Mammalian Review
Mammalian - Questions and Answers
The typical features of mammals are: body (more or less) covered with hair; presence of the diaphragm muscle (that separates the thorax from the abdomen); mammary glands that produce milk (in females); enucleated blood red cells; middle ear with three ossicles.
2. What are the three main groups into which mammals are divided?
The three groups into which mammals are divided are: monotremes (or prototherian, e.g., platypus), marsupials (or metatherian, for example, kangaroos) and placental (or eutherian, such as humans).
3. Do all mammals have a placenta?
Mammals of the monotreme group (platypus, echidnas) are oviparous, egg-laying, and they do not have a placenta. Mammals of the marsupial group (kangaroos, koalas, opossums) do not have a placenta either; females of this group give birth to embryonic young that then continue development within the mother’s pouch. Placenta only forms in female placental mammals.
4. What are the main orders of placental mammals? What are some representative species and distinguishing features of each of those orders?
The orders into which placental mammals are divided are the following:
Artiodactyls, mammals with an even number of fingers in claws or paws like, e.g., cows, sheep, giraffes. Carnivorous, predators with canine teeth like dogs, lions, tigers. Cetaceans, aquatic animals without posterior limbs and similar to fishes, like whales and dolphins. Edentates, creatures with rare or absent teeth, like sloths, armadillos, anteaters. Lagomorphs, small-sized mammals having three pairs of continuously growing incisive teeth specialized in gnawing, like rabbits and hares. Perissodactyls, also known as ungulates (hooved), big-sized animals with an odd number of fingers in each paw, e.g., horses and rhinos. Primates, characterized by the big cranium and well-developed brain, like humans and apes. Proboscideans, big-sized animals whose nose and superior lip form the trunk (snout), e.g., elephants. Chiropterans, flying nocturnal mammals (bats). Rodents, animals with two pairs of continuously growing incisive teeth, e.g., mice, rats, castors, squirrels. Sirenians, aquatic mammals of freshwater, deprived of posterior limbs, like dugongs and manatees.
5. How is gas exchange done in mammals?
Mammals breathe through lungs, their respiration is pulmonary.
6. How is circulation characterized in mammals?
Mammals present a closed and complete circulatory system. The heart has four chambers and the arterial blood does not mix with venous blood.
7. What is the type of nitrogen waste that mammals eliminate?
Like chondrichtian fishes and adult amphibians, mammals are ureotelic, i.e., they excrete urea.
8. How do placental mammals reproduce?
Placental mammals reproduce sexually, they have internal fecundation and they are viviparous, i.e., their embryo develops within the mother’s body and from her it gets the nutrients through the placenta.
9. Is fecundation in mammals internal or external?
Fecundation in mammals is internal, with copulation. In the contemporary world human technology is able to promote artificial external fecundation of human gametes and of gametes of other animals.
10. Is the mammalian embryonic development direct or indirect?
In mammals the embryonic development is direct, without larval stage.
11. Are there aquatic and flying mammals?
Cetaceans (whales, dolphins) and sirenians (dugongs, manatees) are aquatic mammals. Chiropterans (bats) are flying mammals.
12. Are the limbs modified into wings of bats and the wings of birds examples of evolutionary analogy or homology? What about whale fins compared to fish fins?
Bat and bird wings have the same function and the same origin (they are modified limbs) so they are analogous and homologous organs. Whale fins are a modification of the posterior limbs while fish fins although having the same function do not come from modified limbs; so they are analogous but not homologous structures.
Evolutionary homology suggests common ancestry while biological analogy relates to the concept of evolutionary convergence, the appearance of similar features in evolutionarily distant species that explore the same type of environment (in the mentioned case, the aquatic habitat).
13. Mammal identity card. How are mammals characterized according to examples of representing beings, basic morphology, skin, respiration, circulation, nitrogen waste, thermal control and types of reproduction?
Examples of representing beings: dogs, cats, horses, giraffes, elephants, apes, humans, bats, whales, dolphins, opossums, kangaroos, platypus. Basic morphology: hair, diaphragm muscle, mammary glands, enucleated red blood cells. Skin: impermeable, hairy. Respiration: pulmonary. Circulation: closed and complete, heart with four chambers. Nitrogen waste: urea. Thermal control: homeothermic. Types of reproduction: sexual, internal fecundation, oviparous monotremes (prototherians), marsupials (metatherians), placental (eutherians).
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