Sibling competition and cooperation over parental care

Until the 1960s and 1970s, evolutionary biologistsenvisioned family interactions as harmonious, withparents maximizing the number of surviving off-spring (Lack 1947). However, after the developmentof the theories of kin selection and parent–offspringconﬂict (Hamilton 1964; Trivers 1974), it becameevident that family members might have conﬂict-ing interests concerning the allocation of parentalresources and that such conﬂicts may be partic-ularly violent between siblings. Sibling competi-tion refers to rivalry between siblings over accessto limited parental resources (Box 8.1). The causeof sibling rivalry stems from offspring demand-ing more resources from their parents than par-ents are willing to supply. This mis-match betweensupply and demand of resources is the outcomeof three key life history strategies (Stearns 1992). First, because reproductive activities are costly, par-ents are selected to optimally allocate resourcesbetween the different reproductive events ratherthan to maximize effort at the current attempt.Second, parents face a trade-off between offspringnumber and quality, and hence they usually max-imize their ﬁtness by producing several medium-quality offspring rather than by producing fewerhigher-quality offspring. Third, parents often cre-ate more offspring than they can rear to inde-pendence either because resources become scarcerthan anticipated by the parents or because marginaloffspring are created as an insurance against earlyfailure of the core offspring (Forbes 1991). Thelimitation of resources leads to three forms ofconﬂict between family members: siblings com-pete among each other to share resources (thischapter), offspring are in conﬂict with their par-ents over how much parents should invest in pro-viding resources (Chapter 7), and in species withbiparental care the mother and father are in conﬂictover how much effort each party should assume(Chapter 9). The observation that even closely related indi-viduals compete intensely for resources may seemcounterintuitive at ﬁrst sight. The Arabic proverb ‘Iagainst my brothers, my brothers and I against mycousins and I, my brothers, and my cousins againstthe strangers’ perfectly illustrates that even thoughindividuals often support close relatives, siblingsmay compete when conﬁned in a restricted spacewhere shared resources are limited. There is thusa trade-off between behaving altruistically towardsrelatives to derive indirect genetic beneﬁts and com-peting with them to obtain direct material bene-ﬁts (Mock and Parker 1997; West et al. 2002). Thismakes the study of sibling interactions challeng-ing as such interactions range from cooperation toﬁerce competition (Drummond 2001; Roulin 2002),and hence it can help understand the evolution ofselﬁshness and altruism among close relatives (Box8.1). As emphasized by Mock and Parker (1997),Hamilton’s kin selection theory not only sets theconditions promoting altruism but also speciﬁes theevolutionary limits on selﬁsh behaviour. FollowingHamilton’s rule an allele coding for altruism willspread in a population when the beneﬁt of beingaltruistic multiplied by the coefﬁcient of related-ness between the altruistic donor and its recipientexceeds its costs. Conversely, the inverse Hamil-ton’s rule states that an allele coding for selﬁshnesswill spread if the beneﬁts of being selﬁsh exceed thecosts to the victim multiplied by the coefﬁcient of