140 



ANALYSIS OF THE ENVIRONMENT 



thunder, and the swish and patter of rain. 

 Rocks, displaced by frost heaves, may roil 

 noisily down a mountainside, and the 

 sound of a mountain avalanche or of a ma- 

 jor earthquake carries still farther. The roar 

 of a waterfall or of a river in flood is fairly 

 distinctive. Despite such physically pro- 

 duced vibrations to which many animals 

 may react, especially birds and mammals, 

 it is the substratal vibrations and sounds of 

 biotic origin that are of prime importance 

 in ecology, and their discussion is not di- 

 rectly appropriate in connection with the 

 physical environment. 



Hearing is much more important for 

 man than for most other animals, especially 

 because of the use of sound in intercom- 

 munication in our species. This holds true 

 in human society even though our sense of 

 hearing is less keen than that of certain 

 other animals and although our ear muscles 

 are undergoing evolutionary retrogression. 

 In general, sight, the chemical senses, and 

 touch have decidedly greater significance 

 for animals than does phonoreception. The 

 importance of sounds is further limited 

 since they are primarily restricted to the 

 terrestrial environment or, at most, to the 

 surface regions of bodies of water. 



One of the most dramatic uses of aerial 

 vibrations with regard to the relation be- 

 tween animals and their physical environ- 

 ment is their employment by bats in flight 

 as a means of avoiding obstacles. The meth- 

 od by which these dusk or night-flying 

 forms are able to fly successfully through 

 dark forests or in and out of long tortuous 

 caves without injury has long been a mat- 

 ter of controversy. Spallanzani is said to 

 have found in 1794 that bUnded bats could 

 fly as skillfully as those with full vision and 



that they could even successfully avoid silk 

 threads hung about the room. Jurine 

 (1798) discovered that stopping the ears 

 of bats lessened their ability to avoid ob- 

 jects, an observation that Spallanzani con- 

 firmed (Allen, 1939; Jurine, 1798). The 

 fact that a few wires stretched over a water- 

 ing trough at night form a good collecting 

 device for many bats is only apparently in 

 contradiction to these observations, since 

 experiments by Grifiin and Galambos show 

 a minority percentage of contacts made by 

 bats flying through a barrier of wires. 



It now appears tPiat bats emit sound near 

 the upper range of human hearing (be- 

 tween 20,000 and 30,000 cycles per 

 second) (Best and Taylor, 1943) and also 

 give out short bursts of supersonic vibra- 

 tions of from 45,000 to 50,000 vibrations 

 per second. These can be made audible to 

 man and recorded by modern sound-detect- 

 ing and amplifying systems. Flying bats de- 

 tect obstacles in their path by emitting 

 supersonic notes that appear to be reflected 

 back and form what may be called sound 

 shapes and shadows. These are detected by 

 the bilaterally placed ears. A small, active, 

 alert bat, by appropriate maneuvering in 

 full flight, can avoid wires only a millimeter 

 in diameter. Ability to avoid such small ob- 

 jects varies from species to species. It is 

 greater in the smaller insect-catching spe- 

 cies than in larger forms. Different individ- 

 uals within the same species show varying 

 degrees of this power, and the same in- 

 dividual loses its precision of performance 

 with increasing fatigue. Do night-flying in- 

 sectivorous bats find their often minute par- 

 ticles of food by the same device (of. 

 Allen, 1939, p. 136; Griffin and Galambos, 

 1941; and Galambos and Griffin, 1942)? 



9. CURRENTS OF AIR AND OF WATER' 



Currents in air or water are a direct ex- 

 pression of the pushing power of pressure, 

 which, in turn, results from the pull of grav- 

 ity. Other things being equal, such currents 

 run from regions of greater to those of low- 

 er density; and the rate of flow is faster, the 

 steeper the density gradient. Temperature 

 differences produce density differences that 

 have great ecological effects in setting up 

 and maintaining currents of either air or 



water. Heated fluids become lighter and 

 tend to rise and flow over cooler parts of 

 the same mass. Contrariwise, chilled Huids 

 become denser and sink until they are oi 



* For an extension of this discussion, the 

 interested reader is referred to the following: 

 Byers (1944), Humphreys (1942), Kendrew 

 ( 1938 ) , Sverdrup, Johnson, and Fleming 

 (1942), Wenstrom (1942), and Willett (1931. 

 1944). 



