MHC-mediated fetal odourtypes expressed by pregnant females influence male associative behaviour.

Mice can recognize one another by individually characteristic phenotypic body odours (odourtypes) that reflect their genetic constitution at the highly polymorphic major histocompatibility complex (MHC) of genes on chromosome 17. We have shown previously that MHC-determined odours are produced by fetuses: house mice, Mus domesticus, can be trained to discriminate between genetically identical pregnant females carrying 9-18-day-old fetuses of differing MHC type. Theoretically, it should be possible for a mouse to determine the MHC type of the sire based on the odourtype of the pregnant female. In the current study we investigated whether untrained male mice show spontaneous discrimination between such pregnant mice. In experiment 1, sexually inexperienced male mice spent more time near pregnant females that carried fetuses most genetically different from the males themselves. Experiment 2, designed to evaluate possible experiential effects on this preference, tested males that were cohabiting and had impregnated a female that was either genetically identical to the test male (excepting X and Y chromosomes) or differed from him only at the MHC. Males in the former case performed virtually identically to those tested in experiment 1. In contrast males in the latter group did not display this preference. These studies reveal that among untrained male mice, fetal MHC type influences choice behaviour presumably via fetal odourtypes expressed in maternal secretions/excretions and that previous housing and/or mating experience modulates male choice. Copyright 2000 The Association for the Study of Animal Behaviour.