ONLY one drug of every ten successfully tested in laboratory animals ends up working in people. One reason, of course, is that mice are not men. Another, though, might have to do with the fact that whereas human patients are afforded all manner of creature comforts, their animal proxies are not.

Although medical science's favourite critters relish temperatures of a little over 30°C, laboratories routinely keep them at five or ten degrees below that. This is not in order to torture the beasts but, rather, because when kept warm they are unmanageably aggressive. The downside is that they have to eat more than they otherwise would, in order to keep their bodies warm. That changes their physiology. And that in turn alters the way they metabolise drugs, with possibly confusing results.

Joseph Garner, of Stanford University, thinks the answer is to keep the labs cool, but let mice cope with the low temperatures as they do in their natural habitat: not by eating more but by building nests. So far, though, no one has a clear idea of how much nesting material is needed to keep mice happy. Dr Garner and his colleagues therefore decided to find out. They have just reported their results in the Public Library of Science.

Dr Garner and his team let each of their mice, 36 males and as many females from three strains commonly used in trials, roam free in two cages connected by a narrow tube. One cage was kept constant at one of six temperatures between 20°C and 35°C. The other was maintained at 20°C but was stocked with up to ten grams of finely shredded paper, which the mice could use to weave a nest. The idea was to check whether the animals would rather build a nest in the cooler cage or move to the warmer one, possibly tugging nesting material along with them strand by strand.

The researchers found that the rodents' preferences varied slightly between strains, as well as between sexes (with females partial to higher temperatures, possibly because of their thinner protective layer of fat), confirming that there is no single set of conditions in which all mice feel cosy. In general, though, with little nesting material around, the animals laboriously carried strands of paper over to the warmer spot, one or two at a time. But leave at least six grams of paper in the chilly cage, and many mice will prefer instead to brave the cold and build a nest there. That seems a small price to pay for better drug trials.