Abstract
How parents negotiate over parental care is a central issue in evolutionary biology because it affects the evolutionary outcome of sexual conflict. A recent theoretical model shows that ‘turn-taking’ in provisioning visits by the parents is an evolutionarily stable negotiation strategy, and empirical studies have shown that parental nest-visits do indeed alternate more than expected by chance. However, such alternation may also be generated by a refractory period, or by correlated temporal heterogeneity (CTH) in provisioning rates of the two parents driven by temporal environmental variation. Here we use a recently developed measure of alternation and a novel measure of CTH in the provisioning rates of pairs to clarify what can be concluded about the occurrence of turn-taking from the provisioning patterns of pairs. First, we show using a simulation model that turn-taking can, by itself, generate both a refractory period and CTH in provisioning rates. Second, we incorporate this insight into a conceptual framework that combines an existing randomization analysis with a novel analytical approach in which ‘pseudo-pairs’ are created by analytically pairing the provisioning sequence of a parent at one nest with the contemporaneous provisioning sequence of the other-sex parent at a nearby nest. This allows us to partition the alternation score into different components. This approach confirms that isolating a component of alternation that can be unequivocally attributed to turn-taking is probably impossible. However, the pseudo-pairs analysis does isolate a component that can be unequivocally attributed to general temporal environmental variation (environmental variation that causes CTH in provisioning rates across [as well as within] pairs). Third, we use these techniques to partition the alternation score of 17 pairs of great tits Parus major provisioning in the wild. Approximately 8% of the observed alternation score is due to the frequency distribution of the inter-visit intervals, 74% to nest-specific effects on the sequence of inter-visit intervals, and 18% to general effects on the sequence of inter-visit intervals. This last component can be unequivocally attributed to general temporal environmental variation, and is the first empirical demonstration of alternation by free-living provisioning parents being generated by temporal environmental variation.
Highlights
In species with bi-parental care, there is an evolutionary conflict of interest between the two parents (Trivers, 1972)
We investigated whether turn-taking in nest visits affects the frequency distribution of IVIs, including the presence of a refractory period, and provisioning correlated temporal heterogeneity (CTH) by simulating and analyzing sequences of provisioning visits by pairs
(because they were randomly drawn from a negative exponential distribution), the IVIs of the independently provisioning parents follow a negative exponential distribution [mean IVI = 0.995 min ± 0.995 (SD); Figure 2A; because the two parents are exactly equivalent in their provisioning behavior, the distributions of IVIs for males and females have been combined in the calculation of the mean and SD, and in the figures, for both the independently provisioning and turn-taking parents]
Summary
In species with bi-parental care, there is an evolutionary conflict of interest between the two parents (Trivers, 1972). A number of theoretical models have been developed to investigate how sexual conflict affects the evolutionarily stable amount of care that parents devote to offspring. Despite the diversity of patterns of parental investment that these models represent, including both “sealed bids” (Houston and Davies, 1985) and “negotiation” (McNamara et al, 1999; Johnstone and Hinde, 2006; Lessells and McNamara, 2012), they all predict that the evolutionarily stable outcome of sexual conflict is a decrease in parental care and reduction in parent and offspring fitness compared with completely cooperating parents. The evolutionary stable outcome of this strategy is a “turn-taking” rule in which each parent does not provision when it is the last to feed, but only after a visit by the mate, leading to strict alternation of the nest visits. The ESS is remarkable in that it results in completely cooperative behavior, with each parent having maximum fitness
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