NEST CO-OCCUPATION IN THE DIGGER WASP CERCERIS-ARENARIA - COOPERATIONOR USURPATION

The functional significance of nest co-occupation in Cerceris arenaria (Hymenoptera: Sphecidae) was investigated. Willmer (1985a, Behav. Eco l. Sociobiol, 17, 151-160, 1987, Proc. 3rd fur. Congr. Entomol, 2, 229 -240) suggested that in this species co-occupation is a mutualistic re sponse to ecophysiological pressures, and that these pressures might h ave played a general role in the evolution of eusociality. In this stu dy nest-less 'Searcher' females almost always (99%, N=80) accepted (st arted using) pre-existing nests rather than digging new burrows. Burro ws were occupied by sequences of females whose periods of occupation o verlapped briefly. Go-occupations began when Searchers accepted nests that already had owners. The behaviour of Searchers after nest accepta nce depended on time of acceptance. Late in the day, Searchers remaine d inside nests and usually closed them. Early in the day, Searchers st arted provisioning accepted nests. If these nests were already occupie d, the result was joint-provisioning. When a nest was joint-provisione d, it received significantly more prey than the owner could provision alone, but less than twice what she could provision alone. Go-occupati ons rarely (4%, N=47) lasted more than 1 day, and ended when one femal e left the nest for good. Searchers remained in possession of nests af ter 49% (N=47) of co-occupations. Searchers appear functionally to be usurpers: they are known to prefer nests with owners and on average, o wners lose out if Searchers accept their nests. Willmer's hypothesis t hat co-occupations are mutualistic is rejected. An analysis of time co sts suggests that accepting a pre-existing nest has larger payoffs tha n digging a new burrow. The high rate of replacement of nest owners (1 1-16% per day) will reduce the relatedness of offspring emerging from the same nest, presenting a barrier against the kin-selected evolution of nest-sharing. Variation in social interactions in the genus Cercer is may be consistent with the predictions of a recent model of coopera tion based on reciprocity. (C) 1995 The Association for the Study of A nimal Behaviour