The range of orders (Table 2) is consistent with the wide range of gestation times, ranging from a few days (small rodents and rabbits) to a few months (bats, beavers, sloths, hippopotamus, and primates), a year to a year and one-half (horses, zebras, manatees, and rhinoceros), to 22 months for elephants. The number of neonates per gestation ranges from one (olive colobus monkey, grey gibbon, plains zebra, three-toed sloth, hippopotumus, long-nosed bat), to 14 (long-tailed mouse, guinea pig, desert cottontail, antelope ground squirrel, European beaver, nutria, hoary marmot), to 27 or more for mole rats, which can enlarge their vertebrae to increase space in the abdominal cavity to accommodate their prolificacy.[3,6] Metatherian (marsupial) species have short gestation periods similar to those for small eutherian rodents followed by several months of maturation of neonates in their mother's pouch. A female kangaroo can simultaneously support three offspring in different stages of development a zygote in her uterus, a developing neonate on one nipple in her pouch, and a joey that spends most of its time outside the pouch, but returns periodically for suckling. The female kangaroo can arrest development of the zygote in her uterus while a previously conceived joey is nursing in her pouch. When lactation (and the associated energy demand) diminishes, development of the zygote resumes. She can also produce milk of differing composition to each nipple, one milk suited for the developing neonate and the other for the independent joey who is not yet weaned.
Social structures vary from solitary lives except at mating (wombat, sloth, rhinoceros, manatee) to larger groups usually family groups of a dozen or fewer members (beaver, colobus monkey, guinea pig) or large herds (wild asses, zebra, elephant, hippopotamus). Many species are nocturnal, and some hibernate to survive cold winters. Some species are territorial (lemur, gibbon, fruit bat, tammar wallaby) and others are nonterritorial (manatee, swamp wallaby, elephant). A good example of a highly organized society is the inconspicuous and arguably inelegant mole rat.
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