GESTATION PERIOD. At 22 months it is the longest of any land mammal. Calves often weigh 250 - 300 pounds when born. There was considerable difference between the birth weights of Jennie and Obert, consistent with the normal variations between males and females. Jennie weighed in at 275lbs while Obert's weight at birth was 340lbs. The babies are weighed on a special scale housed in the baby barn.