342 Comparative Animal Physiology 



important in determining the thermal properties of animals in relation to their 

 environment. Animals are 70 to 90 per cent water. The heat conductivity of 

 water is low, compared with that of many materials, being only 0.0014 cal./ 

 cm./sec./° C, compared with 0.9 for copper. The heat conductivity of water 

 is higher than that of other liquids, such as ethyl alcohol (0.00042) and olive 

 oil (0.000395). The thermal conductivity of muscle is a little lower than 

 that of water (0.001 cal. /cm./ sec./ ° C), and the conductivity of fat is one- 

 third less. Low heat conductivity results in slow warming or cooling of an 

 aquatic medium, and in limited conduction of heat within an animal. Also, 

 the specific heat of water is high, 1.0 cal./gm./° C. at 0° C, compared with 

 0.09 cal./gm./° C. for copper, 0.535 for ethyl alcohol, and 0.389 for benzene. 

 Animal tissues, except compact bone, require 0.7 to 0.9 calories to raise the 

 temperature of one gram of tissue one degree. Hence animals with much 

 tissue mass are slow to warm or cool. Most transfer of heat in animals is by 

 circulating body fluids, and a sluggish circulation makes for slow heat transfer. 



The normal vital temperature range is based on the thermal properties of 

 water. Natural waters (hot springs excepted), because of their high specific 

 heat and small heat conduction, rarely have a temperature above the upper 

 limit for most aquatic animals— 35-40° C. At the cold extreme, the freezing 

 point of aquatic animals (except marine bony fish and brine shrimps) is 

 normally similar to or lower than the freezing point of the medium (Ch. 2). 

 Since ice has a lower specific gravity than water, aquatic animals do not freeze 

 so long as they remain in water. The thermal properties of water make it an 

 ideal solvent as a medium and as the principal cytoplasmic solvent.'''^ 



Terrestrial animals are subject to a much greater change of temperature than 

 aquatic, but here too body temperature is closely related to water balance. 

 Water has a high heat of fusion (79.7 cal./gm.), aqueous solutions supercool 

 by several degrees, especially in capillary spaces, and bound water is resistant to 

 freezing. Hence partially dehydrated animals can withstand temperatures well 

 below the freezing point of water without their tissues becoming frozen. In 

 high air temperatures animal body temperatures are limited by the high heat 

 of vaporization of water (586 cal./gm. evaporated at 0°, compared with 55 

 cal./gm. lor CO^ or 302 cal./gm. for ammonia). Thus water loss by vaporiza- 

 tion has a marked cooling efl:'ect on any moist surface. An understanding of 

 both water balance in animals and the thermal properties of water is essential 

 to an understanding of the temperature relations of animals. 



TEMPERATURE CHARACTERISTICS 



Chemical reactions are accelerated as temperature rises: decrease in a 

 reaction rate as temperature rises in the natural range does not occur in 

 biological systems. To describe the magnitude of the temperature elfect on a 

 variety ol processes several methods are in use. An approximation to describe 

 the effect ol a narrow specified temperature range is the Qm, which is the 

 factor by which a reaction velocity is increased for a rise in temperature of 10 

 degrees. 



10 



Q,„=(K,/K,) t^ - t^ 

 where Ki and Kj are velocity constants corresponding to temperatures ti and 



