Two coping styles, ‘‘active’’ and ‘‘passive’’, are well defined for some species (see Koolhaas et al., 1999 for a review). Even though the existence of distinct coping styles in pigs is controversial (Jensen et al., 1995), studies on coping abilities have provided insights into piglet behaviour (Hessing et al., 1993; Ruis et al., 2001). Behavioural tests have been used to identify possible behavioural patterns indicative of coping styles. Hessing et al. (1993) introduced the backtest as a general assessment of pigs’ coping styles. According to Benus et al. (1991) passive copers are less social, less aggressive and show longer attack latencies than active copers. Consequently, social challenges, in which piglets are allowed to interact with unfamiliar individuals (D’Eath and Burn, 2002) and social isolation tests (Ruis et al., 2001) have been used as indicators of pigs coping styles. In pigs, the effects of co-mingling litters on coping abilities before weaning are unclear. Thus, the purpose of this study was to determine how co-mingling litters affected piglets’ pre-weaning growth, ear injuries, suckling behaviour and responses to behavioural tests used to measure coping abilities. Thirty sows and their respective litters were housed in standard farrowing crates until day 13 after birth. On day13, the partition between two neighbouring pens was removed for 20 litters allowing piglets to interact (forming 10 co-mingled litters). The remaining 10 control litters were kept in standard farrowing crates throughout the experiment. Three focal piglets from each litter were used for data collection. Focal piglets were weighed and ear injuries recorded on days 2, 4, 9, 12, 15 and 18 after birth. There were no differences in piglets’ weight gain before or after co-mingling. Ear injuries were more abundant in co-mingled litters on day 15 (P < 0.05) but these differences disappeared by day 18. Suckling behaviour was recorded on days 5, 8, 10, 14, 16 and 18 after birth. There were no differences in teat fidelity, suckling frequency and mother fidelity between treatments. Three behavioural tests, social challenge, isolation, and backtest, were performed before and after co-mingling. There were no treatment effects on piglets’ response to the isolation test and backtest. Co-mingled piglets showed longer latency for the first aggressive interaction (P < 0.05), spent more time in proximity to one another (P < 0.05) and performed less single bites (P < 0.05) than control piglets during the social challenge. In addition, the duration and frequency of aggressive interactions (P < 0.05) were lower in co-mingled piglets than control piglets. Co-mingling did not affect the frequency of single head thrusts or oral–nasal contact, but did tend to increase the frequency of escape attempts (P < 0.10). Our results suggest that co-mingling litters during lactation affects piglets’ social behaviour, by primarily decreasing aggressive interactions during social challenges.

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