With the exception of themonotremes, all mammals bear live young. Unlike othervertebrates, they are the only animals that produce milk for their young through their mammary glands. Parental care of the young is universal among mammals, sometimes for long periods.
Mammals became the dominant land animals after non-birddinosaurs were eliminated. Recent work helped to explain their success:epigenetics seems to have started in early mammals.[4]
Mostmarsupial andeutherian mammals have a reproductive cycle known as theoestrous cycle (U.S: estrous cycle). Females are sexually active in the oestrous stage, when they are 'on heat' for a few days each month. If anovum is not fertilized, theendometrium (uterus lining) is resorbed. Oestrus cycles may occur once or twice a year, or many times a year. Each type of mammals has its own frequency.
Humans andprimates, are quite different. They have amenstrual 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. In this system, eggs are released from the ovaries mostly in the middle of the cycle, away from the menstrual period. Thisovulation 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. Keeping males and females together is related to the long learning period of primates.
Onediagnostic feature of mammals is the lowerjaw. Unlike earlier forms, it is a single bone, thedentary. This is one feature which can be seen in fossils, or at least those which are complete enough to have the lower jaw. Another diagnostic feature is theear ossicles. Mammals have three little bones in their inner ear. These 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 in the skull and limbs, so that it is usually possible to identify and describe a mammal from its skeleton alone, or even a partial skeleton.
Another diagnostic feature is theneocortex of thebrain, which no other vertebrate has. This is involved in the kind of flexiblebehaviour andlearning typical of mammals. Reptiles and birds have much of their behaviour controlled by "inherited behaviour chains", which roughly translates as "instincts". Almost all animals can do some learning, but mammals do far more than other vertebrates. Their behaviour is much more flexible thanlizards, 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,[5] 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).
Theemotions 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. Any kind of relationship with a snake or agecko will be less deep because they can't show emotions as easily (for example). This is because reptiles don't have many of the same ways to show emotions (like smiling, wagging tails, etc) that mammals do, but they can still feel happy, and can recognize owners.
Once, most mammals were nocturnal. Today, many (more than two-thirds) are still nocturnal. It makes a lot of sense when you realise that the daytime was ruled by dinosaurs for so long, Primates (except humans) sleep at night in trees. They are active during the daytime, and some come out onto more open ground.
Baboons (and humans!) are rather special in that they come out onto the grassland in the daytime. Humans are fully daytime animals, and can see colours quite well (nocturnal animals have poor colour vision, but can see in the dusk.).
There are about 50 characters which are typical of mammals: 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, and control of body temperature within limits.
Tooth replacement: two sets, and no continuous replacement. Enamel on the tooth surface. Reptile teeth are all alike (apart from size); mammalteeth follow a set pattern (starting at the front):incisors,canines,premolars andmolars.
Occipital condyles. Two knobs at the base of the skull fit into the topmost neckvertebra; mosttetrapods have only one such knob.
With the exception of the five monotreme species, all other mammals have an outlet for food waste which is separate from theurinogenital outlet. Monotremes, reptiles and birds have a commoncloaca at the rear.
Colour vision is defective or absent in most mammals.Primates are the exception, but we know their colour vision re-evolved from ancestors which lacked it.
In reptiles and birds theblood vessel which carries oxygenated blood from the left ventricle of the heart is on the right side of the body; but in mammals it is on the left side.[6]
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, thedentary.
Theirinner ear has three tiny bones, theossicles: malleus, incus and stapes.
All mammals feed milk to their young, and protect and look after them.
The vast majority of mammal species givebirth to live young, these are theplacental mammals, most of which are classified asEutheria and a small number are classified asmarsupials.[7]
Marsupials are mammals with pouches to carry young in, like thekangaroo.
There are only five species (themonotreme mammals) that layeggs, the duck-billedplatypusOrnithorhynchus, and four species of spiny anteaterEchidna. Themonotremes are confined toAustralia andNew Guinea, and are the sole survivors of an earlier group of mammals.
