REVIEW

Sibling competition and cooperation in mammals:challenges, developments and prospects

Robyn Hudson & Fritz Trillmich

Received: 24 April 2006 /Revised: 2 May 2007 /Accepted: 3 May 2007 / Published online: 31 May 2007# Springer-Verlag 2007

Abstract Many vertebrates grow up in the company ofsame or different-age siblings, and relations among themcan be expected to significantly influence individual lifehistories and the development of individual morphological,physiological, and behavioral phenotypes. Although studiesin birds still dominate and have stimulated most theoreticalconsiderations, the increasing number of mammalianstudies promises to broaden our understanding of thiscomplex field by enabling interesting comparisons with therather different bird system. It therefore seems timely tobring together recent studies of sibling relations inmammals and to demonstrate what these can offer in theway of fresh insights. In this brief review, intended toaccompany a series of papers on a diverse range ofmammals, we outline the current state of sibling researchin mammals, comparing it to the better studied birds. Mostobviously, in mammals, mother and young are in muchcloser contact during early life than in birds, and siblingscan influence each other’s development as well as themother’s physiology while still in utero. During nursing,mammalian young also encounter a very different feeding

situation to bird siblings. These contrasts should helpstimulate further debate, as well as provide furtheropportunities to study the relative importance of maternalversus sibling effects on individual development. Finally,we discuss the need to balance studies of sibling compe-tition and conflict with a consideration of the benefitsaccruing to individuals from sibling presence and the needfor long-term studies of the influence of early siblingrelations on individual development and life histories.

Keywords Sibling conflict . Sibling cooperation .

Mammals . Parent–offspring conflict

Introduction

Many animals grow up in the company of same ordifferent-age siblings so that relations among them can beexpected to form an important part of their developmentalenvironment. More specifically, sibling relations can beexpected to play a significant role in shaping individualphenotypes, whether morphological, physiological, orbehavioral. In shared developmental environments, compe-tition for limited resources is considered a particularlyimportant mechanism shaping developmental differencesamong siblings (Mock and Parker 1997).

Interest in sibling competition began in earnest with thepuzzling observation of obligate siblicide in severaltaxonomically diverse bird species (Procter 1975; Brownet al. 1977; Gargett 1978; O’Connor 1978; Stinson 1979;Cooper 1980; review in Mock and Parker 1997). Sincethen, the field has broadened considerably, both in therange of species investigated, including humans, and in thescope of questions asked (Mock and Parker 1997; Sulloway1996, 2001; Borgerhoff-Mulder 1998; Stockley and Parker

Behav Ecol Sociobiol (2008) 62:299–307DOI 10.1007/s00265-007-0417-z

Communicated by A. Schulte-Hostedde

This contribution represents the introduction to the special issue“Sibling competition and cooperation in mammals”.

R. Hudson (*)Instituto de Investigaciones Biomédica,Universidad Nacional Autónoma de México,AP 70228,04510 Mexico, Federal District, Mexicoe-mail: rhudson@biomedicas.unam.mx

F. TrillmichDepartment of Animal Behavior, University Bielefeld,PO Box 10 01 31, 33501 Bielefeld, Germanye-mail: fritz.trillmich@uni-bielefeld.de

2002; Conley 2004; Mock 2004; Forbes 2005; Drummond2006). It has also developed into an interdisciplinary enterprisewhere the interests and expertise of behavioral ecologists,sociobiologists, physiologists, psychobiologists, and develop-mental psychologists increasingly overlap. Presently, birdstudies still dominate and have stimulated most theoreticalconsiderations of the problems arising from parent–offspringinteractions and sibling competition (O’Connor 1978; Parkerand Macnair 1979; Godfray 1995; Mock et al. 1998; Brilotand Johnstone 2003; Kölliker 2005).

