Birth temperature followed by a visual critical period determines cooperative group membership.

Cooperative behavior often arises when a common exploitable resource is generated. Cooperation can provide equitable distribution and protection from raiding of a common resource such as processed food. Under crowded conditions in liquid food, Drosophila larvae adopt synchronized feeding behavior which provides a fitness benefit. A key for this synchronized feeding behavior is the visually guided alignment of a 1-2 s locomotion stride between adjacent larvae in a feeding cluster. The locomotion stride is thought to be set by embryonic incubation temperature. This raises a question as to whether sib larvae will only cluster efficiently if they hatch at the same temperature. To test this, larvae were first collected and incubated in outdoor conditions. Morning hatched lower temperature larvae move slower than their afternoon higher temperature sibs. Both temperature types synchronize but tend to exclude the other type of larvae from their clusters. In addition, fitness, as measured by adult wing size, is highest when larvae cluster with their own temperature type. Thus, the temperature at which an egg is laid sets a type of behavioral stamp or password which locks in membership for later cooperative feeding.