Author(s): Georgia J. Mason (corresponding author) ; Jonathan Cooper ; Catherine Clarebrough 
Animals may suffer in captivity if they are strongly motivated to perform activities that their housing does not permit. Here we investigate to what extent these limitations affect caged mink and find that these animals will not only pay high costs to be able to perform a range of natural behaviours, but they will also release the 'stress' hormone cortisol when prevented from indulging in swimming, their favourite activity. Despite arguments that mink housed in fur farms have successfully adapted to captivity, these animals may suffer by being deprived of resources that exist in the wild.
Fur farming is widespread in North America, Scandinavia and Europe, with some 30 million mink pelts being produced annually worldwide. On these farms, American mink (Mustela vison ) are kept in wire-mesh cages (dimensions are typically 0.9 x 0.4 x 0.3 m), with access provided to a single nest box, drinking water and paste-like food. It has been claimed that this causes frustration to the animals , which in the wild would patrol territories 1-4 km long, use several nest sites, and hunt by following scent trails, investigating burrows, and diving and swimming for aquatic prey . Others argue that the excellent health and breeding success of farmed mink are evidence of successful adaptation to captivity [3, 4].
We have objectively investigated this issue of possible deprivation by measuring the costs paid by farm-raised mink to reach resources that will enable them to behave naturally. Because of the key role of pleasure in...