Swimming helps to cut stress levels of farmed mink

Fur-farmed mink would find captivity far less stressful if they were allowed to go swimming now and again. A preference-analysis technique usually applied to humans found that caged mink suffered stress and frustration in the absence of a pool.

Dr Georgina Mason and colleagues from Oxford University in Britain also found that farmed mink still enjoy the same outdoor pursuits as their wild cousins even after 70 generations in captivity. She reports her findings today in the science journal Nature.

The American mink, Mustelavison, is extensively farmed for its pelt in North America, Europe and Scandinavia, with about 30 million pelts produced annually. Most are kept in small, wire cages little bigger than an elongated shoe box, but in the wild they might range over territories several kilometres long.

While the mink are given all the food they can eat and appear fitt and healthy, the Oxford tea wanted an objective assessment. "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," the researchers suggest.

They turned to well established techniques used to assess human consumer preferences. They measured how high a price in time and energy the mink would pay to gain access to various "resources", including a pool, a cage full of interesting toys, unusual objects such as traffic cones and boxes, a plastic tunnel, an empty cage and an empty nest site.

The team made the mink work for their pleasure by applying increasing weights to one-way cage doors, making them put more effort into getting what they really wanted. The animals' unquestioned favourite was the pool and they would go to great lengths to get there, pushing 1.75 kg weights out of the way to go for a dip. The second and third preferences were for access to the new nest and time in the empty cage.

"We found the animals rated the water pool as the most valuable resource; it attracted the greatest total expenditure and had the highest reservation price, greatest consumer surplus measures of utility and the most inelastic demand," the authors stated.

Dr Mason also wanted a separate indicator of animal response and found it in the levels of the "stress" hormone, cortisol. They found stress levels were highest when the mink were denied food or denied access to the pool. The two stress measurements were "indistinguishable". "These results suggest that caging mink on fur farms does cause the animals frustration, mainly because they are prevented from swimming."

This behaviour pattern was found in the mink "despite being bred in captivity for 70 generations, being raised from birth in farm conditions, and being provided with food ad libitum".