Abstract
In most cooperative breeders, helping is directed at close kin, allowing helpers to gain indirect fitness benefits by increasing the reproductive success of close relatives, usually their parents. Extrapair paternity (EPP) occurs at high rates in some cooperative breeders, reducing the relatedness of helpers to the young they help raise. Even so, a son that helps is related to the brood by at least 0.25 through his mother and to within-pair young by 0.5, whereas a potential helper that has EPP in his own nest is related only to the offspring he sires and unrelated to any extrapair offspring. In birds, EPP often favors older males, which in the extreme case can result in sons being more closely related to young in their parents' nest than to young in their own nests. The fitness benefit of helping will thus be enhanced if helping lightens the workload and increases survival of helpers and their fathers, enabling them to become old, hyper-successful extrapair sires. Here, we develop and analyze a proof-of-concept model, grounded in the western bluebird (Sialia mexicana) system, demonstrating the conditions under which high population levels of EPP can generate inclusive fitness benefits of helping behavior that outweigh the costs. This model provides a new perspective on the relationship between EPP and helping behavior in cooperative breeders and suggests a strong need for empirical work to gather unprecedented data on paternity over the lifetime of helpers and their parents.
Highlights
Cooperative breeding—characterized by joint care of young by 3 or more group members—is taxonomically widespread in animals, occurring at low frequencies in invertebrates, birds, and mammals (Brown 1987; Stacey and Koenig 1990; Solomon and French 1997; Bourke 2011)
We analyze the model with parameters based on data from a population of western bluebirds in which Extrapair paternity (EPP) success is age-biased, 7% (3–16%) of pairs have a helper, and males successful at EPP have double the reproductive success of males that are not (Dickinson et al 1996; Dickinson and Akre 1998; Ferree and Dickinson 2014)
Our results demonstrate that an age bias in paternity success can lead to greater inclusive fitness from helping than from breeding for first-year adult males, and that a reduction in annual adult mortality for adults in groups with a helper adds to this effect, increasing the area of parameter space in which helping is favored over breeding
Summary
Cooperative breeding—characterized by joint care of young by 3 or more group members—is taxonomically widespread in animals, occurring at low frequencies in invertebrates, birds, and mammals (Brown 1987; Stacey and Koenig 1990; Solomon and French 1997; Bourke 2011). Based on inclusive fitness theory, this would reduce the likelihood that Hamilton’s rule (rB > C) will be met (Hamilton 1964) This framing of the problem has led researchers to consider monogamy to be an important factor in the evolution of helping (the “monogamy hypothesis”; Boomsma 2007, 2009), and this is straightforward for female helpers: a female that breeds independently is sure to be the mother of all the offspring in her brood (assuming an absence of brood parasitism), so a high probability that offspring in her parents’ brood are sired by an extrapair male would likely reduce the inclusive fitness benefits of helping relative to those of breeding.
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