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Scavenging by
 Pheidole pallidula
 a key for understanding decision-making systems in ants

The usual evolutionary and ecological approaches to foraging in social insects often lack an investigation at the level of both individual behavioural complexity and social mechanisms ruling the emergence of adaptive collective strategies. The prey scavenging behaviour of the dimorphic ant Pheidole pallidula was used in this study to investigate (1) how individuals estimate the size of prey, (2) how they modulate their behaviour and communication and (3) how these modulations generate the diversity of collective foraging patterns. For a pile of small prey (fruit flies), the recruitment of foragers was slow because of the weak intensity of individual trail-laying behaviour and the long time spent by ants wandering around the food. In contrast, for a large prey item (a cockroach), strong recruitment was induced by ants that dashed back to the nest laying a more continuous chemical trail. Experiments with small immovable prey showed that the tractive resistance of prey was the key parameter the foragers used to estimate prey size and that it ruled their trail-laying intensity. These data allow us to generate a model about decision making in scavenging. The rules leading to collective choice in a foraging or an agonistic context are discussed. On the basis of these findings, some theoretical stances in sociobiology and some shortcomings in current approaches to cooperation in social insects are considered. ? 1997 The Association for the Study of Animal Behaviour