Benefits of extra-pair mating may depend on environmental conditions-an experimental study in the blue tit (Cyanistes caeruleus)

Abstract

Extra-pair mating constitutes a relatively common reproductive strategy in many socially monogamous bird species. This strategy may considerably improve reproductive success of males, but female benefits from extra-pair matings still remain unclear and empirical evidence is scarce. This may be because genetic benefits of extra-pair mating are not always revealed. It is possible that they are shown only in unfavourable environmental conditions and hence problems arise with detecting differences between within- and extra-pair offspring whose performance is measured under favourable conditions. In order to test this prediction, we manipulated environmental conditions by altering brood sizes of blue tits and compared phenotypic characteristics of within- and extra-pair offspring in mixed-paternity broods. We found that extra-pair young exhibited a higher response to phytohemagglutinin in comparison to within-pair young, but this was only observed among nestlings from experimentally enlarged broods. These results indicate that genetic benefits may interact with the environment, and thus benefits of extra-pair mating are likely to become visible only when conditions are relatively unfavourable.

Keywords: Birds; Cell-mediated immunity; Extra-pair paternity; Indirect benefits.

Fig. 1

The immune response to PHA of within-pair offspring (WPO solid line) and extra-pair offspring (EPO dashed line) from control and experimental nests (enlarged broods). Results of post hoc analyses (linear mixed model, see “Material and methods” section for details on statistics). Least squared means with standard errors are presented. Line with asterisk connects significantly different groups (p < 0.05)

Fig. 2

The body mass of within-pair offspring (WPO solid line) and extra-pair offspring (EPO dashed line) in the 3 years of study. Results of post hoc analyses (linear mixed model, see “Material and methods” section for details on statistics). Least squared means with standard errors are shown. Line with asterisk connects significantly different groups (p < 0.05)