
The number and positioning of complex and simple mammary glands varies widely in different mammals. The nipples and glands can occur anywhere along the two milk lines, two roughly-parallel lines along the ventral aspect of the body. In general most mammals develop mammary glands in pairs along these lines, with a number approximating the number of young typically birthed at a time. The number of nipples varies from 2 (in most primates, including humans, for example,) to 16 found in pigs. The Virginia Opossum has 13, one of the few mammals with an odd number[1][2].
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