Despite being bred in captivity for 70 generations, farm-raised mink still long to swim, according to a report published today in the journal Nature. The findings could influence how animals kept for human use are treated.

Assessing animal welfare is no easy task, especially when it comes to evaluating a creature's subjective experiences. But Georgia J. Mason of the University of Oxford and her colleagues were able to do just that by studying how hard the minks were willing to work in order to gain access to things like toys, an alternative nesting site and a water pool. What the minks prized above all, it turns out, was the pool. Moreover, when the researchers blocked access to the pool and measured the levels of the stress hormone cortisol in the minks' urine 24 hours later, they found a 50 percent increase in cortisol productiona rise indistinguishable from that caused by food deprivation over the same period.

Applying the techniques employed by Mason's group to other captive animals should enable scientists to decide whether depriving them of resources and opportunities to behave instinctively causes frustration and stress, Michael Mendl of the University of Bristol remarks in a commentary accompanying the report. "If so, scientific argument for their provision will be strong and should form the basis for decisions to alter housing conditions, even though such decisions will inevitably be coloured by political, economic and practical decisions."