Female marmots 'cheat on' their males

The research data was obtained from a population of alpine marmots (Marmota marmota) from the French Alps studied between the years 1990 and 2010. The greatest number of illegitimate offspring is seen when both dominating male and female are genetically either very similar or very different. "Some 10% of litters contain at least one illegitimate offspring from copulation with a non-dominant male. It is believed that females behave like this in order to have genetically more varied offspring.

Dominant Male, Minimum Requirements
Marmots are mammals which live in small groups of up to 20 individuals, and theoretically within this group only the dominant couple reproduces. Both males and females of the group have a very strong urgency to become dominant, since if they don’t they have to leave the group, and chances of surviving alone in the alpine winter are very slim.

The female’s choice of a male depends on a group of genes related with immunological responses (major histocompatibility complex, or MHC), which should be very different to and compatible with her own. This results in a varied bloodline with a greater chance of resisting parasites. Such compatibility is the dominant male’s best chance to avoid that the female copulates with other individuals. “However, this first selection of a male does not take into account a second genetic aspect, which is consanguinity (something like ‘relatedness’),” says Ferrandiz.

Infidelity as an Option for Increased Genetic Variation
When the dominant couple has a high degree of consanguinity or the individuals have a very similar MHC, the female corrects this by “cheating on” her mate and reproducing with other males that probably are more compatible with her genetically. It is expected that these other males’ genomes will be neither very different nor very similar, and that they will have a MHC which is very different to that of the female. If this did not occur, the offspring produced by a couple with a high consanguinity and similar MHC would tend to have reduced genetic variability, which will probably result in a reduced fitness for survival and a worse immunological response.

MHC Often Related to Body Odour
Is it still unknown how females are able to detect such genetic differences. For other vertebrates such as humans and other primates, it has been observed that MHC plays a role in certain body odours and mating choices. If this is also the case for marmots then body odours may be a signal to females when searching for a mate. “This is the line of research that we want to follow now, to see if there is a relationship between odours of individuals and their genetics, specifically of the major histocompatibility complex,” says the CREAF researcher. She adds, “MHC is the current hot topic of research on the immunological response of vertebrates. This complex influences natural selection because it is directly involved in bodily defences when dealing with parasites.”

Although the dominant male reproduces the most, the litters of the females can include pups from different fathers, and no differences in upbringings have been observed. In fact, Mariona Ferrandiz explains, “in a previous study it was seen that the illegitimate offspring survived more than the legitimate ones, probably due to the fact that they have a greater genetic diversity.”

Reference: Ferrandiz-Rovira, M., Allainé, D., Callait-Cardinal, M.P., Cohas, A. "Mate Choice for Neutral MHC Genetic Characteristics in Alpine Marmots: Different Targets in Different Contexts?" (2016). Ecology and Evolution. DOI: 10.1002/ece3.2189