The nose knows: minnows determine predator proximity and density through detection of predator odours

Abstract

Prey often face a behavioural trade-off between fitness-related activities and costly predator avoidance. Individuals that are able to assess their level of risk should maximize their fitness by allocating appropriate amounts of time and energy to avoid predators. The threat-sensitive predator avoidance hypothesis states that prey should respond to a given threat with an intensity that matches their level of risk. Prey fish are known to assess predation risk using predator odours. However, the level of sophistication of risk assessment based on predator odours is not well understood. We conducted two experiments to investigate whether fathead minnows, Pimephales promelas, assess proximity and density of pike, Esox lucius, predators based on pike odours. In experiment 1, we exposed pike-experienced minnows to either 60 ml of pike odour from 12 pike (i.e. 5 ml/pike), 60 ml of pike odour from two pike (i.e. 30 ml/pike) or a control of 60 ml of water. Minnows exposed to odours from two pike showed a more intense antipredator response than did minnows exposed to odours from 12 pike, demonstrating that minnows can detect individual pike in a mixture of odours from several pike. The fact that minnows responded with a greater intensity as the per-pike concentration increased while the overall concentration of pike odour remained constant, indicates that minnows can use odours to determine their relative proximity to predators. In experiment 2, we exposed pike-experienced minnows to either 5 ml/pike from each of 12 pike, 5 ml/pike from each of two pike or a water control. Minnows in the 12-pike treatment showed a stronger antipredator response than did minnows in the two-pike treatment, demonstrating that minnows can assess the relative density of predators using predator odours. These results demonstrate an amazing level of sophistication of predator odour assessment; they are the first to show that prey fish can use odours to determine predator proximity and relative density to respond to predators in a threat-sensitive manner.