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Main Characteristics of mammals:

  • Endothermy – maintain high, constant body temperature through their metabolism
  • Pelage – hair or fur made of protein called keratin covering all or part of the body for insulation & camouflage
  • Four chambered heart ( two atria & two ventricles) keep oxygenated & deoxygenated blood from mixing; double circulation

Mammal Heart

  • Mammary glands in females are modified sweat glands that make milk containing sugars, proteins, & fats to nourish young
  • Single jawbone
  • Specialized teeth for biting, cutting, & chewing
  • Highly developed brain (large cerebrum)

  • Diaphragm – muscle below lungs that aids respiration
  • Most are viviparous (live birth)
  • Uterus in females where young develop
  • Placenta lines uterus & provides nutrients and gas & waste exchange for developing young
  • Have sweat glands for cooling & scent glands for attracting mates & marking territories

Mammal Ancestors:

  • Fossil records show mammals arose from group of reptiles called therapsids at the end of the Paleozoic era
  • Therapsids were endotherms with specialized teeth like mammals

Lycaenops: drawing by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox
Early Mammals:

  • First mammalian fossil found in Mesozoic era (hair, single jawbone, specialized teeth, & endothermic)
  • Early mammals were small, shrew like, insect eaters that had large eye sockets making them probably nocturnal
  • When dinosaurs became extinct, new habitats & food supplies opened up for mammals
  • “Age of mammals” occurred during Cenozoic era
  • Oviparous (egg laying) monotremes evolved first


sunset on kangaroo island


  • Viviparous (live birth) marsupials with incomplete uterine development appeared next & then placental mammals 


 title illustration 
Tasmanian DevilArmadillo


Specializations of the mouth & digestive system:

  • Single jawbone
  • Incisors – specialized, chisel like front teeth for biting & chewing
  • Canines – pointed teeth or fangs behind incisors to help grip, puncture, & tear prey
  • Bicuspids – teeth with two points behind the canines used to shear & shred food
  • Molars – flattened back teeth to grind & crush
  • Baleen – thin plates in the roof of the mouth of some whales that strain food from water
  • Microorganisms living in the gut help some mammals digest cellulose from plants
  • Hoofed mammals (cows, sheep, giraffes…) have a four-chambered stomach with bacteria living in the first chamber or rumen
  • Cud – digested food in the rumen that is regurgitated, swallowed, & then chewed again to break down plant cellulose
  • Caecum – stomach chamber in elephants, horses, & rabbits that contains bacteria to digest cellulose

Adaptations for Endothermy:

  • High demand for oxygen
  • Right & left sides of heart separated by septum so oxygenated & deoxygenated blood don’t mix
  • Left side of heart pumps blood to lungs & back (pulmonary circulation)
  • Right side of blood pumps oxygenated blood to body cells (systemic circulation)
  • Diaphragm – sheet of muscle below lungs that moves up & down in chest to change air pressure so gas moves into & out of the lungs
  • Alveoli or air sacs in the lungs are surrounded by capillaries and increase the surface area for the absorption of oxygen
  • Hair or fur and a fat layer insulates and prevents heat loss

Nervous System Adaptations:

  • Largest vertebrate brain
  • Cerebrum surface is folded to increase surface area without increasing volume
  • Cerebrum controls sensory organs, coordinates movement, regulates behavior, & is responsible for memory & learning
  • Have five major senses — vision, hearing, olfaction (smell), touch, & taste
  • Bats, whales, dolphins, porpoises  use echolocation (bouncing off of high frequency sounds) to navigate & find prey

Reproductive Adaptations:

  • Each of the 3 mammal groups — monotremes, marsupials, & placentals— has a unique reproductive pattern
  • Monotreme females lay 1-2 leathery-shelled eggs containing yolk & incubates them with her body heat

  • Young monotremes are small & partially developed at hatching so depend on mother for protection and milk from mammary glands
  • Marsupials have short development period inside of the mother & newborns must crawl to the mother’s pouch or marsupium after birth, attach to a nipple for milk, and finish developing

photograph of kangaroo and her joey
Mother Kangaroo & “Joey”

