AGGREGATION IN A DESERT TENEBRIONID BEETLE - A COST BENEFIT ANALYSIS/

Parastizopus armaticeps (Coleoptera: Tenebrionidae), a nocturnal fosso rial detritivore inhabiting southern Kalahari dunes, aggregates in bur rows during the day. Group size increases during drought but 25% of be etles are still found alone or in pairs. During drought, beetles from large groups leave burrows after sunset synchron ously and earlier tha n pairs and single animals and earlier than beetles of any group size after rain. Detritus from the beetles' major foodplant is scarce and f ood competition high. Beetles emerging early preferentially select and carry high-quality transportable items into burrows to eat (forage); late-emerging ones feed on the low-quality large twigs on the surface. Foraging is shown to be a strategy to secure food items against surfa ce competitors, nor one to reduce body water loss during surface expos ure. The costs and benefits of group vs. solitary lifestyles and alter nate hypotheses for early and synchronous emergence were tested experi mentally. Grouped beetles had lower body water loss rates but, due to competition with burrow mates, higher feeding costs than single ones. It is hunger that advances and thus synchronizes emergence time, not s ocial facilitation. Field data support a model intervals.