Within-group kinship and mating system in a cooperative breeder, the northern mole vole (Ellobius talpinus), in the Lower Volga region

Subterranean rodents are key model systems for studying the ultimate and proximate causes of cooperative breeding and reproductive skew. The northern mole vole is a truly subterranean, highly social cooperative breeder with an evolutionary history independent of other subterranean rodents. This makes it a unique system for investigating parallelism and divergence in underground social evolution. By combining microsatellite analysis, low-invasive age estimation, and four-year group monitoring, we identified first-order relationships, elucidated the genetic mating system, and gained new insights into sex-specific social strategies. Most groups consisted of a single breeding pair and their offspring born in the current or previous 1–2 reproductive seasons. However, nearly half of the groups contained multiple adult males unrelated to the breeding female yet closely related to each other and, in known cases, philopatric. Several males shared paternity in some of these groups. Extra-group genetic polyandry was rare or absent, and no evidence of extra-group paternity was detected. Pair bonds persisted for years, likely until the death of one partner. In known cases, the disappearance of breeding females was followed by (i) the emigration of most daughters, resulting in a prevalence of male-biased groups and cooperative polyandry; (ii) incestuous breeding by some daughters; (iii) plural breeding of some sons; and (iv) serial monogamy of some breeding males. The turnover of breeders appears to be a determinant of social dynamics and mating system in the studied population.