  • Placentals are the largest group of mammals
  • Gestation (period of development inside mother) is longer in placental mammals
  • Nutrients, wastes, gases exchanged through membrane lining uterus called the placenta
Section 2 Review

Order Monotremata:

  • Oviparous
  • Not completely endothermic (lower body temperature & it fluctuates)
  • Have a cloaca where wastes, eggs, & sperm are emptied
  • Includes duck-billed platypus & spiny anteaters or echidna
 title illustration  title illustration 
  • Live only in Australia & New Zealand
  • Platypus:
    1. Waterproof fur
    2.Webbed feet
    3. Flattened tail for swimming
    4. Flat, sensitive, rubbery muzzle used to root for worms & crayfish
    5. Digs a den in bank of river to lay eggs
    6. Female curls around eggs & incubates them
    7. Newborns lick milk from nippleless mammary glands    
  • Echidnas:
    1. Terrestrial
    2. Coat of protective spines
    3. Long snout to probe ant hills & termite nests
    4. Incubate eggs in a brood pouch on female’s belly

Order Marsupialia:

  • Found in New Guinea, Australia, & the Americas
  • Dominate animal in Australia due to lack of competition from placental mammals
  • Known as pouched animals
  • Pouch called marsupium
  • Viviparous (live birth)
  • Tiny, immature young must crawl to mother’s pouch after birth
  • Young attach to mammary gland nipple to nurse until able to survive outside of pouch
  • Includes opossum, kangaroo, wombat, & koala


Koala and joey. Photograph © Mick Stevic.Dykiel.A.02.01.27.Opossum.jpg
Koala & babyOpossum


Placental Mammals :

  • Young carried in uterus & nourished by placenta
  • Gestation periods (time of development within uterus) varies among species
  • Adapted for life on land in water, and in air
  • Mammal species make up 95 % of all animals
  • At least 18 orders exist

Order Insectivora:

  • Includes moles, hedgehogs, & shrews
  • Small with high metabolic rate
  • Found in North America, Europe, & Asia
  • Have long, pointed noses to grub for insects & worms
  • Teeth adapted to pick up & pierce prey
  • Adapted to live on & under ground, in trees, and in water
  • Shrews feed above ground & have claws to help sweep invertebrates into their mouths
  • Moles live underground, have reduced eyes & no external ears, and have short limbs to dig tunnels




Order Rodentia:

  • Largest mammal order (40% of all species)
  • Found everywhere except Antarctica
  • Includes squirrels, chipmunks, gophers, rats, mice, & porcupines
  • Have two instead of four incisors
  • Teeth continue to grow throughout their life
  • Feed on hard seeds, twigs, roots, & bark
  • Gnawing keeps incisors sharp
  • High reproductive capacity
  • Guinea pig & capybaras are two rodents found in South America


Seh - Porcupine - Photograph


Order Lagomorpha:

  • includes rabbits, hares, & pikas
  • Found worldwide
  • Have a double row of upper incisors & two large front teeth backed up by two smaller teeth
  • Continuous growing teeth
  • Herbivores


Mountain BeaverA wild hare


Order Edentata:

  • Includes anteaters, armadillos, & sloths
  • Found in North, Central, & South America
  • Means “without teeth”
  • Only anteaters are completely toothless
  • Armadillos & sloths have peg-like teeth without enamel
  • Have long sticky tongues & claws on powerful front paws to open ant hills& termite nests
  • Sloths are herbivores
  • Armadillos eat small reptiles, frogs, mollusks, & dead animals


picture of a cute armadillophoto of baby sloth


Order Chiroptera:

  • Only flying mammals
  • Includes bats found everywhere except polar regions
  • Front limb is modified into a wing with a skin membrane stretching from the finger bones to the hind limb
  • Clawed thumb, extending from top edge of wing, is used for walking, climbing, & grasping
  • Most are nocturnal (night active)
  • Use echolocation (emission of high frequency sounds that bounce off objects) to navigate and locate food
  • Have small eyes & large ears
  • Feed mainly on insects
  • Tropical bats don’t use echolocation, but have large eyes & keen sense of smell to find fruit to feed on & nectar

Order Cetacea:

