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Class Mammalia. more than 5,000 species today presence of the mammary gland – feeding of young production of milk - balanced liquid of fats, proteins, sugars, minerals and vitamins development of hair and a fat layer under the skin for the retention of heat

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class mammalia
Class Mammalia
  • more than 5,000 species today
  • presence of the mammary gland – feeding of young
    • production of milk - balanced liquid of fats, proteins, sugars, minerals and vitamins
  • development of hair and a fat layer under the skin for the retention of heat
  • efficient respiratory and circulatory systems
  • larger brain
    • ability to learn complex tasks
  • differentiation of teeth
    • variety of shapes and sizes
    • adapted for chewing, ripping and grinding
  • Concepts
    • 1. characteristics evolved gradually over 200 million years from the synapsid lineage
    • 2. two subclasses initially evolved – Prototherians and the Therians
    • 3. skin is thick and protective – covered with hair
    • 4. able to exploit a wide variety of feeding habitats
    • 5. efficient circulatory and respiratory systems – support a high metabolic rate
    • 6. greatly expanded brain with cerebral cortex
    • 7. metanephridic kidneys to minimize water loss
    • 8. complex behavioral patterns for survival
    • 9. most are viviparous
  • belong to a group of amniotes known as synapsids
    • non-mammalian synapsids – lacked hair, had a sprawling gait and laid eggs
  • distinctive characteristic of synapsids – temporal fenestra
    • hole behind the eye socket
    • in humans – for the passage of jaw muscles and anchorage onto the temporal bone
  • synapsids evolved into large herbivores and carnivores during the Permian period
  • mammalian synapsids emerged by the end of the Triassic period – 200 MYA
    • not true mammals
  • development into the Prototherians and Therians
    • Subclass Prototheria
      • now only contains extinct species
    • Subclass Theria
      • made up of three infraclasses:
        • Ornithodelphia (monotremes)
        • Metatheria (marsupials)
        • Eutheria (placentals)
first true mammals arose during the Jurassic period
    • small – about the size of a shrew
  • emergence of three lineages: monotremes (egg-laying), marsupials (mammals with a pouch) and eutherians (placental mammals)
  • monotremes: found only in Australia and New Guinea
    • one species of platypus and two species of echidnas (spiny anteaters)
    • egg laying retained as a primitive egg-laying characteristic – unlike other mammals
    • have hair
    • produce milk – lack nipples
    • milk is secreted by glands on the belly of the mother
    • hatchling sucks the milk from the fur
    • possess a cloaca
  • marsupials: opposums, kangaroos and koalas
    • marsupials and eutherians share many characteristics
    • higher metabolic rate than monotremes
    • nipples provide milk
    • embryo develops inside the uterus
    • development in utero is short – female cannot produce sufficient hormones to maintain the uterine lining
    • born early in its development – completes development while nursing in the marsupium
  • eutherians:
    • more complex placenta than marsupials
    • well developed extra-embryonic membranes
    • longer period of pregnancy
    • completion of embryonic/fetal development within the uterus
    • made up of 12 orders including
      • Order Primata - primates
      • Order Lagomorpha – rabbits
      • Order Rodentia – squirrels, chipmunks, rats, mice, beavers, porcupines, woodchucks and lemmings
      • Order Cetecea – whales, dolphins
      • Order Carnivora – dogs, cats, raccoons, minks, sea lions, seals, walruses, otters
      • Order Proboscidea – elephants
      • Order Perissodactyla – ungulates (horses, rhinos, zebras)
      • Order Artiodactyla – hoofed (pigs, hippos, camels, antelope, deer, sheep, giraffes, cattle
external structure and locomotion:
    • like other vertebrates – skin is made up of epidermal and dermal layers
      • epidermis – containing glands
        • sebaceous, sudoriferous, scent
    • development of hair – specialized keratinized structure
      • forms in the dermis – hair follicles
      • many animals have different types of hair forming a pelage or coat
        • e.g. longer hairs protecting a coat of shorter hairs
      • periodically molted in many mammals
        • gradual in the primates (including humans!)
        • many mammals acquire a thicker coat in the fall and shed much in the spring
        • molting is frequently associated with changes in coat color
          • e.g. loss of white hair with snow melt
      • sense of touch – base of the hair is associated with sensory neurons
        • also can be pulled erect or flattened
        • controlled by the autonomic nervous system – in response to danger
      • insulation – air spaces between hairs warms the mammals
        • hair shaft also has air spaces in many mammals
        • pulling the hair upright can also traps more air
      • pigmentation – melanin content
        • wide variety seen in many mammals
        • some colorations are warnings – e.g. skunk
      • reduced in hotter climates – e.g. hippos, elephants, humans
        • e.g. naked mole rat has no pelage
    • claws, hooves or nails – locomotion, manipulation, defense
      • claw = accumulation of keratin over the distal portion of the digit – not a nail!!!
