Among the mammals, there are three
major variations in reproductive systems. This is the basis for dividing
them into subclasses and infraclasses.
| class: | Mammalia | ||
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| subclass: | Prototheria | Theria | |
| infraclass: | Metatheria | Eutheria | |
Members of the subclass Prototheria lay eggs like most non-mammalian vertebrates. However, they feed their newborn with mammary gland secretions like all other mammals. They lack nipples, but the skin over their mammary glands exude milk for their babies. The Prototheria are also referred to as monotremes, which literally means that they have one opening for excretion and reproduction. This is similar to birds. There are only three surviving rare species groups of Prototheria. These are the Australian platypus and 2 echidna (spiny anteater) species of Australia and New Guinea.
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Platypus (subclass Prototheria) |
Echidna (subclass Prototheria) |
Genome of the Platypus--video clip from Nature.com
(length = 7 mins. 30 secs.)
All other living mammalian species, including humans, are in the subclass Theria. They have in common the fact that they give birth to live young. Therian mammals apparently did not evolve from the Prototheria. The relatively primitive prototherian reproductive system evidently evolved after their evolutionary line separated from the other early mammals.
The oldest infraclass of therian mammals is the Metatheria, or the marsupials. Their young are born very immature and cannot live without further development in the mother's pouch. The word marsupial comes from marsupium, the Latin word for purse. Marsupials include kangaroos, koalas, opossums, and many other similar animals. Most of them are native only to Australia and New Guinea.
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| Kangaroo and koala (infraclass Metatheria) | ||
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| Human fetus in utero |
Most mammal species, including humans, are in the infraclass Eutheria. They are also referred to as placental mammals. Eutherian mothers carry their unborn children within the uterus where they are nourished and protected until an advanced stage is reached. This is made possible by the umbilical cord and placenta which connects the fetus to the uterus wall and enables nutrients and oxygen to get to the offspring as well as provides a means of eliminating waste.
Placental mammals have been extremely successful in out-competing monotremes and marsupials for ecological niches. Placental mammals are found on all continents, in the air, and in the seas. Primates, cats, dogs, bears, hoofed animals, rodents, bats, seals, dolphins, and whales are among the dominant placental mammal groups today. Nearly 94% of all mammal species now are placental mammals.
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| Whale, dolphin, monkey, and zebra (infraclass Eutheria) |
The next tutorial in this series, The Primates, investigates all of the Linnaean classification categories below the infraclass level for humans, apes, monkeys, and some other closely related animals. This will take us from the "order" level down to "species."
NEWS: A team of researchers led by Wesley Warren at Washington University School of Medicine reported their completion of a draft of the platypus genome sequence in the May 8, 2008 issue of the journal Nature. This showed that the platypus has about 18,500 genes (about 2/3 as many as humans) and that 82% are shared with humans, mice, dogs, opossums, and chickens. Other platypus genes show links to reptiles, including those related to egg-laying, vision, and venom production. Adult male platypuses can inject their poison with a spur just above the heel of each hind foot. Apparently, they use this as a weapon against other males during the mating season. Platypuses are also unusual in having sensors in their bills that are used to detect faint electrical fields from their prey when they hunt them under water.
NEWS: The results of a 5 year global project sponsored by the Union for Conservation of Nature to survey all living mammals has been completed. The researchers concluded in October 2008 that one half of the 5487 mammal species are declining in numbers and at least 1/4 are now threatened with extinction due primarily to habitat destruction, hunting by humans, and climate change (Jan Schipper et al., Science 1165115, 2008).