Inbreeding-environment interactions throughout the life of a songbird
29 September 2017
Campus Drie Eiken, Promotiezaal Q0.02 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Raïssa De Boer
Wendt Müller & Marcel Eens
PhD defence Raïssa De Boer - Faculty of Science, Department of Biology
It is well-known that reproduction between related individuals negatively affects the offspring (=inbreeding depression). However, the magnitude of inbreeding depression is highly context-dependent and can vary according to the intrinsic state or the environmental conditions. A rarely considered, yet extremely relevant feature of the environment is the social environment, i.e. the presence of and interactions with conspecifics. The social environment changes throughout life, from the early family dynamics to mate choice and sexual signaling in adulthood, and each social context can cause variation in the magnitude of inbreeding depression via ‘inbreeding-environment interactions’.
The aim of this research was to gain a comprehensive understanding of inbreeding and inbreeding-environment interactions, by studying canary offspring originating from unrelated parents (=outbred) and full sibling parents (=inbred) throughout life, in particular the early nestling phase, sexual maturity and reproductive senescence. Within each phase, relevant traits were compared between inbred and outbred birds, including nestling growth, song expression, attractiveness, competitiveness, fecundity, and survival rate. I hypothesized that the magnitude of inbreeding depression becomes more emphasized in stressful contexts. Early in life, late hatched nestlings had an age, size and (egg) quality disadvantage in sibling competition in comparison to first hatched nestlings, and thus experienced a more stressful environment. Consequently, inbreeding depression became apparent in late-hatched nestlings, but under relaxed competition there were no differences in growth between inbred and outbred nestlings. Inbreeding also increased nestling mortality. Furthermore, inbred birds that did survive were still disadvantaged at sexual maturity, since inbreeding affected male song expression. This potentially acted as a cue in mate choice, because outbred females invested less in reproduction with inbred males than with outbred males. Interestingly, inbred males were fiercer competitors than outbred males, which contrasts the findings in early life where inbred nestlings were suffering from being disadvantaged in sibling competition. This discrepancy could be explained by the fact that inbred males had shorter lifespans and lower fertility than outbred males, and that the persistence in male-male competition could therefore reflect an alternative mating strategy and in particular an increased investment in current reproduction.
Similarly, inbred females showed a steeper decline in reproductive output with increasing age than outbred females, indicating enhanced reproductive senescence. Thus, inbred birds have lower future reproductive value than outbred birds, which should skew investment towards early reproduction. Finally, I found that inbreeding depression not only affected the inbred individuals but also the fitness of outbred partners, because pairs in which the male, female, or both birds were inbred had lower annual reproductive success than pairs in which both partners were outbred. In conclusion, this thesis clearly shows the importance of considering the impact of the social environment on the magnitude of inbreeding depression, and that inbreeding depression has severe negative effects that can even impinge on outbred individuals.