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High female mortality due to male aggression in Hawaiian monk seals led us to investi- gate the role of habitat use and social structure on sex ratios and aggression at Laysan Island, Hawaii. The sex ratio was strongly skewed towards males in the early 1980s and this, combined with the social structure, asynchronous reproduction, and terrestrial habitat use patterns, resulted in dramatic sex ratio imbalances in particular areas. Male:female ratios approached 1:1 in the northeast, whereas the overall ratio in the southwest was 5:1 ranging up to 25:1. Most of the aggressive incidents were observed in the southwest and females using this area were more likely to incur injuries. To reduce aggression, we selectively removed 37 males between 1984 and 1994, bringing the adult sex ratio to parity. Here, we evaluate the effect of this correction on aggression and female mortality. Before the removals, aggression accounted for an average annual mortality of adult females of 4.1% (range 0 to 12.9%), with up to 8 females being killed per year. The male removals, together with natural pro- cesses, decreased the absolute sex ratio from 2.1:1 in 1983 to 0.9:1 in 1994. Both the proportion and the absolute number of injuries and deaths declined after this date. Although some adult females still incurred severe mounting injuries, the proportion of females that were lost decreased to 0.3% yr -1 (range 0 to 2.6%), and only 3 females are believed to have been killed through 2005. Thus, sex ratio reduction through selective male removals proved to be a valuable tool in reducing mortality in this Critically Endangered species.

Impacts of sex ratio reduction on male aggression in the Critically Endangered Hawaiian monk seal Monachus schauinslandi