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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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Feeding specializations:
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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Feeding specializations:
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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Feeding specializations:
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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Feeding specializations:
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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Feeding specializations:
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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Feeding specializations:
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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Feeding specializations:
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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Feeding specializations:
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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Craniodental Specializations:
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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Craniodental Specializations:
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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Craniodental Specializations:
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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Terminology related to
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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Terminology related to
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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Terminology related to
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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