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2 wheels, 3 years, 200,000 animals is an average of 3.8 per hour.

200,000 triggers of the animal sensors, 12,000 times that the wheel started spinning.

Also: the researchers hypnotize that the slugs ended up in the wheel by accident. Some mice returned multiple times; that's a fairly good indicator that they found some benefit from it (need not be that they liked it; it's fairly easy to make up perfect direct benefits. For example, it might be that fleas jump of a mouse if it runs in a wheel for a while.

2 wheels => 100000 animals/wheel 1000 days => 100 animals/wheel*day