The mask of seniority? A neglected age indicator in house sparrows Passer domesticus

Abstract

S. Nakagawa, (correspondence) and T. Burke, Department of Animal and Plant Sciences, University of Sheffield, S10 2TN, UK. Email:itchyshin@yahoo.co.nzIn many species, females prefer older males as social and genetic mates, and male secondary sexual traits often act as ageindictor mechanisms. It has been reported that almost all the extra-pair paternity in one population and recently inanother population of house sparrows Passer domesticus, was achieved by males more than one year old. We aimed todetermine whether there is a morphological trait that distinguishes yearlings from older males in house sparrows. Here,we report that such a trait the extent of black around a male’s eye, which we term the ‘mask’ or ‘mask of seniority’, is areasonable indicator of male age, being adequate to assign yearlings and older males correctly 81% of the time. It is wellknown that a male’s black throat patch, often called the ‘badge’ or ‘badge of status’, signals fighting ability. We speculatethat badge size signals dominance, and is therefore often used in male male competition, whereas mask size may play akey role as an age indicator mechanism, potentially used in female choice. It is surprising that this important trait evadedresearchers’ attention during 20 years of house sparrows being a model organism for sexual selection.