The increasing number of studies in mammals, however,promises to broaden our understanding of this complexfield by enabling interesting comparisons with the birdsystem where the laying of an egg finishes the directphysiological interaction between parent and offspring.Mammals represent a very different situation in whichmother and young are in much closer contact during earlylife, and where siblings can influence each other’s devel-opment as well as the mother’s physiology while still inutero (O’Gara 1969; vom Saal 1989; Haig 1993). Further-more, the provision of milk allows a physiologically moredirect regulation of supply and demand between a motherand her offspring than is the case for feeding of the youngin birds (Collier 1999; Neville 1999). While suckling,mammalian young interact closely with the mother whoproduces adjusted quantities of nutritionally adapted milk inresponse to offspring stimulation, often provides immuno-logical support through antibodies contained in the milk(Peri and Rothberg 1986; Adamski and Demmer 2000;Zhou et al. 2000), as well as the thermal brood care that issimilarly provided by bird parents. Thus, the very differentfeeding situation encountered by mammalian compared tobird siblings presents an interesting contrast that shouldhelp stimulate further debate on such general questions asthe evolution of competitive and begging strategies. Inaddition, mammalian brood care and the specific influencesof lactation offer further access to the study of the relativeimportance of maternal versus sibling effects on later lifehistory as well as on the development of personality.

The overwhelming importance of lactation in mamma-lian development and thereby the dominating role of themother has led to a strong focus on the interaction betweenmothers and their litters as a whole (Rheingold 1963;Krasnegor and Bridges 1990), with little attention given torelations among siblings themselves. Similarly, studies ofbehavioral ecologists on mammals beginning with Trivers’seminal paper (1974) have put more emphasis on parent–offspring conflict than on sibling competition (but seeStockley and Parker 2002 for a theoretical treatment). Andfinally, considering the system as a closed-loop feedback(Alberts and Gubernick 1990; Stern 1996), psychologisthave traditionally focused on mother–offspring bondingand the role the mother plays in development so that

particularly in studies of humans, mother–offspring studiesdominate and studies of sibling influences on developmentare much rarer (but see Sulloway 1996; 2001; Borgerhoff-Mulder 1998; Conley 2004). Nevertheless, mammalogistshave become increasingly interested in sibling interactionsto the point that it now seems timely to bring togetherrecent studies and to demonstrate what the field can offer inthe way of fresh insights.

This exciting development in mammalian research wasparticularly evident at two recent international meetings, the2004 meeting of the European Animal Behaviour Societiesin Groningen, the Netherlands, and the 2005 InternationalEthological Congress in Budapest, Hungary. Contributionsat these conferences gave rise to the idea to bring together aseries of papers demonstrating the wide taxonomic range ofcurrent mammalian research in this field (Drake et al. 2007:ungulates; Fey and Trillmich 2007: rodents; Bautista et al.2007: lagomorphs; Hofer and East 2007, White 2007,Trillmich and Wolf 2007: carnivores). The resultingcollection also provides interesting contrasts betweenspecies bearing precocial and altricial young as well asbetween monotocous species bearing one young andpolytocous species bearing several young. Furthermore,the series combines examples of field studies and theirpossibilities for insight into the role of variable environ-ments with studies on both wild and domestic species incaptivity, offering the advantages of more controlledenvironments. The broad comparative approach representedby this series stresses the wide variety among mammals, asamong birds, in mechanisms and functions of siblingrelations and their interaction with parent–offspring con-flict, and provides once more a salutary reminder that thereis no "typical" mammal.

In this introduction it is our aim to point out areas ofinvestigation where mammals offer additional, even unique,possibilities for research on sibling relations, and to suggestquestions that so far have not been addressed broadly. Inthis way, it is hoped that these considerations, together withthe accompanying series of studies, will stimulate furtherempirical work and theoretical developments in the field.

Mammals: comparisons and contrasts

Theory about sibling competition began with Trivers’(1974) and O’Connor’s (1978) seminal papers and hassince given rise to a whole host of sibling competitionmodels, which for reasons of space we can only point tohere (Macnair and Parker 1979; Parker and Macnair 1979;Mock and Parker 1997; Wright and Leonard 2002) mostlytailored with birds in mind (for contest competition, seeDrummond 2006; scramble competition, Harper 1986;supply and demand models, Hussel 1988; Parker et al.

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2002; Royle et al. 2004; Kölliker 2005). These models ofcompetition “compete” to a certain extent with models ofhonest signaling (reviewed in Godfray 1995) in parent–offspring interaction, and it is still far from clear whichmodels best predict offspring behavior under the multitudeof circumstances constituting sibling and parent–offspringinteraction (Maynard-Smith and Harper 2003). Further-more, the ecological circumstances favoring differentmodes of sibling competition are still poorly understood,although models have been suggested for particular casesbased on closely related groups (herons, Mock 1985;boobies, Drummond 2001), and a recent review offers anintegrated approach to understanding different systems ofdominance in vertebrate broods and litters (Drummond2006). Models treating more specifically the case ofinterbrood conflict have been developed by Parker (Mockand Parker 1997).

