IS BIGGER BETTER - WATER-BALANCE IN THE POLYMORPHIC DESERT HARVESTER ANT MESSOR PERGANDEI

Foraging activity of ants in xeric areas may be limited by desiccation stresses. To assess the extent of such stresses on a polymorphic ant species [Messor pergandei (Mayr), body mass range 1-12mg], we measured body water reserves, absolute rates of water loss, and cuticular perm eability over the species' foraging temperature range (15-45 degrees C ). Cuticular permeability of M. pergandei was typical of xeric arthrop ods (17 mu g cm(-2)h(-1)mmHg(-1)). However, the effect of polymorphism on both absolute and relative rates of water loss was unexpected. Lar ge workers lost water more slowly than small workers, even after adjus ting for cuticular surface area. In addition, the body water reserves of large workers were larger, even after adjusting for body mass. Smal l workers of this ant species are therefore disproportionately prone t o dehydration, yet, in spite of this, constitute a large and important component of the foraging force. To assess the practical relevance of desiccation to polymorphic ants, we developed a 'worst-case' model of desiccation-limited foraging parameters (time to complete loss of loc omotor coordination in 5% of the foraging force). In this model, avera ge actual foraging duration was substantially less than would be requi red to incapacitate the ants by dehydration. We discuss direct and ind irect evidence suggesting that desiccation may, nevertheless, impose l imitations on the foraging activity of ants.