GRAVITY, PRESSURE, AND SOUND 



135 



as one phase of the physicochemical envi- 

 ronment. 



Mechanical pressure may impinge locally 

 on a small part of an animal, or the whole 

 body may be subjected to altered pressure. 

 Organisms that live at sea level are ex- 

 posed to a pressure of approximately 15 

 pounds to the square inch. This constitutes 

 a pressure of one atmosphere; it decreases 

 with altitude and increases with increasing 

 depth of water. The pressure to which or- 

 ganisms are exposed ranges approximately 

 from half an atmosphere at an altitude of 

 about 5800 meters to 1000 atmospheres at 

 a depth of 10,000 meters in the ocean. 



Animals are sensitive to uneven pressure 

 on their bodies. The whole set of responses 

 to touch illustrates this general statement. 

 A moving ameba, for example, stops motion 

 if subjected to a slight local pressure. Yet 

 amebae are relatively insensitive to in- 

 creased pressure equally applied to all parts 

 of the body surface. For many animals, 

 touch reactions are important in orientation. 

 Many animals normally respond to touch so 

 that some accustomed region, usually the 

 ventral side, is in contact with the substra- 

 tum, while other body surfaces are more or 

 less free from local pressure. This is an im- 

 portant part of the so-called ventral-earth 

 reaction, which was discussed in connection 

 with responses to gravity; touch may often 

 be more important than gravity in initiating 

 righting behavior patterns. 



There is space only for mention of the 

 whole field of thigmotaxis, as the automatic 

 response of nonsessile animals to tactile 

 stimuli is now called. In general, many ani- 

 mals respond positively to slight local 

 pressures and give a strong negative reac- 

 tion to more intense ones. Touch is espe- 

 cially important for animals that live in 

 weak light or in darkness and may be quite 

 potent even for animals in lighted habitats. 

 It is also an important element in sex recog- 

 nition for many animals (Warden, Jenkins 

 and Warner, 1940). 



ATMOSPHERIC OR SUBATMOSPHERIC 

 PRESSURE 



At sea level at 0° C. the mean atmos- 

 pheric pressure is 1033 gm. per square 

 centimeter; this equals 1.1033 X 10° dynes 

 /cm^. It amounts to about 14.7 pounds per 

 square inch and is sufficient to support a 

 column of mercury 760 mm. high. For 

 rapid approximations, 15 pounds per square 



inch or 1 kg./cm^. may be said to equal 

 one atmosphere's pressure. Ecologists, in 

 common with many meteorologists, usually 

 speak of pressures of less than an atmos- 

 phere in terms of the millimeters (or 

 inches) of mercury that would be sup- 

 ported. Increases in hydrostatic pressure 

 with depth are often recorded in terms of 

 the standard atmosphere as a unit. 



Ecological interest in variations of the 

 total air pressure revolves about phenomena 

 that accompany storms and those con- 

 cerned with higher altitudes, whether en- 

 countered by mountain-climbing or by air- 

 borne organisms. Day-to-day variations in 

 atmospheric pressures differ in different 

 latitudes. In the belt of the trade Avinds, 

 the mean barometric pressure is almost con- 

 stant from month to month, although there 

 is a small diurnal fluctuation of approxi- 

 mately the same ampUtude day after day. 

 In higher latitudes, more or less periodic 

 variations occur during part of the year, and 

 still larger and more sudden changes take 

 place in connection with tornadoes and 

 hurricanes. 



It is not yet clear to what extent the 

 variations in pressure are themselves impor- 

 tant for animals, although there is no doubt 

 that the winds and rains that accompany 

 large scale fluctuations in atmospheric pres- 

 sure have real ecological significance; these 

 will be discussed later. Although there has 

 been a considerable amount of experimen- 

 tation, here, as elsewhere in researches that 

 deal with the effect of pressure on animals, 

 the better analytic experiments have seldom 

 been concerned with the range of values 

 normally found in nature. Aside from some 

 good observation and experiments by Bert 

 (1878), experimenters from Boyle (see p. 

 16) to the present have been primarily 

 interested in subjecting animals to vacua 

 or near vacua or to extremely high pres- 

 sures, wholly impossible even on the high- 

 est mountains or in the deepest ocean. Such 

 experiments test the physiological limits of 

 protoplasmic possibilities without yielding 

 clear indications of ecological properties. 

 They illustrate a significant difference be- 

 tween physiological and ecological ap- 

 proaches to many problems. 



A small amount of evidence connects 

 emergence of pupae with change in baro- 

 metric pressure. Chapman (1931) reports 

 that "adult insects are said to emerge dur- 

 ing times of high barometric pressure." In 



