Asymmetries in physiological state as a possible cause of resident advantage in contests

As is common in defense of resources in many animals, contests on host fruit between female Mediterranean fruit flies (Ceratitis capitata) are generally resolved in favour of the resident individual. Here we offer an interpretation of resident advantage in this species which is derived from a dynamical state-variable perspective on behaviour. We first demonstrated the occurrence of residence advantage. In field-cage assays of freely-foraging and freely-interacting females within a tree bearing host coffee berries, the occurrence of two females on a berry almost always resulted in contests. Approximately half of the contests among females on berries resulted in clear winners; resident females won an overwhelming majority of such contests. Contests tended to occur while the resident fly was currently engaged in egg-laying, either boring into the berry with their ovipositor or marking the berry after laying eggs. Non-residents, by contrast, were uniformly engaged in searching behaviour. We next tested the hypothesis that degree of resident advantage is a function of the degree to which a resident is engaged in egg-laying behaviour just prior to the contest. In experimental manipulations of resident status, which female won depended strongly on what residents were doing at the time contests were initiated. If residents were resting and grooming, non-residents (who were uniformly searching) usually won. If residents were laying eggs or marking the fruit after laying eggs, residents usually won. Only when both residents and non-residents were engaged in searching behaviour did the outcome of a contest not depend on resident status. Finally, we tested the hypothesis that contest outcome is related to the dynamical state of the contestants in terms of their relative age. We staged contests between host-deprived females that differed in age by two weeks, manipulating age independently of resident status. Older females won virtually all contests with younger ones regardless of resident status. In a field-cage assay of freely-foraging flies, older females also engaged in significantly more egg-laying activity. Taken together, these results indicate that older females value a fruit more highly than do younger ones, invest more in defense of that resource and therefore win more contests.