Social rank governs the effective environment of siblings

Abstract

Siblings within the same family often differ dramatically in phenotype. Some differences are attributable to initial maternal handicaps (birth or hatching asynchrony, differences in egg or neonate size and hormonal or antioxidant titre); but differences among siblings may also arise from differences in the brood-rearing environment that offspring experience. Here, I use a model system-a long-term study of nestlings in an altricial bird-to study how an initial maternal handicap, hatching asynchrony, regulates the effective social environment of siblings in the same family as measured by offspring survival. The interaction of family size and structure generated wide differences in the effective environments of siblings living in the same physical space (a nest), and reared by the same parents, in the same family structure. Social rank was the key component of the unshared environment of contemporary siblings, and was alone sufficient to generate near-maximal differences in offspring performance. Nestlings sitting side-by-side effectively lived in different worlds.

Asymmetric contests at the nest

“…Some asymmetry is beneficial for decreasing violence at the nest and increasing overall weight gain, but too much asymmetry would bring about the loss of the smallest chick (Forbes 2011). Possible mechanisms by which parents could impact asymmetry are laying interval, number of eggs, incubation behavior (when to start, how much time to invest in incubation, egg rolling), nestling feeding intervals, and so on.…”

“…A number of recent studies have further shown that in asymmetric nests the nestlings are less aggressive and achieve higher weight gain (Forbes 1991;Wiebe and Bortolotti 1994;Osorno and Drummond 1995;Amundsen and Slagsvold 1998;Cook et al 2000;Vinuela 2000). Nevertheless, the dynamics of growth rates can vary dramatically between core (early hatched) vs. marginal (later hatched) offspring (Forbes and Glassey 2000) as can survival (Forbes 2011). Besides hatching asynchrony, relative relatedness among nestlings may also influence competition and relative growth rates (Royle et al 1999).…”

Parental preference for investment risk incites family strife

“…Though survival to leave the nest is an obviously incomplete measure of evolutionary fitness, it is unreasonable to argue that it is not a useful proxy. Survival among specific classes of offspring show a near maximal range of variation, from the virtually guaranteed survival of core offspring in small broods, to the near certain death of marginal offspring in large broods (Forbes 2011). Weatherhead and Dufour (2000) have shown that fledging success is robust predictor of subsequent recruitment to the breeding population in red-winged blackbirds.…”

“…Hatching asynchrony creates a brood hierarchy dividing the progeny into castes of advantaged 'core' and disadvantaged 'marginal' offspring (Mock and Forbes 1995;Forbes et al 1997;Forbes 2011). Most often the core brood consists of two or more nestlings that are the same size and age, the rough equivalent of multiple births in humans, and one or more marginal offspring hatch one or more days after the core brood.…”

“…To answer these questions I use a long-term data set gathered on red-winged blackbirds (Agelaius phoeniceus), an altricial songbird that serves as a model system for the study of sibling competition (Forbes 2005). These blackbirds create structured families via hatching asynchrony laying the foundation for an asymmetric sibling rivalry among contemporary siblings (Forbes 2009(Forbes , 2011. Older and larger nestlings frequently outcompete younger nestmates in non-aggressive begging competitions for food delivered by parents (Teather 1992;Forbes et al 1997Forbes et al , 2001Glassey and Forbes 2002b;Forbes 2010Forbes , 2011Hall et al 2010).…”