Most marsupial and eutherian mammals have a reproductive cycle known as the oestrous cycle (U.S: estrous cycle). Females are sexually active only during the oestrous stage, when they are 'on heat' for a few days each month. If an ovum is not fertilized, the endometrium (uterus lining) is resorbed. Oestrus cycles may occur once or twice a year, or many times a year. Each group of mammals has its own frequency.
Humans and primates, are quite different. They have a menstrual cycle. In this case, females are sexually receptive at any time, but only fertile when an ovum is released from an ovary. In this case, the endometrium (if not needed for an fertilised egg) is discarded. The endometrium is shed, and takes with it a certain amount of blood. In this system, eggs are released from the ovaries mostly in the middle of the cycle, away from the menstrual period. This ovulation is 'concealed', meaning, it is not obvious when it occurs. This process, so it is thought, tends to keep the male and female together, which is unusual in mammals with the oestrous cycle.
One diagnostic feature is the lower jaw which, unlike earlier forms, is composed of a single bone, the dentary. This is one feature which can be seen in fossils, or at least those which are complete enough to have the lower jaw. Mammals have three little bones in their inner ear, the ear ossicles. The ear ossicles are bones which were, long ago, part of the lower jaw in early proto-mammals.
There are quite a number of other features, particularly in the skull and limbs, so that it is usually possible identify and describe a mammal from its skeleton alone.
Neocortex and behavior
Another diagnostic feature is the neocortex of the brain, which no other vertebrate has. This is involved in the kind of flexible behavior and learning typical of mammals. Reptiles and birds have much of their behavior controlled by "inherited behavior chains", which roughly translates as "instincts". Almost all animals can do some learning, but mammals do far more than other vertebrates. Their behavior is much more flexible than lizards, for example, and that is made possible by their neocortex.
Other things in the life of mammals seem to be connected with this flexibility and learning. Play is a kind of early learning period in which, according to one theory, mammals develop skills which they will need in life. All mammalian young play, and this is very obvious in the more intelligent mammals (primates, cats).
The emotions of mammals are very noticeable, and rather similar to ours. It is possible, and quite common, for humans to have a friendly relationship with another mammal. It is quite impossible for a human to have any kind of relationship with a snake or a gecko (for example). This is because the reptile simply does not have the same basic emotions as a human.
There are about 50 characters which are typical of mammals, and some of the most important are discussed above. A few more examples will make it clear that mammals are very different from reptiles and birds:
- Sweat glands
- Tooth replacement: two sets, and no continuous replacement. Enamel on the tooth surface. Reptile teeth all alike; mammal teeth follow a pattern (incisors, canines, premolars and molars)
- Occipital condyles. Two knobs at the base of the skull fit into the topmost neck vertebra; most tetrapods, in contrast, have only one such knob
- Outlet for food waste separate from urinogenital outlet. Reptiles and birds have a common cloaca at the rear
- Mammals excrete urea; reptiles and birds excrete uric acid
- Colour vision is defective or absent in most mammals (primates are an exception)
- In reptiles and birds the blood vessel which carries oxygenated blood from the left ventricle of the heart lies on the right side of the body; but in mammals it lies on the left side.
- There are many features of the skeleton that mammals share
- Their neck almost always has seven vertebrae, no matter how long it is.
- Their lower jaw is made of just a single bone on each side, the dentary.
- Their inner ear has three tiny bones, the ossicles: malleus, incus and stapes.
In the language of cladistics, these 50 unique characters are apomorphies which prove that mammals are a clade descended from a common ancestor.