Chapter 22

Mammals: Specialized Teeth, Hair, Endothermy, and Vivparity

Evolutionary Perspective

•  Fossil evidence –  Mammals evolved from reptilian ancestors –  Tertiary period (70 mya) - Age of Mammals

•  Extinction of many reptilian lineages •  Allowed for adaptive radiation of mammals •  First mammals were small

–  10 cm –  Delicate skeleton –  Some herbivores, others hunted –  Good hearing –  May have been nocturnal

What is a mammal?

•  Endothermic and homeothermic – Permits high levels of activity at night – Allows them to stay in one place year

round •  Placenta and mammary glands

– Nourish developing young – Give parental care

•  Specialization of teeth / secondary palate – Allows for diversified diet – Palate allows for breathing while chewing

•  Highly evolved brain – Enlarge neocortex – Allows for memory and fast learning – Elaborate sense organs

Diversity of Mammals

•  Characteristics of modern mammals – Hair – Mammary glands – Specialized teeth – endothermy

•  When was the 1st evidence of hair? •  About 60 mya

•  When is it believed that hair actually appeared?

•  130 mya

•  Why is evidence of mammalian relationships hard to study? – Mammals evolved very rapidly after the

extinction of the dinosaurs

•  Kingdom: Animalia •  Phylum: Chordata •  Class: Mammalia

– Order Insectivora: •  Small, primitive mammals, 3rd largest

mammalian order •  Hedgehogs, moles, shrews

•  Order Chiroptera –  Long, slender bones in arms and hands –  Flight membranes extend from body – Mostly insectivorous –  bats

•  Order Carnivora –  Predatory, highly developed sense of smell – Canines well developed – Dogs, cats, bears, raccoons, minks, sea lions, seals,

walruses, otter

•  Order Perissodactyla – Hoofed, elongate skull – Odd number of toes – Horses, rhino, zebra, tapir

•  Order Artiodactyla – Hoofed, grazing and browsing mammals –  Even number of toes –  Pigs, hippos, camels, deer, giraffes, cattle

•  Order Cetacea – Streamlined, nearly hairless –  Insulated by blubber – Paddlelike flippers – Whales, dolphins, porpoises

•  Order Xenarthra –  Incisors and canines absent –  Prominent toes and claws on forelimb –  Limbs adapted for climbing or digging –  Anteaters, tree sloths, armadillos

•  Order Lagomorpha –  2 pairs of upper incisors, one pair lower –  Teeth never stop growing –  Feed on vegetation – Rabbits, pikas

•  Order Rodentia –  Largest mammalian order – Upper and lower jaws have one pair of ever-growing

incisors –  Squirrels, chipmunks, mice, rats, beavers,

porcupines, woodchucks, lemmings

– Order Primates •  Adapted for agility and tree-dwelling •  Unspecialized teeth •  Grasping digits •  Nails on digits •  Lemurs, tarsiers, monkeys, gibbons, apes,

humans

•  Order Proboscidea –  Long, muscular proboscis (trunk) –  Second incisor on each side of upper jaw modified

into tusks –  African and Indian Elephants

•  Order Sirenia –  Large, aquatic herbivores – Nearly hairless with thick skin –  Flipperlike forelimbs, no hindlimbs – manatees

•  Order Monotremata –  Egg-laying – Duck-bill platypus, spiny anteater, echidna

•  Order Marsupialia –  Viviparous pouched mammals – Opposums, kangaroos, koalas, Tasmanian wolves,

wombats, bandicoots, numbats

Mammal Characteristics

•  Body covered with hair •  Integument with sweat, scent,

sebaceous and mammary glands •  Skull with 2 occipital condyles,

secondary bony palate, middle ear with 3 ossicles, 7 cervical vertebrae, pelvic bones fused

•  Mouth with diphyodont teeth (no permanent), teeth heterodone in most, single enlarged jaw bone

•  Moveable eyelides and fleshy external ears

•  4 limbs adapted for many forms of locomtion

•  Circulatory system with 4 chambered heart

•  Respiratory system with lungs, voice box and muscular diaphragm

•  Excretory system with metanephric kidneys and bladder

•  Brain highly developed

•  Endothermic and homeothermic •  Cloaca present only in monotremes •  Separate sexes •  Internal fertilization •  Young nourished by mammary glands

Tibetan Fox

Mammal Adaptations

•  What is the purpose of: – Endothermy? – Hair? – Glands? – Hibernation? – Teeth?

