Each year, according to the U.S. Department of Agriculture, roughly 820,800 guinea pigs, dogs, cats, and other animals covered by the Animal Welfare Act are used in research in the United States; of those, about 71,370 are subjected to unalleviated pain. These stats don’t track the millions of mice and rats that are used in lab experiments and excluded from the animal protection law (although the rodents are covered by other federal regulations). Scientists and their institutions say they’re committed to keeping pain or distress to a minimum in lab animals where they can. But how do you know how much pain a mouse or a zebrafish feels? And who decides how much pain is too much?