132 



ANALYSIS OF THE ENVIRONMENT 



its to the size of land vertebrates, since the 

 Weight increases by cubes, while strength 

 length, the so-called cube rule, while the 

 strength of a leg, as of any other structural 

 support, is related to its cross section. 

 Weight increases by cubes, while strength 

 of support increases by squares. In large 

 land animals, the bulk of the leg must in- 

 crease out of proportion to the increase in 

 weight of the remainder of the body. 



The size that skeletal animals may attain 

 with safety varies with structural mechanics 

 and with the surroimding medium. Water 

 has more power to support the weight of 

 organisms than does air. Largely as a result 

 of this relationship among animals in which 

 skeletal support is important, aquatic forms 

 may be larger than their terrestrial relatives 

 when both follow a similar structural pat- 

 tern. The Hercules beetle {Dynastes), 

 which reaches 15 cm. in length, or giant 

 grasshoppers (Palophus) that may attain 

 a length of 30 cm., or, for that matter, 

 the larger land crabs, are much smaller 

 than the lobster (Homarus), which may 

 be 60 cm. long, or the really giant crab 

 {Kampfferia) , whose appendages may 

 reach a spread of more than 10 feet. The 

 giant eurypterids of Paleozoic seas were far 

 larger than their descendants, the terrestrial 

 scorpions. 



Similar conditions hold among the verte- 

 brates. Modem whales, 30 meters in length 

 and weighing up to 108,000 kilograms, 

 dwarf Hving elephants, 3.5 meters in length 

 and weighing only 4000 kilograms. The 

 extinct reptile, Brontosaurus, which was 

 20 meters long and weighed perhaps 

 38,000 kilograms, is also dwarfed by mod- 

 em whales. It was smaller than the simi- 

 larly extinct, water- dwelling Brachiosaurus, 

 whose periscope-like neck could easily have 

 looked over a three-story building, if such 

 had been present. It may have had a Hving 

 weight of 45,000 kilograms (Romer, 1933). 



Another efiFect of increasing weight of 

 body brings in an application of Euler's 

 principle that the capacity of a column 

 to support weight varies inversely as the 

 square of its length. In accordance with this 

 principle, the leg bones of a heavy verte- 

 brate tend to be shorter than those of re- 

 lated lighter species. 



The various mechanical principles that 

 are illustrated by the vertebrate skeleton, 

 particularly for terrestrial forms, are closely 

 comparable to many of those used in build- 



ing houses or bridges. Considered from this 

 point of view, functional osteology is close- 

 ly related to ecology. This theme is weU 

 developed by D'Arcy Thompson (1917: 

 see also Boker, 1935). 



Among terrestrial animals, birds have a 

 diflEerent ratio between supporting bones 

 and body bulk from that found in mammals. 

 They are able to carry more weight per unit 

 of the supporting skeleton. Their support- 

 ing bones are excellent examples of the 

 strength to be found in paper-thin struc- 

 tures formed into cyHnders or with stiffened 

 ridges. The frigate bird, with a wing ex- 

 panse of 7 feet, weighs in all about 2 

 pounds; the skeleton weighs 4 ounces, 

 somewhat less than the feathers. 



The hollow tubular bones of birds con- 

 tain air cavities connected with the lungs. 

 Other air sacs, in addition to the relatively 

 large lungs, are found in the body, and aU 

 are filled with air which, the lungs ex- 

 cepted, is usually warmer than that in the 

 surrounding atmosphere. Often the iimer 

 air is much warmer than that outside; 

 and the greater the difference, the 

 greater is its lifting power. A considerable 

 amount of somewhat warmer air is also 

 trapped within the feathery covering of 

 the body. All these mechanisms help lower 

 the specific gravity of the whole bird. 



The weight-saving mechanisms that re- 

 duce the specific gravity of birds are re- 

 lated to their powers of flight. The support- 

 ing planes formed by wings and tail also 

 assist birds to maintain themselves in the 

 air against the pull of gravity. The various 

 devices are sufficiently effective so that 

 large birds can maintain or gain altitude in 

 soaring flight in uprising currents of air of 

 such slight power that they will barely 

 support dust particles or tiny winged 

 insects. 



Gravity exerts its persistent pull on 

 aquatic animals. Other things being equal, 

 these tend to be slightly heavier than sea 

 water; most recorded values for the specific 

 gravity of different types of cells lie be- 

 tween 1.02 and 1.08. Aquatic organisms 

 have evolved certain flotation devices 

 which, acting with the supporting power of 

 the water, help to offset the tendency to 

 sink. More than one device may be present 

 in a given organism, and those of diverse 

 evolutionary relationships may show con- 

 vergent adaptations for floating. 



Some of the flotation mechanisms are: 



