Animals and their Environment
At the top of the thermoneutral zone the upper critical temperature
marks the point at which whole body conductance is maximum. As the
upper critical temperature is approached latent heat loss is increased by
some energy-requiring process such as panting or sweating. (Sweating
is not a passive process; sweat has about half the concentration of salts
as normal body fluids so salts must be removed before secreting the
sweat.) Since evaporative cooling requires some energy, the metabolic
rate must increase somewhat as is shown in the diagram. As environmental
temperature increases, body temperature also increases somewhat and
this increases metabolic rate. At these high temperatures the only avenue
available for balancing the energy budget is the latent heat term. When air
temperature is equal to body temperature, the latent heat loss must equal
the metabolic heat production. If the air temperature is higher than body
temperature, this additional heat gain must also be dissipated as latent
heat. The rate of latent heat loss depends heavily on the vapor pressure
of the environment, and possibly on the boundary layer conductance, but
maximum values of latent heat loss for fairly arid environments range
from 200 to 400 W m-2.
The strategy for balancing the energy budget over a wide range of
environmental conditions should now be clear. The preferred mode is by
varying conductance. Below the thermoneutral zone, metabolic energy
is required to balance the energy budget, while above the thermoneutral
zone evaporation of water is required.
12.6 Operative Temperature
We returnnow to the definition of the operative temperature T, that was introduced inEq. (12.1 1). There we explainedthat the operative temperature
combines air temperature and radiation in a single equivalent temperature. This is a convenient way to represent the animal environment for
at least two reasons. First, temperature is intuitively useful because it is
easier for us to picture how an animal would respond to a 20°C change
in temperature than it is to picture how it would respond to a 400 W m-*
change in absorbed radiation. Second, a lot of the knowledge we have on
thermal physiology of animals comes from experiments conducted in environmental chambers. The operative temperature allows us to use results
of these experiments directly in outdoor situations where the combined
radiation and temperature produce an operative temperature equivalent
to conditions in the environmental chamber.
The operative temperature (sometimes called the equivalent blackbody
temperature) is the temperature of a blackbody cavity (with air temperature equal to wall temperature) that provides the same heat load (or cold
stress) as is present in the natural environment of the animal. Another
way of saying that the heat load is the same in the two environments
is to say that M - hE for the animal is the same in the two environments. Therefore a mathematical definition of the operative temperature
can be obtained by substituting Eq. (12.1 1) for M - AE in Eq. (12.10)
At the top of the thermoneutral zone the upper critical temperature
marks the point at which whole body conductance is maximum. As the
upper critical temperature is approached latent heat loss is increased by
some energy-requiring process such as panting or sweating. (Sweating
is not a passive process; sweat has about half the concentration of salts
as normal body fluids so salts must be removed before secreting the
sweat.) Since evaporative cooling requires some energy, the metabolic
rate must increase somewhat as is shown in the diagram. As environmental
temperature increases, body temperature also increases somewhat and
this increases metabolic rate. At these high temperatures the only avenue
available for balancing the energy budget is the latent heat term. When air
temperature is equal to body temperature, the latent heat loss must equal
the metabolic heat production. If the air temperature is higher than body
temperature, this additional heat gain must also be dissipated as latent
heat. The rate of latent heat loss depends heavily on the vapor pressure
of the environment, and possibly on the boundary layer conductance, but
maximum values of latent heat loss for fairly arid environments range
from 200 to 400 W m-2.
The strategy for balancing the energy budget over a wide range of
environmental conditions should now be clear. The preferred mode is by
varying conductance. Below the thermoneutral zone, metabolic energy
is required to balance the energy budget, while above the thermoneutral
zone evaporation of water is required.
12.6 Operative Temperature
We returnnow to the definition of the operative temperature T, that was introduced inEq. (12.1 1). There we explainedthat the operative temperature
combines air temperature and radiation in a single equivalent temperature. This is a convenient way to represent the animal environment for
at least two reasons. First, temperature is intuitively useful because it is
easier for us to picture how an animal would respond to a 20°C change
in temperature than it is to picture how it would respond to a 400 W m-*
change in absorbed radiation. Second, a lot of the knowledge we have on
thermal physiology of animals comes from experiments conducted in environmental chambers. The operative temperature allows us to use results
of these experiments directly in outdoor situations where the combined
radiation and temperature produce an operative temperature equivalent
to conditions in the environmental chamber.
The operative temperature (sometimes called the equivalent blackbody
temperature) is the temperature of a blackbody cavity (with air temperature equal to wall temperature) that provides the same heat load (or cold
stress) as is present in the natural environment of the animal. Another
way of saying that the heat load is the same in the two environments
is to say that M - hE for the animal is the same in the two environments. Therefore a mathematical definition of the operative temperature
can be obtained by substituting Eq. (12.1 1) for M - AE in Eq. (12.10)