Abstract

Dispersal is an important mechanism used to
avoid inbreeding. However, dispersal may only
be effective for part of an individual’s lifespan
since, post-dispersal individuals that breed over
multiple reproductive events may risk mating
with kin of the philopatric sex as they age. We
tested this hypothesis in black grouse Tetrao
tetrix, and show that yearling females never
mated with close relatives whereas older females
did. However, matings were not with direct kin
suggesting that short-distance dispersal to sites
containing kin and subsequent overlap of reproductive
lifespans between males and females
were causing this pattern. Chick mass was lower
when kinship was high, suggesting important
fitness costs associated with inbred matings.
This study shows that increased inbreeding risk
might be a widespread yet rarely considered
cost of ageing.