SIBLING SYMBIOSIS IN NESTLING BIRDS

Auk, The, Jan 2007 by Forbes, Scott

Gonzaga: Once more, brothers in arms.

Bertoldo: I'll live and die so.

Philip Massinger, Maid of Honor (1632)

IF THE TITLE of this perspective were "Sibling rivalry in nestling birds," readers could hardly be faulted for expecting a traversal of welltrodden ground. We have come to think of conflict among avian nestlings as natural and to some extent inevitable, which is a long way from the traditional view of families as harmonious social units. It is de rigueur to state that the theory of kin selection opened our eyes to the once-surprising possibility that the closest of relatives may benefit at one another's expense. The Cain-and-Abel battles of Black Eagles (Aquila verreauxi) and other large predatory birds, which had been described as "an inexplicable example of apparent biological waste" (Brown et al. 1977), became potentially explicable as extreme forms of sibling competition in cases where close relatives were also close rivals for limited food and tight space. We learned that nestling egrets, boobies, and ospreys fight to secure food, with sometimes fatal consequences; that nestling bee-eaters can use a modified egg-tooth to slash nestmates; and that kestrels and owls sometimes cannibalize kin. These and many more spectacular examples of sibling rivalry were chronicled in Mock and Parker's (1997) masterful Evolution of Sibling Rivalry, which provided a comprehensive overview of theory and data bearing on conflict within families, especially conflict among siblings. Birds were featured prominently in the book, and dramas within the nest captured widespread interest both within and outside ornithology. Mock and Parker left little doubt that siblings within a nest are often important, and sometimes lethal, competitors for food and space. This focus on conflict is an understandable reflection of changing notions of the family and, in particular, revision of the earlier, somewhat romantic notion that close kin must live harmoniously because of their shared genetic interests. But have we gone too far?

If one uses the scientific literature to gauge relative interest in sibling relations, it is clear that the pendulum has swung sharply to the side of conflict, competition, and rivalry. A recent (November 2006) search of the Web of Science revealed 334 citations for the keywords "nestling conflict" or "nestling competition," and only 11 for "nestling cooperation" or "nestling mutualism." Though I doubt that any serious worker has forgotten that cooperation is also expected among close kin, I suspect that many find conflict a more compelling topic of study than cooperation. Here, to help pull the pendulum back from the pole of conflict among siblings, I will focus on the social benefits that accrue to nestlings during life within a brood.

SIBLING SYMBIOSIS

In fact, the conflict-cooperation duality is a bit too narrow for my present purposes, and in its place I will borrow from the language of population ecology to refer to symbiosis (Wilson 1975), not among different species, but among siblings. There is a trio of potential symbiotic interactions (Table 1): parasitism, where one benefits and another suffers (as in siblicide); mutualism, where both parties benefit (as when two individuals cooperate to resist a predator); and commensalism, where one benefits and the other neither gains nor loses (as when one individual sits in the shade of another).

BROTHERS IN ARMS: THE BENEFITS OF BROODMATES

The term "brothers in arms" made its first appearance in English literature in Philip Massinger's 17th-century play Maid of Honor. Though it refers to the camaraderie of two Knights of Malta taking up arms in military service, "brothers in arms" seems appropriate for describing alliances that form among siblings during life in the nest. Though propinquity can make siblings rivals for food or space, it can also make them (I) close allies in battles with parents over the level of parental investment and (2) partners in producing and distributing thermal resources within the nest. It is these symbiotic interactions that I will focus on below.

I will mention only briefly the now-familiar benefits of indirect fitness derived from the success of siblings (i.e., if siblings survive and thrive, which results in more descendents, because of an individual's actions, an inclusive fitness benefit accrues to the latter; Hamilton 1964), because these are well studied and widely understood. And I will not directly address the intriguing issue of multilevel selection within families, whereby broods potentially become targets of selection, because that topic has been ably addressed by Wilson and Clark (2002).

THE CONCEPT OF THE STRUCTURED FAMILY

My purpose is to examine the forces that push siblings toward increased generosity as opposed to selfishness, and much of what follows, curiously, revolves around parentally imposed competitive asymmetries among contemporary offspring. These occur when parents impose phenotypic handicaps on certain of their progeny and not on others; resulting differences in egg size, hormonal titre, immune-system complement, or birth-hatching asynchrony render some offspring "more equal" than others (Lack 1947, Magrath 1990, Williams 1994, Schwabl et al. 1997, Forbes and Glassey 2000, Royle et al. 2003, Groothuis et al. 2005).