Sand goby males guarding eggs from one female without access to food lost 13.9% of their body weight over a caring period of 10 days. This was more than twice the body weight lost by non-guarding males, who had nests but no eggs (weight loss 5.8%). Caring for the eggs therefore imposes an energetic cost on the male. Egg losses in the field were on average 3.3% of the egg mass. Males with egg losses were on average smaller than males without losses and they also had smaller egg masses. If a male was removed and his nest taken by a new male, 31.0 to 100.0% of the original eggs were eaten within 24 hr. In guarding males we never observed hetero-cannibalism. The proportion of time a male spent fanning was positively correlated to his standard length, larger males spending more time fanning. It is concluded that larger males are better fathers.