Abstract
Cooperative breeding groups often involve “helpers-at-the-nest”; indeed, such behavior typically defines this intriguing breeding system. In few cases, however, has it been demonstrated that feeding nestlings by helpers, rather than some other behavior associated with helpers’ presence, leads to greater reproductive success. One prediction of the hypothesis that feeding behavior per se is responsible for the fitness benefits conferred by helpers is that there should be close congruence between the patterns of helping-at-the-nest and the fitness effects of helpers. Here we look for such a relationship in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) in order to begin to identify the behaviors of helpers that drive the increased fitness benefits they confer. In terms of young fledged, a helper male confers approximately the same fitness benefits to a group as does a helper female; more dramatically, the effects of helper males increases with increasing food supply, most importantly the prior year’s acorn crop on which this species depends, whereas that of helper females does not. These patterns do not match the nest-feeding patterns of helpers, which are greater for females than males and do not increase with a larger acorn crop the prior autumn. In contrast, the proportion of time helpers spend tending acorn-storage facilities (granaries) and are present in or near their home territory is greater for males than females and, at least for males, positively related to the size of the acorn crop. These results fail to support the hypothesis that the primary benefit conferred by helpers is feeding young in the nest; rather, they suggest that behaviors such as territorial defense and predator detection are more important. Understanding exactly what those behaviors are in this, and most other cooperatively breeding systems, remain to be determined.
Highlights
A central problem in the field of evolutionary biology is to understand why helpers help (Pennisi, 2005)
Because helpers are typically offspring of the breeders (Emlen, 1991), and since cooperative breeding is generally defined by the presence of more than a pair of individuals feeding at a nest, the assumption is typically that such helping behavior increases the reproductive success of the group, and that the additional offspring that helpers help raise, above and beyond what would be produced in the absence of helpers, confers inclusive fitness benefits
One is that there are a sizeable number of cooperative breeding species that are not kin-based (Riehl, 2013), and despite evidence supporting the importance of kin selection in many cooperative breeding systems (Russell and Hatchwell, 2001; Griffin and West, 2003; Browning et al, 2012), helpers are in some cases presumably gaining direct, rather than indirect, fitness benefits
Summary
A central problem in the field of evolutionary biology is to understand why helpers help (Pennisi, 2005). Because helpers are typically offspring of the breeders (Emlen, 1991), and since cooperative breeding is generally defined by the presence of more than a pair of individuals feeding at a nest, the assumption is typically that such helping behavior increases the reproductive success of the group, and that the additional offspring that helpers help raise, above and beyond what would be produced in the absence of helpers, confers inclusive fitness benefits. Experimental studies have demonstrated that, despite considerable load-lightening (Koenig and Walters, 2012a), the feeding rate of both helpers and breeders is primarily determined by brood size rather than the converse (Koenig and Walters, 2012b)
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