Size-assortative mating in the absence of mate choice

Size-assortative mating is one of the most common mating patterns in nature. Nevertheless, the underlying behavioural mechanisms have received little attention. Assortment is typically assumed to result from mate choice, which can be coupled with differences in competitive potential. We investigated the behavioural mechanisms underlying size-assortative mating in a monogamous, biparental goby cichlid, where mutual mate choice should be expected. We performed three field experiments with females and males of Eretmodus cyanostictus to test for the existence of mate preferences in general and with regard to size: (1) a sequential presentation of differently sized potential partners; (2) a removal of partners combined with surveillance until re-pairing with a new partner; and (3) the simultaneous release of new and original partners on the experimental territories. In the removal experiment, we found evidence for weak preferences for large partners relative to own size and to the original partner's size, but pairs were formed irrespective of these preferences. The ecological importance of being paired appears to reduce choosiness and to override mate preferences. Territory ownership was quickly decided by aggressive interactions between original and new partners, and in both sexes the larger contestant won and was immediately accepted as partner by the resident. Our results suggest that strong intrasexual competition can be a powerful promoter of size-assortative mating even in the absence of active mate choice.