134 



ANALYSIS OF THE ENVIRONMENT 



tive substrate. This may be a complex re- 

 sponse that is only partially, if at all, con- 

 trolled by gravity. It has elements of a 

 so-called dorsal-hght reflex, of reaction to 

 touch, and is also related to "righting be- 

 havior." It may be given to vertical planes 

 and even to the roof of a cave, by climbing 

 animals. 



Distribution 



Gravity, among other factors, contributes 

 to the difiFerential vertical distribution of 

 animals. The supporting power of vv^ater, 

 which is primarily a function of its density 

 and viscosity, aids organisms in resisting 

 sinking. In the ocean, such support becomes 

 steadily greater at lower temperatures, as 

 in polar seas or in ocean depths; in fresh 

 water, the density component increases as 

 the temperature falls to 4° C. and then 

 rises somewhat with further chilHng of the 

 water. Salinity decidedly increases both 

 density and viscosity. Equivalent organisms 

 sink less readily in the sea under the pull 

 of gravity than do those in fresh water (see 

 p. 133). 



Radiolarians, for example, of the family 

 Challengeridae, show a vertical distribu- 

 tion apparently determined by the inter- 

 action of the pull of gravity and the support 

 given by water. Those in the upper 400 

 meters tend to be smaller, 0.11 to 0.28 

 mm. in diameter; in intermediate depths, 

 the radiolarian size tends to be interme- 

 diate, 0.21 to 0.28 mm. for the group under 

 consideration; the largest ones sink to 

 depths at which they can float, and below 

 1500 meters they range from 0.33 to 0.58 

 mm. Sagitta, the arrow worm, is larger and 

 more mature in deeper water. The relation 

 of buoyancy of water to cyclomorphosis in 

 Cladocera has already been mentioned (p. 

 118). 



The vertical distribution of both marine 

 and fresh- water animals allows a generaliza- 

 tion that approaches the dignity of an eco- 

 logical rule: The younger stages in the life 

 cycle occur nearer the surface than do the 

 older, more mature forms. This applies both 

 to animal plankton and to the more activelv 

 swimming nekton; the smallest leptocephali 

 of eels furnish one exception to this rule. 

 Newly hatched cladocerans (Daphnia) 

 sink more slowlv than do adults and so re- 

 quire less energv to keep afloat in the sur- 

 face waters of lakes and ponds. In the 

 ocean, active swimmers, such as sharks or 



fat-laden floaters like the ocean sunrisn, 

 MoJa mola, are found in the surface waters. 

 Such adults give other exceptions to the 

 general rule that surface-dwelUng marine 

 fishes tend to be juveniles. 



A geographic test of the ecological appli- 

 cation of the interaction of principles con- 

 cerned wdth flotation, despite the pull ol 

 gravity, comes from a diverse lot of organ- 

 isms. Thus Clione, a pteropod (Gastro- 

 poda), the marine copepod, Calantis, and 

 jellyfish, Aglantha, live in the surface 

 waters off the Norwegian coast. They are 

 absent in the warmer Atlantic, but live at 

 a depth of 750 to 1000 meters. The sup- 

 port offered by the water is similar through- 

 out this distribution (Hesse, Alice, and 

 Schmidt, 1937). 



Adaptive Behavior 



Animals show adaptive responses to grav- 

 ity other than those that are primarily 

 concerned \\dth the maintenance of level. 

 The geonegative reaction of the caterpillars 

 of the monarch butterfly (Danaus plexip- 

 pus) and of other insects, aids them in find- 

 ing their natural food. When fully fed, or 

 in environmental stress, the organisms often 

 become geopositive. The Colorado potato 

 beetle (Leptinotarsa) becomes geopositive 

 with desiccation. A final, familiar instance 

 of the adaptive behavior of animals in re- 

 lation to gravity must suffice. Juvenile 

 spiders frequently travel to some vantage 

 point where the air flow is httle interrupted. 

 They then spin a gossamer thread that is 

 carried by the wind until at length the tiny 

 spinner balloons away, at the mercy of the 

 currents of air, to some new, wholly un- 

 chosen spot. Death, or a new lease on life, 

 lies at the end of this aerial journey. Many 

 spiders are broadcast so, although each in- 

 dividual organism is heavier than air and 

 each is always being pulled toward the 

 earth by the persistent, powerful force of 

 gravity. The other side of this story, the 

 distributing power of air currents, will be 

 considered in the follo^ving chapter. 



PRESSURE 



Pressure acts as a mechanical process and 

 as an osmotic phenomenon. Although these 

 two forms of pressure have much in com- 

 mon, it is convenient, and probably logical, 

 to consider the former in close connection 

 with gravity and to treat osmotic pressure 



