Homeothermy and temperature regulation

Why is body temperature (T) important? Relationship between metabolic enzymes and temperature

1. Definitions
   1. Ectothermy: Internal heat sources are small; body T largely dependent on external sources of heat
   2. Endothermy: Internal heat sources are a significant determinant of body T
   3. Homeothermy: The ability to maintain a steady body T that is independent of environmental (ambient) T
      Endothermy and homeothermy are distinct concepts (see Fig. 1), but they are tightly linked in mammals. Homeothermic endotherms defend a temperature "set point" (metabolic thermoregulation).

2. Costs and benefits of homeothermy
   1. Benefits
      1. Faster diffusion (greater biochemical efficiency)
      2. More rapid cellular resposes (e.g., neurons)
      3. More rapid and stronger muscle contractions
      4. Ability to be active in otherwise inhospitable environments (e.g., polar regions)
      5. Freedom from dependence on sunlight to regulate body T
      6. Allow for coordination of biochemical processes, since they're all optimized to operate in the same range of temperatures. This in turn enables a more complex physiology.
      7. Increased ability to sustain a high level of activity
   
   
   
   2. Costs
      1. Increased energy requirements (roughly an order of magnitude higher at room temperature)
         • Greater risk of starvation
         • Fewer resources to devote to growth and reproduction
      2. Increased evaporative water loss because of higher respiratory rate
      3. Increased susceptibility to thermal stress - because enzymes have evolved to operate within a narrow range of temperatures

3. How is body T maintained ( thermoregulation )?
   1. Behavioral thermoregulation
   2. Metabolic thermoregulation
   1. Environmental heat loss/gain
      • Human example
      • Non-human example (see Fig. 1)
      • High resting metabolic rates
      • Thermal inertia leads to systematic differences in metabolic rate in mammals of different sizes (metabolic scaling)
      • Video on metabolic scaling: How to make an elephant explode with science
      • Tactics to balance internal heat production with environmental heat gain/loss

4. Characteristics of endothermy (suggested reading)
   
   
   
   1. Thermoneutral zone (TNZ): The range of temperatures at which no extra energy is required to maintain homeothermy.
   2. Lower critical temperature (LCT): The lower temperature threshhold at which the animal has to increase its metabolism to maintain body temperature.
   3. Upper critical temperature (UCT) The upper temperature threshhold at which the animal has to increase its metabolism to cool the body.

   Homeothermy worksheet #1

   
   
   

5. Special temperature regulation strategies:
   1. Hypothermia
      1. Benefits
      2. Can be daily, seasonal
      3. Examples: hedgehogs, some bats, some primates, carnivores, rodents
      4. Characteristics of the hypothermic state
         1. Shivering is suppressed
         2. Body T falls to within a few degrees of ambient T (unless ambient T falls below freezing)
         3. Metabolic rate decreases, leading to
            • Reduced respiration rate
            • Reduced heart rate
      5. Warming back up
         1. Metabolism of brown fat
            • distribution of brown fat
            • Mitochondrial content of white and brown fat cells
            • Heat production through uncoupling proteins
         2. Shivering

      6. Hypothermia occurs in bouts (see Fig. 1), with interspersed warming periods
         
         

   Think-Pair-Share Why might they periodically warm up during hibernation?

   2. Heat loading
      • Daily fluctuations in body T (see Fig. 1)
      • Effect of water availability on body T
      • Countercurrent heat exchange in blood vessels leading to and coming from the brain
      • Video (skip to 33:50)

Basic skeletal anatomy

Skeletal structure

1. Basic parts
   1. Skull=cranium + mandible
   2. Postcranial skeleton=axial skeleton + appendicular skeleton
   
   
2. Skull
   1. Landmarks
      1. Foramen magnum
      2. Rostrum
      3. Turbinal bones (AKA turbinates)
      4. External auditory meatus
      5. Auditory bullae
      6. Maxilla
      7. Mandible
      8. Zygomatic arch
      9. Sagittal crest
      
      
   2. Views
      • Ventral view
      • Close-up ventral view
      • Dorsal view
      • Lateral view
      • Mandible
   
   
3. Axial skeleton (shown in pink)
   1. Vertebrae
   2. Ribs
   
   
4. Appendicular skeleton
   1. Pectoral girdle + forelimbs + manus + pollex + phalanges
   2. Pelvic girdle + hindlimbs + pes + hallux + phalanges
   
   

Structures associated with homeothermy (*diagnostic)

1. Skeletal features
   1. Single bone in mandible*
   2. Squamosal-dentary articulation*
   3. Heterodont, diphyodont dentition
   
