INTRODUCTION: HUMAN ADAPTATION
An adaptation is any response to environmental conditions (stresses) that improves an organism's chance of survival and reproduction. Adaptation is an on-going process that continues as environmental conditions change; therefore, there is no such thing as an "end product" in adaptation. Types of environmental stresses include extreme temperatures and/or humidity, high altitudes, diseases, overpopulation, and ultraviolet radiation, among a myriad of other conditions. We must always clearly stipulate environmental stresses when studying adaptations.
There are five ways in which humans adapt to environmental stresses. Physiological adaptations are reversible, nonhereditary biological responses to environmental stresses that operate at the individual level. Developmental adaptations are reversible or irreversible, nonhereditary biological responses to environmental stresses that operate on the individual level. Genetic adaptations irreversible, hereditary biological responses to environmental stresses that operate at the population level. Behavioral adaptations are reversible, behavioral responses to environmental stresses that operate on the individual level. Finally, cultural adaptations are reversible, behavioral responses to environmental stresses that operate on the population level.
Physical anthropologists have investigated extensively human
adaptations
(and population-level variations in adaptations) to three environmental
stresses: heat stress, cold stress, and high altitude. In this
lab
we consider heat stress and cold stress.
OBJECTIVES
Upon successful completion of this lab, students will know how to
READINGS
Review lecture notes on adaptation.
HEAT STRESS
Heat stress can affect anyone anywhere in the world during any season. Regardless of climate, individuals have the potential of overheating as a result of intensive activity or extended periods of movement.
There is no significant variation among human populations in our ability to cope with heat stress, which may be due to our tropical origins (our species evolved in a tropical environment). While it may take Arctic dwellers longer to acclimate to hot conditions, they are no less able to adapt physiologically than other individuals and can do so in about one week.
Heat dissipation is the primary biological problem related to heat. It is the process of getting rid of excess body heat in order to avoid overheating. Overheating can be deadly for humans if an individual's core body temperature rises dramatically, such as to 104 degrees or more. Humans can tolerate less of a rise in internal body temperature than many other mammals.
Humans respond to heat stress physiologically, genetically, behaviorally, and culturally. Vasodilation, or the expansion of blood vessels, especially those near the surface of the skin or in the appendages, due to increased heart rate, is a physiological response. Vasodilation increases blood flow and, when accompanied by increased pulse rate, can lower internal body temperature by transferring heat to the body extremities and by spreading heat more evenly throughout the body. Heat transferred to the skin surface is lost to the surroundings through convection and radiation. The effectiveness of vasodilation depends on air temperature (the cooler the air, the more effectively heat is lost) and the amount of body movement (the less movement, the more effectively heat is lost).
Sweating, another physiological response to heat stress, is the release of water and minerals through the pores of the skin. Sweating is triggered by increases in body temperature that are not relieved by vasodilation. Sweating makes the skin wet, and when sweat is evaporated heat is released, thus reducing heat stress. The effectiveness of sweating as a cooling mechanism depends on humidity (the less humid, the more effectively heat is lost) and air movement (the more air movement, the more effectively heat is lost); it doesn't work in high humidity.
Genetic responses to heat stress include long linear body form (Bergmann's rule), long narrow appendage form (Allen's rule), and bipedalism. According to Pete Wheeler, bipedalism aids in body temperature regulation because it reduces the surface area exposed to the warming sun's rays and it increases body surface area exposed to cooling winds to make sweating more effective.
Behaviorally, individuals may engage in specific cooling behaviors -
such as swimming and eating ice cubes - in order to prevent
overheating.
Culturally, human responses to heat stress include light clothing,
construction
of shelters to block the sun and maximize the flow of breezes, cooling
systems like fans and air conditioners, and scheduling of daily
activities
to avoid the hottest times of the day.
