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

I describe the results of an experimental manipulation of resource availability (nest substrate) and distribution (nest size), leading to effects on the opportunity for sexual selection and the survival of male sandgobies Pomatoschistus minutus competing for these resources. This study represents one of few such experimental manipulations. It shows a clear-cut effect of male-male competition on the survival of males, and it shows temporal variation in the opportunity for sexual selection, suggesting that short-term assessment of sexual selection could be misleading. Males breeding under male-male competition for nest sites lived on average 18 d less than males in the high-nest-availability treatment. This considerable cost of reproduction probably 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 nest availability, it changed in a complex manner over the season. In the latter case, sexual selection was initially low but increased during midseason to values nearly as high as in the nest-site-limited environment and then decreased again toward the end of the season. Previous studies have shown that temporal variation in sexual selection follows variation in population density and sex ratio. This study demonstrates that there can be considerable temporal variation in the opportunity for sexual selection without changes in population parameters.