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Our Friends, the Mammals


  • Synapsida: The Mammals

    • In mammals, the ancestral vertebrate jaw bones have become modified to form the bones of the inner ear.
    • Mammalian synapomorphies
      • mammary glands
      • fleshy lips for suckling (marsupials and placentals only)
      • hair (grows between the analog of the reptilian scute/scale)

    • There are about 4500 extant species in more than 14 orders.
  • Monotremes - Egg-Laying Mammals (Platypus and Echidna)
      These are the only living mammals that lay eggs, which are reptilian in structure and development.

  • Marsupials - Pouched Mammals
      These are born at an extremely altricial state, crawl to the mother's marsupium (pouch), crawl in, latch onto a teat, and undergo the rest of their development in the marsupium.

      Note: the opposite of altricial is precocial. (What does it mean?)

  • Eutherians - Placental Mammals
      These are the mammals that give birth to live young that develop in the mother's uterus, and are nourished during development via the connection to the mother's bloodstream, the placenta. The placental membranes are homologous to the amniotic egg membranes. YOUR ASSIGNMENT: Find out which membrane (amnion, chorion and allantois) has developed into WHAT in the mammalian system. (HINT: Review the Amniotic Egg link at the UC Berkeley Museum of Paleontology)

    The Synapsid vertebrates are probably the most familiar to all of us, as we happen to be one of the many species of synapsids. What's so special about the synapsids?

  • They were the first vertebrates to radiate into a wide diversity of terrestrial habitats

  • They diversified into herbivorous, carnivorous forms known as PELYCOSAURS (they looked somewhat like lizards, but were NOT! Lizards are diapsids.) You've probably seen pictures of one of the most famous pelycosaurs, Dimetrodon.

  • One group of carnivorous pelycosaurs gave rise to the THERAPSIDS, the only synapsids to survive beyond the Paleozoic great extinction (about 245 mya).

  • Therapsids had limbs positioned directly beneath the body, allowing for a more energy-efficient gait and movement (Think of the sideways limb attachment of crocodilians and other reptiles for comparison.)

  • Most of the therapsids vanished during the great extinction at the end of the Permian (245 mya)

  • The last surviving therapsids, the CYNODONTS, were the dominant mammal ancestors in the Mesozoic
    • high rate of metabolism
    • jaw musculature increased in size and complexity
    • bony plate (now seen as the hard and soft palates) separates the mouth and nasal cavities, allowing the animal to hold food in its mouth and still breathe
    • This inovation would also allow young to breathe while suckling, a major mammalian inovation

    The Importance of Teeth

    The earliest true mammals showed up in the late Triassic, and were mouse-sized carnivores.

    They had diphyodont dentition, meaning they were unable to constantly replace lost teeth (as their amniote ancestors could), but rather replaced them only once over the span of a lifetime.

    • deciduous ("baby") teeth
    • permanent teeth

    Humans and all other mammals show this pattern today.

    Another dental inovation: differentiated teeth (we are HETERODONTS, as opposed to HOMODONTS as all other vertebrates are.)

    • incisors
    • canines
    • premolars
    • molars
    The number of each of these types of teeth varies with species, and the "dental formula" is one diagnostic character of mammal species.

    In humans, the adult dental formula is 2:1:2:3/2:1:2:3 - This is the number of teeth in half of the upper jaw (2 incisors, one canine, 2 premolars, and 3 molars) and in half of the lower jaw (two incisors, 1 canine, 2 premolars and 3 molars).

    Note that a few mammals have secondarily reverted to the homodont condition, but this is derived with respect to other mammals.

    Some mammals are HYPSODONTS. This means that their teeth grow continually throughout their lives, and must grind against each other to remain a normal, manageable shape. Some examples: rodents, lagomorphs (hares, rabbits, and pikas), horses, elephants and many other herbivores. What do you suppose is the advantage of having continually growing teeth? What could be some disadvantages?

    What makes us mammals?

  • Hair (a derivative of the integument)
    • undercoat - provides most insulation
    • guard hairs - coarser and longer, this provides protection against wear, and is usually the part of the PELAGE that has color patterns
    • hair length is genetically determined, as we already have discussed
    • hair usually sheds annually or twice a year
    • specialized hairs called VIBRISSAE ("whiskers") are used for touch sensation

    • Horns and antlers (another derivative of the integument)
      • HORNS are sheaths of keratinized epidermis (keratin is the protein that makes up hair) wrapped around a bone core rising from the skull. They are permanent, and never shed. (Note: Rhino horn isn't a true horn: it'sa swirl of hair-like filaments rising from the dermis and firmly "cemented" together with glycoproteins to form the structure, which is not attached to the skull.
      • ANTLERS are composed of solid bone at maturity, but are shed annually.
  • Integumentary glands
    • sweat glands
      • eccrine - secrete water and salts, and function in cooling
      • apocrine - secrete a milky substance associated with reproduction and emotion; not involved in thermoregulation

    • scent glands - vary greatly in number, function, and location across species

    • sebaceous (oil) glands - function to keep skin and hair soft and pliable with its secretion, SEBUM.

    • mammary glands - the gland that gives our class its name, evolved for feeding the young. Our fleshy, muscular lips have evolved for the function of suckling.

    (FINAL EXAM MATERIAL ENDS HERE)

    MAMMAL NUTRITION

    Mammals may be
    • herbivores (including frugivores)
      • browsers
      • grazers
      • gnawers
      Such mammals often have a complex intestinal flora of specialized microorganisms that can digest cellose (a complex plant starch), which is not digestible by mammals.

      Many herbivores (horses, rabbits, elephants, some primates), have a CECUM, a side pocket that branches off the junction between small and large intestine. This contains a complex ecosystem of microorganisms that provides essential nutrients via fermentation of the herbivore's food.

      Lagomorphs (hares and rabbits) and some rodents re-ingest their CECOTROPES, strong-smelling pellets delivered from the cecum via the anus. Your text calls this "coprophagy", which means "ingestion of feces. This is not truly correct, as cecotropes are NOT feces. They are an essential part of the lagomorph diet. Hence, your bunny engages, not in coprophagy, but in CECOTROPHY.

      Other herbivores (bovids (cow family) and ovids (sheep family)) are RUMINANTS which have a large, four-chambered stomach. Ruminants swallow food, which passes to the RUMEN, where it is partially digested and regurgitated as CUD. This is re-chewed to break down fiber, and then sent back to the rumen for further digestion.

  • insectivores - feed on insects
  • carnivores -feed primarily on herbivores
      Let's talk about the "carnivore intelligence" myth....
  • omnivores - feed on a variety of plant and animal matter

    MAMMAL REPRODUCTION

  • Egg-laying: Monotremes
  • Live birth of altricial young which are then raised in a pouch: Marsupials
  • Internal brooding of young via a placenta - Eutherians (placental mammals) Most mammals have defined mating seasons, with females accepting male advances only during her estrous cycle. When not in "heat" (estrus), the female is unreceptive to male sexual advances.
    • One estrus per breeding season: monoestrous
    • Multiple estrus per breeding season: polyestrous
    • Induced ovulators: ovulate upon the act of mating

    Let's meet some interesting mammalian orders.