Post by djoser-xyyman on May 26, 2010 9:37:50 GMT -5
In 1847, Carl Bergmann observed that within the same species of warm-blooded animals, populations having less massive individuals are more often found in warm climates near the equator, while those with greater bulk, or mass, are found further from the equator in colder regions. This is due to the fact that big animals generally have larger body mass which results in more heat being produced. A normal byproduct of metabolism in cells is heat production. Subsequently, the more cells an animal has, the more internal heat it will produce. In addition, larger animals usually have smaller surface areas relative to their body masses and, therefore, are comparatively inefficient at radiating their body heat off into the surrounding environment. In 1877, Joel Allen noted that among warm-blooded animals, individuals in populations of the same species living in warm climates near the equator tend to have longer limbs than do populations living further away from the equator in colder environments. This is due to the fact that a thin body with relatively long appendages is less compact and subsequently has more surface area. The greater the surface area, the faster body heat will be lost to the environment. In extremely cold environments, a stocky body with short appendages would be more efficient at maintaining body heat because it would have relatively less surface area compared to body mass.
This same phenomenon can be observed among humans. Members of the Masai tribe of East Africa are normally tall and have slender bodies with long limbs that assist in the loss of body heat. This is an optimal body shape in the hot tropical parts of the world but would be at a disadvantage in subarctic regions. In such extremely cold environments, a stocky body with short appendages would be more efficient at maintaining body heat because it would have relatively less surface area compared to body mass.
The Ju/'hoansi of Southwestern Africa and the Aborigines of Australia usually respond physiologically to the cold in a different way. Thick fat insulation develops around the vital organs of the chest and abdomen. In addition, their skin cools due to vasoconstriction at night. As a result, heat loss is reduced and the core body temperature remains at normal levels. However, the skin feels very cold.
This response would not be adaptive if the Kung and the Aborigines lived in consistently freezing environments because the concentration of body heat in their torsos would allow the loss of fingers, toes, and other appendages from frostbite. Their physiological adaptation is to environments that rarely stay below freezing long and that do not have abundant high calorie fatty foods.
Hot Climate Responses
Adapting to hot environments is as complex as adapting to cold ones. However, cold adaptation is usually more difficult physiologically for humans since we are not subarctic animals by nature. We do not grow dense fur coats nor do we usually have thick layers of fat insulation like polar bears.
The effect of heat on our bodies varies with the relative humidity of the air. High temperatures with high humidity makes it harder to lose excess body heat. This is due to the fact that when the moisture content of air goes up, it becomes increasingly more difficult for sweat to evaporate. The sweat stays on our skin and we feel clammy. As a result, we do not get the cooling effect of rapid evaporation.
In dry hot weather, humidity is low and sweat evaporates readily. As a result, we usually feel reasonably comfortable in deserts at temperatures that are unbearable in tropical rain forests. The higher the desert temperatures, the more significant of a cooling effect we get from evaporation. This relationship between relative humidity and air temperature is quantified below. When the apparent temperature is in the light yellow range, heat exhaustion and cramps are likely for humans. In the bright yellow range, life threatening heat stroke is likely.
While evaporative cooling is very effective in dry climates, there is a major drawback. That is the rapid loss of water and salts from the body through sweat. This can be fatal in less than a day if they are not replaced. It is common to lose a quart or more of water through sweating each hour in harsh summer desert conditions. Commercial "sport drinks" are designed to help people in these situations rehydrate and replenish lost mineral salts. It is easy and inexpensive to create your own equivalent drink without the unnecessary food coloring and sugar that the commercial drinks often include to make them more appealing to customers.
Most people have the ability to physiologically acclimatize to hot conditions over a period of days to weeks. The salt concentration of sweat progressively decreases while the volume of sweat increases. Urine volume also reduces. In addition, vasodilation of peripheral blood vessels results in increased heat loss through radiation. Vasodilation also causes flushing, or reddening, of the skin since more blood is close to the surface.
This same phenomenon can be observed among humans. Members of the Masai tribe of East Africa are normally tall and have slender bodies with long limbs that assist in the loss of body heat. This is an optimal body shape in the hot tropical parts of the world but would be at a disadvantage in subarctic regions. In such extremely cold environments, a stocky body with short appendages would be more efficient at maintaining body heat because it would have relatively less surface area compared to body mass.
The Ju/'hoansi of Southwestern Africa and the Aborigines of Australia usually respond physiologically to the cold in a different way. Thick fat insulation develops around the vital organs of the chest and abdomen. In addition, their skin cools due to vasoconstriction at night. As a result, heat loss is reduced and the core body temperature remains at normal levels. However, the skin feels very cold.
This response would not be adaptive if the Kung and the Aborigines lived in consistently freezing environments because the concentration of body heat in their torsos would allow the loss of fingers, toes, and other appendages from frostbite. Their physiological adaptation is to environments that rarely stay below freezing long and that do not have abundant high calorie fatty foods.
Hot Climate Responses
Adapting to hot environments is as complex as adapting to cold ones. However, cold adaptation is usually more difficult physiologically for humans since we are not subarctic animals by nature. We do not grow dense fur coats nor do we usually have thick layers of fat insulation like polar bears.
The effect of heat on our bodies varies with the relative humidity of the air. High temperatures with high humidity makes it harder to lose excess body heat. This is due to the fact that when the moisture content of air goes up, it becomes increasingly more difficult for sweat to evaporate. The sweat stays on our skin and we feel clammy. As a result, we do not get the cooling effect of rapid evaporation.
In dry hot weather, humidity is low and sweat evaporates readily. As a result, we usually feel reasonably comfortable in deserts at temperatures that are unbearable in tropical rain forests. The higher the desert temperatures, the more significant of a cooling effect we get from evaporation. This relationship between relative humidity and air temperature is quantified below. When the apparent temperature is in the light yellow range, heat exhaustion and cramps are likely for humans. In the bright yellow range, life threatening heat stroke is likely.
While evaporative cooling is very effective in dry climates, there is a major drawback. That is the rapid loss of water and salts from the body through sweat. This can be fatal in less than a day if they are not replaced. It is common to lose a quart or more of water through sweating each hour in harsh summer desert conditions. Commercial "sport drinks" are designed to help people in these situations rehydrate and replenish lost mineral salts. It is easy and inexpensive to create your own equivalent drink without the unnecessary food coloring and sugar that the commercial drinks often include to make them more appealing to customers.
Most people have the ability to physiologically acclimatize to hot conditions over a period of days to weeks. The salt concentration of sweat progressively decreases while the volume of sweat increases. Urine volume also reduces. In addition, vasodilation of peripheral blood vessels results in increased heat loss through radiation. Vasodilation also causes flushing, or reddening, of the skin since more blood is close to the surface.
