When to leave: the timing of natal dispersal in a large, monogamous rodent, the Eurasian beaver

- Individuals are more likely to disperse at lower population densities.
- Beavers delay dispersal with increasing age of the same sex parent.
- Possibly, older parents are more tolerant of their offspring.
- Individuals benefit from delaying dispersal.
- They increase competitive ability while remaining in their natal territory.

As dispersal is a dangerous part of an individual's life, its timing is important to increase the chances of survival and successful establishment of a territory. We investigated factors affecting the timing of natal dispersal in the Eurasian beaver, Castor fiber, a territorial, monogamous, long-lived mammal, using data from an 18-year individual-based study (1998-2015). We tested hypotheses about the causes of dispersal onset, namely competitive ability, kin competition (sibling competition and offspring-parent competition), population density and intolerance by an incoming, unrelated dominant individual. Only 9% of individuals remained philopatric and became dominant after both of their parents disappeared. Average age at dispersal was 3.5 years, with some individuals delaying dispersal up to age 7 years. Beavers dispersed more frequently with increasing age (i.e. with increasing competitive ability and possibly experience) and when population density was lower. Further, both females and males delayed dispersal with increasing same-sex parental age. Older parents were either more tolerant towards philopatric subordinates, or subordinates awaited the disappearance of their senescing parents to take over the natal territory. From comparisons with other populations, we conclude that the high population density in our area was possibly the ultimate driver of dispersal with individuals delaying dispersal to increase their competitive ability.