Inbreeding depression of sexually selected traits and attractiveness in the zebra finch

An underexploited approach to study condition dependence of sexually selected ornaments is to use inbreeding to manipulate individual genetic quality. Because of differences in genetic architecture, sexually selected traits are expected to suffer stronger inbreeding depression than morphological traits. We used full-sib mating to investigate the effects of inbreeding on viability, attractiveness, morphology and potentially secondary sexual traits in male and female zebra finches, Taeniopygia guttata. Unexpectedly, the genetic load from lethal alleles (leading to early mortality) was low. In males, we found strong negative effects of inbreeding on song rate, beak colour, body size and choice chamber attractiveness, whereas body condition, fat deposition, song structure and plumage ornaments were unaffected. In females, beak colour, body size, fat deposition and choice chamber behaviour showed significant inbreeding depression. These results indicate that some but not all adult phenotypes are affected by inbreeding, most probably because of the homozygous expression of slightly deleterious mutations. Our findings complement alternative approaches to study the condition dependence of traits that may be relevant in mate choice or competition.