takswan mamalia

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Mammalian Specializations

Chapter 21


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Mammalian groups Monotremes

Marsupials

Eutherians

Have varying reproductive modes Egg laying in monotremes

Eutherians have long gestations Marsupials have very short gestation

lengths


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Common reproductive aspects Blastocyts

An embryonic ball of cells that forms the embryo

All mammals grow from this blastocyst Trophoblast

An embryonic tissue of mammals specialized forimplanting the the embryo onto the uterine wall

(in Therians), obtaining nutrients from the mother,and secreting hormones to signal the state ofpregnancy to the mother


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Common reproductive aspects Endometrium

Glandular uterine epithelium of the

mammals that secrete materials thatnourish the embryo in uterus

Presence of corpus luteum Formed by the ruptures follicles after

releasing egg

Secretes hormones that sustain earlystages of pregnancy


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Monotreme Reproduction Primitive reproductive tract

2 oviducts remain separate, do not fuse

during development except at the basewhere they join with urethra to formurogenital sinus (Fig 21.4 a)

Oviducts swell to form uterus that retains

the fertilized egg Fertilization occurs in the anterior portion

of the oviduct (fallopian tube)


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Monotreme Reproduction Ovaries larger in compares to Therians

Monotremes provide embryo with moreyolk

Produce smaller eggs at ovulation

Eggs retained in uterus & nourished bymaternal secretions, increase in sizeafter which the shell is secreted.


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Monotreme Reproduction Egg shell is leathery

1-2 eggs laid at hatching

Hatching is rapid (7-10 days) In platypus only left oviduct is functional and

hatching is ~ 12 days

Lay eggs in burrows, but echidnas lay eggs in

a ventral pouch Young hatch as embryos, and brooding has

to continue for about 16 weeks (fig. 21.1)


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Reproduction in Therians All have placentation: 2 types

Choriovitelline placenta

Placentas developed from the yolk sac seen in allTherian animals during early development

Chorioallatontoic placenta Developed from the chorionic & allantoic extra-

embryonic membranes Grows out & takes over from the CV placenta

Typical trait of all eutherians

Most marsupials have only one CV placenta, butsome show a transitory CA placenta at end ofgestation


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Reproduction in Therians Embryonic diapause

Maintaining eggs in a state of arrested

development before implantation as in Kangaroos; Carnivores; Rodents; Bats

Enables mating and birth of young to occur

at optimal times of the year


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Reproduction in Therians Male reproductive anatomy

Monotremes retain testes in abdomen

In therians, testes descend into scrotum

Descent is genetically controlled inmarsupials and hormonally controlled in

eutherians Scrotum in front of penis in marsupials and

behind in most eutherians.


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Reproduction of Eutherians Ureters enter into the bladder rather than the

cloaca

Oviducts fuse anterior to the urogenital sinusto form a uterus

All have a single midline vaginam but only afew have a single midline uterus as seen in

humans Some have a bipartite uterus for some or all

of its length. Bipartite uterus is abnormal inhumans


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Reproduction of Eutherians Urogenital sinus and alimentary canal

have separate openings

Space between them is perineum(space between anus and vagina)

In primates the urogenital sinus

separates into distinct vaginal andurethral openings


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Reproduction of Eutherians Corpus luteum is maintained for a larger period than

one estrus cycle

Allows for larger gestation lengths Some young are altricial (rodents & insectivorous)

Other young are precocial (most ungulates)

All young require lactation for transfer of essential

antibodies Almost all ungulates bear one precocial young

Parturition and lactation are hormonal


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Reproduction of Marsupials Females

Female oviducts do not join on midline becauseureters pass medial to reproductive ducts to enterbladder

2 separate uteri

2 vaginae, Lateral one for sperm passage only

Pseduovaginal canal for parturition Corpus luteum is not maintained

Young ejected at end of estrus cycle


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Reproduction of Marsupials Young ones are neonates

Well developed limbs, jaws, secondary palate,

large lungs, tongue and facial muscles Climb up the pouch and attach to the nipples

Some ejected directly into the pouch or mammaryarea of pouchless animals

Pouch absent in some: mice & Opposums Lactation continues after young ones detach from

the pouch


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Feeding specializations:

Dentition Incisors to seize food

Canines to stab prey

Premolars to pierce and crash food

Molars: to break down food into fineparticles

Therians have tribosphenic molars


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Dentition Canines

Lost in herbivores or modified

Tusks of pigs and walruses: modifiedcanines

Upper canines larger in male primates

Male horses have small functionlesscanines

Maybe used in male fighting and display


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Dentition Incisors

Tusks of elephants= modified incisors

Enlarged in gnawing mammals and growcontinuously throughout life (rabbits,rodents)

Rodent incisors have only enamel in theanterior


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Dentition Premolars

Single cusped for slicing food

Molars:

3- cusps for thorough food processing

In many herbivores both molars and

premolars are the same as in horses


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Dentition Molars of Omnivorous & Fruit eating

mammals

Cusps are rounded, flattened structuresideal for crushing

Upper molars: 4th cusp

Molars called bunodonts (fig 21.6 e) since they appear like a square rather than

triangular and also the rounded nature


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Dentition Molars of herbivores

Teeth have ridges called lophs that help to phone

some kind of shearing blades Lophodont teeth

Straight lophs (kangaroos, rabbits)

Selenodont teeth

Molars have crescent lophs as in of artiodactyls (deer) Multilophed teeth: lamellar

Wombarts, warthogs, rodents, elephants


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Dentition Dental durability

Diphyodont condition: adult dentition must

last a life time Problem for herbivores who have to deal

with more abrasive vegetation

Grazers also have to deal with high toothwear due to silica in grasses


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Dentition Solutions

Hyposodont teeth

Highly crowned teeth. Crown extends deep intothe jaw bone

Deep lower jaws & deep cheek regions Brachyodont: low crowned teeth- primitive mode

Larger hyposodont mammals (ungulates) Layer of cementum to cover whole tooth

Usually covers only root & base of crown

Cementum is a bone-like material, fills the highlophs of teeth


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Dentition Still teeth get worn out

Animals cant eat anymore

Horses (20-30) should be fed soft food No molars left

Hypselodont mammals

Molar teeth with evergrowing crowns Roots do not close

Unique in small mammals: rodents and rabbits


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Carnivorous mammals Have large canines to subdue prey

Specialized post-canine teeth for

shearing

E.g Carnassials

A pair of teeth specialized as tearing blades

Formed by last premolar in upper jaw andist molar in lower jaw


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Craniodental Specializations Generalized mammals: Primitive mode

Molars triangular

Pointed individual cusps

E.g in insectivorous & opossum

Anteaters Most elongated jaws

Progressively reduced teeth Highly elongated tongue

Enlarged salivary glands

Teeth reduction in nectar sucking mammals


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Craniodental SpecializationsAquatic feeders

Highly elongated jaws

Anterior-most teeth lost (dolphins,porpoises)

Teeth single cusped, pointed and increased

in number


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Craniodental SpecializationsAquatic Feeders

Baleen Whales

Teeth replaced by baleen Sheets of fibrous hornlike epidermal tissue that

extend from downward from the upper jaw

Used for filter feeding

Walruses Flat postcanine teeth

For crushing shells of sea food


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Craniodental Specializations:

Carnivores vs Herbivores Jaw closing muscles

Masseter

Temporalis

Pterygoideus

Jaw opening muscles in therians

Digastric


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Carnivores vs Herbivores Carnivores

Large temporalis

muscles to allow aforceful bite tosubdue prey

Herbivores

Reduced size oftemporalis muscles

Large size of masseterto create force requiredto grind large amountsof fibrous materials withback teeth and to allowside to side movementof jaws. Skull & teethmodified to grind toughresistant food in large

quantities


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Carnivores vs Herbivores Large coronoid process

of the jaw for insertionof temporalis muscles

Temporal fossa is large.(area from which thetemporalis originates)

