22 - Mammals

Endothermy

  • Definition: Endotherms are organisms that generate their own body heat.

  • Requirements: Evolved changes enhance efficiency in:

    • Prey acquisition (e.g., catching prey)

    • Prey processing (chewing and digesting)

    • Oxygen delivery (to muscles for pursuit)

Mammal Skulls

Skeletal Synapomorphies

  • First: Lower jaw (dentary) composed of one bone; loss of the joint between articular and quadrate.

  • Second: New roles for quadrate and articular in hearing.

    • Hyomandibula: stapes

    • Articular: malleus

    • Quadrate: incus

  • Adaptation satisfies the conflict between hearing and feeding due to increased metabolic demands in endothermic mammals.

Mammal Teeth

  • Diphyodont: Two sets of teeth (deciduous and permanent).

  • Heterodont: Four types of teeth (incisors, canines, premolars, molars) adapted to diet.

  • Occlusion: Precise cusps interdigitate for efficient chewing, increasing surface area for digestive enzymes.

Mammal Skeletons

General Features

  • Posture: Upright, with few exceptions.

  • Pectoral Girdle: Clavicle and coracoid reduction; flexible scapula.

  • Rib Structure: Only thoracic ribs; 1st 7 pairs attached to the sternum (true ribs).

  • Spine Flexibility: Enhanced dorsoventral flexion; typically 7 cervical vertebrae.

Limb Adaptations

  • Trade-off: Speed vs. power in locomotion modifications.

  • Locomotion Types: Plantigrade, digitigrade, unguligrade.

Fossorial Specializations in Armadillos

  • Scapula: Long posterior angle for muscle leverage.

  • Humerus: Thick and specialized for digging.

  • Forearm Adaptation: Shorter out-lever increases power.

Flight in Bats

  • Bats are the only mammals capable of powered flight.

  • Unique hand bone structure differing from birds and mammals.

Semi-Aquatic Mammals

  • Adaptive characteristics for swimming and agility on land.

  • Different shoulder and limb musculature among different seal species.

Modes of Locomotion

  • Different Modes: Cursorial, arboreal, saltatorial, fossorial, aerial, bipedal, swimming.

Soft Structures

Functions of Adipose Tissue

  • Storage: Beneath skin and around organs for insulation, energy, and cushioning.

  • Types of Adipose Tissue:

    • White adipose tissue (WAT) – typical body fat.

    • Brown adipose tissue (BAT) – metabolically active; generates heat for endothermy.

Respiratory System

  • Lungs with Alveoli: Gas exchange via alveolar sacs.

  • Oxygen and CO2 Exchange: Efficient transfer into and out of blood.

Blood Characteristics

  • Red Blood Cells: Lack nuclei.

  • Platelet Presence: Essential for clotting; unique among vertebrates.

  • Heart Structure: Separates oxygenated and deoxygenated blood.

Brain and Cognition

  • Brain Size: Largest relative to body size among vertebrates.

  • Olfactory System: Enhanced olfactory bulb for strong sense of smell.

Integumentary System

  • Skin Functions: Camouflage, communication, and sensation.

  • Gland Types:

    • Sebaceous, apocrine (communication), and eccrine (sweat for thermoregulation).

Visual System

  • Retinas: Dominated by rod cells; good night vision but poor in color.

  • Tapetum Lucidum: Reflective layer enhancing visual sensitivity in low light.