Adaptive Advantages of Reversed Sexual Size Dimorphism in European Owls

Abstract

Assumptions and predictions of previous hypotheses about reversed sexual size dimorphism, as found in owls and raptors, were tested by data pertaining to European owl species. Degree of dimorphism in wing length was found not to correlate with degree of dimorphism in weight. Degree of dimorphism in wing length correlates with percent birds in diet while dimorphism in mass correlates with breeding latitude. Given division of labour, I suggest selection acts on males to become efficient foragers (shorter wings) while on females for fasting endurance (larger mass). I advance a new hypothesis for sex dimorphism in terms of mass, namely that females are larger because larger bodies take longer to starve and can stand lower ambient temperatures without increasing heat production. This seems highly adaptive if breeding begins under harsh environmental conditions, if prey size is large and prey (delivered by males) is brought at long and unpredictable intervals. This hypothesis has the advantage over previous ones in that, given division of labour, it can explain why females are larger than males in owls and raptors rather than in other birds.