But now here is where things get interesting. Obviously some medical research requires mass numbers of animal test subjects to die. Given that monkeys are expensive and their numbers relatively limited, we can't risk using them for initial tests. Which is why other animals are used, they may not be as close as monkeys, but they will weed out most of the dangerous stuff and they are more numerous, such as pigs and rats. Anything that is going to kill a pig or a rat is almost certainly going to be dangerous to humans, so they and some other animals are used to weed out the worst of the worst so that the limited monkeys aren't all killed off. A monkey might seem like the obvious choice, but would a pig?