Mammals!!!NB #159

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Mammal TaxonomyKingdom:

Animalia◦ Phylum: Chordata

Subphylum: Craniata Infraphylum:

Vertebrata Class: Mammalia

Evolution of Mammals Began evolving 320 mya “Age of Mammals” began 70

mya◦ Flourished after the extinction of

many reptiles The early group of mammal

ancestors were part of the “synapsid branch”◦ Large sails- raised body

temperature at night◦ Lacked hair◦ Egg layers (oviparous)◦ Either herbivores or carnivores◦ Large anterior teeth

Gap between these teeth and the posterior teeth allowed the jaw to close

◦ Arched palate- strengthened the upper jaw and allowed air to pass over prey in the mouth

Evolution of Mammals Therapsids

◦ Predators- teeth mostly in the front of the mouth to hold & tear prey

◦ Herbivores- large space separating anterior & posterior teeth Posterior teeth specialized to shred plants

◦ Hindlimbs directly beneath body◦ Changes in ribs allowed for mammalian

breathing methods Over 185 my, mammals evolved as

small (size of domestic cats) and nocturnal◦ Development of hair and endothermy◦ Teeth specialized to facilitate digestion◦ Brain specialized for hearing and

smelling◦ First “true mammals” present in the

Jurassic Period (oldest fossil is 160 myo)◦ Mammals began to flourish 65 mya after

the Cretaceous-Tertiary mass extinction (caused by the meteorite)

Characteristics of Mammals

Mammary glandsHairDiaphragmThree middle-ear

ossiclesHeterodont dentitionSweat glandsSebaceous glands (oil)Scent glandsFour-chambered heartLarge cerebral cortex

Diversity of Mammals5,400 speciesDominant terrestrial

animals on every continent (even extended to the oceans)

Two lineages◦ Prototheria-

monotremes◦ Theria- marsupials &

plancental mammals

External StructureBody covered in two-layered

skin (epidermis & dermis)◦ Used for protection &

temperature regulationHair- unique to mammals

◦ Grows from a follicle in the skin◦ Pelage- coat of hair◦ 2 Types:

Long guard hairs- protection Short underhairs- insulation

◦ Periodically molt old hairs◦ May grow a thicker pelage in

the winter and/or change pelage color

External StructureOther hair functions:

◦ Sense of touch◦ Insulation

Air spaces Arrector pili muscle

Muscle around hair follicle that contracts & allows the hair to stand upright

Amount of air trapped in the pelage increases, which provides more insulation

Also contract in response to threat (fight-or-flight) Allows the animal to look bigger

◦ Camouflage- color due to melanin

◦ Hair reduced in large mammals in hot climates (hippo) and aquatic mammals (whales)

External Structure Claws- used for locomotion &

offensive/defensive behavior◦ May modify to form nails or hooves

Sebaceous glands- secretes oil to waterproof & lubricate the hair & skin

Sudoriferous glands- secretes sweat that aids in cooling

Scent or musk glands- secrete pheromones for defense, recognition, & territory

Mammary glands- present in all mammals, but only functional in females◦ Secrete milk to nourish young◦ Secreted through nipples or teats in most

mammals◦ Monotremes secrete milk from the skin

and into depressions on the belly

Skull & TeethHeterodont- teeth are

specialized for different functionsTeeth are set in sockets of the

jab2 sets of teeth throughout life

◦ 1st- deciduous or milk teeth◦ 2nd- adult/permanent teeth

4 kinds of teeth-◦ Incisors- front teeth, used for

gnawing or nipping◦ Canines- long & conical, used for

catching, killing, and tearing prey◦ Premolars- truncated for chewing◦ Molars- broad surface, used for

chewingDental formula- each mammal

species has a set # of teeth

Dental Formula

Teeth of the upper jaw above teeth of the lower jaw in the order: incisors, canine, premolars, & molars

Nutrition & DigestionDigestive tract

specialized for dietMany different diets

◦ Insectivores- anteaters◦ Omnivores- bears◦ Carnivores- lions◦ Herbivores- horses

Enlarged cecum in the intestines to digest cellulose (thanks to microorganisms)

Circulation, Gas Exchange, & Temperature Regulation

Four chambered heartCirculatory system allows

maternal and fetal blood exchanges across the placenta

Breathe with lungsDiaphragm allow air to

enter the lungsMaintain temperature

with insulating fat layers & cool with longer appendages and/or sweating

Winter Sleep & Hibernation

Winter sleep – burrowing under the snow and become less active (still alert)

Hibernation- hypothalamus of the brain slows the metabolic, heart, & respiratory rates◦ “True hibernators”-

Monotremes, moles, shrews, rodents, & bats

◦ Lose around 1/3-1/2 of body weight during hibernation

◦ Aroused from hibernation occurs by metabolic heating (takes several hours)

Nerves & SensesComplex brainMay communicate

over long distances through olfaction (smelling)

Vision is an important sense◦ Color vision not as

developed as reptiles (except for primates)

Reproduction & Development Internal fertilization

Most mammals are viviparous (give birth to live young)◦ Advantage: allows the mother to remove

ties to a nest◦ Disadvantage: growing a fetus requires a

lot of energy Oviducts are modified into one or two

uteri Reproductive cycles: Most have

definite times during the year to mate◦ Usually determined by the season◦ Estrus cycle- when females are receptive

to males Determined by hormones

◦ Some mammals begin ovulating post copulation

Gestation period- length of time young develop within the female reproductive tract

Altricial young- helpless at birth Precocial young- walk and run shortly

after birth