Figure 1. Reciprocal social interactions.

a | The Noldus PhenoTyper 3000 apparatus containing two unfamiliar juvenile male C57BL/6J (B6) mice engaged in social interaction. b | Nose-to-nose sniffing between two unfamiliar juvenile male B6 mice. A video camera records the 10-minute session. A human observer, uninformed of the treatment condition, scores parameters of social interaction and non-social exploration of the arena using Noldus Observer event-recording software. Social parameters scored include following (one mouse walks closely behind the other, keeping pace) and push–crawl (physical contact includes pushing the snout or head underneath the partner's body, squeezing between the partner and the arena wall or floor, and crawling over or under the partner's body). Non-social parameters include self-grooming (the mouse grooms its face and body regions in a normal sequential pattern) and arena exploration (walking around the arena, sniffing the walls, floor and bedding, and digging in the bedding). Detailed scoring methods are described in Refs 44,81,90,91,111. c | Representative data for reciprocal social interactions in pairs of juvenile males of two high-sociability inbred strains of mice, B6 and FVB/Ant, and a low-sociability strain, BTBR T+tf/J (BTBR). BTBR mice exhibited lower levels of following and push–crawl and higher levels of self-grooming and arena exploration than B6 mice, as previously reported^81,90,111. FVB/Ant exhibited high levels of following and push–crawl similar to B6, low self-grooming similar to B6, and arena exploration similar to BTBR. These data further support the interpretation of a specific social deficit and unusual repetitive behaviour in BTBR mice. n = 12 B6 mice, 16 FVB/Ant mice and 12 BTBR mice. *p < 0.05 compared with B6 mice.