|MadSci Network: Evolution|
Are elephants and primates the only animals with pectoral mammary glands? Mammary glands differ in location and number. During embryological development, there is a mammary ridge that runs from the armpits (axilla) to the groin. Mammary glands can develop anywhere along this ridge, and occasionally people are born that have accessory nipples associated with this ridge. Primates and elephants are not the only mammals to have mammary glands “oddly” placed along the mammary ridge. Manatees have axillary nipples. Bats have them located along the thoracic wall. The mole rat of Africa has nipples running along the entire ridge, with up to 12 pairs. Location of the nipple seems to be related to posture, anatomy, and the way young animals are carried. Many mammals have abdominal nipples. The abdomen is usually the lowest part of the four-legged animal when that animal stands, and is therefore the most easily accessed area for the infant. Animals that give birth to litters (cats, dogs, pigs, etc) tend to nurse their young while lying down. The nipples on these animals are arranged in parallel rows in the center of the torso – again the most accessible area for the infants when the mother is lying down. Baby elephants have trunks, and so to nurse they have to get their trunk out of the way of their mouth and their mother’s body. For this arrangement, forward placed nipples makes the most sense. Primates carry their infants clutched to the chest, the baby’s head points in the same direction as its mother’s head. For these animals, pectoral nipples work best to facilitate nursing and carrying the infant.
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