Mammals may offer a fresh view on sibling competitionand parent–offspring conflict as their interactions differfrom those of birds. Siblings in a mammalian litter findthemselves in a very different feeding situation from thatencountered by birds. Jostling for a position at the teat islargely left to the young, and the mother—usually adoptinga specific, largely immobile nursing posture—appears tohave little possibility to favor or rebuff a specific offspringduring a suckling bout. As milk ejection occurs simulta-neously at all teats, the young have to end agonisticinteractions to get milk when it becomes available. Thismay make honest signaling of need before and duringfeeding less relevant than in bird chicks where the parentfeeds from a position that, at least in principle, allows achoice among offspring. Nevertheless, although most of theevidence is anecdotal, a similar situation may exist incarnivores at weaning where there is potential for differen-tial treatment of littermates when parents start to bring foodback to the den (Naidenko et al. 2004). A case is alsopresented by competition among different-age sibs or halfsibs, where the mother may rebuff or favor either theyounger or older offspring depending on her condition andenvironmental circumstances (Trillmich and Wolf 2007).

A further complication for a general interpretation ofmother-young and sibling interaction is presented by thedifference in life history between precocial and altricialspecies. This major life history dichotomy may be reflectedin systematic differences in sibling interactions, as precocialyoung seem to have more options and fewer constraints forcoercing parents than less mobile and sensorily limitedaltricial young. There is presently little information onsibling interactions in precocial birds (but see Kalas 1977;review in Drummond 2006), a lack that may relate to theimpression that since precocial birds largely feed them-selves (except in specialized feeders like oystercatcherswhere food continues to be channeled through parents;

Safriel 1981) they need not compete much for limitedparental resources. However, this need not be so, particu-larly with respect to thermal resources. Small precocialmammals, just as precocial birds, are confronted with highmetabolic costs of thermoregulation and small species canreduce these costs by huddling under the parent (Hill 1992).For the parent, it might be cheaper to provide this heat thanto let the energy-limited offspring pay themselves. Similar-ly, proximity to the parent may be of vital importance in theface of predation.

In mammals, most precocial species are monotocous andwean before arrival of the next young. However, asmentioned above, even in some of these species successiveoffspring may compete (Clutton-Brock et al. 1983; Trillmich1986, 1990; Clutton-Brock 1991; Leippert et al. 2000;Trillmich and Wolf 2007). Furthermore, the precocialcaviomorphs, for example the guinea pig, sometimesproduce large litters, in which competition for maternalresources like milk and warmth may well play a major roleearly in life (Fey and Trillmich 2007), and in both birds andmammals, altricial young may compete among themselvesfor thermally advantageous positions within the litter orbrood huddle (Alberts 1978; Webb 1993; Bautista et al.2007). Lack of a sufficient comparative base, however,precludes broad generalizations at this stage about system-atic differences in the mode and outcome of competitiveinteractions among siblings in altricials and precocials.

Monotocous mammals offer an opportunity to lookexclusively at interbrood competition without interferingintrabrood conflict. Continuing requests for maternal inputby older offspring may interfere strongly with maternaleffort directed to the next (potential) offspring (Fuchs 1982;Clutton-Brock et al. 1983; Trillmich 1986; Clutton-Brock1991; Trillmich and Wolf 2007). In some mammals,including humans, lactation results in temporary suppres-sion of the mother’s fertility (Simpson et al. 1981; Clutton-Brock 1991; Banulis and Schlaff 1999; Stevenson 1999),and in kangaroos, in deferred implantation of the waitingembryo (Tyndale-Biscoe and Renfree 1987), thus influenc-ing spacing of reproductive events for the mother andthereby potentially inducing parent–offspring conflict.