  • Includes whales, dolphins, & porpoises
  • Most inhabit oceans, but some dolphins live in freshwater rivers
  • Have a fish shaped body
  • Forelimbs modified as flippers
  • No hind limbs
  • Broad, flat tails to propel through water
  • Breathe through a blowhole on top of the head
  • Divided into two groups — toothed whales & baleen whales
  • Toothed whales:
    1. Includes beaked, sperm, beluga, & killer whales; narwhals; dolphins; porpoises
    2. Have 1 to more than 100 teeth
    3. Prey on fish, squid, seals, & other whales




  • Baleen whales:
    1. Lack teeth
    2. Includes blue, grey, right & humpbacked whales
    2. Have baleen or thin plates of fingernail like material that hangs from the roof of the mouth
    3. Baleen strain shrimp & other invertebrates from water as food


Blue WhaleHumpbacked Whale


Order Sirenia:

  • Includes manatees & dugongs
  • Large herbivores
  • Inhabit tropical seas, estuaries, & rivers
  • Front limbs modified into flippers
  • No hind limbs
  • Flattened tail for propulsion


Manatee photograph by Greg Geffner


Order Carnivora:

  • Found worldwide
  • Includes cats, dogs, raccoons, bears, hyenas, & otters
  • Meat eaters (carnivores) mainly
  • Many feed on both plants & animals (omnivores)
  • Have long canine teeth & strong jaws
  • Clawed toes for seizing & holding prey
  • Keen sense of sight & smell
  • Long limbs for running fast


raccoon photographHyena Cub, Ngorongoro Crater


Order Pinnipedia:

  • Aquatic carnivores
  • Includes sea lions, seals, & walruses
  • Streamlined bodies adapted for swimming
  • Steer & propel through water using broad, flattened tail
  • Called pinnipeds
  • Return to land to feed & give birth
  • Spend much of their time in cold water
  • Large land carnivores so this helps maintain endothermy
  • Can remain under water for 5 minutes to an hour for some species

Order Artiodactyla:

  • Known as ungulates or hoofed mammals
  • Have an even number of toes
  • Includes deer, elk, bison, moose, sheep, cows, caribou, goats, pigs, & camels
  • Herbivores
  • Have large flat molars for grinding plants
  • Found everywhere except Antarctica
  • Cloven or split hooves
  • Fast runners (used for defense)
  • Have storage chamber called rumen in stomach where bacteria break down cellulose
  • Stored food called cud is chewed again & then swallowed to go through digestive system a second time

Order Perissodactyla:

  • Odd toed ungulates
  • Includes horses, zebras, rhinoceroses, & tapir
  • Most are native to Africa & Asia
  • Tapirs are found in Central & South America
  • Have a large, convoluted caecum or blind sac near the small intestine where bacteria digest cellulose


Malaysian Tapir (<I>Tapirus indicus</I>), Sumatra, Indonesia
Caribou (even-toed)Tapir (odd-toed)


Order Proboscidea:

  • Have a boneless trunk or proboscis
  • Includes the African & Asian elephant
  • Wooly mammoth is an extinct member of this order
  • Largest terrestrial mammal
  • Weigh more than 6 tons
  • Feed on plants up to 18 hours a day
  • Proboscis used to gather leaves from high branches & to suck water without lowering the head
  • Modified incisors called tusks help dig for roots & strip bark
  • Jagged molars up to 30 cm long grind plants
  • Have the longest gestation period (20 months for females & 22 months for males)
  • Females can continue to have calves until they are 70 years old


African Elephant Pictureasian elephant
African ElephantAsian Elephant


Order Primates:

  • Includes 2 main groups — Prosimians & Anthropoids
  • Most are omnivores
  • Have teeth suitable for a varied diet
  • Prosimians include lemurs, tarsiers, & lorises
  • Anthropoids include monkeys, apes, & humans
  • Anthropoids have a larger brain
  • Show more complex behaviors than other animals
  • Highly organized social groups
  • Gorilla is the largest primate
  • Have 2 forward-facing eyes for depth perception
  • Have grasping hands & most with grasping feet
  • Some have a grasping tail for life in trees
  • Live in a variety of habitats


Ring Tailed Lemur PhotographGorilla Photograph


Section 3 Review