Size-assortative nest choice in the absence of competition in males of the sand goby,Pomatoschistus minutus

Synopsis Size-assortative choice of nest site is commonly found among fish with paternal care. This pattern can be explained either by the superior competitive ability of large males for preferred large nests, or by a choice of nest size that is related to the male's own body size. Choice of nest size was studied in males of a small marine fish, the sand goby Pomatoschistus minutus, by providing artificial nest sites in laboratory and field experiments. A positive correlation between nest size and male size, indicating a size-assortative nest choice process, was found in both aquarium and field experiments. This is argued to be the outcome of a preference for an optimal nest size and not to be due to competition, as (a) a surplus of nest sites was available to the individuals in nature; (b) in the aquarium experiments, single males of all sizes, when offered a choice between a smaller and a larger nest site, showed a preference for nests in proportion to their body size and not for the largest available nests; and (c) in the aquaria, when single males were offered only a small or only a large nest site, small males abstained from building nests in the large sites, whereas in the small nest sites males of all sizes built nests. My conclusion is that size assortative nest choice occurred in the absence of competition, which thus is not a necessary condition for size-assortative nesting, contrary to what is commonly assumed in many studies. This preference may represent a compromise (or trade-off) between the advantage of a nest large enough to accommodate many eggs and a smaller one with lower maintenance and defence costs.