Sex play and behavioural sexualization in the pig.

The patterns of adult copulatory behaviour can be observed in piglets. This sex play is more frequent in young boars than in young sows and reaches a peak during the second neonatal month. The castration of young boars within 5 min after birth (7 animals) eliminated the intense sex play of the second month. Gonadectomy at 30 days (3 animals) did not change mounting frequency during the second month, but reduced it during the third month. Castration at 60 days (7 animals) did not influence later mounting frequency. As adults (6 to 8 months of age), boars castrated at birth or at 30 or 60 days displayed typical female immobilization after an injection of 1 mg of oestradiol benzoate intramuscularly. In those castrated at 120 or 150 days of age (4 animals in each case) the female reaction was observed in only 35% of cases. No female response was observed in boars castrated at 180 days (3 animals). Sexualization of the nervous mechanisms of sexual behaviour in boars develops slowly and progressively during the prepubertal period and is independent of the evolution of sex play.