Behavioural observations We observed our subjects for two hours, twice a day, during five time-slots covering the day-light period: 0700–1000 hours, 1000– 1300 hours, 1300–1530 hours, 1530–1800 hours and 1800– 2100 hours. During each observation session, 10-minute scan- samples recorded the whole group, and each focal horse was observed continuously for 10-minute sampling sessions [34]. Each horse was observed for 10 hours in all. The two observers involved each year (M. B. & M. M. in 2004, M. B. & A. B.R. in 2005) recorded 25% of the observation sessions simultaneously to improve data reliability, which was controlled using Cohen’s [35] kappa coefficient, that was k = 0.95. Group scan-samples recorded the identity of, and distance to, nearest neighbour as spatial proximity is commonly used to estimate affinities between horses [25]. Distances to nearest neighbour were scored by classes of 0.5 Horse Body-Lengths (HBL) from 0.5 to 3.5 HBL and by a class (i.e. far) when distances were greater than 3.5 HBL. ‘‘Close’’ distances to the nearest neighbour corresponded to 0.5 HBL. ‘‘Far’’ distances to the nearest neighbour were scored for all distances exceeding 3.5 HBL from the nearest neighbour. Times spent in each distance class and near different neighbours were calculated in percentages of scan- samples, before analysis. Social interactions were recorded continuously during focal sampling and were expressed as number of occurrences for 10 hours. Social interactions were divided into two categories: (i) aggressive interactions, including head-threat, kick-threat, bite, kick and chase and (ii) positive contacts in the group, including approach, olfactory investigation, mutual groom- ing and head-body contact. References 1. West MJ, King AP, White DJ (2003) The case for developmental ecology. Anim Behav 66: 617–622. 2. Bradshaw GA, Schore AN, Brown JL, Poole JH, Moss CJ (2005) Elephant breakdown. Nature 433: 807. 3. Thorallsdottir AG, Provenza FD, Balph DF (1990) Ability of lambs to learn novel food while observing or participating with social models. Appl Anim Behav Sci 25: 25–33. 4. Voelkl B, Schrauf C, Huber L (2006) Social contact influences the response of infant marmosets towards novel food. Anim Behav 72: 365–372. 5. Seyfarth RM, Cheney DL (1986) Vocal development in vervet monkeys. Anim Behav 34: 1640–1658. 6. McComb K, Moss C, Durant SM, Baker L, Sayialel S (2001) Matriarchs As Repositories of Social Knowledge in African Elephants. Science 292: 491–494. 7. West MJ, King AP, Freeberg TM (1997) Building a social agenda for the study of bird song. In: Snowdon CT, Hausberger M, eds. Social influences on vocal development. Cambridge: Cambridge University Press. pp 157–177. 8. White DJ, King AP, West MJ (2002) Facultative development of courtship and communication in juvenile male cowbirds (Molothrus ater). Behav Ecol 13: 487–496. 9. Kuhl PK (2003) Human speech and birdsong: communication and the social brain. Proc Natl Acad Sci USA 100(17): 9645–9646. 10. Pettit GS, Bates JE, Dodge KA, Meece DW (1999) The impact of after-school peer contact on early adolescent externalizing problems is moderated by parental monitoring, perceived neighborhood safety, and prior adjustment. Child Dev 70: 768–78. 11. White DJ, King AP, Cole A, West MJ (2002) Opening the social gateway: early vocal and social sensitivities in brown-headed cowbirds (Molothrus ater). Ethology 108: 23–37. 12. Hausberger M, Henry L, Cousillas H, Mathelier M, Bourjade M (2006) Social experience, vocal learning and social cognition in the European starling, Sturnus vulgaris. Acta Zool Sin 52(supplement): 618–621. 13. Slotow R, van Dyk G, Poole J, Page B, Klocke A (2000) Older bull elephants control young males. Nature 408: 425–426. 14. Bourjade M, Moulinot M, Henry S, Richard-Yris MA, Hausberger M (2008) Could adults be used to improve social skills of young horses, Equus caballus? Dev Psychobiol 50: 408–417. 