COLD STRESS
It is harder for humans to adapt to cold stress than to heat stress because we didn't evolve in a cold environment. Unlike heat stress, there are differences in the degree of cold stress in the world. Near the tropics, where cold stress is not a major problem, adaptations are mostly cultural. Elsewhere, adaptations may be physiological or genetic. There are population-level variations in adaptation to cold stress, most of which are evident in the response of extremities to cold stress because they often are exposed, hard to protect, have more surface area, produce little heat, have little fat, and are far from the body core.
There are two major biological problems related to extreme cold temperatures: heat conservation and heat production. Heat conservation is retention of body heat to maintain the core body temperature in the acceptable range. Heat production is the generation of body heat to maintain the core body temperature in the acceptable range.
Humans respond to cold stress physiologically, genetically, behaviorally, and culturally. Vasoconstriction is a physiological response to the problem of heat conservation. It is the shrinking of the blood vessels, especially those in the extremities, in order to reduce heat flow away from the body trunk. This is the best response if temperatures are cold but not below freezing. One danger of vasoconstriction is the possibility of frostbite of the appendages, which can lead to death of tissues, gangrene, infection, loss of digits, death. Another physiological response related to heat conservation is insulation, which involves increasing the fat layer under the skin.
Shivering is a physiological response to the problem of heat production. It is the involuntary contraction of the muscles in order to generate heat. Shivering can increase heat production by as much as three times. Increasing one's basal metabolic rate (BMR) also leads to increased heat energy output and is another physiological response to heat production.
Genetic responses to cold stress that address the problem of heat conservation include short round body form (Bergmann's rule), short appendage form (Allen's rule), long and narrow nose form (Thomson's rule) to warm air, and cold-induced vasodilation (CIVD). CIVD is cyclical rewarming of the extremities through vasodilation. There is an alternation between vasoconstriction, which warms the body trunk, and vasodilation, which warms the extremities. CIVD functions to reduce the dangers of frostbite of the extremities. It is common in Arctic dwellers such as the Inuit ("Eskimos"), other Native Americans, and European-Americans, but it is not found among African and African-American populations. A genetic response to cold stress that addresses the problem of heat production is increasing one's basal metabolic rate (BMR), which leads to increased heat energy output.
Movement and exercise are behavioral responses to the problem of heat production. This may involve rubbing one's hands together, jumping, walking, and swaying.
Cultural responses to cold stress include clothing to conserve body
heat, use of shelters (insulation) to conserve body heat, use of fire
and
heating devices to produce heat, and increased fat consumption to
produce
heat.
LAB ASSIGNMENT
In this lab we'll document and analyze individual variation in
physiological
responses to environmental stress. The two adaptations we're
concerned
with are sweating as a response to heat stress and vasoconstriction as
a response to cold stress.
STEP ONE: HEAT STRESS
After normalizing everyone's body temperature, we'll all enter a
very
warm room and exercise until we begin to sweat. We'll record how
long it takes for each of us to begin sweating.
STEP TWO: COLD STRESS
We will submerge one arm to the elbow in ice water and record how
long
each of us can keep our arm submerged.
STEP THREE: ANALYSIS
All data (subject number, sex, body mass index, sweating time,
submersion time) will be recorded in pencil by each student on the
answer sheet provided.
We'll also calculate correlation coefficients for (a) body index and sweating and (b) body index and submersion for all subjects, males only, and females only. How strong are the correlations? Why?
For your analyses, please write in pencil and round answers to
hundredths
place. You either need to show your calculations if you calculate
the
statistics
by hand or turn in your spreadsheet if you run the statistics on the
computer.
WHAT TO TURN IN
Turn in the following items.
DUE DATE
The due date will be announced in class.
TERMS
REFERENCES
Brainard, Jean
1991Human Adaptation. The Ohio State
University,
Columbus.
Molnar, Stephen
2002 Human Variation: Races, Types and Ethnic Groups
(5th
ed.). Prentice Hall, Upper
Saddle River, NJ.