Evolutionary Ecology Research, 2001, 3: 331–344 © 2001 Adam R. Smith Correlated evolution of colony defence and social structure: A comparative analysis in eusocial wasps (Hymenoptera: Vespidae) Adam R. Smith, 1 * Sean O’Donnell 1 and Robert L. Jeanne 2 1 Animal Behavior Area, Department of Psychology, University of Washington, Box 351525, Seattle, WA 98195 and 2 Department of Entomology, University of Wisconsin, 1630 Linden Drive, Madison, WI 53706, USA ABSTRACT Animal societies depend on effective defence of group resources. Defensive mechanisms can be costly and may constrain the evolution of social structure. We analysed how exocrine mechanisms of colony defence were affected by the evolution of social complexity and of nest architecture in paper wasps (Vespidae). Eusocial paper wasp species exhibit two discrete grades of eusociality, with new colonies founded either by queens or by coordinated swarms of queens and workers. Swarm-founding shows multiple evolutionary origins from independent-founding ancestors within the Vespidae. Nest architecture also varies among paper wasps. Nests with covering envelopes evolved from naked combs several times. We hypothesized that: (1) evolutionary transitions from independent- to swarm-founding would obviate the need for chemical defence against ants and (2) transitions from naked combs to enveloped nests would have a similar effect on chemical defence. In support of the first hypothesis, we found that all independent-founding species possess ant-repellent glands (Van der Vecht’s gland), while many swarm-founders do not. Furthermore, phylogenetic analysis suggested that evolutionary loss of this gland was statistically more likely to follow transitions to swarm-founding. Evolution of nest envelopes was less strongly associated with losses of the ant repellent gland. These patterns suggest that maintenance of defensive exocrine glands is costly. The patterns also suggest that group behavioural defence against ants is a key adaptive feature associated with the evolution of swarm-founding. The hypothesis that the evolution of nest envelopes obviated chemical defence against ants was not as well supported. Keywords: ant predation, correlated evolution, independent-founding, social behaviour, swarm-founding, sternal glands. INTRODUCTION Living in social groups imposes a number of costs on members (Alexander, 1974). For example, many animal societies accumulate resources (brood and food stores) that attract *Author to whom all correspondence should be addressed. e-mail: arsmith@u.washington.edu Consult the copyright statement on the inside front cover for non-commercial copying policies.