Males use time whereas females prefer harmony: individual call recognition in the dimorphic blue-footed booby

Abstract

In breeding birds, acoustic signalling is often an important cue for reunion between sexual partners. However, in spite of its potential interest, mate recognition has rarely been examined by comparing the two sexes. We studied the blue-footed booby, Sula nebouxii, a socially monogamous seabird, with a dramatic call sexual dimorphism suggesting two different strategies for identity coding: the female call is an amplitude-modulated sound with a harmonic series slowly modulated in frequency, while the male call is a noisy whistle strongly modulated in frequency. To compare acoustic strategies between the sexes, we (1) recorded calls of both males and females, (2) searched for an individual signature and characterized it, and (3) tested, using a playback experiment, whether the calls of males and females were equally efficient for mate recognition. Results showed that an individual signature was present in the calls of both sexes. However, the acoustic parameters involved differed: female individual identification was principally achieved by a spectral analysis of the call whereas males' identity relied mostly on temporal cues. More than 70% of both females and males tested in playback experiments successfully recognized their mate. This suggests that the coding strategies are equally efficient in terms of individual recognition between mates. From a broader point of view, our results underline the importance of assessing both males and females within the same investigative framework.