MadSci Network: Evolution

Re: Are elephants the only animals, apart from primates, with mammary gland in

Date: Fri Jul 11 14:00:13 2003
Posted By: Thomas M. Greiner, Associate Professor of Anatomy / Physical Anthropology
Area of science: Evolution
ID: 1056644187.Ev

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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