Parentage outcomes in response to estrogen exposure are modified by social grouping in zebrafish. 
Evidence has recently emerged that endocrine-disrupting chemicals (EDCs) can affect various behaviors, including dominance and aggression in social groups, including fish.  This study investigated the effect of short-term exposure of male adult zebrafish to 17alpha-ethinylestradiol (EE(2)) on subsequent reproductive output and parentage in colonies with differing numbers of competing males.  It was predicted that impacts of EDCs might differ in social groups of fish of differing size because of the greater costs of maintaining dominance hierarchies in large groups.  Adult male zebrafish were exposed for 14 days to clean water, 2 ng/L EE(2) or 10 ng/L via the water, prior to placement into colonies in clean water with unexposed females.  Exposure to EE(2) at the concentrations adopted prior to the breeding trials did not significantly affect subsequent colony reproductive output.  The reproductive success of the most reproductively successful (MRS) male within colonies containing two males (relative to controls) was also unaffected.  There was, however, a significant impact of previous EE(2) exposure in tanks containing four males, resulting in a reduction in paternity for the most successful male.  Hence, nonlethal behavioral impacts of even short-term exposure to EDCs can have significant impacts on social dominance hierarchies and population genetic diversity.