•  How have these things increased the survival of mammals?

Evolutionary Pressures

•  Mammals naturally found on all continents (except Antarctica) and live in all oceans

•  Adaptive radiation made this possible

•  External Structure and Locomotion – Skin – Consists of epidermal and dermal layers – Protects from mechanical injury, invasion

by microorganisms and sun – Also important in temperature regulation,

sensory perception, excretion, water regulation

– Hair •  Keratinized derivative of the epidermis •  Held in hair follicle •  Pelage (coat of hair) consists of 2 kinds of hair

–  Short, insulating hair –  Longer, outer hairs

•  Hair must be shed (molting) •  Important for sense of touch •  Guard hairs (vibrissae)

–  Thick –  Found around legs, nose, mouth, eyes –  Very sensitive to movement

•  Almost no hair: – Naked mole rats

•  Reduced hair: –  Large mammals in warm climates –  Aquatic mammals

•  Hair color – Depends on amount of melanin in the hair – Most coats are dark above and light underneath

Homework

•  Take Notes on Skin – Page 390-391

•  The skin of mammals – Features

•  Hair •  Variety of epidermal glands •  Stratified, cornified epidermis •  Dermis may be thicker than epidermis

– Epidermis •  Composed of stratified squamous epithelium •  Rapid cell divisions push skin to surface •  Dead cells become keratinized •  Keratinized cells make out outer layer (stratum

corneum) •  Keratin is water-proof

• Dermis – Thickest portion – Contains blood vessels, lymphatic vessels,

nerve endings, hair follicles, small muscles and glands

•  Hypodermis – Loose connective tissue, adipose tissue

and skeletal muscles

•  Sweat pores – Regulates body temperature – Found in humans and horses – Sudoriferous glands (sweat)

•  Produce sweat for cooling •  Can produce pheromones

•  Sebaceous glands (oil) – Connected to hair follicles – Secrete sebum – Keep skin soft – Secrete protective layer from

microorganisms – Can also act as pheromone

•  Hair – Keratin-filled cells from the epidermis – 2 parts

•  Shaft – outside of skin •  Root – holds hair to skin

– Arrector pili muscles controls hair position

•  Nails – Modifications of epidermis – Variety of forms

•  Nail – flat plate of dorsal surface of digit •  Claw •  Hoof •  Horns •  Baleen plates (found in toothless whales)

Homework

•  P. 403 •  Read the summary of chapter 23 •  Write down the points that pertain to

mammals •  Do this on a separate sheet of paper to

be turned in

•  Complete Ch 22 notes outlines

•  Glands – develop from the epidermis of the skin – Sebaceous glands are associated with hair

follicles and lubricate and waterproof the skin

– Sudoriferous (sweat) glands release water secretions for evaporative cooling

– Scent or musk glands located around face, feet or anus, secrete pheromones

– Mammary glands functional in females, produce milk

•  The Skull and Teeth – Jaw articulation distinguishes reptile from

mammal skulls – a single bone articulates the mammalian

lower jaw – mammal secondary palate extends

posteriorly by a fold of skin called the soft palate •  almost completely separates nasal passages

from mouth cavity – mammal teeth are specialized for different

functions. heterodonts

– most mammals have 2 sets of teeth in their life •  1st set called deciduous or milk teeth •  these teeth are lost, permanent teeth replace

them – adult mammals have up to 4 kinds of teeth

•  Incisors most anterior teeth, used for gnawing •  Canines long, stout, conical, used for catching,

killing, tearing •  Premolars next to canines, have 1 or 2 roots,

used for chewing •  Molars have broad chewing surface, 2 to 3

roots

– Zoologists use a dental formula to characterize mammals

– Human Beaver • 

– some mammals have no teeth, example armadillo and giant anteaters

– omnivores feed on a variety of plants and animals •  Posterior teeth flattened for grinding •  Anterior teeth with sharp ripping surface

