Using motivation to assess welfare. Are we measuring what we think we're measuring?

Abstract

The measurement of motivation has become an important tool used to assess the resources and experiences that an animal values, and subsequently to aid in determining its welfare. However, the relationship between these measurements and welfare state is not well defined, with conclusions normally being based on the assumption that an animal's welfare is reduced if a 'highly valued' resource is not provided. The aim of the research reported here was to investigate the mechanism/s that drive motivation in a behavioural demand feed test and investigate the effect of energy availability on motivational strength, as well as potentially, to provide a better understanding of the relationship between motivation and welfare state. The experiment undertaken involved investigating the relationship between net energy and motivation for food in a behavioural demand test i.e. is the reason motivation decreases in a feed-based demand test due to the animal reaching a point of energetic balance (energy expended = energy consumed)? Sheep were tested to see how many times in a 23-hour period they would walk a specific distance for a small (5g) food reward and it was hypothesised that they would stop walking (lack motivation) once they had reached a zero energy balance. Eight sheep were trained in a 50m laneway to access a double-sided feeder and gained a food reward with each access event. The distance the sheep walked in the laneway to access this reward was increased progressively on a logarithmic scale (1.5, 6.1, 12.3, 24.8, 50.0, 60.2, 72.5, 87.5, 105.5m) with each test period. Sheep were randomly allocated to one of two treatments (14-hr restriction and an unrestricted control). The results indicate that both control and restricted sheep worked beyond a zero energy balance and the level of prior food availability had no effect on the distance they were willing to walk (p>0.05). Interestingly, the zero balance occurred at approximately the same point as motivation first began to decrease (Pmax). These results indicate that energy input alone does not fully explain changes in motivation for food in a behavioural demand test. Further studies will investigate other possible mechanisms that may contribute to motivation in this specific situation.