Males with high levels of oxidative damage form weak pair bonds in a gregarious bird species

The number and quality of social bonds can have major consequences for fitness. For example, in socially monogamous species with biparental care, pair bond quality has been linked to the latency to breed as well as the number and survival of offspring. Given these benefits, what mechanisms prevent some individuals from forming strong pair bonds? Markers of physiological stress and ageing, such as oxidative stress and telomere length, might mediate individual differences in behavioural performance. However, the possibility that physiological stress could also constrain the strength of the pair bond has rarely been investigated. We show that in captive colonies of the socially monogamous, gregarious zebra finch, Taeniopygia guttata, individuals with higher levels of plasma lipid oxidative damage formed weaker pair bonds. This effect was sex specific: while males with more oxidative damage spent less time in bodily contact with their prospective breeding partners, no such link was found in females. Although females experienced higher absolute levels of plasma oxidative damage, pair bond investment may have been more constrained in males due to the costly expression of their sexually selected traits. Pair bond strength was not associated with levels of the key antioxidant glutathione or with telomere length. Individuals’ ability to form strong pair bonds may thus be constrained by their levels of oxidative damage, with potential downstream effects on fitness.