Artificial insemination (AI) is the reproductive biotechnology most widely used in pig production systems. However, determining the optimal time to inseminate still relies on a subjective evaluation of behavioural and physical signs of the sow to detect the onset of oestrus by technicians. Sexual behaviour of the sows during oestrus is affected by oestrogen concentration and is characterized by a variety of signals, such as vocalizations, riding of pen mates when group-housed, changes in physical activity and/or feed intake decrease (Soede & Kemp, 1997). However, the most important signal is immobilization or 'standing' in response to back pressure from a teaser boar, another gilt