   
2. Soft tissues
   1. Hair*
      1. Anatomy (See also Fig 6.2)
      2. Functions (insulation, protection, communication)
      3. Types (based on function and how they grow: definitive vs. angora)
         • Vibrissae
         • Guard hairs (spines, bristles, awns)
         • Under hairs (wool, fur, velli)
         
         
      4. Color patterns
         • Cryptic
         • Reflective/filtering
         • Countershading
         • Disruptive
         • Warning
         
         
      5. Pigments
         • Eumelanin (blacks to browns)
         • Pheomelanin (reds to yellows)
         • Combinations
           • agouti
           • Close up of hairs (Fig. 2) comparing agouti and non-agouti
           and genetic regulation of coloring patterns
         
         
      6. Moulting and hair replacement
      
      
   2. Glands associated with hair
      1. Sebaceous glands*
      2. Sweat glands* (not all mammals)
      
      
   3. Muscular diaphragm*
      
      
      • Associated with the loss of lumbar ribs (see slide 9) (also see comparison with modern reptiles)
      • Allows abdominal organs to be displaced outwards as thoracic cavity expands
      
      
   4. Four-chambered heart with left aortic arch*
      • Evolution of the four chambered heart -> high oxygenation and high pressure

   5. Enucleate erythrocytes*
      image
      colored scanning electron micrograph

Dentition and feeding

I. Teeth

1. Heterodont dentition
   1. Incisors: cutting, gathering food
   2. Canines: capturing food, communication displays
   3. Premolars: grinding or slicing
   4. Molars: grinding or slicing
   5. Premolars + molars often hard to differentiate, so referred to as "cheek teeth"

2. Tooth structure
   1. crown
   2. root
   3. enamel
   4. dentine
   5. pulp cavity
   6. cementum
   
   
3. Tooth growth
   1. closed-rooted
   2. open-rooted (ever-growing)
   
   
4. Tooth replacement
   1. Deciduous (milk) teeth
   2. Horizontal vs. vertical replacement
   
   
5. Occlusal surfaces
   • For grinding plant material
   • For gnawing plant material
   • For mashing soft food
   • For slicing off hunks of flesh
   • In some species, only a subset of teeth occlude at any given time and the animal must adjust its jaw position to allow other teeth to occlude, e.g., rodents
   
   
6. Variations on the basic pattern
   1. Homodont dentition
   2. Edentate
   3. Tusks
   4. Diastema
   
   
7. Dental formulae (e.g., 3/3, 1/1, 4/3, 1/1)

II. Foods and Feeding

1. Anatomy of the digestive system
2. Modes of foraging
   1. Insectivory
      1. aerial
      2. terrestrial
      3. myrmecophagy (ant-eating; see first minute or so of video)
      
      
   2. Carnivory
      1. terrestrial
      2. aquatic
      3. sanguinivory (blood feeding; see video starting at about 1:00)
      
      
   3. Herbivory
      1. ruminants
      2. hind gut fermenters
      3. granivory (chipmunk stuffing cheek pouches with seeds; start at 1:05)
      4. folivory
      5. frugivory
      6. nectivory
      7. gummivory
      
      
   4. Filter feeding
   5. Omnivory
   
   
3. Anatomical specializations associated with different modes of foraging
   

   Mode of foraging	Teeth	Tongue	Stomach	Intestines	Cecum
   Insectivory	numerous, spiky, incisors procumbent Example: mole Example: shrew	--	simple	short	mostly lacking
   Myrmecophagy	absent or reduced in numbers, peg-like Example: tamandua anteater	extremely long	simple, often roughened	short	small or lacking
   Terrestrial carnivory	sharp incisors; long, conical canines; often carnassial cheek teeth; may have crushing molars Example: dog	--	simple	short	small
   Aquatic carnivory	homodont, spiky, numerous Example: common dolphin	--	simple or multichambered (cetaceans only)	variable	small or absent
   Sanguinivory	very sharp upper incisors; reduced cheek teeth Example: vampire bat	grooved	tubular, highly extensible	long	small or lacking
   Herbivory (except nectivores)	incisors robust or absent; canines reduced or absent; diastema; cheek teeth enlarged with complex occlusal surfaces Example: beaver	--	simple (hindgut fermenters) or multichambered (ruminants)	long	large
   Filter feeding	none (baleen) Example: blue whale	--	multichambered	--	present (for digesting chitin)
   Omnivory	sharp incisors and canines; flat cheek teeth with rounded cusps Example: bear	--	simple	long	small