naturally-formed groups with various adult-young ratios and hypothesized that adult-young ratios would influence aggression rates and social cohesion of young Przewalski horses, Equus f. przewalskii. Young horses live in parental families (including one adult stallion, usually their father, two to five mares including their mother and their one- to three-year old offspring) until sexual maturity (two to three years) [22]. Pioneer studies showed that young domestic horses are sensitive to adult influences (e.g. mothers [23,24], unrelated adults [14]). Here, we show for the first time that adult-young ratios are directly correlated to the expression of aggression and to the social cohesion of young horses in their family group, aspects that affect the social skills of young horses. Results The 12 focal animals were one- and two-year-old Przewalski horses belonging to five families with adult-young ratios (AYR = - number of horses over three years old in the group / number of horses three years old and under) ranging from 0.67 to 1.33. Aggressive and positive interactions and spatial proximity to the nearest neighbour were recorded during direct observations. Spatial relationships were then assessed by calculating the proportion of time spent with each nearest neighbour and the corresponding inter-individual distances, to estimate affinities between individuals [25, see methods]. Aggression rates and spatial relationships varied significantly with adult-young ratios when the factor group size effect was kept constant (see methods). Adult-young ratios were negatively correlated with aggression rates (Kendall partial coefficient: T = 20.54, p,0.02, Fig. 1), which were up to four times higher in groups with the lowest proportions of adults (Na: mean number of aggressive interactions; Na (AYR = 1.33) = 3.2561.65; Na (AYR = 0.67) = 13.6763.71). Adult-young ratios were also negatively correlated to time spent close to young nearest neighbours: the higher the proportion of Adult-Young Ratio Effects young, the longer young remained close to their nearest neighbours, especially young neighbours to the detriment of adults (Fig. 2ab; Kendall partial coefficients: time spent at less than 0.5 horse body-length from nearest neighbour, T = 20.44, p,0.05; time spent with a nearest young neighbour, T = 20.44, p,0.05; time spent at more than 3.5 horse body-length from nearest neighbour, T = 0.66, p,0.01; time spent with a nearest adult neighbour, T = 0.44, p,0.05). We therefore verified that the preferences/avoidances of young for partners were not merely proportional to the availability of partners in their group (see methods for calculations). Analyses revealed that the lower the adult-young ratio, the more young avoided some partners (Kendall partial coefficient: T = 20.53, p,0.02), especially adults: young had an adult as their nearest neighbour consistently less often than expected by chance (in all groups: mean number of young partners avoided = 0.5860.23, mean number of adult partners avoided = 2.4260.26; one-sample permutation test: p = 0.002). Figure 1. Aggression by young in relation to adult-young ratios in their family groups. N: number of aggressive interactions performed by young in 10 hours. Kendall partial coefficient correlation, p,0.02. doi:10.1371/journal.pone.0004888.g001 Figure 2. Spatial relationships of young Przewalski horses with their nearest neighbour in relation to adult-young ratios. a- Time spent with either a young neighbour or an adult neighbour, in relation to the adult-young ratio, b- Time spent close to or far from the nearest neighbour, in relation to the adult-young ratio. Time is expressed in percentage of scan-samples recorded in the field. Black triangles: adults; white circles: young; white squares: far from nearest neighbour (farther than 3.5 Horse Body-Length); stars: close to nearest neighbour (less than 0.5 Horse Body-Length). Categories ‘‘close’’ and ‘‘far’’ are not exclusive alternatives. Kendall partial coefficient correla- tion: * p,0.05, *** p,0.01. doi:10.1371/journal.pone.0004888.g002 PLoS ONE | www.plosone.org 2 March 2009 | Volume 4 | Issue 3 | e4888