On the Lateralised Motor Behaviour of Insects

Lateralization is the localisation or preference of function on one side of an organism, such asthe human tendency to have a dominant left or right hand. This is an example of individual-level lateralization, but populations can be lateralized as well. A population that deviates froma 1:1 ratio of left-lateralized to right-lateralized individuals is said to exhibit population-levellateralization, such as humans' ~9:1 right:left-handed ratio. Individualand population-level lateralization is very common in vertebrates, and some invertebrate species have recentlybeen identified to exhibit such biases. However, groups or subpopulations (such as colonies)of any species have not previously been shown to possess directional biases of their own,distinct from other conspecific groups. Remarkably, the red wood ant, Formica rufa, exhibitssuch `colony-level' lateralization in forelimb preference during a gap crossing task. Of the fourcolonies tested, one colony has more workers with a bias towards the left forelimb, the otherthree colonies having more workers with a right forelimb bias. Colony-level lateralization is notpredicted by established evolutionary theories of population-level lateralization. We offer anevolutionary account of colony-level lateralization in eusocial insects by consideringinteractions among workers within colonies, between colonies, and between workers and their