Motivational and physiological analysis of the causes and consequences of non-nutritive sucking by calves

The issue of behavioural deprivation is central to animal welfare but can be dealt with only once the motivational basis of the behaviour is understood. The occurrence of apparently functionless vacuum behaviours by farm animals has renewed interest in ''Lorenzian'' models of motivation, which describe motivation as being reduced by the performance of behaviour rather than by achievement of the normal functional goals. Non-nutritive sucking by calves of an artificial, dry teat was examined. Most non-nutritive sucking occurred directly after a meal and was elicited by the taste of milk. Increasing the volume of milk drunk did not reduce the amount of non-nutritive sucking, suggesting that the consumption of milk did not immediately reduce the motivation. However, non-nutritive sucking was not totally independent of hunger: non-nutritive sucking was increased when calves skipped a meal, or were placed on a low feed level. Performance of non-nutritive sucking behaviour itself reduced its underlying motivation, but this motivation declined spontaneously within 10 min even if the calves could not suck, and there is no evidence that sucking motivation accumulated across meals. Cross-sucking within groups of calves occurred most often after meals and was reduced when the calves were given a teat to suck. The apparent satiating effect of non-nutritive sucking may be due to its effect on metabolic hormones: levels of insulin and cholecystokinin after the meal were increased when the calves could suck a teat after drinking milk. Non-nutritive sucking by calves displays some but not all of the properties ascribed to ''Lorenzian'' motivational processes: performance of the behaviour is more effective than achievement of functional consequences in reducing the motivation. However, the behaviour is elicited when milk is drunk, and the underlying motivation does not continue to accumulate when the calves cannot suck. Deprivation of sucking behaviour may interfere with digestive processes. (C) 1997 Elsevier Science B.V.