Addressing the behavioural priorities of captive animals and the development of practical, objective measures of the value of environmental resources is a principal objective of animal welfare research. In theory, consumer demand approaches derived from human microeconomics should provide valid measures of the value of environmental [2]. Price elasticity (the perseverance of resource consumption in the face of rising cost) and income elasticity (the perseverance of resource consumption in the face of lower income) are most commonly suggested by ethologists. In practice, however, a number of empirical and theoretical problems render these measures difficult to interpret in studies with animals, particularly where a cost is imposed on access to the resource but time with the resource is used as a measure of consumption [3]. This may be a relatively straightforward protocol for recording resource use by automatic means, but a number of studies have found that animals can compensate for increased cost of access with longer visit length. Furthermore, direct observation of the test animals' behaviour have shown that resource interaction is more intense having overcome higher costs on access [1]. As a consequence measures based on time with the resource may underestimate resource consumption at higher access costs and demand curves derived from these measures may not be a true reflection of the value of different resources. An alternative approach to demand curves is "reservation price", which is the maximum price individual animals are prepared to pay to gain access to resources. This is not only a simple means of recording resource use, but maximum price paid is also analogous to measures used in human economics to investigate the value of providing additional resources on human welfare [4].

In studies using this approach, farmed mink (Mustela vison) paid higher prices in terms of pushing open weighted doors for access to food and for access to swimming water, than for access to resources such as tunnels, water bowls, pet-toys and empty compartments. These indicate that the mink placed a high value on food (as expected) and also on swimming water, and viewed these as more important than other resources. This was supported by the mink's response to denial of resources, as they showed an increase in urinary cortisol production and persevered with attempts to interact with resources when denied access to food and the swimming water, but not when denied access to other resources.

Figure 1. Mink in water bath on experimental mink farm.

In conclusion, the maximum price paid to use a resource appears to be a valid means of assessing resource value, that is simpler to use and less prone to artefacts created by the animal's abilities to alter rates and bout lengths than measures based on elasticity, and whose findings are supported by independent measures of animal well-being.

References

1. Cooper, J.J.; Mason, G.J. (2000). Increasing costs of access to resources cause re-scheduling of behaviours in American mink (Mustela vison): implications for the assessment of behavioural priorities. Applied Animal Behaviour Science, 66, 135-151.
2. Dawkins, M.S. (1983). Battery hens name their price: consumer demand theory and the measurement of ethological 'needs'. Animal Behaviour, 31, 1195-1205.
3. Mason, G.; McFarland, D.; Garner, J. (1998). A demanding task: assessing the needs of captive animals. Animal Behaviour, 55, 1071-1075.
4. Varian, H.R. (1996). Intermediate Microeconomics. A Modern Approach. Norton, London.

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