On the evolution of claustral colony-founding in ants

Abstract

The dispersal and survival of offspring is a key step in an organism?s life history. In ants, the main strategy used to complete this step is known as independent colony-founding. Here, individual young queens leave their natal nest, find a suitable area to dig a new nest and found their own colony. There are two versions of this strategy. First, in the primitive version, queens forage for food on which to raise their brood; such queens are semiclaustral. Second, in the derived version, queens do not forage but seal their nest and raise their brood by metabolizing specially laid-down body reserves; this is the claustral (meaning `cloistered?) strategy. Claustral queens exhibit a suite of traits that have evolved to favour this derived colony founding strategy. Mortality during foraging has been suggested as the main selective pressure behind the evolution of claustral colony founding. This shift, from semi-claustral to claustral founding, is an essential step in the evolution of ants. However, the explicit conditions under which it might occur have not been examined. Furthermore, in some derived species the primitive semi-claustral founding strategy has re-appeared. We review the occurrence of semi-claustral founding and develop a simple model to determine when foraging mortality might favour claustral colony founding. Claustral founding is favoured when mortality is high or costs of provisioning claustral queens are low. Surprisingly, our model shows that semi-claustral colony founding may be favoured under biologically realistic parameter values. This may explain the increasing number of reports of semi-claustral founding in derived ant taxa. However, in the general absence of data on foraging and provisioning costs, it remains unclear how frequently semi-claustral founding will be the preferred strategy.