Abstract
BackgroundStudies of birds have a disproportionate representation in the literature on life-history evolution, because of the (apparent) ease with which the costs and benefits can be quantified and manipulated. During reproduction, birds frequently show a highly conserved pattern of mass change and changes in mass loss during breeding have been widely considered to be a valid short-term measure of the costs of reproduction. Experimental manipulations of the breeding attempts of birds usually argue that the presence of a response shows that a cost of reproduction exists, but there is little consensus as to how the size of these costs can be measured.ResultsWe model this mass loss by considering how a parent can maximise its lifetime reproductive success, using a theoretical framework that is particularly suited to modelling parental care in altricial birds. If lifetime reproductive success is taken to be the sum of a parent's current and future reproductive success, we show that the exact forms of these components will influence the optimal amount of mass a parent should lose. In particular, we demonstrate that the shape of the relationship between parental investment and chick survival will lead to differing degrees of investment between parents of different initial qualities: parents with initially high levels of energy reserves could conceivably invested a lesser, similar or greater amount of resources than parents with initially low reserves, and these initially 'heavy' parents could potentially end up being lighter than the initially 'lighter' individuals.ConclusionWe argue that it is difficult to make predictions about the dependence of a parent's final mass on its initial mass, and therefore mass loss should only be used as a short-term measure of the costs of reproduction with caution. The model demonstrates that we require a better understanding of the relationship between mass loss and both current and future reproductive success of the parent, before predictions about mass loss can be made and tested. We discuss steps that could be taken to increase the accuracy of our predictions.
Highlights
Studies of birds have a disproportionate representation in the literature on lifehistory evolution, because of the ease with which the costs and benefits can be quantified and manipulated
In this paper we describe a simple model that suggests that if the parent is optimising its total reproductive success, quantifiable differences in mass loss and final mass may occur for reasons that have not yet been explored experimentally
The model shows that it is vital to have a clear understanding of the effects that a manipulation or environmental change has upon both the current and future reproductive success of a parent, before we can state whether an observed change in the mass of the parent reflects an optimal response to the manipulation
Summary
Studies of birds have a disproportionate representation in the literature on lifehistory evolution, because of the (apparent) ease with which the costs and benefits can be quantified and manipulated. Most species of bird show stereotypical patterns of mass loss during their breeding period, and studies looking at these changes in body mass have been appearing in the literature since the 1930s [1,2]. Shown are the fitness functions for parents that start off with high (heavy) and low (light) values of x0 – the resulting offspring fitness at any final mass (such as that at which future reproductive success fitness is maximised, xpers) is greater for the parent that starts off with higher reserves, as it has allocated a larger amount of reserves to the offspring to reach this mass. In this paper we describe a simple model that suggests that if the parent is optimising its total reproductive success, quantifiable differences (or their absence) in mass loss and final mass may occur for reasons that have not yet been explored experimentally
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