    • mammary glands – modified sudoriferous gland
      • modification in structure results in a teat (not a nipple!)
skull and teeth:
    • used to classify the mammalian class
    • method of jaw articulation distinguishes the mammal from the reptile
      • reptiles – jaw articulates at two small bones (quadrate bones) at the rear of the skull
      • mammals - these bones move into the middle ear along with the stapes = ossicles
        • have a single articulation between the mandible and the temporal bone
    • secondary palate seen in the reptiles extends back further into the oral cavity and forms the soft palate
      • almost completely separates the oral from the nasal cavity
      • allows mammals to breath while they eat – therefore they can chew and don’t have to swallow whole
    • arrangement of teeth
      • reptiles are homodonts
      • mammals are heterodonts
        • set into sockets in the jaw
        • most have two sets of teeth – deciduous and permanent dentition
        • adults have up to four types of teeth
        • shape and size of the teeth vary with diet
        • zoologists use a dental formula to divide mammals into their groups
        • # of teeth in one half of the upper and lower jaws in the order of incisor, canine, premolar and molar
          • e.g. human 2>1>2>3
          • 2>1>2>3
axial skeleton
    • vertebral column divided into cervical, thoracic and lumbar – like birds
    • only the thoracic regions articulates with ribs
    • ribs cage is a protective structure
    • rib articulation with the vertebrae is very flexible – allows for twisting of the trunk
  • appendicular skeleton
    • rotation of the limbs under the body
    • reorientation of the pelvic and pectoral girdles
    • therefore the bulk of the body’s weight is borne easily by the limbs
    • joints usually limit the range of motion of these limbs
    • bones of the pelvic girdle are fused in mammals
      • advantageous for locomotion
      • problem for birth
digestion and nutrition:
    • similar to other vertebrates
    • specializations seen in the teeth
    • other specializations reflect feeding habitats
      • e.g. size of the intestinal tract correlates with diet
circulation and gas exchange:
    • development of a four chambered heart
    • similar to birds – pulmonary and systemic circuits
    • birth to live young required adaptations in the circulatory system of the pregnant female
      • fetal circulation – blood entering the right atrium can bypass the pulmonary circuit
      • gas exchange occurs across the placental surface
    • gas exchange:
      • high metabolic rates of mammals require increased O2
      • larger snouts
      • separate nasal and oral cavities
      • highly branched respiratory passages
      • well-developed lungs with alveoli for gas exchange
      • development of a diaphragm for negative-pressure ventilation
temperature regulation:
  • ectothermy vs. endothermy
  • thermoregulation: maintenance of an internal temperature within a tolerable range
    • critical to survival
    • determines rates of enzymatic reactions, structures of proteins and nucleic acids etc….
  • thermoregulation is an integral part of homeostasis
    • each species has adapted an optimal temperature range
  • metabolic heat is used extensively by many animals to determine internal temperature
    • metabolic processes generate heat as a byproduct
  • ectotherms: low internal body temperature
    • low metabolic rate – heat does not affect body temperature
    • rely on the environment for gains in body temperature
      • e.g. basking in the sun
    • more variable internal temp = poikilothermic
    • amphibians and reptiles
  • endotherms: high internal body temperature
    • higher metabolic rate
    • can be used to directly regulate internal body temperature
    • internal temp is more stable = homeothermic
    • leads to the development of more complicated respiratory and circulatory systems
    • birds and mammals
  • common misconception is the classification of ectotherms and endotherms
    • NOT based on whether they have constant or variable body temps
    • it is the source of heat used to maintain body temp
    • “cold-blooded” vs. “hot-blooded” is also correct
    • many ectotherms can have quite high body temps – e.g. lizards basking in the sun
  • many advantages
    • higher body temp and better CV and respiratory systems allows for vigorous activities over a sustained time point
    • e.g. flying, long-distance running
    • also solves certain thermal problems living on land enabling terrestrial animals to maintain a stable body temp during environmental fluctuations
    • but tolerance of large swings in external temp are better tolerated by ectotherms
  • but it is energetically expensive
    • metabolic rate = sum of all the energy-requiring biochemical reactions occurring over a given time interval
      • measured in calories
      • calorie =
    • the MR of a human at rest is between 1,300 to 1,800 kcal per day
    • but a resting ectotherm might have a MR of only 60 kcal per day
    • so endotherms need to consume larger amounts of food more often to keep up with their metabolic “demands”
heat exchange
Heat exchange
  • whether an ectotherm or an endotherm – heat is exchanged through 4 processes
    • 1. conduction – direct transfer of heat between objects directly in contact with each other
    • 2. convection – transfer of heat to the air
