Female reed warblers in social pairs with low MHC dissimilarity achieve higher MHC compatibility through random extra-pair matings

Major Histocompatibility Complex (MHC) polymorphism is maintained by balancing selection through host-pathogen interactions and mate choice. The likelihood of mate choice based on MHC-dissimilarity in songbirds has been questioned because of their rather poorly developed olfactory sense, a trait considered crucial in pre-copulatory mate choice to determine both own MHC and MHC of putative partners. However, our results suggest a mechanism where extra-pair matings can maintain high MHC polymorphism also without the need for olfactory cues. In our study population of reed warblers, Acrocephalus scirpaceus, females with extra-pair young in their broods have a higher MHC similarity with their social mate than females without extra-pair young in their broods. We suggest that when MHC similarity is high between social mates, females can improve MHC diversity in their offspring through extra-pair mating. A post-copulatory mate choice mechanism that relies on multiple copulations does not demand that females identify and actively choose MHC-dissimilar mating partners. In line with this reasoning the MHC-dissimilarity of successfully siring extra-pair males were neither different from that of social pairs without extra-pair young nor from putative extra-pair males that were breeding close to the female's social nest. ### Competing Interest Statement The authors have declared no competing interest.