Superiority of extra-pair offspring: maternal but not genetic effects as revealed by a mixed cross-fostering design

Abstract

Extra-pair copulations (EPC) are the rule rather than an exception in socially monogamous birds, but despite widespread occurrences, the benefits of female infidelity remain elusive. Most attention has been paid to the possibility that females gain genetic benefits from EPC, and fitness comparisons between maternal half-siblings are considered to be a defining test of this hypothesis. Recently, it was shown that these comparisons may be confounded by within-brood maternal effects where one such effect may be the distribution of half-siblings in the laying order. However, this possibility is difficult to study as it would be necessary to detect the egg from which each chick hatched. In this study, we used a new approach for egg-chick assignment and cross-fostered eggs on an individual basis among a set of nests of the collared flycatcher Ficedula albicollis. After hatching, chicks were ascribed to mothers and therefore to individual eggs by molecular genetic methods. Extra-pair young predominated early in the laying order. Under natural conditions, this should give them a competitive advantage over their half-siblings, mediated by hatching asynchrony. However, we experimentally synchronized hatching, and after this treatment, extra-pair young did not outperform within-pair young in any studied trait including survival up to recruitment and several indicators of reproductive success and attractiveness. We obtained only modest sample sizes for the last two traits and did not test for extra-pair success of male offspring. Thus, we cannot exclude the possibility of advantages of extra-pair young during the adult phase of life. However, our data tentatively suggest that the more likely reason for females' EPCs is the insurance against the infertility of a social mate.

Female collared flycatchers choose neighbouring and older extra‐pair partners from the pool of males around their nests

“…However this phenotype‐linked fertility hypothesis has weak empirical support since ejaculate or sperm traits are usually uncorrelated to male phenotypes (Mautz et al ). In line with this we also found no difference in ornament size of extra‐pair and social males despite our previous study tentatively suggested that females might engage in extra‐pair copulations to insure clutch fertility (Krist and Munclinger ). Useful steps would be to take into consideration sperm traits to test if they are linked to male phenotype and have effect on paternity in the collared flycatcher.…”

“…There was no difference in the wing patches or forehead patches between social and extra‐pair males. Moreover, we did not find evidence of superiority of extra‐pair chicks in a previous study which also suggested that females did not obtain good or compatible genes for their offspring from extra‐pair copulation (Krist and Munclinger ). However, that study was aimed mainly on short‐term fitness benefits and we were unable to exclude superiority of extra‐pair young in adult phase of life (Krist and Munclinger ).…”

“…Mixed clutches with eggs from different nests were compiled. When the last eggs had been laid and incubation had started, the dummy eggs were replaced with a new clutch with the same size as the original (see Krist and Munclinger for more details). We refrained from checking the nests every day to avoid disturbance and desertion by females during the incubation period.…”

“…We also tested if the extra‐pair males were neighbours and whether the traits of females, social or extra‐pair males predicted the distance between the nests of cuckolded males and their cuckolders. We used collared flycatchers Ficedula albicollis as the study species due to frequent extra‐pair paternity (Sheldon et al , Sheldon and Ellegren ), as was also found in our population (Krist et al , Krist and Munclinger ). Males of this species possess two secondary sexual ornaments that could play a role in sexual selection: a forehead patch (Sheldon and Ellegren ) and a wing patch (de Heij et al ).…”

Moderate heritability and low evolvability of sperm morphology in a species with high risk of sperm competition, the collared flycatcher Ficedula albicollis

“…We conducted paternity analyses for part of the population in 2001–2002 (Krist et al., ), 2006–2009 (Krist & Munclinger, ) and for the whole population in 2013 (Edme et al., ). We did not find any case of intra‐specific brood parasitism.…”