Reversed sexual size dimorphism in the Ryukyu Scops Owl Otus elegans on Minami-daito Island

Abstract

One of the consequences of selections acting on body size is the difference in body size between the sexes or sexual size dimorphism. Although many hypotheses have been proposed for reversed sexual size dimorphism (RSD) in raptors, ornithologists have rarely paid attention to temporal aspects of RSD when testing their hypotheses. Because selection pressures may vary temporally, describing temporal variation in RSD is a first step towards understanding evolutionary mechanism which shape and maintain the dimorphism. Here, we describe RSD in the population of Ryukyu Scops Owls Otus elegans on Minami-daito Island using a dataset of the external measurements of 770 individuals obtained during a 17-year long-term population monitoring project. Females were larger than males in body mass, culmen length, bill depth, bill width, tail length and flattened wing length, whereas males were larger than females in tarsus and head lengths. Among these traits, the degree and direction of RSD of body mass, tarsus length, bill depth and flattened wing length varied across years. There were neither increasing nor decreasing trends in RSD. This is a rare study which addressed temporal variation in RSD in a raptor species.