    • 3. radiation – transfer of heat from the sun
    • 4. evaporation – loss of heat through evaporation of liquids off the surface of the animal
    • in endotherms – heat is retained by the pelage
      • without a pelage – mammals can conserve heat by allowing the temp of surfaces to drop
      • well-vascularized appendages allow for the evaporation of excess heat during hotter climate periods
        • these are a problem is heat is to be conserved
      • counter-current heat exchange systems – regulates heat loss from exposed areas
        • arteries passing peripherally through the core of an appendage are surrounded by veins that carry blood back to the heart
        • heat transfers from arterial blood to the vein and vice versa
      • heat evaporation is not a problem in moist environments – evaporative cooling requires water loss
        • e.g. hot climates – jack rabbits – have large vascularized ears – heat loss through convection – no water loss
nervous and sensory:
    • active lifestyle accompanies by development of more complex nervous system and sensory structures
    • sense of touch becomes well developed
      • sensory neurons located in epidermis and dermis
      • proprioception becomes well developed
    • olfaction well developed in many mammals
      • larger snouts allow for more olfactory epithelium
      • larger areas of the brain for identifying smells
    • auditory senses well developed
      • development of a pinna and external ear canal
      • middle ear with three ossicles
      • development of a coiled cochlea for hearing
      • sense of balance/equilibrium becomes better developed – inner ear
    • vision
      • similar eye structure to birds
      • focusing through changing the shape of the lens
      • color vision less developed in many mammals
        • rods more numerous
excretion and osmoregulation:
    • like all amniotes – metanephric kidney
    • excretion of urea as the main product
      • reptile and birds – excrete uric acid which requires less water to form
      • urea requires conversion of ammonia through addition of CO2
      • urea is highly soluble and requires relatively large amounts of water to excrete
    • nephron – filtration through the glomerulus
      • highly efficient tubular network for reabsorption of water and salts
      • primary adaptation to the nephron is the loop of Henle
      • allows for the production of concentrated urine
        • e.g. beaver produces urine 2X more concentrated in salts than blood
    • water loss can vary greatly in mammals
    • development of behaviors that minimize water loss
    • to enhance survival in the terrestrial environment
    • visual cues – communication
    • habitat behaviors – minimize water loss, maximize food, thermoregulation
    • pheromones – species and sex recognition
      • attraction of opposite sex
      • signals reproductive state
      • may also induce sexual behaviors
        • e.g. male bull elk – urinates on his underbelly to advertise their reproductive status to the females during mating season
    • auditory communication and vocalization
      • herds stay together and remain calm through familiar sounds
      • also capable of warning others and scaring predators off
      • vocalizations in primates
        • variety of uses
        • social interactions and bonding
    • territoriality
      • marked and defended
      • e.g. cats rub their face – not affection, spreading of scent from facial scent glands
      • e.g. sea lions – 2 weeks the males engage in vocalizations, displays and fights to stake claim to favorable areas along the beach
        • females select the site that appeals to them for giving birth
        • this ensures that the male will get to mate with the female and will father next year’s offspring
        • development is arrested for three months after initial fertilization
    • development of very rigid reproductive cycles
      • many eggs are incapable of being fertilized or are not produced out of cycle
    • usually coincides with climate and resource improvements
      • in climates with few seasonal changes or in an environment that can be controlled – reproduce at any time of the year
    • most females undergo estrus = time during which the female is behaviorally and physiologically receptive to the male (fertile egg is produced)
      • complex series of hormonal changes that affect egg production, uterine and vaginal environments and sperm production
    • many mammals are monoestrus
      • only once a year
      • strictly regulated by the environment
      • e.g. wild dogs, bears and sea lions
      • e.g. domestic dogs – biestrus
      • other animals have cycles that can repeat on a cyclical basis
        • e.g. rats – every 4 to 6 days
    • fertilization is internal – upper 1/3 of the oviduct
    • in a few mammals – fertilization is delayed after coitus (delayed fertilization)
      • e.g. bats – coitus in fall, fertilization in spring
        • can store sperm in utero for up to two months!!
        • adaptation to winter dormancy
    • most mammals have direct fertilization after coitus
      • but development of the embryo can be arrested for one to two weeks = embryonic diapause
      • e.g. sea lions, some bats, bears and marsupials
      • may be due to presence of resources – allows mother time to feed before having to nurse
      • also allows young to be born during a time of good resources