Almost faithful: SNP markers reveal low levels of extra-pair paternity in the Eurasian beavers

Abstract

Mating systems largely affect individual reproductive strategies which further drives evolution. Monogamy, where males and females form exclusive pairs for more than one breeding season, is particularly intriguing in this context, as there are real and potential costs of genetic monogamy to both sexes. However, molecular studies in a variety of species have revealed that social monogamy does not necessarily imply genetic monogamy due to occurrence of extra-pair copulations resulting in extra-pair offspring. Although common in birds, <10% of mammals are monogamous. Here we use single nucleotide polymorphism (SNP) to investigate the genetic mating system of the Eurasian beaver (Castor fiber), a species traditionally considered to be not only socially but also genetically monogamous. We found evidence for low frequency of extra-pair paternity (EPP) and multiple paternity within litter. Only 5.4% young were produced by EPP and only 7% of litter contain at least one extra-pair young. Moreover, we found indications that only pairings of old individuals engaged in EPP. None of these pairs produced more than one litter as a result of EPP and none of the EPP events resulted in mate change. Our findings suggest that EPP in beavers might be the consequence of a lapse in mate guarding ability of old males.

Landscape structure and population density affect intraspecific aggression in beavers

“…This period coincides with the beavers' mating season, suggesting that mate guarding by male territory owners in the ice-free Eurasian population and conflicts resulting from intruders that seek extra-pair copulation lead to the observed sex differences. This hypothesis is partly supported by findings that extra-pair copulations occur in the Norwegian population (Nimje et al, 2019), whereas no extra-pair paternity was detected in a Russian population that experienced extensive ice cover during winter (Syrůčková et al, 2015). Moreover, Crawford et al (2015) found no sex differences in conspecific aggression in areas that were also (at least in parts) covered by ice.…”

Genetic monogamy and mate choice in a pair-living primate

“…Opportunities for EPC are also affected by population density, with the lower densities making the encounters between individuals and, consequently, EPC less likely 63 . The positive relationship between population density and EPP rates was demonstrated, e.g., in Eurasian beavers, Castor fiber , and in many bird species 15 , 16 , 21 , 64 . At our study site, population density was estimated at 34 individuals/km 2 (unpublished data).…”

“…For example, in Azara’s owl monkeys who are very similar to titis in all aspects of their social system, mated individuals experience intense intra-sexual competition from floaters of both sexes 56 , 57 . However, the evidence from Azara’s owl monkeys and many other bird and mammal species indicate that floaters do not copulate with the mated animals as often as might be intuitively expected, and EPP are attributed to the neighboring individuals in most cases (e.g., 1 , 16 , 19 ; but see 18 , 58 ). In titis, only anecdotal reports of replacements by intruders exist 59 , 60 , but given the difficulty of detecting floaters, it is possible that they are present in titi populations, too.…”

Genetic monogamy and mate choice in a pair-living primate

“…Opportunities for EPC are also affected by population density, with the higher densities making the encounters between individuals and, consequently, EPC less likely 53 . The positive relationship between population density and EPP rates was demonstrated, e.g., in Eurasian beavers, Castor fiber, and in many bird species 8,45,54,55 . Notably, the only published record of EPC in titis comes from a population of Plecturocebus ornatus (previously C. moloch) living in a disturbed habitat with exceptionally high density of 406 individuals/km 2 56 .…”