Elephants have miles of unbroken savanna to roam inside Uganda's Queen Elizabeth Park, where their numbers total 2,500, a dramatic rise after heavy poaching in the 1980s. Outside the preserve villagers kill elephants that trample and eat crops, though attacks have diminished with the digging of trenches to protect fields from wild trespassers. Elephants have longer pregnancies than almost any other mammal. They carry their calves for 22 months, and cows usually only bear one calf every two to four years. Reproduction rates are not sufficient to sustain population numbers at the current poaching rates.