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Extra-pair reproduction is widely hypothesised to allow females to avoid
inbreeding with related socially-paired males. Consequently, numerous
field studies have tested the key predictions that extra-pair offspring
are less inbred than females’ alternative within-pair offspring, and that
the probability of extra-pair reproduction increases with a female's
relatedness to her socially-paired male. However such studies rarely
measure inbreeding or relatedness sufficiently precisely to detect subtle
effects, or consider biases stemming from failure to observe inbred
offspring that die during early development. Analyses of
multi-generational song sparrow (Melospiza melodia) pedigree data showed
that most females had opportunity to increase or decrease the coefficient
of inbreeding of their offspring through extra-pair reproduction with
neighbouring males. In practice, observed extra-pair offspring had lower
inbreeding coefficients than females’ within-pair offspring on average,
while the probability of extra-pair reproduction increased substantially
with the coefficient of kinship between a female and her socially-paired
male. However, simulations showed that such effects could simply reflect
bias stemming from inbreeding depression in early offspring survival. The
null hypothesis that extra-pair reproduction is random with respect to
kinship therefore cannot be definitively rejected in song sparrows, and
existing general evidence that females avoid inbreeding through extra-pair
reproduction requires re-evaluation given such biases.
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