Mammalian Characteristics
I. Success due to a suite of important (and diagnostic) characteristics
A. Enhance intelligence and sensory ability
B. Promote endothermy → allowed mammals to survive under a wide array of environ. conditions
C. Increase efficiency of reproduction
D. Securing and processing food → considerable variation in dentition
E. Basic mammalian body inherited > 200 mya
I. Soft anatomy and internal systems
A. Hair → the most distinguishing, and obvious, characteristic
1. The coat of hair covering body is called pelage
2. A new epidermal structure in the evolution of the outer covering
3. Primary function → thermal insulation
a. Homeothermy → maintain relatively stable internal body temperature
b. Endothermy → shared with birds
c. These two conditions permit
1) High level of activity at night
2) Year round penetration into low temperature habitats
4. Other functions
a. Protective coloration and concealment
b. Waterproofing and buoyancy
c. Behavioral signaling
B. Skin glands
1. Skin contains several kinds of glands unique to mammals
2. Mammary glands (mammae) → nourishment of young
a. Key feature which give mammals their name
c. Provides milk for young
d. Long period of parental care
3. Sweat glands
a. Help regulate body temperature → evaporative cooling
b. Eliminates some waste
4. Sebaceous glands
a. Located at hair follicles
b. Secrete oily secretion → lubricates hair and skin; form of waterproofing
c. Abundant on semiaquatic mammals → beaver, mink, etc.
5. Scent and musk glands
a. Attracting mates
b. Marking territories
c. Commication during social interactions
c. Protection → skunk
C. Fat and energy storage
1. Not unique but vital to mammals
2. Three major functions
a. Energy storage
b. Source of heat and water
c. Thermal insulation
3. Many mammals rely on fat stores to survive on during limited supply of food or high
metabolic demand
a. Hibernation
b. Reproduction and lactation
D. Circulatory system
1. Highly efficient
a. Closed system → no mixing of blood with coelomic fluid
b. Double circulation
2. 4 chambered heart → double pump
a. Right side receives oxygen low blood and pumps it to lungs
b. Left side receives oxygenated blood and pumps to body
c. With functional left aortic arch (birds have right aortic arch)
3. Biconcave enucleate blood cells → increases oxygen carrying capacity
E. Respiratory system
1. Lungs → large, filling thoracic cavity along with heart
2. Gas exchange takes place across the membrane of lungs
3. Muscular diaphragm → helps in respiration; unique to mammals
F. Reproductive system
1. Internal fertilization
2. In females, both ovaries are functional
3. Testes lie outside of coelomic cavity in scrotum
2. Give birth to live young → viviparous
G. Muscular system
1. Limb and trunk musculature highly adaptable
2. Great development of dermal musculature → skin moves independently of underlying
tissues
H. Very advanced nervous system
1. Brain unusually large compared to other vertebrates relative to rest of body
2. Highly elaborate sense organs
a. Highly developed hearing → in 18% of mammals hearing substitutes for vision
b. Acute smell → muscosal surfaces of the skull (nose area)
3. Provides an unequaled level of environmental awareness and responsiveness
II. The skeleton
A. Important morphological trend → skeletal simplification
1. Bones more completely ossified
2. Considerable fusion of bones
3. Greater flexibility of axial skeleton → limbs greater speed and range of movement
4. May be advantageous in terms of metabolic economy
B. Subdivided into three sections
1. Skull (cranium)
2. Axial skeleton → spinal column and rib cage
3. Appendicular skeleton → pectoral and pelvic girdles, forelimbs (arm), and hind limbs
D. Skull → cranium and rostral region
1. No joints → exceptions are jaw articulation and middle ear joints
2. Functions
a. Protect brain → primary function
b. Provides surface from which temporal muscles originate → close jaws
3. Rostral (facial) region
a. Single dentary bone → lower jaw or mandible
b. Secondary palate → allows eating and breathing at same time
c. Upper jaw → maxilla
4. Secondary palate (also in crocs) and epiglottis
a. Bony extension capable of closing the external nasal openings from the interior
nasal passages and allowing simultaneous chewing and breathing.
b. Structure that closes off windpipe when we swallow
5. Three middle ear bones
a. Incus, spates, an malleus
b. Birds and reptiles have only one → stapes
6. Two occipital condyles to articulate with axial skeleton
E. Axial skeleton
1. Vertebral column
a. Allows for greater freedom of head movement than reptiles
b. Powerful dorsoventral (instead of lateral) flexion of the spine
2. Five well-differentiated sections
3. Sternum → anchors the ventral ends of the rids → rigid rib cage
F. Appendicular skeleton → limbs and girdles
1. Limbs beneath organism instead of out to side
2. Positioned according to type of locomotion
3. Pelvic girdle
a. Ilium elongated and projecting forward
b. Ischium and pubis extend backward
4. Pectoral girdle → fusion and lose of some bones
G. Dentition
1. Teeth are one of the most important characteristics → major keys to mammalian success
2. Determines feeding mode of mammal → reflects trophic level and feeding specialization
3. Some mammals lack teeth → edentate
a. Monotremes
b. Mammals that feed on termites and ants
1) Anteaters (Xenarthra: Myrmecophagidae)
2) Pangolins (Pholidota)
3) Baleen whales (Cetacea: Mysticetes)
4. Heterodont dentition → vary in form and function
a. Occur on
1) Premaxillary bone
2) Maxillary bone
3) Dentary bone
b. Some mammals have homodont dentition
1) Toothed whales (Odontocetes)
2) Armadillos (Xenarthra: Dasypodidae)
c. Diphyodonty → two generations of teeth
1) Deciduous dentition → baby teeth
2) Permanent set
5. Tribosphenic pattern → basic modern mammalian tooth
a. All modified teeth derived from this pattern
b. Early mammals → cusps arranged longitudinally
c. Three cusps arranged in triangle
d. Arrangement gives crushing and shearing function