feed on grasses and other vegetation form 2 groups: browsers and grazers, such as ungulates (hooved mammals, including horses, deer, antelope, cattle, sheep, and goats) and gnawers, including many rodents as well as rabbits and hares. Canines are absent or reduced in size, whereas molars, are adapted for grinding, are broad and usually high-crowned. Rodents have chisel-sharp incisors that grow throughout life and must be worn away to keep pace with their continual growth use both plants and animals for food. Examples are pigs, raccoons, many rodents, bears, and most primates, including humans. Also eat fruits, berries, and grasses when hard pressed. Foxes, which usually feed on mice, small rodents, and birds, eat frozen apples, beechnuts, and corn when their normal food sources are scarce Monotremes are oviparous mammals, such as the duck-billed platypus, which has one breeding season each year. Embryos develop for 10-12 days in the uterus, where they are nourished by yolk supplies deposited prior to ovulation and by secretions from the mother. A thin, leathery shell is secreted around the embryos before the eggs are laid. Because monotremes have no nipples, the young lap milk secreted onto the belly fur of the mother.
Marsupials are pouched, viviparous mammals that exhibit a second pattern of reproduction. Gestation is brief in marsupials, and therefore all marsupials give birth to tiny young that are effectively still embryos, both anatomically and physiologically. However, early birth is followed by prolonged interval of lactation and parental care.
Eutherians. The reproductive investment is in prolonged gestation, unlike marsupials in which the reproductive investment is in prolonged lactation.