Badgers are facultatively social, forming large groups at high density. Group-living appears
to have high reproductive costs for females, and may lead to increased levels of inbreeding.
The extent of female competition for reproduction has been estimated from field data, but
knowledge of male reproductive success and the extent of extra-group paternity remains
limited. Combining field data with genetic data (16 microsatellite loci), we studied the mating
system of 10 badger social groups across 14 years in a high-density population. From 923
badgers, including 425 cubs, we were able to assign maternity to 307 cubs, with both parents
assigned to 199 cubs (47%) with 80% confidence, and 14% with 95% confidence. Age had a
significant effect on the probability of reproduction, seemingly as a result of a deficit of
individuals aged two years and greater than eight years attaining parentage. We estimate
that approximately 30% of the female population successfully reproduced in any given
year, with a similar proportion of the male population gaining paternity across the same
area. While it was known there was a cost to female reproduction in high density populations,
it appears that males suffer similar, but not greater, costs. Roughly half of assigned paternity
was attributed to extra-group males, the majority of which were from neighbouring social
groups. Few successful matings occurred between individuals born in the same social group
(22%). The high rate of extra-group mating, previously unquantified, may help reduce inbreeding,
potentially making philopatry a less costly strategy
