A field demonstration of the costs and benefits of group living to edible and defended prey.

Both theoretical and laboratory research suggests that many prey animals should live in a solitary, dispersed distribution unless they lack repellent defences such as toxins, venoms and stings. Chemically defended prey may, by contrast, benefit substantially from aggregation because spatial localization may cause rapid predator satiation on prey toxins, protecting many individuals from attack. If repellent defences promote aggregation of prey, they also provide opportunities for new social interactions; hence the consequences of defence may be far reaching for the behavioural biology of the animal species. There is an absence of field data to support predictions about the relative costs and benefits of aggregation. We show here for the first time using wild predators that edible, undefended artificial prey do indeed suffer heightened death rates if they are aggregated; whereas chemically defended prey may benefit substantially by grouping. We argue that since many chemical defences are costly to prey, aggregation may be favoured because it makes expensive defences much more effective, and perhaps allows grouped individuals to invest less in chemical defences.