Understandably, studies have concentrated on the usuallymore conspicuous, competitive aspect of sibling interac-tions in broods or litters. However, this seems to be anoversimplification since at the same time that offspringwithin a nest compete for limited resources they alsoprovide an environment that enables normal development.This is perhaps clearest when considering the thermalrequirements of chicks or pups of altricial species, whichcan only maintain an adequate body temperature if theyhuddle together (Hull 1973; Hill and Beaver 1982; Hill1992; Blumberg and Sokoloff 1998; Bautista et al. 2003;Bautista et al. 2007). In these species single offspring

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quickly cool and may die directly from hypothermia orindirectly by no longer being able to stimulate furtherparental care (Stern 1996). Indeed, the thermoregulatoryimportance of brood- or littermates, particularly in earlylife, might also help explain why parents so often undergothe costs of producing doomed, supernumery young(reviewed in Mock and Forbes 1995). Additionally,sufficient stimulation of parents, be it via sucking inmammals or begging in birds, is vital for maintainingparental care and such stimulation can sometimes beachieved only by a group of siblings. Notable in thisrespect are the tricks cuckoo chicks have been proposed toplay after eliminating the host brood; to maintain parentaleffort in feeding the single chick, the parasite simulates thebegging sounds of an entire brood (Kilner et al. 1999).Cowbirds apparently follow the alternative strategy ofallowing host chicks to survive to keep parental feedingrate sufficiently high to allow normal development andgrowth of the parasitic chick (Kilner 2003).

Similar effects also occur in utero. Embryos may have tosend strong signals to allow implantation and to maintain apregnancy, and several fetuses may jointly create asufficient signal in multiple pregnancies (Haig 1993). Thisapplies particularly if mothers tend to abort small litters asin pigs (Polge et al. 1966), and continues when severaloffspring are needed to maintain maternal care later in lifeas shown for bears (Tait 1980; Dahle and Swenson 2003).An interesting variation is provided by egrets, which willabandon unprofitably small broods early in a breedingseason in favor of a larger brood later on. Later in thebreeding season, however, parents support small broods, asit is either a small brood or none at all (Mock and Parker1986). Interesting here is that this may provide a check onthe siblicidal behavior of the nestlings—if they kill a sibearly in the season it may result in parents abandoningthem. Such cases of sibling “cooperation” can perhaps beclassified as byproduct mutualism (Dugatkin 2002) asbenefits accrue to all members of the sibship (see alsoBautista et al. 2007).

Mechanisms

Behavioral mechanisms of sibling competition have beenstudied in considerable detail in a variety of species (Mockand Parker 1997; Drummond 2006), but the physiologicalprocesses associated with such competition have onlyrecently received attention (Fey and Trillmich 2007).

Behavioral

Behavioral mechanisms of sibling competition range fromspectacular aggressive interactions, sometimes leading tosiblicide, through various milder agonistic interactions, to

scramble competition not involving overt aggression butwhich nevertheless can have indirect lethal consequences(Drummond et al. 2000; Bautista et al. 2005; Drummond2006). In some cases these behaviors resemble territoriality,for example, where kittens, piglets and young hyraxesdefend specific teats (Ewer 1959; Fraser and Jones 1975;Hoeck 1977; Drake et al. 2007), as well as possiblenegotiation among siblings in the absence of parents(Roulin et al. 2000). These struggles generally run theircourse without obvious interference by parents (but seeJohnstone 1996; Lougheed and Anderson 1999; Naidenkoet al. 2004; White 2007; Trillmich and Wolf 2007).

One difference between birds and mammals in the wayoffspring interact with their parents is begging. Whether anindividual attempts to make itself more obvious to a parentby giving visual and acoustic displays as in altricial birdchicks, or in attempting to approach a teat may make a bigdifference in the options of the parent to influencedistribution of food among squabbling offspring (White2007; Trillmich and Wolf 2007). A bird parent approachingthe nest can in principle choose from which side toapproach and who to feed a given morsel of food. Incontrast, a mammalian mother such as a mouse, rabbit orguinea pig standing over a litter, or a cat or sow lying downto nurse has little possibility to influence sibling interac-tions (but see Trillmich and Wolf 2007). Thus, at least inrelation to suckling, begging in mammals seems to be agroup effort that induces the mother to take up the nursingposture rather than an individual attempt to obtain fooddirectly—except for the cases of monotocous mammalswhere begging may even take the form of tantrums as inindividual primate young (Simpson et al. 1981; Gomendio1991; Hrdy 1999). Once the mother adopts an appropriateposture, sibling competition can run its course largelywithout maternal interference. It is then only at a stagewhen pups are fed solid food—as in canids—that actualbegging behavior similar to that observed in birds is seen.Thus, food supply by mammalian mothers is largelyregulated in a manner corresponding to the “restauranthypothesis” (Algers 1993; Jensen et al. 1998) whereby theintensity and duration of suckling by the young willdetermine milk yield through a positive feedback onmaternal physiology.

