Sibling Rivalry in the Birds and Bees

Abstract

Families are complex social structures that are forums for often-spectacular cooperation and altruism among close kin. We find myriad examples of selfsacrificial behavior across the full spectrum of nature, from simple social insects where sisters forgo their own reproduction, and sometimes forfeit their lives to aid their close relatives, to mother hippos, bears, and crocodiles that are both fearless and lethal in the protection of their progeny. Nurturing parents and affectionate siblings match our expectations of family harmony, and biologists on the basis of shared genetic interests expect cooperation and altruism among close relatives.Inclusive fitness theory, first proposed by biologist William D. Hamilton (Hamilton, 1964a(Hamilton, , 1964b, provides the conceptual framework for understanding interactions among kin. Close relatives have a high probability of sharing identical copies of genes by recent common ancestry, and promoting the interests of relatives also promotes one's own (shared) genetic interest; the closer the relationship, the higher the probability of sharing identical genes. Thus from a genetic perspective, altruism within the family is easy to understand. But families are also venues for extremes of violence and selfishness, and Robert Trivers (1974), building upon Hamilton's work, showed how parents and offspring might disagree over how resources within the family should be shared, defining the concept of parent-offspring conflict (Trivers, 1974)

Why Offspring in Nonhuman Families Differ

“…Bet-hedging strategies involve sacrificing arithmetic mean fitness to reduce variance in fitness ( Cohen, 1966 ; Donaldson-Matasci, Lachmann, and Bergstrom, 2008 ; Seger and Brockmann, 1987 ). By creating a diversified offspring portfolio, parents may reduce variance in fitness and increase geometric mean fitness ( Crean and Marshall, 2009 ; Forbes, 2009 , 2010b ). Laaksonen (2004) , for example, suggests that hatching asynchrony in birds – a maternal effect – may represent an offspring diversification strategy that yields bet-hedging benefits by inducing phenotypic variation among progeny.…”

“…This has been linked to recurrent spontaneous abortion following a first-born son ( Nielsen, 2011 ), and reduced birth weight of later-born sons ( Nielsen et al, 2008 ). Such birth order effects could serve to modulate the extent of sibling competition via maternal effects, though they could be manifestations of a cryptic sibling rivalry ( Forbes, 2010b ) that uses the mother as a conduit for “sibling effects.” The effect of birth order on traits such as sexual orientation, handedness and birth weight likely represent the tip of a phenotypic iceberg, and the current evidence suggests that they are mediated by the early prenatal environment, when synaptic connections are being established and brain circuits are highly plastic ( Champagne and Curely, 2005 ).…”

“…-507-sibling rivalry (Forbes, 2010b) that uses the mother as a conduit for "sibling effects." The effect of birth order on traits such as sexual orientation, handedness and birth weight likely represent the tip of a phenotypic iceberg, and the current evidence suggests that they are mediated by the early prenatal environment, when synaptic connections are being established and brain circuits are highly plastic (Champagne and Curely, 2005).…”

Transition to siblinghood causes substantial and long-lasting physiological stress reactions in wild bonobos

“…TTS-related stress responses could result from sibling rivalry, nutritional and social weaning from the mother, and /or other changes in the socioecological environment of the older offspring. In contrast to species such as rodents and birds in which sibling rivalry among same-aged litter mates can be severe or even lethal (Forbes, 2010), humans and non-human primates tend to give birth to singletons. Therefore, siblings differ in age and their dependence on mothers for food, transport, and social support, and thus, weaning status.…”

“…In contrast to species such as rodents and birds in which sibling rivalry among same-aged litter mates can be severe or even lethal (Forbes, 2010), humans and non-human primates tend to give birth to singletons. Therefore, siblings differ in age and their dependence on mothers for food, transport, and social support, and thus, weaning status.…”