Unit 5: Aves and Mammals

1. Class Aves (Birds)

Aves are highly specialized vertebrates adapted for aerial life. They are often described as "glorified reptiles" due to their direct evolutionary lineage from theropod dinosaurs.

1.1 General Characteristics and Classification

General Characteristics

• Feathers: The unique diagnostic feature of birds. Feathers are modified epidermal scales, essential for flight and insulation.
• Forelimbs Modified into Wings: The primary adaptation for flight.
• Bones: Lightweight and strong. Many bones are pneumatic (hollow and filled with air sacs) to reduce weight.
• Beak: Jaws are modified into a toothless, horny beak (bill).
• Heart: Four-chambered (two atria, two ventricles), ensuring complete separation of oxygenated and deoxygenated blood. This supports their high metabolic rate.
• Thermoregulation: Homeothermic (warm-blooded), maintaining a constant, high body temperature.
• Respiratory System: Highly efficient. Lungs are connected to a system of air sacs that allows for a one-way flow of air, ensuring a constant supply of fresh oxygen.
• Reproduction: Internal fertilization. Oviparous (lay hard-shelled amniotic eggs).

Classification (Up to Order)

• Order Passeriformes: "Perching birds" or songbirds. Largest order. (e.g., Sparrow, Crow, Myna).
• Order Columbiformes: Pigeons and Doves.
• Order Anseriformes: Waterfowl. (e.g., Ducks, Geese, Swans).
• Order Galliformes: "Fowl." (e.g., Chicken, Peacock, Quail).
• Order Struthioniformes: Flightless "Ratites." (e.g., Ostrich).

1.2 Archaeopteryx: Characteristics and Phylogenetic Importance

Archaeopteryx is a famous fossil from the Jurassic period. It is a perfect connecting link, demonstrating the evolutionary transition from reptiles to birds.

Reptilian Characteristics (Ancestral)

• Teeth: Jaws contained sharp, reptilian teeth (modern birds are toothless).
• Long Bony Tail: Had a long, reptilian tail with many vertebrae (modern birds have a short pygostyle).
• Claws on Fingers: Had three clawed fingers on its "hand" (the wing).
• Solid Bones: Lacked pneumatic bones.

Avian (Bird-like) Characteristics (Derived)

• Feathers: Had well-developed flight feathers on its wings and tail, identical to modern birds.
• Wings: Forelimbs were modified into wings.
• Furcula: Possessed a fused clavicle or "wishbone."

Phylogenetic Importance

Archaeopteryx provides undeniable proof that birds evolved from reptiles (specifically, theropod dinosaurs). It shows how reptilian features were gradually modified into the avian features we see today, with feathers evolving long before other flight adaptations like a keeled sternum or pneumatic bones.

1.3 Migration in Birds

Bird migration is the regular, seasonal, large-scale movement of birds between their breeding grounds and their wintering grounds. This is driven by the need to find food and suitable breeding conditions.

• Triggers: Changes in day length (photoperiod) are the primary trigger, which in turn affects hormone levels.
• Navigation: Birds use a sophisticated combination of cues:
  • Sun Compass: Using the position of the sun.
  • Star Navigation: Using constellations at night.
  • Geomagnetism: Sensing the Earth's magnetic field.
  • Visual Landmarks: Following coastlines, rivers, and mountain ranges.
• Example: The Arctic Tern holds the record, migrating from its Arctic breeding grounds to the Antarctic and back each year.

1.4 Flying and Perching Mechanism in Birds

Flying Mechanism

Flight involves two main strokes:

1. Downstroke (Power Stroke):
   • The wing is pulled downwards and forwards by the massive pectoralis major muscle.
   • The wing is fully extended, and the feathers lock together to form an airtight surface, pushing down on the air to generate lift.
2. Upstroke (Recovery Stroke):
   • The wing is raised by the smaller pectoralis minor (supracoracoideus) muscle.
   • This muscle is located *under* the wing but pulls it up via a "rope-and-pulley" system, attaching to the top of the humerus via a tendon that loops through the shoulder.
   • The wing is partially folded, and the feathers twist to let air pass through, reducing drag.

Perching Mechanism

This is an automatic, passive mechanism that allows birds to grip a branch without conscious effort, even while sleeping.

• The bird has long flexor tendons that run from the leg muscles, down the back of the ankle joint, and into the toes.
• When the bird bends its ankle to squat on a perch, the tendons are automatically pulled tight.
• This tightening flexes the toes, causing them to lock around the perch. The bird's own weight maintains the grip.
• The toes will not release until the bird straightens its legs to fly away.

2. Class Mammalia

Mammals are a highly successful class of homeothermic vertebrates, known for their intelligence, complex social behavior, and adaptability.

2.1 General Characters and Classification

General Characters

• Mammary Glands: The unique diagnostic feature. Females produce milk to nourish their young.
• Hair: The body is covered in hair (or fur) for insulation.
• Thermoregulation: Homeothermic (warm-blooded).
• Heart: Four-chambered, with a left aortic arch.
• Ear Ossicles: Three bones in the middle ear (malleus, incus, stapes) that transmit sound.
• Diaphragm: A muscular sheet separating the thoracic and abdominal cavities, which aids in breathing.
• Teeth: Heterodont (different types: incisors, canines, premolars, molars) and Thecodont (set in sockets).
• Reproduction: Most are viviparous (give birth to live young).

Classification (Up to Order)

• Subclass Prototheria (Monotremes): Egg-laying mammals. (e.g., Order Monotremata: Platypus, Echidna).
• Subclass Metatheria (Marsupials): Pouched mammals. Give birth to very immature young that complete development in a pouch (marsupium). (e.g., Order Marsupialia: Kangaroo, Koala).
• Subclass Eutheria (Placental Mammals): Young develop fully inside the uterus, nourished by a complex placenta.
  • Order Primates: (e.g., Monkey, Ape, Human).
  • Order Rodentia: (e.g., Rat, Squirrel).
  • Order Carnivora: (e.g., Tiger, Dog, Bear).
  • Order Cetacea: (e.g., Whale, Dolphin).
  • Order Chiroptera: (e.g., Bats).
  • Order Proboscidea: (e.g., Elephant).

2.2 Echolocation in Bats

Echolocation is a biological sonar used by bats (and dolphins) to navigate and hunt in complete darkness.

Mechanism:

1. Emission: The bat produces extremely high-frequency, ultrasonic sound pulses. These are generated in the larynx and emitted through the mouth or nose.
2. Detection: The sound waves travel outwards, hit an object (like a moth), and bounce back as an echo.
3. Reception: The bat's large, complex ears detect the returning echo.
4. Interpretation: The bat's brain instantly processes the time delay, direction, and change in frequency of the echo.
   • Time Delay indicates distance.
   • Difference in sound between the two ears indicates direction.
   • Doppler Shift (change in pitch) indicates the speed and direction of the prey.

This system is so precise that a bat can determine the size, shape, texture, and movement of a tiny insect from several meters away, allowing it to "see" its world using sound.