Inbreeding and disease avoidance in a free‐ranging koala population

Habitat destruction and fragmentation are increasing globally, forcing
surviving species into small, isolated populations. Isolated populations
typically experience heightened inbreeding risk, and associated inbreeding
depression and population decline; although individuals in these
populations may mitigate these risks through inbreeding avoidance
strategies. For koalas, as dietary specialists already under threat in the
northern parts of their range, increased habitat fragmentation and
associated inbreeding costs are of great conservation concern. Koalas are
known to display passive inbreeding avoidance through sex-biased
dispersal, although population isolation will reduce dispersal pathways.
We tested whether free-ranging koalas display active inbreeding avoidance
behaviours. We used VHF tracking data, parentage reconstruction, and
veterinary examination results to test whether female koalas make mate
choices based on [1] relatedness, and [2] chlamydial disease in available
mates. We found no evidence that female koalas based their mate choice on
the relative relatedness of available mates. In fact, as the relatedness
of potential mates increases, so did inbreeding events. We also found no
evidence that female koalas based their mate choice on the chlamydial
disease status of available mates. The absence of active inbreeding
avoidance mechanisms in koalas is concerning from a conservation
perspective, as small, isolated populations may be at even higher risk of
inbreeding depression than expected. At-risk koala populations may require
urgent conservation interventions to augment gene flow and reduce
inbreeding risks. Similarly, if koalas are not avoiding mating with
individuals with chlamydial disease, populations may be at higher risk
from disease than anticipated, further impacting population viability.