In farm animals, salivary cortisol has become a widely used parameter for measuring stress responses. However, only few studies
have dealt with basal levels of concentration of cortisol in pigs and its circadian rhythm. The aim of this study was to examine the
effects of ambient temperature and thermoregulatory behaviour on the circadian rhythm of salivary cortisol levels in fattening pigs.
Subjects were 30 fattening pigs of different weight (60 to 100 kg), kept in six groups in an uninsulated building in pens with partly
slatted floors. Saliva samples were taken every 2 h over periods of 24 h at different ambient temperatures at two times in winter
and four times in summer. Thermoregulatory behaviour was recorded in the same 24-h time periods. The effect of time of day,
body weight, ambient temperature and behaviour on the cortisol level was analysed using a mixed-effects model. Two peaks of
cortisol levels per day were found. This circadian pattern became more pronounced with increasing weight and on days where
thermoregulatory behaviour was shown. Mean cortisol levels per day were affected by weight but not by thermoregulatory
behaviour. From our data, we conclude that long-term variations in cortisol concentration may be influenced by increasing age
and weight more than by the respective experimental situation. In assessing animal welfare, it seems more reliable to consider
the circadian pattern of cortisol concentration instead of only one value per day.