Lecture 15 BIO120

Animal Ecophysiology

  • Heat balance/thermal ecology of animals

  • Modes of heat gain and loss; homeostasis

  • Size, shape, insulation, evaporative cooling, behavioural thermoregulation

  • Trade-off principle and adaptive compromises (example of weasel body shape)   


Biodiversity is more than just the number of species at a site; it’s also the diversity of their morphologies, physiologies, and behaviours


Physiological ideology

  • Physiologists study how organisms acquire energy and nutrients and tolerate physical conditions 

  • Ecologists study how organisms deal w their environment and how the environment limits where they live

  • Physiological ecology or “ecophysiology” is simply the study of physiology in the context of an organism’s ecology 


Core ideas of physiological ecology

  • Ranges of tolerance (lectures 13 & 14) ultimately limit distribution

  • Organisms are complex chemical reactions

  • Reactions occur (enzymes function) best at optimum temperature and osmotic conditions, where fitness is maximized 

  • Many mechanisms for homeostasis have evolved to challenge hostile environments

  • Maintenance of homeostasis requires energy and is often limited by constraints and tradeoffs


Organisms as adaptive solutions to environmental challenges:

  • Organisms physiology reflects climate and other conditions to which organism is adapted

  • Diff. environments lead to diff. Solutions (diff. physiologies)

  • Similar environments often lead to similar adaptations (even in diff. Taxa, a phenomenon called convergent evolution)

  • Example: Animals that live in cold places tolerate colder temperatures than animals that live in warm places 


Temperate animals withstand colder temperatures than tropical animals 

  • Data points are terrestrial arthropods, reptiles, and amphibians 

  • Jennifer Sunday → McGill → GRAPH (absolute latitude vs. Temp. tolerance limit) 


Temperate animals (animals that live at high latitudes) also tolerate a wider range of temperatures than tropical animals

  • Remember seasonal temp. Variation is low near the equator and increases w latitude

  • GRAPH → Sunday et al. → latitude vs. thermal tolerance breadth 


Heat balance especially important to homeotherms (birds, mammals) 

  • Poikilotherms (most reptiles, amphibians, fish, invertebrates) lack physiological means to deviate from environmental temperature (although they use behavioural means): their temperatures fluctuate

  • Homeotherms must regulate heat balance to keep internal temperature within a narrow range: many traits contribute 

  • Because maintaining a constant internal temperature requires energy, poikilotherms have lower energy requirements than similarly sized homeotherms 


Modes of heat gain or loss

  • Radiation → heat transfer by electromagnetic radiation

  • Conduction → transfer by direct contact w substrate (e.g., feet lose heat to ground) 

  • Convection → heat transfer mediated by moving fluid (usually air or water) 

  • Evaporation → efficient cooling from wet surfaces

  • Redistribution → circulatory system redistributes heat among body parts, esp. Core to appendages 


Size matters to hear balance (and other balances of gains and losses) 

  • Homeostasis and surface area: volume (SA:V)

  • Surface Area determines equilibration rate 

  • Volume provides the inertia


Size matters

  • Bergmann’s rule: Homeotherms tend to be larger at higher latitudes (colder) 

  • Sun Bear 65kg short fur, Black bear 275kg medium fur, Polar bear 650kg long fur. 

  • Bergmann’s rule and climatic adaptation in woodrats → woodrats in warmer places tend to be smaller than woodrats in colder places

  • An exception… What about elephants? They are big tropical animals. Not really an exception, woolly mammoths very closely related to african and asian elephants alive today, were very very large and lived in cold places. 


Shape matters: 

  • A sphere has the smallest SA:V, so why aren't homeotherms always spheres in cold climates 

  • Sometimes SA is needed for function

  • Sometimes particular shapes are needed for function 

  • Tradeoffs and adaptive compromises


Who has the maximum SA:V ratio? 

  • Chrysopelea gliding snake, Borneo; restricted to warm tropics


Who has the minimum SA:V ratio?

  • American Pika, Ochotona princeps: alpine tundra rabbit: restricted to cold habitats; note spherical shape, reduced ears (for a rabbit)


Allen’s rule: homeotherms tend to have smaller appendages at higher colder latitudes 

  • Big ears facilitate heat loss to the environment

  • Arctic fox: smallest ears, Red fox medium ears neither super warm nor super cold → Toronto, Fennec fox massive ears lives in sahara.

  • Arctic hares vs. desert hares → appendages reduced in cold climates


What else matters? Insulation is even more important than size/shape 

  • Fur on muskoxen

  • Blubber on seals 

Thick insulation:

 half-sheared sheep → see picture 

  • Took 30 tries, wool on one side of the body only was so heavy the sheep kept falling over when they were sheared on only one side

Blubber of seal makes up 58% of cross-section, 42% are guts and musculature.


What else matters? Convective cooling enhanced by vascularization 


What else matters? Countercurrent circulation to libs conserves heat 

  • Arteries and veins should be appressed in appendages to conserve heat; separated in appendages to conserve heat; separated in appendages designed to shed heat

  • Countercurrent flow maintains gradient, so heat is always flowing from outgoing blood to incoming blood


What else matters? Evaporative cooling → humans sweat, cools us down, dogs stick out tongues and cool bc tongue is wet surface


Behaviour counts too, animals will seek out cooler places (shade) to cool off, etc. 


Evaporative cooling, kangaroo licks paws, sticks them out, elephants spray water on themselves


Reconciling an apparent paradox: weasels are small predators, short furred, very long and thin. 


Weasel in winter = ermine: active all year; camouflaged for snowy environment 


Metabolism of weasels: the costs of being long and thin. 

  • Metabolism of weasels is 50-100 percent more expensive than other animals of the same size roughly

  • Weasels are predators, typical weasel prey: pocket gopher thomomys talpoides, seldom leaves underground burrows; mostly eats roots, tubers, so, the paradox involve the requirements of the weasel’s predatory lifestyle 

  • Helps weasels to be long and thin for hunting


Skinny weasel in cold climates = example of a trade-off: 

  • Being long and thin makes weasels subject to thermal stresses (costly)...

  • … but allows them to be better predators (beneficial) 

  • Because they are long and thin, we infer that the fitness gains of being a good hunter offset the fitness costs of an expensive metabolism

  • If they can get enough prey, they can stay warm despite their heat-wasting shape

  • Phenotypes of all organisms are riddled with compromises dictated by trade-offs


Two reasons why natural selection produces deeply imperfect organisms

  • Trade offs

→ Being good at x may necessarily imply being bad at y

  • Constraints

→ selection builds on what is already there, especially existing developmental programs

→ Tinkering, yes; fundamentally fresh redesign, no