REDUCED DISEASE IN OFFSPRING - A BENEFIT OF COLONIALITY IN SUNFISH

Increased disease and parasitism are a well-documented cost of group l iving for colonial birds and mammals, but we now show that disease in offspring of fish may be reduced by nesting in colonies. The aquatic f ungus Saprolegnia sp., which is a common cause of egg mortality among freshwater fishes, is more prevalent in the nests of solitary than col onial male bluegill sunfish (Lepomis macrochirus). Moreover, fungal in fection decreases with nest density in colonies. This may be due in pa rt to a behavioural advantage since colonial males can devote less tim e to defending eggs and more time to fanning them, which reduces funga l infection. In addition, we demonstrate experimentally that solitary nests become infected at higher rates than colonial nests, even in the absence of parental males. This suggests that colonies are encountere d by spores at a lower rate and/or that the large number of nests in c olonies dilutes the number of fungal spores per nest. Through one or a ll of these mechanisms, egg mortality in colonial nests is lowered sig nificantly. Therefore, in some cases, disease may select for group liv ing.