With only a few exceptions, each mammal species has a characteristic gestation period that shows remarkably little variation. In comparisons between species, gestation periods tend to increase as body size increases. However, effective comparisons of gestation periods among mammals must take into account a fundamental distinction in the state of the neonate at birth. As a general rule, it is possible to distinguish fairly clearly between mammals that give birth to several poorly developed (altricial) offspring and those that give birth to a few (usually just one) well-
Altricial offspring are largely hairless at birth and their eyes and ears are sealed with membranes. They are relatively helpless at birth and are typically deposited in a nest. By contrast, precocial offspring, which are usually born with a welldeveloped coat of hair and with their eyes and ears open, are typically able to move around quite actively at birth and are rarely kept in a nest. Other things being equal, it is obvious that the gestation period should be relatively longer for precocial offspring than for altricial offspring. In principle, it might be expected there would be a smooth continuum between altricial and precocial offspring. In practice, however, there is a fairly sharp division between them. When the relationship between gestation period and body size is examined for altricial and precocial mammals separately, it is found that there is a wide gap between them.
At any given maternal body size, the gestation period for precocial offspring is about three times as long as that for altricial offspring. Furthermore, each main mammal group (order or suborder) is distinguished by the typical condition of the neonate and the relative length of the gestation period. Most insectivores, tree shrews, carnivores, and many rodents (myomorphs and sciuromorphs) give birth to altricial offspring after a relatively short gestation period, whereas hoofed mammals, hyraxes, elephants, cetaceans, pinnipeds, primates, and hystricomorph rodents give birth to precocial offspring after a relatively long gestation period. This is one of the few reproductive characters for which there is supporting evidence. Pregnant fossil horses from the Eocene and Miocene have consistently been found to have only one fetus, while an Eocene fossil bat has been found with twin fetuses. This shows that the small litter size of horses and bats, at least, have characterized those groups for at least 45 million years.
An inverse relationship between the average number of offspring produced at birth (litter size) and the gestation period is only to be expected. For a given uterus volume, there is clearly a trade-