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Anyone who’s ever owned pet rodents knows just how much they love running in their wheels. But a team of physiologists in Madison, Wisconsin, have taken this a step further. They wondered how generations of house mice would develop if they were selected for their ability to run long distances. After 23 generations, it seems that the mice have adopted the same approach that the Romans used over 2000 years ago for covering long distances in short times: run fast, but little and often. Ted Garland Jr began the experiment over 7 years ago, when his team set up a massive selection program to breed mouse superrunners. The experiment was designed so that there were eight separate mouse populations, four that were selected for running, and four control populations. Each population started off with ten families. At six weeks of age, each mouse was given a cage with an exercise wheel. Then they got a week to run whenever the fancy took them, before they ran the keenest-runner-test that decided whether they got to pass their genes on. Each family sent its best male and female runner to found the next generation. Of course, they weren’t always the best runners in the whole population, in fact ‘some were real duffers’ says Girard, but it was important to make sure that close relatives never mated. After the mice had been selected for ten generations, they were tested to see whether their performance had changed. The mice from these selected populations easily out-strode their unselected rivals. After a total of 23 generations had passed through the banks of running wheels, Girard and Matt McAleer watched over 1000 hours of mouse running footage. They chose each mouse’s top performance for detailed analysis of their running behaviour. Girard says, ‘we all had the impression that the mice were running more intermittently’. In the final analysis Girard found that the experiment hadn’t produced a super breed of muscle bound mice that ran fast for extended periods; the mice had found another way round the problem. They were running at top speed in frequent short bursts. Girard thinks that the mice probably evolved a new running behaviour because they may have maximised their physiological response at an earlier stage of selection. The only way they could continue to improve, once they’d optimised their speed, was to combine their improved physiology with a new style of behaviour. She says ‘I think normal mice could do it too, but these mice want to run more’. Justin Rhodes believes that this is because the mice process dopamine differently, which increases their motivation to get their jogger’s high. This is similar to the neurological bases of Attention Deficit Hyperactivity Disorder (ADHD) in children. Having started off asking a physiological question, Garland’s team discovered that neuropsychology is a big part of the answer. Understanding the neurological behaviour of these marathon-mice could eventually produce improved therapies to relieve the disturbing symptoms of ADHD. Venom With Velocity (p. 4345)