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Reed Warbler Pairs With Similar MHC Immune Genes Were More Likely to Have Extra-Pair Young

Wild Common Reed Warbler perched on a reed in Wrocław, Poland
Research: Behaviour, Genetics & genomics

Female common reed warblers (Acrocephalus scirpaceus) paired with males that were unusually similar to them at MHC class I immune genes were more likely to have extra-pair young — offspring sired by a male other than the female’s social partner. The extra-pair fathers were often more MHC-dissimilar from the female than her social partner, but there was little evidence that females actively chose the most dissimilar available male.

The six-year study in southwestern Poland combined MHC-I genotypes from 128 females and 127 males with parentage data from 501 nestlings. Thirty-five social pairs produced at least one extra-pair young.

MHC dissimilarity describes functional differences in immune genes

MHC-I genes help the adaptive immune system recognise infected cells by presenting peptide fragments from inside cells to T cells. Different MHC variants can recognise different sets of pathogen-derived peptides.

Here, MHC-I dissimilarity measured the functional distance between the peptide-binding regions carried by the female and male. It was not a measure of their overall genetic relatedness. A more MHC-dissimilar partner can potentially give offspring a broader combination of immune-gene variants.

Figure comparing MHC-I dissimilarity in three types of reed warbler pairs
Social pairs with extra-pair young were the only group with lower MHC-I dissimilarity than expected from random pairing. Figure 1 from Halupka et al. (2026), Evolution, CC BY 4.0. Source.

Pairs with extra-pair young were unusually MHC-similar

Social pairs that produced extra-pair young had lower MHC-I dissimilarity than expected from random pairing. They were also more MHC-similar than many of the alternative pairings females could potentially have formed with nearby males.

The males that actually sired extra-pair young tended to be more MHC-dissimilar from the female than her social partner. Extra-pair mating could therefore increase the MHC difference between the parents contributing genes to some offspring.

The females showed little sign of choosing the most MHC-dissimilar male

The striking part was that the extra-pair fathers did not look strongly selected for their MHC genotype. Among 21 cases where a genotyped extra-pair sire could be compared with other males available within 60 metres of the nest, only three sires were more MHC-dissimilar from the female than expected under random choice.

The authors therefore propose a simpler explanation. If a female’s social partner is already exceptionally MHC-similar to her, almost any other local male may offer greater MHC dissimilarity. Opportunistic extra-pair mating can then produce the same genetic pattern that deliberate MHC-based mate choice might create.

The pattern was specific to MHC rather than overall relatedness

Pairs with and without extra-pair young did not differ in the proportion of shared microsatellite alleles, and their eggs had similar hatching success. These results make a broad inbreeding-avoidance explanation less likely, although the authors note that only a limited number of microsatellite markers were available.

The study therefore suggests that extra-pair mating can compensate for an unusually MHC-similar social pairing without requiring females to identify and deliberately select the most MHC-dissimilar male.

Sources and image credit

  1. Halupka, L., Strandh, M., Sztwiertnia, H., Klimczuk, E., Hasselquist, D., O’Connor, E. A., & Westerdahl, H. (2026). Female reed warblers in social pairs with low MHC dissimilarity achieve higher MHC dissimilarity through random extra-pair mating. Evolution, 80(4), 725–734. https://doi.org/10.1093/evolut/qpag001Primary open-access research article first published 19 January 2026 under CC BY 4.0. A six-year field study in the Stawy Milickie nature reserve, southwestern Poland, combined 128 females, 127 males, parentage data from 501 nestlings and MHC-I genotyping. Social pairs with extra-pair young had lower MHC-I dissimilarity than expected from random pairing, while actual extra-pair sires showed little evidence of active MHC-based selection.

Image: hedera.baltica (Pawel Wiecek) — wild common reed warbler (Acrocephalus scirpaceus) perched on a reed in Wrocław, Poland, photographed 26 May 2016; Wikimedia Commons / Flickr; representative species image, not from the study population · CC BY-SA 2.0 · Source

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