Welcome to the Alpine Marmot Project
Social and Genetic Pairing – The mating system determines how individuals mate. Recent studies in birds and mammals show that the social mating system may differ from the genetic mating system. Thus, in the majority of bird species, individuals live in pairs, and each pair raises its offspring in a nest the pair built. But this social monogamy (formation of a stable social couple that stays together after mating) does not necessarily imply genetic monogamy (mating only between the two members of the social couple). Indeed, in the majority of socially monogamous birds, genetic monogamy is NOT the norm; some or all of the chicks are often fathered by a male different from the social partner.
In mammals, social monogamy is uncommon, but even when present, it is rarely associated with genetic monogamy. However, in the Alpine marmot, a dominant social pair can persist for many years. But even if in most cases genetic monogamy is observed, exceptions do exist. Our research indicates that approximately 20% of litters contain young who are not fathered by the dominant male of the group, and that 10% of young are not fathered by the dominant male.
Lack of Genetic Monogamy: Why? – What are the causes of this lack of genetic monogamy among socially monogamous birds and mammals? There is an obvious benefit for males. A male can mate with one or morefemales in addition to its social partner, a strategy that increases the number of the male’s genetic offspring without increasing its paternal investment. But what benefits do females receive for mating with males who are not their social partner? Females do not appear to gain direct benefits (parental care, food resources …), but three main types of indirect benefits have been proposed:
The formation of a couple is often the result of a competitive process. Females could thus be constrained not to mate with the best males (from a genetic point of view). Yet, the “cuckoldry” could allow the females to mate with a male bearing better genes than their social partner (known as the “good genes hypothesis”).
Similarly, this competitive preocess could results in females constrained to mate with genetically incompatible males. Indeed, mating with a partner who is too genetically similar (inbreeding) increases the chances of producing homozygous young, who may have difficulty surviving and reproducing (inbreeding depression). The “cuckoldry” could allow the female to mate with a partner who is more genetically compatible (less similar genetically) to produce more heterozygous offspring (known as the “genetic compatibility hypothesis”).
Finally, mating with several males allows females to diversify the genomes of their offspring, an outcome that is especially desirable when environmental or other conditions are changing. This additional genetic diversity increases the chances that some descendants will be able to adapt to the changing environment (known as the “genetic diversity hypothesis”).
Conditions Favoring Extra-Pair Mating – In the Alpine marmot, our results show that dominant females may engage in extra-pair copulations under certain conditions. For example, the rate of extra-pair paternity increases as the number of sexually mature subordinate males in the family group increases. It seems that only females whose mates have to control the reproduction of a large number of sexually mature subordinate males engage in extra-pair copulations. In addition, the amount of extra-pair paternity depends on the genetic characteristics of the dominant male. Males who are genetically very similar or very dissimilar to the dominant female are most likely to be cuckolded. This finding suggests there is an optimal range of genetic diversity for Alpine marmot pairs.
Extra-pair matings affect both the genetic characteristics and the phenotypic characteristics of the offspring. Even though we have not been able to show directly that the young with extra-pair paternity are more heterozygous than offspring of the dominant male, we have demonstrated that extra-pair young are genetically less close to the dominant female than are the young of the dominant male. In addition, we have shown that offspring from extra-pair matings are more likely to survive their first two years than are their half-siblings fathered by the dominant male, suggesting that the extra-pair offspring are more fit.
Our findings support the genetic compatibility hypothesis , with genetic compatibility measured using microsatellite markers randomly distributed throughout the genome. Our results suggest that females avoid inbreeding and that inbreeding avoidance is advantageous to females.
Identifying Males with the Right Genes – It is possible that the female seeks a male who is genetically different from her at some specific genes. The genes of the major histocompatibility complex (MHC), which is part of the immune system, could be relevant. If the female mates with a male with different MHC genes than her own, their young would be heterozygous for MHC, and could possibly better defend themselves against a wider range of pathogens than otherwise.
How can the female recognize advantageous genetic characteristics in potential sexual partners? One possibility is to use a unique characteristic of the male. Many laboratory studies suggest that odors depend on genes of the MHC. For example, mice are able to distinguish the scents of other mice whose only genetic differences occur in MHC genes. Recently, we characterized the genetic architecture of the MHC in the Alpine marmot. Currently, we are studying these genes to learn (1) if the MHC genes are involved in mate choice and (2) what mechanisms exist for choosing sexual partners based on their genetic characteristics.