Effects of resource distribution on sexual selection and the cost of reproduction in sandgobies

I describe the results of an experimental manipulation of resource availabi lity (nest substrate) and distribution (nest size), leading to effects on t he opportunity for sexual selection and the survival of male sandgobies Pom atoschistus minutus competing for these resources. This study represents on e of few such experimental manipulations. It shows a clear-cut effect of ma le-male competition on the survival of males, and it shows temporal variati on in the opportunity for sexual selection, suggesting that short-term asse ssment of sexual selection could be misleading. Males breeding under male-m ale competition for nest sites lived on average 18 d less than males in the high-nest-availability treatment. This considerable cost of reproduction p robably stems from increased stress levels as a result of higher levels of aggression. The opportunity for sexual selection was high throughout the 2- mo experiment in the low-nest-availability treatment, while in the high nes t availability, it changed in a complex manner over the season. In the latt er case, sexual selection was initially low but increased during midseason to values nearly as high as in the nest-site-limited environment and then d ecreased again toward the end of the season. Previous studies have shown th at temporal variation in sexual selection follows variation in population d ensity and sex ratio. This study demonstrates that there can be considerabl e temporal variation in the opportunity for sexual selection without change s in population parameters.