Adult-Young Ratio, a Major Factor Regulating Social Behaviour of Young: A Horse Study Marie Bourjade1,2*, Alice de Boyer des Roches1, Martine Hausberger1 1 Universite´ de Rennes 1, Laboratoire d’Ethologie Animale et Humaine, Centre National de la Recherche Scientifique, Rennes, France, 2 Association Takh pour le cheval de Przewalski, Station Biologique de la Tour du Valat, Arles, France Abstract Background: Adults play an important role in regulating the social behaviour of young individuals. However, a few pioneer studies suggest that, more than the mere presence of adults, their proportions in social groups affect the social development of young. Here, we hypothesized that aggression rates and social cohesion were correlated to adult-young ratios. Our biological model was naturally-formed groups of Przewalski horses, Equus f. przewalskii, varying in composition. Methodology/Principal Findings: We investigated the social interactions and spatial relationships of 12 one- and two-year- old Przewalski horses belonging to five families with adult-young ratios (AYR) ranging from 0.67 to 1.33. We found striking variations of aggression rates and spatial relationships related to the adult-young ratio: the lower this ratio, the more the young were aggressive, the more young and adults segregated and the tighter the young bonded to other young. Conclusion/Significance: This is the first study demonstrating a correlation between adult-young ratios and aggression rates and social cohesion of young individuals in a naturalistic setting. The increase of aggression and the emergence of social segregation in groups with lower proportions of adults could reflect a related decrease of the influence of adults as regulators of the behaviour of young. This social regulation has both theoretical and practical implications for understanding the modalities of the influence of adults during ontogeny and for recommending optimal settings, as for instance, for schooling or animal group management. Citation: Bourjade M, de Boyer des Roches A, Hausberger M (2009) Adult-Young Ratio, a Major Factor Regulating Social Behaviour of Young: A Horse Study. PLoS ONE 4(3): e4888. doi:10.1371/journal.pone.0004888 Editor: Martin Giurfa, Centre de Recherches su la Cognition Animale - Centre National de la Recherche Scientifique and Universite´ Paul Sabatier, France Received November 25, 2008; Accepted February 15, 2009; Published March 18, 2009 Copyright: 2009 Bourjade et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The Association Takh was supported financially by the MAVA foundation but this work did not receive specific funding. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: marie.bourjade@gmail.com Introduction Evidence of the vast role played by human adults and adults of some animal species in regulating the social behaviour of young is now widespread [1,2]. Experienced adults can be essential in providing models for learning behaviour socially (e.g. response of young to novel food: lambs, Ovis aries [3], common marmosets, Callithrix jacchus [4]) and above all, as models for producing and using adequate social signals efficiently [1,5,6]. For example, young brown-headed cowbird males, Molothrus ater, raised without adults were able to produce potent songs, but were unsuccessful when courting females because they failed to direct songs towards females and formed less consortships than males raised with adults [7,8]. The absence of adults also affects language development of human children, Homo s. sapiens, and associates with externalizing problems in human adolescents [9,10]. Such deleterious effects on the development of social behaviour due to the absence of adults have been widely reported in songbirds (e.g. brown-headed cowbirds [8,11], European starlings, Sturnus vulgaris [12]) and, to a lesser extent, in mammals (e.g. African elephants, Loxodonta Africana [13], domestic horses, Equus ferus caballus [14]). Indeed, direct evidence shows that the lack of older elephants in a population triggers the expression of aberrant behaviour and hyper-aggression in young males that, however, can be successfully reduced by re-introducing adult males [2,13]. Likewise, the introduction of adult domestic horses in same age - same sex groups of young appeared to decrease aggression rates and increase positive social interactions, compared to controls without similar introductions [14]. Beyond the mere presence or absence of adults, the adult-young ratio (i.e. the relative proportion of adults in a group) appears to be an important factor for social development, both in animals and in humans. Thus, this ratio plays an essential role in song acquisition in European starlings [15,16] and in human language develop- ment [17]. A high adult-child ratio in child-care centres is one of the strongest predictors of positive care-giving and high cognitive developmental rates of children [18,19]. Children are more responsive to their environment, have higher language acquisition rates and better global communication skills when adult-child ratios are high [20,17]. An anecdotal report of the effects of adult- young ratios on social behaviour is also given by Campbell monkeys, Cercopithecus c. campbelli: the removal of two adult females from a captive group led, among others, to the increase of aggression rates between young cage-mates [21]. However, no studies have systematically investigated the potential link between adult-young ratios and other aspects of social behaviour of young animals, such as aggression rates and social cohesion, especially in natural settings. Here, we observed PLoS ONE | www.plosone.org 1 March 2009 | Volume 4 | Issue 3 | e4888