Eutheria: Life History & Ecology
Birth and early development in Eutheria
Eutherians, like their closest relatives the marsupials, give birth to live young. In eutherians, however, the young are nurtured within the body of the mother by the placenta, which allows nutrients to pass from the blood of the mother almost directly into the blood of the young. The placenta also allows oxygen to reach the developing young, thereby making more energy available than in marsupials.
Marsupial young are born hairless and helpless, and must crawl to a special pouch where they will continue their development. In some groups of eutherian mammals, such as the rodents and some carnivores, this is not very different from the marsupials the young are born hairless and blind, and must be nurtured by their mother for a time before they can begin to live on their own. The primary difference is that the young are not raised in a special pouch, as in marsupials.
The degree of development at birth varies greatly among different eutherian groups. Young ungulates may be able to walk within minutes of being born, while human children may take a couple of years to accomplish this. The length of parental care following birth also varies greatly, from about a month to several years.
Diet and body temperature in Eutheria
The different groups of eutheria differ greatly in their choice of foods. Some, such as the ungulates and elephants are herbivorous, feeding on plants. These may feed on grasses or low herbs (grazing) or on the leaves of shrubs and trees (browsing). Others, such as the Carnivora, are primarily carnivores, feeding on other animals as their primary source of nourishment; some of these carnivores may specialize on particular prey, such as insects or fish. Still other eutherians, such as raccoons and bears, are omnivores, eating both meat and plant material.
There has been a great deal of study in eutherian mammals on the correlation of size and the amount of food required to maintian constant body temperature. The general finding is that as size decreases, metabolic resting rate (the rate at which energy is burned while at rest) increases exponentially. In other words, mice eat more food in proportion to the size of their bodies than elephants do, in order to maintain their bodies. This may become a problem for species which live in temperate or arctic regions where food is scarce for several months each year. This may be solved by collecting food for winter storage, by building up a layer of fat and hibernating, or by seasonal migration to more favorable habitats.