Nevertheless, it remains unclear in what way short- andlong-term need can be communicated by a litter in such asystem if no obvious begging signals are available(Laurien-Kehnen and Trillmich 2003). Here mammalianyoung may have to depend on the feedback systemprovided by the mother’s lactational physiology to achievetheir ends. However, it is not clear to what extentmammalian species in general correspond in their lactationalphysiology to the better studied primate (human), lagomorph(rabbit), rodent (rat) and ungulate pattern of milk flow

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regulation (Fraser 1980; Mena et al. 1990; Cameron 1998;Collier 1999; Neville 1999).

Genetic

In contrast to birds, there is no evidence so far of obligatesiblicide in any mammal, at least postnatally. In pronghornantelope, however, it is reported to occur prenatally(O’Gara 1969), and Forbes (2005) has also argued thathumans meet the statistical definition for parental obligatebrood reduction in twin pregnancies, although whether thisis a result of sibling competition remains unclear. Consis-tent with theory, this may relate to the ability of mammalianmothers, via lactation, to provide a steadier flow ofnutriments to their young (Dall and Boyd 2002) than isoften the case in birds (Dall and Boyd 2004), therebybridging nutritional bottlenecks, which might otherwiseoblige littermates to kill sibs to ensure their own survival.Nevertheless, if mothers are unable to maintain a sufficientmilk supply, competition among littermates may result infacultative siblicide (Hofer and East 2007; Trillmich andWolf 2007). The genetics of parent–offspring and siblinginteractions become highly complex in any case as siblingsform a selective environment for each other as well as fortheir parent(s) and parents through brood care obviouslyrepresent an important component of the offspring’senvironment (Mousseau and Fox 1998). Thus, there isselection going on among all of these players andcoadaptation via linkage disequilibrium is to be expectedin the long run (Kölliker 2005).

Physiological

Best documented are hormonal mechanisms. Notable is thedeposition by mother birds of varying quantities of hormones,for example testosterone, in eggs (Schwabl 1993), the effectsof which on development and sibling interactions arecurrently being keenly investigated (Groothuis et al. 2005).Similar effects have also been shown in mammals where theinteraction between mother and offspring occurs in uterousually for a much longer period and may influenceoffspring phenotype quite strongly (Kaiser and Sachser1998; 2001). Further, mammalian offspring are known toinfluence each others’ development in utero via thedifferential production of steroid hormones, best knownbeing the masculinization of females by adjacent maleembryos (vom Saal 1989; Zielinski et al. 1992). In all thesecases, however, it is an open question whether thephenotypic changes observed are adaptive for the mother,for the offspring, for both or for neither. Although there ismuch speculation, hard data are lacking for mammals as wellas for birds (see discussion in Groothuis et al. 2005).

Sequential siblings or half siblings may also influenceeach others’ development. This is most obvious inmarsupials where a young present in the pouch may delaythe development of further offspring in utero by stimulatingthe production of prolactin, resulting in delayed implanta-tion of the embryo in waiting (Tyndale-Biscoe and Renfree1987; and Fuchs 1982; Eisen and Saxton 1984 for a similarphenomenon in rodents). However, in many other mam-mals, including humans, that produce overlapping litters oroverlapping single young, negative effects on pre- andpostnatal growth and survival of offspring associated withcompetition for the mother’s resources are also well known(reviewed in Hausfater and Hrdy 1984; Martínez-Gómezet al. 2004; see also Trillmich and Wolf 2007).

Furthermore, mammals born in larger litters are generallysmaller than those from smaller litters and grow more slowlysince mothers do not seem to compensate fully withincreased milk yield for the increased demands of a largerlitter (Drummond et al. 2000; Laurien-Kehnen and Trillmich2003). This implies slower development and sometimescatch-up growth after independence from maternal milk.Given that such catch-up growth is suspected to havenegative long-term influences on health and fertility(Metcalfe and Monaghan 2001; Mangle and Munch2005), it is unclear how growing up in a larger litter willinfluence life history. In this context, the physiology-lifehistory nexus described by Ricklefs and Wikelski (2002) isimportant, as is the ‘thrifty phenotype’ hypothesis(reviewed in Bateson et al. 2004), which proposes thatyoung growing up under nutritionally limited conditionsmay be better adapted metabolically to nutritional hardshipin the future than those developing without such con-straints. Given these considerations, it is surprising that solittle investigation has been directed to the effects of litter orbrood size and the concomitant competition among siblingson the stress-associated physiological state of the individ-uals (but see Naguib et al. 2004; Blas et al. 2005; Fey andTrillmich 2007) and the possible short- and long-termconsequences on the immune system, metabolic rate,development of temperament (or personality traits), andlater survival and fertility. Whether such stressors arenecessarily negative or may imply beneficial levels ofactivation remains to be seen.

