Human Adaptation III
- Quiz is next Tuesday
Humans and the Environment
- In this lecture we will review three examples of how humans respond physiologically and
morphologically to environmental challenges.
- Physiological response to temperature stress.
- Climate and morphological variation.
- Hypoxia and adaptation to high altitude.
Physiological Response to Temperature Stress
- Humans, as most other mammals, have very efficient systems to regulate internal body
o Optimal temperature is around 37
o It is dangerous for the internal body temperature to be higher than 40
(hyperthermia), and if below 35 (hypothermia)
- Let’s review some physiological responses to cold and heat stress
Physiological Responses to Cold Stress
- Vasoconstriction: Narrowing of blood vessels to reduce blood flow and heat loss.
o Tissue and organ damage
- Vasodilation: Opening of blood vessels to increase blood flow and heat loss.
- Lewis hunting phenomenon: After initial exposure to cold stress, there are alternating
cycles of vasoconstriction and vasodilation. After some time the cycles become more
frequent and less extreme, maximizing the efficiency of the process.
- First you have more dramatic cycles, that eventually stabilize and become less extreme
o This is a way in which the efficiency maximizing
Physiological Responses to Heat Stress
- Heat is lost from the body in four different ways.
o Radiation. Heat flow in the form of electromagnetic radiation
o Convection. Heat flow through air molecules
o Conduction. Heat exchange through physical contact with other objects
o Evaporation. Heat loss through conversion of water to vapor. In the process of
sweat evaporation, heat energy is consumed. This is the only process that
results in heat loss without heat gain.
Coping With Heat Stress
- At high temperatures, our bodies’ response is mainly based on two mechanisms: o Vasodilation. Opening of blood vessels means increased transfer of internal heat
to the surface, where is lost by radiation, convection and evaporation.
Blood vessels dilate, and the you exchange more heat to the environment
o Sweat Evaporation. This is the most effective mechanism of heat loss.
Normally, it accounts for only 23% of total lost, but at hot temperatures it can
represent 90% of the total loss. Sweat evaporation is more effective in dry
conditions than in humid conditions.
In hot conditions, this will represent a high percentage of heat
- Mechanisms of heat loss in a moderate climate. At hot temperatures (35°C), sweat
evaporation can constitute 90% of total heat loss.
Climate and Morphological Variation
- Body size and proportions are important for the regulation of the internal body
- There is a general relationship between body size and shape with latitude in many
mammals and birds.
- The relationship between body size and temperature was first noted by a 19 century
German scientist, Karl Christian Bergmann.
o He noticed animals living in high latittudes tend to be bigger and larger than
animals living in the tropics
- An American scientist, Joel Allen, noted the importance of body shape, particularly the
o He noticed the same thing as Bergmann but for body shape (appendages)
- Let’s review Bergmann’s and Allen’s rules
Bergmann’s Rule (Will ask on the exam)
- Body size tends to be greater in populations living in colder climates.
- As body mass increases, the relative surface area decreases proportionately.
- Because heat is lost at the surface, increased mass allows for greater heat retention, and
reduced heat loss.
o The ratio of surface and volume will be much larger: there is more surface than
volume with an animal of smaller size
o There is more efficient way reduce heat loss and that is why animals (mammals
or birds) tend to be larger in cold environments
Allen’s Rule (Will ask on the exam)
- Shorter appendages are adaptive in colder climates, because the decreased
surface/volume area is more effective reducing heat loss.
- Longer appendages are adaptive in warmer climates, because increased surface/volume
area promote heat loss.
o There is more efficient transferring of heat because there is a high ratio
o You tend to see this (with exceptions) in many animal groups
Do Bergmann’s and Allen’s Rules Hold For Humans?
- As you can see in the picture bellow, in general, human populations conform to those two