Benefit asymmetry in aggregations of a desert beetle: the costs of being small

During summer drought, the desert tenebrionid beetle Parastizopus armaticep s forms dynamic aggregations in burrows which individuals leave at interval s to live solitarily. Aggregation reduces individual water loss rate but be etles compete there for the sparse detritus they or others collect and carr y in. Courtship groups form to breed after the first heavy rain following d rought but these contain fewer small females than expected. To explain this discrepancy we simulated conditions in a drought aggregation and compared the transpiratory water loss rates, feeding potentials and fat reserve depl etion rates of large and small individuals. Small beetles lost slightly mor e body water than large ones, had a lower food intake in competitive situat ions with them and less total body fat. They depleted their fat reserves af ter 7 days starvation while large ones did not reach depletion at 10 days. Aggregation is thus energetically more costly for small individuals, despit e its benefits in reducing water loss rate. They can only maintain their fa t reserves by leaving regularly to feed to satiation alone but then risk de siccation. Many small females may thus be physiologically incapable of egg production after prolonged drought. The relative costs and benefits of aggr egation in P. armaticeps therefore depend on body size and aggregation, alt hough enhancing survival, can reduce the lifetime reproductive success of s mall females.