Mammals

Evolutionary Relationship of Mammals

  • Mammals evolved around 220 million years ago alongside dinosaurs.
  • Early mammals were small, likely nocturnal, while dinosaurs dominated the terrestrial landscape.
  • A significant mass extinction event 65 million years ago allowed mammals to thrive, with critical insulation provided by hair possibly aiding their survival.
  • Post-extinction, mammals diversified, filled ecological niches left by dinosaurs, with maximum diversity reached around 15 million years ago.

Evolution of Mammals

  • Key ancestors:
    • Therapsids: Early synapsids whose limbs were positioned beneath their bodies and featured enhanced jaw muscle attachments.
    • Cynodonts: Showed stronger jaws and a higher metabolic rate, some characteristics of early mammals.
    • Early mammals: Exhibited features such as hair, mammary glands, and specialized molars for shearing food.

Characteristics of Modern Mammals (Class Mammalia)

  • Diversity: Approximately 6495 species, including humans.
  • Habitat: Mammals are found in all habitats across the globe.
  • Defining Features:
    • Mammary Glands: Present in all females; specialized tissue secreting milk, with newborns born toothless and reliant on suckling for nourishment.
    • Hair: Almost all mammals have hair, serving multiple functions such as insulation, camouflage, or sensory functions (e.g., whales and dolphins have reduced but present hair).
    • Endothermy: Mammals are warm-blooded, allowing them to maintain consistent body temperature and be active in varied climates.

Grouping of Modern Mammals

  • Three Major Groups:
    1. Placental Mammals: Nourish embryos fully in utero; the placenta forms from the chorion and allantois, facilitating nutrient and waste exchange between maternal and fetal blood.
    2. Marsupials: Includes around 350 species; fertilized eggs develop briefly before the immature young crawl into a pouch to nurse and continue development.
    3. Monotremes: Only five species (including the platypus and echidnas); they lay shelled eggs and show reptile-like characteristics while possessing mammary glands without nipples. Milk is excreted onto fur for the young to lap up.

Structure and Function of Mammary Glands

  • Mammary glands are unique to mammals, providing nutrition to young post-birth.
  • They consist of specialized tissue that secretes milk, essential for the growth and development of newborns.

Structure and Function of Hair

  • Hair is formed of dead cells filled with keratin, distinct from the skin structure.
  • Functions include temperature regulation and sensory perception, as well as camouflage in various environments.

Understand Endothermy and Its Benefits

  • Endothermy allows mammals to regulate their body temperature independently of the environment.
  • This capability enables activity throughout the day and in a wider range of environments, granting a survival advantage.

Function of the Placenta vs. Amniotic Egg

  • Placenta: Provides ongoing nourishment and waste management during the critical development phase before birth.
  • Amniotic Egg: Historical reproductive method; provided a way for embryos to develop in a controlled aqueous environment, crucial for early vertebrate life.

Reproduction and Parental Care

  • Placental Mammals: Extended gestation periods with significant maternal care.
  • Marsupials: Short gestation with additional nurturing in the pouch.
  • Monotremes: Oviparous (egg-laying) with minimal postnatal care; young are highly dependent on maternal care.