Assortative pairing, and its relation to mate choice, has rarely been documented in mammals. Using data collected during 1998-2007, we investigated size-assortative pairing as it relates to discrimination amongst potential mates in humpback whale, Megaptera novaeangliae, dyads in the Hawaiian breeding grounds. Across 67 male-female dyads in which both individuals were measured using underwater videogrammetry, male length was positively correlated with female length. Detailed analyses on the assessment of maturity by comparisons with whaling data revealed that mature-sized females associated almost exclusively with mature-sized males and had a significant preference for large mature-sized males. In contrast, mature-sized males were less discriminating in their associations with females and showed no significant preference for mature-sized females. However, mature-sized males that associated with immature-sized females were significantly smaller than males that associated with mature-sized females. Finally, immature-sized males tended to associate with immature-sized females. The sex differences in size preference by mature whales probably reflect the relatively high costs of mature females mating with small or immature males compared to the lower costs of mature males mating with small or immature females. Body size appears to influence the adoption of alternative mating tactics by males such that smaller mature males avoid the costs of competing for the highest-quality females and instead focus their attentions on smaller females that may or may not be mature. Overall, our results provide the first quantitative evidence of size-assortative pairing and female discrimination amongst potential mates in humpback whales and indeed in any cetacean species. © 2012 The Association for the Study of Animal Behaviour.
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