Perspectives

Increasing information on sibling relations in mammals andthe possibilities this offers for comparison with the better-studied birds raises many unresolved questions and promis-ing research topics, and among them are the following.

– Are there systematic differences in the competitivestrategies used and their outcome between well-armed

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young able to inflict serious injury (hyenas, seals, pigs,this special issue) and less well-armed young (rabbits,guinea pigs, this special issue)? Here, we expectevolutionarily stable strategy modeling to providetheoretical guidance as to when we might expect armedyoung to evolve and how we might expect armament toinfluence the form and outcome of sibling conflicts.

– Why do we sometimes observe maternal intervention(Naidenko et al. 2004; White 2007; Trillmich and Wolf2007, and in humans) as originally predicted by themodel of O’Connor (1978) and not in other cases? Whatare the circumstances—environmental and social—or life-history strategies that make such interferencelikely? Why does such behavior apparently not existamong birds where parents act as passive bystanders(but see Lougheed and Anderson 1999 for blue-footedboobies)? To what extent might parent–offspringinterests be congruent given the potential for parentalretaliation by curbing investment if siblicidal offspringmake the brood less valuable to parents (Forbes 1993)?

– What are the benefits of having (same or different age)siblings? Despite the obvious potential for kin selectionamong siblings sharing a common nursery this aspecthas been somewhat neglected in the literature in favorof the consideration of costly competitive and outrightaggressive interactions (but see Kilner 2003; Bautista etal. 2003). However, from prenatal effects such as theneed to have siblings to produce strong enough signalsto maintain a pregnancy, to postnatal thermoregulatoryefficiency or joint stimulation of parental feedingefforts, through to the benefits of sibling presence inlater reproductive life, for example, by the formation ofreproductively supportive matrilines or coalitions forhunting and territorial defense (Cheney and Seyfarth1983; Caro and Collins 1987; Gilbert et al. 1991;Colmenares 1992; Gomper and Wayne 1996; Hrdy1999), positive effects of sibling presence may be justas important and widespread as the effects of compe-tition. In this regard, there is a need for long-termstudies of the effect of sibling relations extending wellbeyond the period of parental dependence.

– Related to the above, is being bigger and moredominant necessarily better in life history terms ormight siblings who occupied socially different nurseryniches be equally successful in later reproductive life(see Drummond et al. 2003 for a possible example inblue-footed boobies), possibly by pursuing differentbehavioral and physiological strategies (for example,Bateson et al. 2004)?

– There is a need for the analysis of sibling effects notonly among same-age individuals (twins, littermates),but also between successive litters or individual youngof the same mother (interbrood conflicts). In this

situation, the potential for conflict between motherand offspring seems most obvious and may help toprovide examples of real conflict about fitness out-comes between mother and offspring (questioned, forexample by Mock and Forbes 1992) in the sense of theoriginal formulation of Trivers (1974).

– And finally, in relation to our own species, the effort todescribe, classify, and explain individual differences inpersonality (temperament) is one of the principalinterests of human psychology. Traditionally, thecontribution of the early social environment to thiscomplex process has focused on the role of parent–(particularly mother–) child relations, the work ofSigmund Freud being one (in-) famous example.However, interest among psychologists in the role ofsibling relations is growing in recognition of the fact thatin the large families typical of many societies andpresumably of our evolutionary past, contact amongsiblings is at least as intense as that between parents andtheir children (Sulloway 1996; 2001; Conley 2004).

Acknowledgements We would like to thank Tatiana Czeschlickwho enthusiastically took up the idea of producing this special issueand helped it along from the time of its conception, through gestationand to the final version. A. Schulte-Hostedde put a tremendousamount of work into editing this special issue and helped in manyways to see the manuscripts through the sometimes erratic process in afriendly and efficient manner. Robyn Hudson and Fritz Trillmichcontributed equally and have been arranged in alphabetical order.

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