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Odonata (dragonflies and damselflies) exhibit a range of sexual size dimorphism (SSD) that includes species with male‐biased (males > females) or female‐biased SSD (males < females) and species exhibiting nonterritorial or territorial mating strategies. Here, we use phylogenetic comparative analyses to investigate the influence of sexual selection on SSD in both suborders: dragonflies (Anisoptera) and damselflies (Zygoptera). First, we show that damselflies have male‐biased SSD, and exhibit an allometric relationship between body size and SSD, that is consistent with Rensch’s rule. Second, SSD of dragonflies is not different from unit, and this suborder does not exhibit Rensch’s rule. Third, we test the influence of sexual selection on SSD using proxy variables of territorial mating strategy and male agility. Using generalized least squares to account for phylogenetic relationships between species, we show that male‐biased SSD increases with territoriality in damselflies, but not in dragonflies. Finally, we show that nonagile territorial odonates exhibit male‐biased SSD, whereas male agility is not related to SSD in nonterritorial odonates. These results suggest that sexual selection acting on male sizes influences SSD in Odonata. Taken together, our results, along with avian studies (bustards and shorebirds), suggest that male agility influences SSD, although this influence is modulated by territorial mating strategy and thus the likely advantage of being large. Other evolutionary processes, such as fecundity selection and viability selection, however, need further investigation.

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Serrano-Meneses, M. A. and Székely, T. 2006. Sexual size dimorphism in seabirds: sexual selection, fecundity selection and differential niche-utilisation. Á Oikos 113: 385 Á394.Seabirds exhibit a range of sexual size dimorphism (SSD) that includes both malebiased (males/females) and female-biased SSD (malesB/females). Here we use phylogenetic comparative methods to test the selective processes that may influence their SSD. Using phylogenetically independent contrasts we show that the sizes of males and females are scaled isometrically in seabirds. We also test three functional hypotheses of SSD: sexual selection, fecundity selection and differential nicheutilisation. First, we found support for the sexual selection hypothesis, even though seabirds are socially monogamous and, as a consequence one might expect sexual selection to be weak. We show that SSD is correlated with an aspect of sexual selection, the agility of male displays, since in species that exhibit aerial displays the males are smaller (relative to the female) than in species in which the males display on the ground. Second, our results are not consistent with the fecundity selection hypothesis, since contrary to the predicted trend, female seabirds lay larger eggs in male-biased species than in female-biased ones. Finally, our results are not consistent with a previous study of the differential niche-utilisation hypothesis, since we found no relationship between SSD and ocean primary productivity in the breeding areas. Taken together, we suggest that seabird SSD is most consistent with the sexual selection hypothesis via the agility of male displays. Nevertheless, further data and tests are required to establish whether different resource utilisation by males and females may also select for SSD.

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