Giving help to a stranger without expecting anything in return is supposed to be a problem for evolution--if this kind of altruism doesn't help your family and it has no immediate reward, surely it costs the individual animal too much to bother? We know humans do it, but it's thought our behavior is shaped more by culture than biology. It turns out that rats pay it forward too. Researchers have shown that a rat who experiences a random act of kindness is more likely than a rat who hasn't to help a strange rat get food--even when it will get none itself. Science Daily, PLoS.