15. Bertin A, Hausberger M, Henry L, Richard-Yris MA (2007) Adult and peer influences on starling song development. Dev Psychobiol 49: 362–374. Adult-Young Ratio Effects Statistical analyses Preferential spatial partners were identified within family groups and based on spatial proximity to nearest neighbours. Spatial preferences were not necessarily reciprocal; for instance, although individual A is B’s nearest spatial partner, A’s nearest neighbour could be either B (reciprocity) or C, another individual closer to it than B (non-reciprocity). Individual A’s preferential spatial partners were individuals that were the closest to A more frequently than expected by chance (partitioned chi-square goodness-of-fit test, Siegel & Castellan, 1988). Avoidance of spatial partners was calculated following the same method. As group size varied among families, Kendall partial correlation coefficients (Kendall partial coefficient) analysed our data by keeping group size constant [36]. A one-sample permutation test compared the number of young and adult partners significantly avoided by the young in all groups. All tests were two-tailed, with a significance threshold of 0.05, and performed under StatXact 4.0.1 (Cytel Software Corporation). All means are given 6SE. Acknowledgments We are grateful to the Mava foundation and the Station Biologique de la Tour du Valat for logistic support and to Ma¨ıc Moulinot and Elodie Vigneron for their field and computing work. We also greatly thank Claudia Feh, Bernard Thierry, Marie-Annick Richard-Yris, Raphae¨l Cle´ment and the two anonymous reviewers for their helpful comments as well as Dr Ann Cloarec for correcting English. Author Contributions Conceived and designed the experiments: MB MH. Performed the experiments: MB AdBdR. Analyzed the data: MB AdBdR. Contributed reagents/materials/analysis tools: MB AdBdR MH. Wrote the paper: MB AdBdR MH. 16. Bertin A, Hausberger M, Henry L, Richard-Yris MA (2008) Adult/young ratio influences song acquisition in a female European starling (Sturnus vulgaris). J Comp Psychol; In press. 17. Burchinal MR, Roberts JE, Riggins RJ, Zeisel SA, Neebe E, et al. (2000) Relating quality of center-based child care to early cognitive and language development longitudinally. Child Dev 71: 339–357. 18. NICHD Early Child Care Research Network (1996) Characteristics of Infant Child Care: Factors Contributing to Positive Caregiving. Early Childhood Research Quarterly 11: 269–306. 19. Howes C, Rubenstein J (1985) Determinants of toddlers’ experiences in care: Age of entry and quality of setting. Child Care Quarterly 14: 140–151. 20. Burchinal MR, Campbell FA, Bryant DM, Wasik BA, Ramey CT (1997) Early intervention and mediating processes in cognitive performance of children of low-income African-American families. Child Dev 68: 935–954. 21. Lemasson A, Gautier JP, Hausberger M (2005) A brief note on the effects of the removal of individuals on social behaviour in a captive group of campbell’s monkeys (Cercopithecus campbelli campbelli): a case study. Appl Anim Behav Sci 91: 289–296. 22. Boyd L, Keiper R (2005) Behavioural ecology of feral horses. In: Mills D, McDonnell S, eds. The domestic horse. The evolution, development and management of its behaviour. Cambridge: Cambridge University Press. pp 55–82. 23. Henry S, Hemery D, Richard M-A, Hausberger M (2005) Human-mare relationships and behaviour of foals toward humans. Appl Anim Behav Sci 93: 341–362. 24. Henry S, Briefer S, Richard-Yris M-A, Hausberger M (2007) Are 6-month-old foals sensitive to dam’s influence? Dev Psychobiol 49: 514–521. 25. Feh C (2005) Relationships and communication in socially natural horse herds. In: Mills D, McDonnell S, eds. The domestic horse. The evolution, development and management of its behaviour. Cambridge: Cambridge University Press. pp 83–93. 26. Feist JD (1971) Behavior of feral horses in the Pryor Mountain wild horse range. PhD Thesis, University of Michigan. 130 p. 27. Coussi-Korbel S, Fragaszy DM (1995) On the relation between social dynamics and social learning. Anim Behav 50: 1441–1453. 28. Hausberger M, Richard MA, Henry L, Lepage L, Schmidt I (1995) Song sharing reflects the social organisation in a captive group of European starlings (Sturnus vulgaris). J Comp Psychol 109: 222–241. PLoS ONE | www.plosone.org 4 March 2009 | Volume 4 | Issue 3 | e4888