Inbreeding accelerates reproductive senescence, but not survival senescence, in a precocial bird

Inbreeding depression is predicted to increase with age because natural
selection is less efficient at purging deleterious alleles that are only
expressed later in life. However, empirical results are scarce, and
equivocal between studies. Here we performed controlled matings between
related and unrelated individuals of domesticated Japanese Quail (Coturnix
japonica) and monitored the performance of their offspring for all fitness
components over their complete life course. We found rapid senescence in
adult survival and egg-laying performance, and inbreeding depression at
all life stages (reduced embryo viability, increased age at maturity, as
well as reduced adult survival and reproduction). Inbreeding depression
did not increase at later ages for survival, but did so for egg laying,
thereby accelerating reproductive senescence. Moreover, the effect of
inbreeding on egg laying persisted after correcting for lifespan,
indicating that both survival and reproduction were independently affected
by inbreeding. We suggest that in heterogeneous populations
intra-generational purging may at earlier ages already select out the
individuals that are homozygous for the specific alleles responsible for
depressed survival, preventing the appearance of increased inbreeding
depression in survival with age. Given that inbreeding affects
reproduction independent of survival this should not apply to reproductive
senescence or homogeneous populations, which may explain equivocal results
between traits and studies.