Is the Bill of the Male Oystercatcher a Better Tool for Attacking Mussels Than the Bill of the Female?

Abstract

Male Oystercatchers, Haematopus ostralegus, take relatively more large and thick-shelled molluscs, like cockles and mussels, while female Oystercatchers take relatively more deeply buried clams and polychaetes. Opening sturdy prey requires considerable muscular effort and the bill must be strong to resist the stresses. Sexual dimorphism in bill morphology is subtle and consists of the following: the bill of the male is shorter in total length and the distal half (gonys-region) is deeper and wider, suggesting it is stronger in resisting forces operating on the bill when opening mussels. Skull dimensions and the weight of the depressor muscle of the beak Musculus depressor mandibulae do not differ between the sexes. It is argued that with equal effort of the head, neck and jaw muscles, greater forces are exerted at the contact point between the bill and the prey in the male bill than in the female to overcome the resistance required to open mussels.