Presence of a

postglenoid process toprevent dislocation ofjaw muscles

Reduced size ofcoronoid process

and temporal fossafor insertion oftemporalis

Absence of

postglenoid process


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Carnivores vs Herbivores Large occipital

region to reflect

extensivemusculature linkinghead to neck. Idealfor resisting

struggling prey

Small occipitalregion except for

pigs that root withtheir snouts

Elongated snouts

Diastema: gapbetween cheekteeth and incisors


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Digestion in Herbivores Plant cell walls: cellulose, require cellulase

enzymes which cannot be produced by any

mammal Thus mammals unable to digest cellulose

Microbes in gut: symbiotic microorganisms,produce enzymes that degrade cellulose andlignin into digestible nutrients


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Digestion Two types of fermentative digestion

Hindgut fermentation &

Foregut fermentation


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Monogastric Animals Hindgut fermentors

Horses, elephants, wombarts, koalas, rabbits, rodents, otherperissodactyls

Simple stomach Enlarged colon and cecum

Chew food thoroughly to release cell contents

Cell contents digested & absorbed in stomach and smallintestine

Cellulose digested in the cecum and colon by microorganisms

Products of fermentation are volatile fatty acids

Most eat large quantities to get enough nutrients


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Hindgut (Monogastric)

Digestion Coprophagy

Eating the first set of feces that are produced

thereby recycling nutrients that would beotherwise be lost

Characteristic of small monogastric animals

such as rabbits and rodents Ferment food in cecum, but do not absorb

much, thus eat the feces


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Foregut (Ruminant)

Fermentors E.g. cows and other ruminant artiodactyls

Camels lack an omasum

Forestomach: 3 chambers store & processfood Rumen :1st chamber

Reticulum: 2nd chamber

Omasum: 3rd

Chamber Fourth chamber:

abomasum: for digestion

Figure 21.9


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Foregut (Ruminant)

Fermentors Food initially retained in the rumen and reticulum.

Degraded by microorganisms

Microorganisms breakdown cellulose

Food regurgitated and re-chewed (cud)

Food in small particles then passes to omasum andthen abomasum (true stomach)

Digestion in abomasum similar to monogastricanimals

Note: all cellulose is broken down before reachingsmall intestines


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Advantages in foregut

fermentation Absorption occurs in small intestine, thus

absorb most of the energy from plantmaterials. Hindgut fermentors rely on cecum& large intestine for breakdown of celluloseand lignin. But absorption is not as efficientas in the small intestines, thus loose energy isfecal matter

Microorganisms attack plant material beforereaching small intestines-which is anadvantage vs hindgut fermentors


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Advantages in foregut

fermentation Microorganisms are themselves a source of nutrients

to the ruminant animals

Microorganisms play a role in nitrogen cycling, since

they can convert urea into microbial protein that canbe used by the animals. Thus, microbes make allessential amino acids required by the animal

A ruminant animal can be more limited in its selectionfor plant species than a monogastric animal whichhas to eat a wide variety of plant spp to get its aminoacids

Detoxify chemical compounds No such benefit for monogastrics


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Disadvantages in foregut

fermentation Foregut system is slow

Movnt thru a cows gut takes 70-100

hrs whereas thru a horse its 30-45 hrs

Do not thrive well on fibrous diets sinceits takes time to finish the processing in

rumen and reticulum (slows passagerate)


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Specializations for Locomotion Scansorial

Generalized form as seen in shrews and squirrels

Limbs and back are flexed during locomotion See figure 21-10 a & b

Larger animals move with a stiffer back andstraighter legs and gallop rather than bound


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Cursorial Limb Morphology Cursorial means specialized for running

Specializations include

Elongated legs

to maximize strides

Long legs provide a long outlever arm for forthe major locomotor muscles such as triceps inforelimbs and gastrocnemius in hindlimb

Enhance speed of motion


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Cursorial Limb Morphology Only certain portions of the limb are

elongated primarily the lower limb

portions Radius & ulna in the forelimb

Tibia and fibula in hind limb

Humerus and femur and phalanges arenot elongated


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Cursorial Limb Morphology Muscles concentrated to the proximal

portions of the limb to reduce the mass in thelower limb No muscles below horses knee (wrist) joint or

ankle (hock) joint

Foot is light

Long elastic tendon transmit force of musclecontraction from upper limb to the lowerlimb. Tendons are long to increase stretch &recoil


