Territorial male gobies respond aggressively to sneakers but do not adjust their sperm expenditure

We investigated whether mating behavior (sperm expenditure, courtship rate, and nest guarding) varied according to different levels of sperm competition in territorial males of two goby species, the grass and the black gobies. We measured sperm expenditure (sperm released after 30 min from the beginning of the spawning), male courtship rate, and nest-guarding behavior in territorial males of both species during simulated spawnings, in which we varied the number of attending sneakers. Our results showed that, in both species, territorial males adjusted their effort in nest guarding to the presence of rival sneakers by increasing the time spent patrolling the territory and attacking the sneakers. In contrast, sperm expenditure and male courtship rate were not influenced by the number of attending sneakers. These results are in agreement with those reported for other fish with alternative mating tactics and help to interpret previous inconsistencies between theoretical predictions and measured levels of sperm released at different levels of sperm competition by sneakers of the two gobiids studied here.