398 



POPULATIONS 



larger masses fail because of poor internal 

 respiration. 



The successful evolution of colonial pro- 

 tozoans from their single-celled progenitors 

 was probably similarly a£Fected both by 

 better distribution of food stored in some 

 of the cells and by the reduced surface- 

 bulk ratio of the colonial forms. Such evo- 

 lution occurred many times within the 

 phylum Protozoa. Similar forces probably 

 acted in the evolution of the many-celled 

 primitive metazoan stock from their one- 

 celled ancestors. Partially organized aggre- 

 gations of protozoan cells supplied the 

 basis from which all higher animals have 

 evolved. Natural selection was a screening 

 agency throughout this evolution, and an 

 important basis for selection— in addition 

 to more highly involved sexual reproduc- 

 tion and other advances in specialized phys- 

 iology and anatomy— was the added safety 

 in the presence of optimal numbers of cells 

 with the concomitant values furnished by 

 a favorable ratio of surface to mass. 



MODIFICA-nON OF THE ENVIRONMENT 



A biologically conditioned medium such 

 as water in which fish have lived may pro- 

 duce beneficial results on later inhabitants, 

 even of the same species (p. 355; Alice, 

 1938). Contrariwise, media surrounding 

 populations, especially around dense popu- 

 lations, may become deleterious (p. 352). 

 Sometimes the results produced are specific 

 and not necessarily simple in interpretation 

 so far as survival values are concerned. 

 Thus the release into culture medium of an 

 antibiotic "killer" substance by certain 

 stocks of Paramecium aurelia has positive 

 survival value for the "killers" and negative 

 value for associated sensitive stocks (Sonne- 

 born, 1947; Austin, 1948 and citations). 



Many animal aggregations retard the 

 rate of temperature changes in their midst; 

 other aggregations generate, as well as 

 conserve, heat, even though they are com- 

 posed of partially or wholly poikilothermal 

 forms. The well-known winter cluster of 

 honeybees is a case in point. Protection 

 against cold for aggregations of warm- 

 blooded animals is diagrammatically illus- 

 trated by the better survival of closely 

 massed coveys of the bobwhite quail 

 (Colinus virginianus) . At night, in cold 

 weather, or when not feeding in daytime, 

 the birds form a compact circle facing out- 



ward with their tails toward the center; 

 they sit wing to wing (Gerstell, 1939). 



The quail huddle is tighter in colder 

 weather; with large coveys, some perch on 

 the backs of others, forming a two-story 

 circle. Laboratory tests under artificially 

 produced weather conditions show that iso- 

 lated birds and those in small coveys are 

 killed by low temperatures, and that mem- 

 bers of larger groups withstand severe tem- 

 peratures in fair to good condition. In addi- 

 tion to the possible survival values of the 

 supposed confusion eflFect when a bobwhite 

 covey suddenly takes wing— explodes in all 

 directions, so to speak— huddling behavior 

 is warmth saving and, in severe winter 

 weather, has definite life-saving values. 

 In heat conservation, huddling of bob- 

 whites gives another illustration of the 

 bulk-surface rule, applied this time to an 

 aggregation of birds. 



Light-sensitive animals survive exposure 

 to lethal illumination longer, other things 

 being equal, if present in sufiicient num- 

 bers so that now one, now another, is 

 shaded. The situation is not simple, since 

 fresh-water planarian worms withstand 

 ultraviolet radiation better if several are ex- 

 posed together even though no shading oc- 

 curs (Alice and Wilder, 1939). 



The protective value of relatively re- 

 duced surface in relation to increased mass 

 has been demonstrated repeatedly for ag- 

 gregated animals exposed to toxic mate- 

 rials (p. 360). Similar relations hold with 

 increased numbers— up to optimal popula- 

 tions—even when the exposed animals do 

 not collect in a compact, surface-reducing 

 mass. 



OTHER SURFACE-MASS RELATIONS 



Much more complicated examples of the 

 importance of surface-mass relations are 

 found in the reaction of certain ungulates 

 when attacked by potential predators. An 

 illustration will make the point. When a 

 group of the American pronghorn is at- 

 tacked by wolves, if twelve to fifteen or 

 more pronghorns are present, the animals 

 form a defensive band that presents a min- 

 imum group surface, and the bucks are en- 

 abled to fight off the attackers. When the 

 local population falls below this critical 

 level, the animals fail to collect when at- 

 tacked, but rather stampede, presenting 

 much exposed surface to their enemies, 

 and the weaker prongbuck are readily 