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Cursorial Limb Morphology Number of digits reduced to decrease

weight of foot, some lost completely

while others are compressed See slides below on Artiodactyls and

Perissodactyls


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Terminology related to

locomotion Plantigrade

Type of locomotion in which the entire sole

of the foot contacts the groundAs in humans and primates who have

retained all the 5 digits

These mammals called pentadactyls


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locomotion Digitigrade

Condition in which an animal walks on the ends ofits metacarpals and metatarsals; only the toescontact the ground in walking

Their wrists and ankles are elevated and thethumb has been reduced or lost

Run or walk faster than plantigrade animals, walkmore silently and more agile

Common in rabbits, rodents and many carnivores


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locomotion: Ungulates Unguligrade

Type of locomotion in which only the tips of thedigits contact the ground

These animals have reduced number of digits

Possess either 4, 3, 2 or 1 and thus walk on tips ofremaining fingers & toes

Weight of body is borne on hoofed whichrepresent modified claws that have becomehardened and thickened.


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locomotion: Ungulates The metacarpals corresponding to the

missing digits have been either reduced in

size or lost and those that are remainingare elongated and often united, amodification that greatly strengthens thelower leg and foot

Limbs of unguligrades are only capable offorward and backward motion, no twistingor rotation is capable


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Ungulates Muscles activating the lower portion of the

limbs are located closer to the body to lessen

the weight of the limb each time it is raised. The appendicular muscles attach to the limb

bones by long lightweight tendons

Thus the limbs and feet of hoofed mammals

which are long and light and only capable ofonly aft movements are highly specialized forrunning and/ maneuvering on rocky terrain


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Two groups of ungulates1. Artiodactyls (even toed)

Retained digits 3 & 4 as functional digits

Digits 2 & 5 reduced or lost in others Digit 1 is lost in all

Pigs & hippopotamus: 4 digits (3, 4, 2, 5)

Camels, deer, elk, giraffes, antelopes,bisons, buffalo, cattle, gazelles, goats,sheep: 2 digits (3& 4) (digits 1, 2 & 5 lost)


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Two groups of ungulates2.Perissodactyls (odd # of digits)

Digit 3 retained as primary functional

digit Bears all of the weight

Digits 2 & 4 are reduced

Digits 1 & 5 usually lost

Horses, zebras, rhinoceros.


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Fossorial Limb Morphology Limbs specialized for burrowing underground or

digging

Digging limbs maximize power at the expense of

speed Short forearm with a long olecranon process (elbow)

Retain all five digits, tipped with stout claws

Large bone projections on limbs for attachment of

strong muscles E.g large acromion on scapula for attachment of deltoid

muscles

Examples are: African golden mole; Australianmarsupial mole; ferrets


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Semiaquatic mammals

(Amphibious) Have paddle-like limbs, use limbs to swim (paraxial

swimming) as we do ourselves.

Denser fur; webbing between their toes

Examples are: Platypus (monotreme)

Marsupials (water opossum, yapok)

Water shrews, desmans, river otter, beavers, muskrat andmink

Hippopotamus

Inhabit a variety of waterways and associatedwetlands

Require both aquatic and shoreline habitats for

feeding


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Aquatic mammals Use undulations of the body for swimming

(axial swimming) via dorso-ventral flexion

Do not use lateral undulations

Swimming enabled by flexion of the vertebralcolumn

Have short paddle-like limbs

Limbs have short proximal ends Have elongated phalanges

Limbs used for breaking & steering


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Aquatic mammals

In summary, the front limbs of aquaticmammals are modified for life in the

sea and superficially resemble themodified appendages of of sea turtlesand penguins. Appendages become

flattened, short and stout and may havea greatly increased number ofphalanges.


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Aquatic mammals: Examples

Order: Cetacea Whales and dolphins

Have lost hind limbs Short necks

Forelimbs modified into paddles

Order: Sirenia Dugongs & manatees:

have lost hind limbs


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Aquatic mammals: Examples

Order Carnivora: Seals, sea lions, andwalruses

Have large naked front flippers andreversible hind flippers that can be broughtunder the body for locomotion on land

In hair seals (earless) front flippers are smaller

than hind flippers, which are not reversible.Thus in these seals, hind flippers are notreversible.


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