Mammals
Mammals are a group of warm-blooded, air breathing vertebrates. The common mammal has the characteristic of fur, or hair, and a three-boned middle ear; but the most remarkable element of group identity is an advanced brain element known as the neocortex, that functions as a center of complex cognition; no species except mammals have this well defined brain structure, meaning that mammals are smarter than any other class of animal. Mammals span a size range from the three centimeter Bumblebee Bat to the 33 meter long Blue Whale. Feeding habits vary widely among species, including carnivores and insectivores who prey on animals, to frugivores and granivores who eat fruit or seeds. Earliest mammals arose approximately 200 to 130 million years ago. All female mammals possess mammary glands used for producing milk for the young which are used to feed and develop the young.
- Brain size: Brain size tends to vary according to body size.
- Mammary Glans: these gland secrete milk for the offspring to drink. This is where the offspring gets most of its nutrients.
- Monotremes: are the only mammals that can lay eggs.
- Marsupials: can carry their offspring in an pouch.
- Placental: Placental mammals all bear live young, which are nourished before birth in the mother's uterus through a specialized embryonic organ attached to the uterus wall, the placenta. The placenta is derived from the same membranes that surround the embryos in the amniote eggs of reptiles, birds, and monotreme mammals.