A mouse finds the bait before you do. It is curious by nature, drawn to anything new in its territory, so the little block of poison tucked behind the refrigerator is less a threat than an invitation. It nibbles. In a city street a few doors down, a rat does the calculus differently. It circles the same bait for days, sniffs, retreats, comes back, sniffs again. That difference in temperament, it turns out, may be quietly reshaping which rodents live and which ones die in the cities of the...