Body size variation in a tropical seabird along a latitude-productivity gradient

Body size can vary across geographical gradients, and these clines have been mainly attributed to temperature (i.e., the heat conservation hypothesis). However, in tropical areas, this pattern is not necessarily fulfilled. Furthermore, it is not known whether a body size cline is sex-biased in dimorphic species. Therefore, we aimed to evaluate the intra-specific variation in body size in a tropical seabird, the Red-billed Tropicbird (Phaethon aethereus), at six breeding colonies in the Pacific Ocean (17–31° N) and to relate body traits to environmental variables in each colony. Further, we examined sexual size dimorphism (SSD) in the species and its possible influence on the pattern of inter-colony variation in body size. We measured body traits in 187 adults. SSD was evaluated using culmen, ulna, and tarsus lengths and body mass. Chlorophyll-a, air temperature, and sea surface temperature (SST) values within an 80-km radius of each breeding site and the latitude of each island were used to create an environmental index. The relationships between body traits and the environmental index were assessed using regression models. Red-billed tropicbirds exhibited a positive, south-to-north body-size cline (between 1 and 9%) related to environmental conditions, and SSD was evident at only two northern colonies (males > females). The body size cline in the species could be influenced by a set of abiotic and biotic factors, which has likely led to phenotypic plasticity. The sexual dimorphism detected in colonies with larger body sizes along with high chlorophyll-a values and low SST values suggest that environmental-mediated variation in body size is a crucial mediator of SSD.