Is bigger better? Water balance in the polymorphic desert harvester ant Messor pergandei

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Abstract
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 permeability over the species' foraging temperature range (15–45d̀C). Cuticular permeability of M. pergandei was typical of xeric arthropods (17μgcm‐2h‐1mmHg‐1). However, the effect of polymorphism on both absolute and relative rates of water loss was unexpected. Large workers lost water more slowly than small workers, even after adjusting for cuticular surface area. In addition, the body water reserves of large workers were larger, even after adjusting for body mass. Small workers of this ant species are therefore disproportionately prone to 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 locomotor coordination in 5% of the foraging force). In this model, average actual foraging duration was substantially less than would be required to incapacitate the ants by dehydration. We discuss direct and indirect evidence suggesting that desiccation may, nevertheless, impose limitations on the foraging activity of ants.