and Chivers conclude that when charr detect disturbance cues, they pay attention to other odors in the vicinity and thereafter treat those odors as suspicious, and so they are more alert to potential predators and can survive longer. ("Behavioural responses to conspecific disturbance chemicals enhance survival of juvenile brook charr, Salvelinus fontinalis, during encounters with predators," Behaviour 139:1099-1109, 2002)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).