WEEK 9 HANDOUT FOR VERTEBRATE ZOOLOGY: MAMMALS 3/16/00
(7th ed.): Ch. 31 Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://www.ucmp.berkeley.edu/mammal/mammal.html
(mammal-like reptiles and the mammals)
possible HVCC audio/visual resources:
VT 3379:"Eyewitness: Cat," VT 3376:"Eyewitness: Dog," VT 3375:"Eyewitness: Horse"
(these will be seen in recitations)
VT 1189:"Life on Earth; 18: The Rise of Mammals" (pelycosaurs, platypuses, and 'possums)
Mammals are derived from the synapsid lineage of reptiles, e.g. pelycosaurs and therapsids. Variety in teeth form was one indication of their relatedness to mammals. Although small, nocturnal creatures during the Mesozoic Era (time of the dinosaurs), mammals have become the dominant vertebrate life form during the Cenozoic Era. Today, they range in body size from a 2 gm, 4 cm pygmy shrew to 100 ton, 30 m blue whale.
Major characteristics of mammals include:2
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skin: |
hair; mammary glands; sweat, sebaceous, and scent glands |
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skeleton: |
specialized teeth (heterodont); three middle-ear bones (ossicles) derived from the bones of a reptile's jaw articulation1; one piece mandible; double occipital condyle on skull to articulate with neck |
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circulation and respiration: |
four chamber heart with left aortic arch; non-nucleated, biconcave red blood cells; diaphragm
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brain: |
large, convoluted cerebral cortex. The convolutions (gyri and sulci) increase the surface area for neurons (nerve cells). |
Mammalian skin has epidermal and dermal layers and the unique keratinized derivative of hair seated in epidermally-derived hair follicles. Hair is dead and shed occasionally (e.g., your pet dog). While hair is usually for insulation, it may also be for sense, e.g. the vibrissae or whiskers.2 Some animals are almost hairless and have blubber for insulation (whales). Mammals have claws, nails, or hooves (keratinized digit coverings). Horns have a bony core. Antlers are all bone without a covering of keratin, but unlike horns, are shed every year.
Different types of glands are present in the skin:
1) sebaceous glands are associated with hair follicles
2) eccrine and apocrine glands produce sweat (and apocrine glands also produce odor)
3) scent (musk) glands are involved in sexual identification
4) mammary glands are enlarged sweat glands that produce milk
Mammalian teeth are highly variable or heterodont. Most mammals have two sets of teeth (milk teeth vs. permanent teeth). There are four types of teeth:2
incisors: cutting, gnawing, snipping
canines: flesh-tearing, piercing
premolars: crushing, shearing
molars: crushing, grinding
The types of teeth vary with the animal. A diastema is a gap between the anterior and posterior teeth, allowing for an elongated snout, e.g., in deer.
Mammalian backbones have five different types of vertebrae:
cervical vertebrae: atlas, axis first two, 7 cervical vertebrae total (even in giraffes and whales)
thoracic vertebrae: bears ribs
lumbar vertebrae: support
sacral vertebrae: attachment for the hip bone and hindlimb
caudal vertebrae: tail
The hip bones (ilium, ischium, and pubis) are more definitely fused than in the reptiles.
Appendages (appendicular skeleton) are directly underneath the body, in contrast to the sprawling posture of amphibians and (many extant) reptiles.
Muscles are typically attached to bones by connective tissue: tendon if narrow, aponeurosis if wide. Muscles attach at an origin and an insertion, and produce an action. Facial muscles, attached to skin, are unique to mammals (need for suckling, expression).1,3
Digestive system. Will discuss more next week. Many herbivores (horses and rabbits) have an enlarged cecum at the junction of the large and small intestines as a fermentation pouch. Sheep and cattle are ruminants with specialized four-chamber stomachs that also function as fermentation pouches to digest the cellulose in plants.2
Circulatory system. The mammalian heart has four separate chambers (R, L atria and R, L ventricles) with a leftward aortic arch. In a fetus, the placenta is the site of gas exchange and not the lungs. As a result, there are openings between the atria (foramen ovale) and between the aorta and pulmonary artery (ductus arteriosus) to minimize blood flow through the fetal lungs. Both of these close soon after birth.
Mammalian lungs are large and complex, with many clusters of alveoli. The lungs are expanded by a muscular diaphragm between the thoracic and abdominal cavities. With the recoil of the diaphragm, exhalation shouldn't expend much energy.
Mammals are endotherms, maintaining a high internal temperature by such means as shivering, as necessary. Hibernation or a slowed metabolism is an approach taken by many mammals around here for winter.3
Mammals have a metanephric kidney, as do birds and reptiles. A metanephric kidney means that the kidney has many units or nephrons. A tuft of capillaries or glomerulus is surrounded by a capsule that leads into a tubule where the water is resorbed. Mammals excrete urea in the watery form of urine.
Mammals are very intelligent, with complex behaviors, and often live in packs. Mammals can learn to adapt and change in response to experiences. They may also evolve complicated social structures within species populations. These social groupings are more than just collections of animals, but have certain characteristics:3
a) social hierarchy
b) division of labor
c) communication and recognition of individuals
d) cohesion of group and rejection of strangers
Most female mammals undergo estrous cycles ("being in heat") at which time they are receptive to sex and reproduction. The gestation period after successful intercourse varies from 8 days in some marsupials to 22 months in elephants.2,3
Overall survey of mammals: the platypus and echidna (spiny anteater) are monotremes--possessing a cloaca and laying eggs (Australia and New Zealand).
Marsupials have a very short gestation period and so the young are born relatively undeveloped. The young crawl into a pouch (marsupium) after birth to complete development. Kangaroos, koalas, wombats, and opossums.
The Eutherians (placental mammals) nourish their young in their uterus via a placenta. Major orders of eutherian mammals include: Order Perissodactyla: odd-toed, hoofed animals, (horses, rhinos); Order Artiodactyla: even-toed, hoofed animals (sheep, cattle); Order Rodentia (mice, rats, squirrels); Order Carnivora (cats, dogs); Order Primata (primates).
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Please send comments and questions to: cronewil@hvcc.edu
Copyright 1999, 2000 by Wilson Crone
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This web page last updated on March 14, 2000