Effects of communal rearing and group size on breeding rabbits' post-grouping behaviour and its relation to ano-genital distance

Group housing is becoming the standard for many farm animal species, as it is seen as a more welfare friendly way of keeping gregarious animals. Aggression between female breeding rabbits currently obstructs the implementation of group housing for this species. Lack of social experience during the rearing period may be one reason why breeding rabbits can act (excessively) aggressive when grouped as adults. To study this, we either reared breeding rabbits with their same-litter siblings and mother only (“litter-only”) or reared four litters and their mothers together from 18 days of life on (“communal rearing”). The litter-only rabbits were born from individually housed mothers, whereas the communally reared rabbits were born from mothers that were group housed during the last three weeks of gestation. After their first kindling, female rabbits from both rearing strategies were housed in groups of four or eight individuals (at an equal space allowance per doe) to assess rearing and group size effects on post-grouping behaviour. Within both treatments we also measured the ano-genital distance at birth (an indicator of masculinization) and studied its relation to adult agonistic behaviour. Communal rearing and larger groups were expected to decrease agonistic behaviour and wounding, whereas rabbits with a longer ano-genital distance were expected to show more offensive agonistic behaviour. The first two hypotheses were not confirmed. Communally reared and litter-only rabbits did not differ significantly in the frequency of their offensive or defensive agonistic behaviour directly post-grouping, or in the severity of wounds sustained in the first three days after grouping (P > 0.10). Communally reared rabbits sniffed/groomed their pen mates less often than litter-only rabbits (P < 0.05). A tendency for a higher frequency of offensive agonistic behaviour in larger groups was found (P < 0.10) and the decrease in defensive agonistic behaviour over time that was observed for the smaller groups was lacking in the larger groups. The third hypothesis was confirmed: female rabbits with a greater ano-genital distance at birth showed more offensive agonistic behaviour upon grouping as adults (r_s = 0.5, P < 0.05). When this relation was studied within the separate rearing strategies, it was only confirmed within the communal rearing treatment, possibly suggesting that prenatal social stress increased both ano-genital distance and aggression. To conclude, communal rearing and larger groups did not decrease agonistic behaviour in breeding rabbits, whereas ano-genital distance measurements may be useful when selecting less aggressive breeding rabbits (although this method may be specifically applicable to communally reared rabbits).