Female attractiveness affects paternal investment: experimental evidence for male differential allocation in blue tits

We reduced the UV reflectance in a sample of females and compared parental care by their mates with that of males paired to sham-manipulated control females. As predicted by the DAH our results demonstrate that males paired with UV-reduced females invested less in feeding effort but did not defend the chicks less than males paired with control females.To our knowledge, this is one of the first studies providing support for male differential allocation in response to female ornamentation.Females frequently choose males on the basis of traits [1] that may signal individual quality [2]. As a consequence they gain direct benefits, e.g. through high-quality territories and paternal investment, or indirect benefits, because attractive mates may provide genes for passing viability and attractiveness to the offspring [2]. On the other hand little is known about why females of several species also possess elaborate traits [3,4]. For a long time the presence of female ornaments was interpreted as being the consequence of genetic correlation with male ornamentation [3,5]. However, recent studies have suggested that female ornaments play a role in female - female competition (intrasexual selection) [6-10] or are sexually selected by males (intersexual selection) [11-17]. In species with biparental care males may gain benefits from choosing “high quality females” and adjust their parental investment to female quality. This may happen if there is much variance in female quality, if the latter affects offspring survival and if the males parental provide some parental investment and/or remating opportunities are low [2]. The idea of adjusting parental effort in response to the aesthetic traits of partners, when they represent honest signals of quality, is known as the Differential Allocation Hypothesis (DAH). Differential allocation is expected whenever individuals face a trade-off between current and future reproduction and the reproductive value of the offspring is connected to t