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Mammalian Specializations

Mammalian Specializations

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Mammalian Specializations

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    1. Mammalian Specializations Chapter 21

    2. Mammalian groups Monotremes Marsupials Eutherians Have varying reproductive modes Egg laying in monotremes Eutherians have long gestations Marsupials have very short gestation lengths

    3. 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 for implanting the the embryo onto the uterine wall (in Therians), obtaining nutrients from the mother, and secreting hormones to signal the state of pregnancy to the mother

    4. Common reproductive aspects Endometrium Glandular uterine epithelium of the mammals that secrete materials that nourish the embryo in uterus Presence of corpus luteum Formed by the ruptures follicles after releasing egg Secretes hormones that sustain early stages of pregnancy

    5. Monotreme Reproduction Primitive reproductive tract 2 oviducts remain separate, do not fuse during development except at the base where they join with urethra to form urogenital 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)

    6. Monotreme Reproduction Ovaries larger in compares to Therians Monotremes provide embryo with more yolk Produce smaller eggs at ovulation Eggs retained in uterus & nourished by maternal secretions, increase in size after which the shell is secreted.

    7. 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)

    8. Reproduction in Therians All have placentation: 2 types Choriovitelline placenta Placentas developed from the yolk sac seen in all Therian 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, but some show a transitory CA placenta at end of gestation

    9. 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

    10. Reproduction in Therians Male reproductive anatomy Monotremes retain testes in abdomen In therians, testes descend into scrotum Descent is genetically controlled in marsupials and hormonally controlled in eutherians Scrotum in front of penis in marsupials and behind in most eutherians.

    11. Reproduction of Eutherians Ureters enter into the bladder rather than the cloaca Oviducts fuse anterior to the urogenital sinus to form a uterus All have a single midline vaginam but only a few 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 in humans

    12. 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 and urethral openings

    13. 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

    14. Reproduction of Marsupials Females Female oviducts do not join on midline because ureters pass medial to reproductive ducts to enter bladder 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

    15. 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 mammary area of pouchless animals Pouch absent in some: mice & Opposums Lactation continues after young ones detach from the pouch

    16. Feeding specializations: Dentition Incisors to seize food Canines to stab prey Premolars to pierce and crash food Molars: to break down food into fine particles Therians have tribosphenic molars

    17. Feeding specializations: Dentition Canines Lost in herbivores or modified Tusks of pigs and walruses: modified canines Upper canines larger in male primates Male horses have small functionless canines Maybe used in male fighting and display

    18. Feeding specializations: Dentition Incisors Tusks of elephants= modified incisors Enlarged in gnawing mammals and grow continuously throughout life (rabbits, rodents) Rodent incisors have only enamel in the anterior

    19. 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

    20. Feeding specializations: Dentition Molars of Omnivorous & Fruit eating mammals Cusps are rounded, flattened structures ideal 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

    21. 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

    22. 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 tooth wear due to silica in grasses

    23. Feeding specializations: Dentition Solutions Hyposodont teeth Highly crowned teeth. Crown extends deep into the 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 high lophs of teeth

    24. 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

    25. 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 and ist molar in lower jaw

    26. 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

    27. Craniodental Specializations Aquatic feeders Highly elongated jaws Anterior-most teeth lost (dolphins, porpoises) Teeth single cusped, pointed and increased in number

    28. Craniodental Specializations Aquatic 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

    29. Craniodental Specializations: Carnivores vs Herbivores Jaw closing muscles Masseter Temporalis Pterygoideus Jaw opening muscles in therians Digastric

    30. Craniodental Specializations: Carnivores vs Herbivores Carnivores Large temporalis muscles to allow a forceful bite to subdue prey Herbivores Reduced size of temporalis muscles Large size of masseter to create force required to grind large amounts of fibrous materials with back teeth and to allow side to side movement of jaws. Skull & teeth modified to grind tough resistant food in large quantities

    31. Craniodental Specializations: Carnivores vs Herbivores Large coronoid process of the jaw for insertion of temporalis muscles Temporal fossa is large. (area from which the temporalis originates) Presence of a postglenoid process to prevent dislocation of jaw muscles Reduced size of coronoid process and temporal fossa for insertion of temporalis Absence of postglenoid process

    32. Craniodental Specializations: Carnivores vs Herbivores Large occipital region to reflect extensive musculature linking head to neck. Ideal for resisting struggling prey Small occipital region except for pigs that root with their snouts Elongated snouts Diastema: gap between cheek teeth and incisors

    33. 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 and lignin into digestible nutrients

