OSR, density and mating competition: insights from the two-spotted goby

In a small marine fish, the two-spotted goby, males compete for females early in the mating season, but later on females compete for males. This provides an ideal opportunity to study the effect of sex ratio on competitive behavior. It is generally agreed that the more abundant sex is expected to compete for the limiting sex (only counting ready-to-mate individuals). Field studies generally find this relation, but laboratory studies often find either opposite results or no effects. We studied the same species both in the laboratory and in the field, and found completely different results in the different studies. We suggest that this inconclusiveness may arise from differences in how behavior is quantified in field versus laboratory studies. The propensity to behave competitively in a given situation (most easily recorded in field studies) could be more indicative of the level of competition than frequencies of behavior (more easily recorded in laboratory studies).