Songbirds show odour-based discrimination of similarity and diversity at the major histocompatibility complex

Abstract

The major histocompatibility complex (MHC) is the most polymorphic region of the vertebrate genome. Individuals with more MHC alleles can respond to a broader suite of pathogens, suggesting that selection should favour the ability to assess the MHC genotype of potential mates. Indeed, MHC-based mate choice, particularly preferences for MHC-dissimilar or MHC-diverse partners, appears widespread among vertebrates. Mammals, fish and seabirds assess MHC through odour cues. However, despite the prominence of songbirds in mate choice studies, the mechanisms by which this group might assess MHC remain speculative. Motivated by the discovery that chemical similarity in the preen oil of song sparrows, Melospiza melodia, predicts similarity at MHC class II, we examined preen oil odour preferences for MHC dissimilarity and diversity. We presented breeding-condition song sparrows with preen oil from opposite-sex conspecifics using a two-choice design. We compared time spent with odour from MHC-dissimilar versus MHC-similar birds, and MHC-diverse versus less MHC-diverse birds. Both sexes spent more time with odour from MHC-dissimilar compared to MHC-similar birds and with odour from more MHC-diverse than less MHC-diverse birds. We conclude that song sparrows, and presumably other passerines, can use preen oil odour to discriminate MHC similarity and diversity of potential mates.