Chill out: cooling promotes aggressive behavior in the ant Formica xerophila

Studies of insect behavior often involve cooling individuals prior to performing experiments or making observations. For such studies, it is important to know how cooling might affect subsequent behavior. Here I investigate how anesthetization by cooling (relative to anesthetization by carbon dioxide or control individuals) affects the aggressive behaviors of two closely related ant species, Formica xerophila and F. integroides. These species regularly compete for resources, and individuals of both species regularly experience nighttime temperatures similar to those of cold-induced anesthetization. I show that cooling increases aggression (toward both interspecific competitors as well as nestmates) for F. xerophila, but has no effect on F. integroides behavior. Although previously cooled F. xerophila are more aggressive during both intraspecific and interspecific interactions, intraspecific fights are much shorter in length than interspecific fights. Furthermore, aggression by F. xerophila decreases as time since being cooled increases. These data suggest that cooling produces a short-term reduction in the ants’ discriminatory abilities, which leads to mistaken identities during encounters. The difference in species’ behaviors after cooling can be attributed to their differences in strategies when encountering potential competitors. Results here confirm that cold anesthesia can affect subsequent behavior, and needs to be tested in behavioral studies with insects. Received 6 September 2008; revised 29 October 2008; accepted 4 November 2008.