istukkalliset

Istukkalliset, also known as "placental mammals," refer to a group of mammals that give birth to relatively undeveloped young, which then continue to develop outside the womb while attached to the mother via the placenta. This reproductive strategy is characteristic of the vast majority of mammalian species, including humans, dogs, cats, and most other placental mammals.

The term "istukkalliset" is primarily used in Finnish, where it directly translates to "placental mammals" or "placenta-bearing mammals." This classification contrasts with monotremes (egg-laying mammals like the platypus) and marsupials (pouched mammals such as kangaroos and koalas), which have different reproductive methods. In placental mammals, the embryo develops within the uterus, receiving nutrients and oxygen through the placenta, a temporary organ that connects the fetus to the mother’s circulatory system.

Istukkalliset exhibit a wide range of adaptations, including varied gestation periods, birth sizes, and parental care behaviors. For example, elephants have long gestation periods and give birth to relatively large, well-developed calves, while rodents like mice have short gestation periods and produce numerous small offspring. This diversity reflects the evolutionary success of placental mammals, which dominate terrestrial ecosystems.

In taxonomy, placental mammals belong to the clade *Eutheria*, a group that includes over 5,400 species. Their evolutionary history traces back to the late Jurassic or early Cretaceous period, with significant diversification occurring during the Cenozoic era. The term "istukkalliset" is rarely used in international scientific literature but remains relevant in Finnish-language biology and zoology contexts.