– mammals that eat plants have flat, grinding posterior teeth and incisors, sometimes have canines

– predatory mammals use canines and incisors for catching, killing and tearing prey

– order Carnivora: 4th upper premolars and 1st lower molar for shearing surface called carnassial apparatus

•  The Vertebral Column and Appendicular Skeleton – vertebral column divided into 5 regions –  first 2 cervical vertebrae: atlas and axis – exceptions: tree sloth has 6 or 9 cervical

vertebrae, manatee has 6

–  trunk divided into thoracic and lumbar regions •  thoracic region contains ribs •  ribs protect the heart and lungs •  Lumbar vertebrae interlock for support, but give

little freedom of movement – appendicular skeleton rotates so

appendages are usually directly beneath the body

–  joints limit movement of appendages – bones of pelvic girdle fused in adult

•  Muscles – skeleton bears the weight of the body – muscle mass concentrated in upper

appendages and girdles – many running mammals have little muscle

in lower leg –  tendons run from muscles high in leg to

cause movement in joints

•  Nutrition and the Digestive System –  feeding habits of mammals reflect the

ecological specializations that have evolved

– carnivores: •  Order Insectivora (shrews) •  Order Chiroptera (bats) •  Order Edentata (anteaters)

•  herbivores – Order Artiodactyla (deer) – Order Perssodactyla (zebras)

– specializations in digestive tracts •  herbivores digest food rich in cellulose •  some have enlarged cecum at junction of large

and small intestine •  cecum is a fermentation pouch where

microorganisms aid in cellulose digestion •  ruminants include sheep, cattle, and deer •  ruminant stomachs modified into 4 chambers

–  1st 3 chambers for storage and fermentation

•  Circulation, Gas Exchange, and Temperature Regulation – eutherian mammals must distribute gases

and nutrients to fetus – maternal and fetal blood are exchanged

across the placenta – nutrients, gases and wastes diffuse

between fetal and maternal blood supplies

– Gas Exchange •  high metabolic rates require efficient gas

exchange •  mammals have a muscular diaphragm that

separates the thoracic and abdominal cavities •  inspiration results from contraction of

diaphragm and expansion of rib cage •  expiration is from elastic recoil of lungs and

relaxation of inspiratory muscles

– Temperature Regulation •  mammals are widely distributed •  nearly all face temperatures that require them

to dissipate excess heat and conserve and generate heat at other times

•  2 categories of heat-producing mechanisms –  Shivering thermogenesis is a muscluar activity – Non-shivering thermogenesis involves heat

production by general cellular metabolism and the metabolism of brown fat

– mammals are insulated by pelage and/or fat deposits

– counter current heat-exchange systems help regulate heat loss from exposed areas

– excess heat is radiated into the air from vessels near the surface of the skin

– evaporative cooling occurs in sweat glands or surfaces during panting

– animals in hot environments can radiate heat from ears(jackrabbits and elephants)

– small mammals avoid heat by burrowing

•  Winter Sleep and Hibernation – Winter sleep is when an mammal becomes

less active, but could easily be woken up •  example: bears and racoons retreat to dens in

the winter – Hibernation is a period of winter inactivity in

which the hypothalamus of the brain slows the metabolic, heart and respiratory rates •  example: Orders monotremata, insectivora,

rodentia and chiroptera have true hibernators

Nervous and Sensory Functions

•  Senses – Well developed sense of touch

•  Receptors associated with hair follicles – Olfaction

•  Locates food, recognize members of same species, avoid predators

– Auditory •  External ear (pinna) developed •  Sensitive to pitch and volume

– Vision •  Color vision poorly developed •  Early mammals may have been nocturnal •  Primates, squirrels and few others have well-

developed color vision

Homework

•  Take notes – Hearing p 423 – Smell p 427 – Vision p 429

– Remember that you are focusing on MAMMALS!!!!!