Facultative adjustment of paternal care in the face of female infidelity in Dunnocks

A much-debated issue is whether or not males should reduce parental care when they lose paternity (i.e. the certainty of paternity hypothesis). While there is general support for this relationship across species, within-population evidence is still contentious. Among the main reasons behind such problem is the confusion discerning between- from within-individual mechanisms. Here, we tested this hypothesis empirically by investigating the parental care of male dunnocks (Prunella modularis) in relation to paternity. We used a thorough dataset of observations in a wild population, genetic parentage, and a within-subject centering statistical approach to disentangle paternal care adjustment within-male and between males. We found support for the certainty of paternity hypothesis, as there was evidence for within-male adjustment in paternal care when socially monogamous males lost paternity to extra-pair sires. There was little evidence of a between-male effect overall. Our findings show that monogamous males adjust paternal care when paired to the same female partner. We also show that – in monogamous broods – the proportion of provisioning visits made by males yields fitness benefits in terms of fledging success. Our results suggest that socially monogamous females that engage in extra-pair behavior may suffer fitness costs, as their partners’ reduction in paternal care can negatively affect fledging success.