    34. Digestion Two types of fermentative digestion Hindgut fermentation & Foregut fermentation

    35. Monogastric Animals Hindgut fermentors Horses, elephants, wombarts, koalas, rabbits, rodents, other perissodactyls Simple stomach Enlarged colon and cecum Chew food thoroughly to release cell contents Cell contents digested & absorbed in stomach and small intestine Cellulose digested in the cecum and colon by microorganisms Products of fermentation are volatile fatty acids Most eat large quantities to get enough nutrients

    36. Hindgut (Monogastric) Digestion Coprophagy Eating the first set of feces that are produced thereby recycling nutrients that would be otherwise 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

    37. Foregut (Ruminant) Fermentors E.g. cows and other ruminant artiodactyls Camels lack an omasum Forestomach: 3 chambers store & process food Rumen :1st chamber Reticulum: 2nd chamber Omasum: 3rd Chamber Fourth chamber: abomasum: for digestion Figure 21.9

    38. 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 and then abomasum (true stomach) Digestion in abomasum similar to monogastric animals Note: all cellulose is broken down before reaching small intestines

    39. Advantages in foregut fermentation Absorption occurs in small intestine, thus absorb most of the energy from plant materials. Hindgut fermentors rely on cecum & large intestine for breakdown of cellulose and lignin. But absorption is not as efficient as in the small intestines, thus loose energy is fecal matter Microorganisms attack plant material before reaching small intestines-which is an advantage vs hindgut fermentors

    40. 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 can be used by the animals. Thus, microbes make all essential amino acids required by the animal A ruminant animal can be more limited in its selection for plant species than a monogastric animal which has to eat a wide variety of plant spp to get its amino acids Detoxify chemical compounds No such benefit for monogastrics

    41. 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 since its takes time to finish the processing in rumen and reticulum (slows passage rate)

    42. 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 and straighter legs and gallop rather than bound

    43. Cursorial Limb Morphology Cursorial means specialized for running Specializations include Elongated legs to maximize strides Long legs provide a long outlever arm for for the major locomotor muscles such as triceps in forelimbs and gastrocnemius in hindlimb Enhance speed of motion

    44. 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 are not elongated

    45. Cursorial Limb Morphology Muscles concentrated to the proximal portions of the limb to reduce the mass in the lower limb No muscles below horses knee (wrist) joint or ankle (hock) joint Foot is light Long elastic tendon transmit force of muscle contraction from upper limb to the lower limb. Tendons are long to increase stretch & recoil

    46. 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

    47. Terminology related to locomotion Plantigrade Type of locomotion in which the entire sole of the foot contacts the ground As in humans and primates who have retained all the 5 digits These mammals called pentadactyls

    48. Terminology related to locomotion Digitigrade Condition in which an animal walks on the ends of its metacarpals and metatarsals; only the toes contact the ground in walking Their wrists and ankles are elevated and the thumb has been reduced or lost Run or walk faster than plantigrade animals, walk more silently and more agile Common in rabbits, rodents and many carnivores

    49. Terminology related to locomotion: Ungulates Unguligrade Type of locomotion in which only the tips of the digits contact the ground These animals have reduced number of digits Possess either 4, 3, 2 or 1 and thus walk on tips of remaining fingers & toes Weight of body is borne on hoofed which represent modified claws that have become hardened and thickened.

    50. Terminology related to locomotion: Ungulates The metacarpals corresponding to the missing digits have been either reduced in size or lost and those that are remaining are elongated and often united, a modification that greatly strengthens the lower leg and foot Limbs of unguligrades are only capable of forward and backward motion, no twisting or rotation is capable

    51. 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 of only aft movements are highly specialized for running and/ maneuvering on rocky terrain

    52. Two groups of ungulates 1. 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)

    53. Two groups of ungulates 2.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.

    54. 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; Australian marsupial mole; ferrets

    55. 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 and mink Hippopotamus Inhabit a variety of waterways and associated wetlands Require both aquatic and shoreline habitats for feeding

    56. 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 vertebral column Have short paddle-like limbs Limbs have short proximal ends Have elongated phalanges Limbs used for breaking & steering

    57. Aquatic mammals In summary, the front limbs of aquatic mammals are modified for life in the sea and superficially resemble the modified appendages of of sea turtles and penguins. Appendages become flattened, short and stout and may have a greatly increased number of phalanges.

    58. 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

    59. Aquatic mammals: Examples Order Carnivora: Seals, sea lions, and walruses Have large naked front flippers and reversible hind flippers that can be brought under 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 not reversible.