Abstract
Recent studies have proposed that conditional cooperation may resolve sexual conflict over the amount of care provided by each parent. Such conditional cooperation may allow parents to equalize their investment by alternating their provisioning visits. This alternated pattern of male and female visits, that is, alternation, is thought to stimulate each other’s investment leading to higher levels of provisioning and potential benefits for offspring development. However, experimental studies testing the role of alternation as an adaptive parental strategy to negotiate the level of investment are still absent. Therefore, we manipulated blue tit (Cyanistes caeruleus) parents by temporarily changing their brood sizes to induce changes in demand and thus visit rates. Parents were expected to visit more—assuming that prey sizes were constant—and alternate at higher levels when confronted with an enlarged brood given the greater potential for sexual conflict. In contrast, in reduced broods visit rates and alternation may become lower due to the smaller investment that is needed for reduced broods. We show that the level of alternation did not differ in response to the manipulated brood sizes, despite a directional change in visit rates for enlarged and reduced broods as expected. Nestlings did not benefit from high levels of alternation as no effects on nestling mass gain were present in either of the different manipulations. These findings indicate that alternation does not serve as a mechanism to motivate each other to feed at higher rates. Parents hence appeared to be inflexible in their level of alternation. We therefore suggest that the level of alternation might reflect a fixed agreement about the relative investment by each of the caring parents.
Highlights
Biparental care is relatively rare among species in the animal kingdom, except among bird species, with more than 81% of the species showing this kind of care (Cockburn, 2006)
To further explore the effects of alternation we investigated whether visit rate increased with a higher level of alternation
Our study shows that blue tit parents adjust their visit rates in response to experimentally manipulated brood sizes
Summary
Biparental care is relatively rare among species in the animal kingdom, except among bird species, with more than 81% of the species showing this kind of care (Cockburn, 2006). It is in both parents’ interest that their partner provides more care during the breeding attempt so that they themselves can retain energy for self-maintenance and future reproduction (Stearns, 1989). Whereas more recent models implemented the possibility that parents are able to adjust their investment in response to their partner on a behavioral time scale (Mcnamara, Gasson & Houston, 1999; Johnstone & Hinde, 2006; Lessells & McNamara, 2012), which better captures the natural situation. A more cooperative strategy has been proposed for the resolution of sexual conflict, namely conditional cooperation, which may avoid or reduce such costs of negotiation (Johnstone et al, 2014)
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