Defensive tolerance to parasitism is correlated with sexual selection in swallows

Abstract

Parasite-mediated sexual selection has been the topic of extensive research and enthusiastic debate for more than three decades. Here, we suggest that secondary sexual characters may not only signal parasite resistance but also defensive tolerance. We exemplify this possibility by analysing information on two sexually selected traits, annual reproductive success, and ectoparasitism in a barn swallow Hirundo rustica population followed for more than 30 years. For each individual, we estimated the slope of the association between reproductive success and parasitism as an index of tolerance and subsequently explored the association with the expression of the sexually selected traits. In accordance with expectations of parasites playing a role in sexual selection, tail length was negatively related to load of chewing lice and nest size was positively related to tolerance to chewing lice. We discuss the importance of considering defensive tolerance for understanding the role of parasite-mediated sexual selection.

Keywords: Host-parasite interactions; Mate choice; Sexual selection; Tolerance to parasitism.

Fig. 1

Relationship between tolerance to chewing lice and tail length A and log₁₀ –transformed volume of nest material B. Lines are regression lines shown separately for males (solid) and females (dotted lines). Size of points is proportional to the number of years with available information used to estimate tolerance to chewing lice