The importance of being gravid: egg load and contest outcome in a parasitoid wasp.

Owners have often been found to have an advantage in animal contests. One explanation of this is that the resource under dispute is of greater value to the owner than to the intruder (nonowner). Such 'resource value asymmetries' may be caused by intrinsic factors, such as the physiological state of the contestants. Females of the bethylid wasp Goniozus nephantidis fight for possession of paralysed hosts. A recent study found that larger females tend to win but owner status also confers an advantage. We tested the hypothesis that ownership advantage is linked to differences in egg load (the number of unlaid ripe eggs) between contestant females. Egg loads were highly variable and positively influenced by age, nutrition, body size and host ownership. Owners almost always won owner-intruder contests. The outcomes of owner-owner contests (in which size and status asymmetries were absent) were strongly influenced by differences in egg load. Copyright 2000 The Association for the Study of Animal Behaviour.