Nipple Enhancers

Marsupials and eutherian mammals typically have an even number of nipples arranged bilaterally, from as few as two to as many as 20. They develop in the embryo, along the 'milk lines'. Most mammals develop multiple nipples along each milk line, with the total number approximating the maximum litter size, and half the total number (i.e. the number on one side) approximating the average litter size for that species. Monotremes, such as the platypus, lack teats; their young drink milk directly from pores in the skin (similar to sweat glands), or by sucking it off of hairs surrounding the pores.
In cetaceans such as whales, the infant cannot form a suction-seal to nurse, due to its mouth structure; the whale's nipple is therefore unlike that of any other mammal. Rather than requiring a sucking action, the discharge of milk is powered by maternal muscles. The calf takes the extended nipple into its mouth, and the mother ejects or expels her milk into the mouth of the calf.
Teats vary in size, location, and structure in different mammalian species. Teats, on occasion, become so plump and filled that milk may escape without suckling. Female goats and ewes have two teats, each with a single mammary gland, located between the hindlegs. Mares have two teats, each with two mammary glands. The teats of the sow can be quite variable in number, from six to thirty, and are located on two parallel lines along the belly. Cows have four teats, each with one mammary gland in the udder. Humans have two teats. Extra teats occur often, and are known as supernumerary teats. They are nonfunctional.
In most eutherian mammals, both males and females have teats. In the male, nipples are often not considered functional with regard to breastfeeding, although male lactation is possible. Notable exceptions to this are rats and horses, neither of which has teats in the male.