Gestation Period

Neonate Type

Although the length of the gestation period is generally correlated to an animal's body size, it is also determined by the state of development at birth for offspring of that species. Among mammals, there are three different groups that can be defined by the level of gestational development of their offspring at birth, and these groupings have proven (through fossil evidence) to be remarkably consistent throughout evolutionary history. Marsupials, which have the shortest gestation periods of all animals, are considered “superaltri-cial,” with offspring that are very underdeveloped at birth and usually crawl into a pouch of some kind to continue their development before being able to survive detached from their mother's body. A second group of mammals includes insectivores,

carnivores, and rodents. They have somewhat longer gestational periods and give birth to “altri-cial” offspring, which are generally poorly developed and typically must develop in a nest or burrow for some period after birth so that they can continue to grow and learn how to move around independently. Altricial offspring are still underdeveloped at birth, being usually hairless and with their eyes and ears sealed off by membranes. Altricial animals bear multiple offspring in litters and tend to have shorter gestation periods, reflecting the fact that the mother has limited space in her womb for completing development prior to birth. In contrast, a third group of mammals (primates, whales, dolphins, and hoofed mammals) has very long gestational periods and usually gives birth to only one or two well-developed “precocial” offspring that are relatively active soon after birth; they are born with fully developed vision and hearing, and are protected by fur and hair. This type of gestation is most common in animals that live out in the open, exposed to the elements, and are unable to conceal their young in burrows or caves.

Each main mammal group is therefore characterized by the type of neonate (altricial, superaltri-cial, or precocial) that its members produce, and by the length of gestational period typical of that group. Insectivores, tree shrews, carnivores, and many rodents have short gestational periods and bear altricial offspring; hoofed mammals, hyraxes, elephants, cetaceans (whales), pinnipeds (walruses, seals, and sea lions), primates, and hystricomorph rodents (Old World porcupines) bear precocial offspring after a fairly long gestational period. In fact, phylogenetic groupings have more impact on the length of the gestational period than animal body size does; the gestation period for an animal that bears precocial offspring will typically be three times longer than that for an animal of the same body size that bears altricial offspring.

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