By number of species, mammals (with 5488 species),[8] are not the most successful vertebrates. Birds, with about 10,000 species have nearly twice as many,[9][10] and reptiles have just as many as birds.[11] Fish have even more species. There are 27,000 species of fish, of which nearly 26,000 are bony fish.[12] However, the word "fish" covers more than one class of animal.
Most zoologists regard mammals as a successful group of animals. One reason is that they are successful in allhabitats on Earth. In the air, in the water, inforests, in the colder regions of the world, and above all ongrasslands, where they are outstandingly successful.
In the air, thebats (Chiroptera) are the mammalian order with the most species. They 'own' the nighttime, since birds are largelydiurnal (daytime) animals. Bats are hugely successful, mostly as nighttime predators of insects.
The terrestrial mammals are fewer in number of species thanlizards, but they are huge in individual numbers, and far more important in the life of the terrestrialbiomes. Their ability to move from place to place and adapt has made them a most effective group. Many mammals live in cold places. These mammals have thick hair orblubber to keep them warm. Others may live in rainforests. On land therodents (rats,mice) are hugely successful, more common in numbers than any other mammals. Large mammals on land have been hunted toextinction in some parts of the world, but the ones which remain are now better protected.
Last, but certainly not least, are theprimates. Their naturalhabitat, with few exceptions, are theforests. Most live in the trees, with hands that grasp, goodcolour vision, andintelligence. In thePliocene period some moved out onto thesavannas asgrassland replaced forests.Mankind is the result of this shift into the savannas.
A somewhat standardized classification system has been adopted by most currentmammalogy classroom textbooks. It is based on living animals. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. 2000.[14]
Because two quite different dates are given in the taxobox, an explanation is needed. Rowe defines mammals as "the taxon originating with the most recent common ancestor of extant (living)Monotremata andTheria". That puts the emphasis heavily on living mammals, where, as Rowe points out, the database of characters is extensive.[2]
Kemp explains the problem with that approach: "If the definition of a mammal is based rigorously upon possession of all the characters of living mammals, then some fossil forms that are extremely mammalian in anatomy... are excluded".[1]
"An altogether different perspective on defining Mammalia is based on traditional palaeobiological practice. An arbitrary decision is made about which characters to select as defining characters... Characters deemed appropriate are those reflecting the... fundamental mammalian biology. The essence of mammalian life is to be found in theirendothermic temperature physiology, greatly enlargedbrain,dentition capable of chewing food, highly agile, energetic locomotion, and so on. The organisms that achieved this grade of overall organisation are deemed to be Mammalia... Around the end of theTriassic period, about 205 mya, a number of fossils are found of very small animals that have [most] of the skeletal characters of modern mammals".[1]
This difference in outlook explains the difference in the two dates given in the taxobox.
1234Kemp T.S. 2005.The origin and evolution of mammals. Oxford University Press, p2.ISBN0-19-850761-5
12Rowe T. 1988. Definition, diagnosis, and origin of Mammalia.Journal of Vertebrate Paleontology8 (3): 241–264.
↑Szalay F.S. 1999. Classification of mammals above the species level: Review.Journal of Vertebrate Paleontology19 (1): 191–195.doi:10.1080/02724634.1999.10011133.JSTOR 4523980.
↑Butler 2009.Journal of Assisted Reproduction and Genetics. p477–486.
↑Burghardt, Gordon M. 2006.The genesis of animal play: testing the limits. MIT Press, p11.ISBN0-262-02543-4
↑Nelson, Joseph S. 2006.Fishes of the World. 4th ed, John Wiley.ISBN9780471756446
↑Clack Jennifer A. 2012.Gaining Ground: the origin and early evolution of tetrapods. 2nd ed, Indiana University Press, Bloomington IN.ISBN978-0-253-35675-8
↑Vaughan, Terry A., James M. Ryan, and Nicholas J. Capzaplewski. 2000.Mammalogy: 4th ed, Saunders.ISBN0-03-025034-X Brooks Cole, 1999.
