However, adult-young ratios significantly influenced neither spatial partner preference (i.e. number of preferred spatial partners, see methods), nor quantity of positive contacts, nor distribution in the other distance classes (Kendall partial coefficient: p.0.05 in all cases). Discussion This study clearly demonstrates how the ratio of adults to young in social groups of horses correlates with aggression rates and social cohesion. Thus, we showed that when, in a group, adult- young ratios were low, young horses were more aggressive, segregated more from adults and established tighter bonds with other young. This is, to our knowledge, the first study investigating the effects of adult-young ratios on aggression rates and social cohesion. The adult-young ratios observed in this Przewalski horse population are similar to those observed in feral horse populations (e.g. from 0.50 to 1.30 [22,26]). The fact that aggression rates of young and their spatial relationships varied with this ratio means that developmental pathways experienced naturally by young can differ significantly in relation to group composition. These findings highlight the major role adults play in channelling aggressive behaviour (e.g. African elephants [6,13], cowbirds [11], domestic horses [14]), but they also suggest strongly that the influence of adults depends on their relative numbers. This therefore raises the important question of the consequences of early-stage variations of adult-young ratios on the long-term acquisition of social skills as well as on the functioning and the efficiency of groups. Only long- term experimental manipulations of adult-young ratios in groups could fill this gap in our understanding. In addition, we found that young in groups with lower proportions of adults formed closer relationships and more selective bonds with peers than did young in groups with higher proportions of adults, who spent more time with an adult nearest neighbour, although it was farther away. Although the presence of adults sometimes seems to induce closer associations between young (e.g. cowbirds [11], domestic horses [14]), other studies [15,16], including this one, specify that associations between young could depend on the adult-young ratio: low proportions of adults in social groups induce more social segregation between young and adults than do high proportion of adults. These results are of interest for understanding developmental processes because social learning usually mirrors social preferences in groups [27]. Songbirds provide a good example of this process. Young starlings learn more from their preferred associates [28], and adult-young segregation, even in the presence of adults, induces young to neglect and to learn less songs from adult tutors [29,15]. The best adult-young ratio to induce song learning from adults appears to be 1:1 [15,16]. Lower attention paid by young to adults induces deficiencies in their central sensory area similar to those observed in socially-deprived animals, thus social segregation and physical separation from adults can have similar effects [30]. Interestingly, a decrease of selective attention appears spontane- ously in children too: toddlers pay less attention to their mothers during visits by a same-age peer [31]. Adult-child ratios in child-care centres are known to affect the cognitive and language development of young through the quantity and quality of adult-child interactions [17]. When adults are proportionally fewer in relation to the number of children, caregivers tend to interact less with the children and to use more authoritarian and restrictive speech that does not stimulate language development and learning [19,32]. In horses, the exclusive relationship between foal-mare dyads induces foals to learn from their mothers since birth [23], but the Adult-Young Ratio Effects strength of their mothers’ influence decreases as the foals grow older [24]. However, one- and two-year-old horses are susceptible to the influence of unrelated adults [14]. Globally, these findings suggest that partners other than mothers could act as social models for older young, but that the proportion of young present in a group could regulate this influence by impairing either attention paid to adults or accessibility to adult partners [11,15]. In a sense, tighter bonds between young in groups with low proportions of adults could be a factor decreasing attention paid to adults and could probably reduce their influence as regulators of the behaviour of young, in particular their aggressive behaviour. However, at this point more research is needed to explain how adult partners, considered as social stimulations of the develop- ment of young, are shared by young members of a social group and how young horses establish their social preferences. Furthermore, this study raises fundamental questions about the impact of adults on the development of young and opens a new line of research investigating the influence of various factors implied. Thus, adult-young ratios appear to be an important feature of social settings that must be taken into account as a potential modulator of social influence when evaluating develop- mental processes. Furthermore, this is true for child-adult ratios in classes, which is a concern of American [17] or European legislations, but not some countries such as France (Eurydice: http://eacea.ec.europa.eu/portal/page/portal/Eurydice/). This insight therefore has practical implications as it highlights the importance of taking adult-young ratios and their effects on learning and social behaviour into account, both for management of animal groups and potentially child schooling. Materials and Methods Focal animals and housing conditions The studied population of Przewalski horses (Equus ferus przewalskii) lives in a 380 ha enclosure of highland steppe at Le Villaret on the Causse Me´jean (southern France). This population grew from 11 individuals brought from zoos in 1993 and 1994. Groups then formed naturally without human intervention until 2003, when there were 55 individuals. Observations were recorded during two periods (May and June 2004; April and May 2005) and focused on five families of seven to 12 horses with adult-young ratios similar to the ratios previously observed at the study site and in their natural environment [22,26]. We calculated adult-young ratios by dividing the number of horses that were over three years old in the group by the number of horses that were three years old and under, as Przewalski horses are adult when they are three years old [33]. Our focal animals were 12 juveniles from these five families: three one-year-old males, three one-year-old females, three two-year-old males and three two-year-old females (Table 1). Table 1. Focal horses in their family groups. Family number Focal horse Family 1 Family 2 Family 3 Family 4 Family 5 1f5, 1f6, 1m5 (N total = 12) 1f7, 1m11 (N total = 9) 1m6, 2f2, 2f3 (N total = 10) 2f4, 2m4 (N total = 7) 2m12, 2m7 (N total = 7) Each horse was given an individual code indicating its age, sex and individual number. Ex: 1f5 is a one-year-old female, number five. doi:10.1371/journal.pone.0004888.t001 PLoS ONE | www.plosone.org 3 March 2009 | Volume 4 | Issue 3 | e4888