ANIMAL AGGREGATIONS 



397 



eflFects resulting from overdense popula- 

 tions. There can be no doubt that over- 

 crowding is normally harmful, both under 

 natural and experimental conditions. 



Modem interest centers as well on the 

 importance of the phenomena associated 

 with undercrowding. It is this set of phe- 

 nomena that Allee has steadily interpreted 

 as providing evidence for the broad prin- 

 ciples of nonconscious proto-cooperation 

 (1945 and citations, 1947). These matters 

 will be summarized at some length in the 

 following pages. 



NATURAL COOPERATION 



Much of the physiological determination 

 of unconscious disoperation, as contrasted 

 with proto-cooperation, depends on surface- 

 mass relations roughly similar to those that 

 underly the operation of Bergmann's rule 

 concerning body size of warm-blooded ani- 

 mals in relation to environmental tempera- 

 ture (p. 119). The surface of a globular 

 object increases as the square, while the 

 mass increases as the cube of its diameter. 

 These surface-mass ratios hold for animal 

 aggregations as well as for individual or- 

 ganisms. The disoperations of overcrowding 

 take place when the mass is overlarge for 

 its surface, even when suitable divisions of 

 labor have evolved. Contrariwise, coopera- 

 tion develops \vith increasing bulk as long 

 as the resulting decrease in surface-to-mass 

 ratio is beneficial. Many primitive coopera- 

 tions of animal groups result from the 

 operation of this relatively simple principle. 



EVIDENCE FROM PHYLOGENY 

 AND EMBRYOLOGY 



Protozoans usually separate when they 

 divide asexually. The asexually produced 

 descendents of a single cell (energid) may 

 be regarded as being comparable with the 

 whole body of a many-celled animal, ex- 

 cept that among most Protozoa, each cell is 

 free from all others. With some protozoans, 

 like Volvox, the cells do not separate, and 

 colonies result. Among other changed re- 

 lations that accompany such a relatively 

 simple collection of attached cells, the ratio 

 of surface to bulk of the colonies differs de- 

 cidedly from that of separate cells. Each 

 cell in a temporary or permanent aggre- 

 gation or in a multicellular organism pre- 

 sents less surface to the outside world than 



does one that leads an independent exist- 

 ence. As a result, the danger of harmful 

 exposure to environmental effects is de- 

 creased, and, on the other hand, the diflB- 

 culty of respiration, of individual food get- 

 ting, and of receiving external stimuli is in- 

 creased. Enlarged bulk, beyond some 

 threshold, requires the functional differen- 

 tiation of parts to become more varied and 

 effective. 



The evidence for automatic cooperation 

 from evolution and normal embryology 

 tends to be circumstantial. Direct evidence 

 is readily derived from studies in experi- 

 mental embrvology and regeneration. One 

 common embryological experiment is to 

 transplant a small piece cut from a young 

 chick blastoderm onto a blood-rich mem- 

 brane of an older embryo. The egg is 

 sealed and incubation is continued. Before 

 hatching time, the egg is again opened, 

 and the transplant is recovered and stud- 

 ied. If the grafted piece is too large, it does 

 not receive sufficient blood supply and de- 

 generates. Up to an optimal size, the larger 

 the transplanted part, the higher the per- 

 centage of successful grafts and the better 

 they grow. There is much similar evidence 

 from comparable experiments on diverse 

 species from varied phyla. 



Manv of the simnler animals have re- 

 markable powers of regeneration of lost 

 parts. Usually a fairly large number of cells 

 must be left together in order for the oper- 

 ated animals to survive. Some sponges and 

 other relatively uncomplicated forms can 

 reconstitute multicellular oreanisms after 

 the complete mechanical separation of their 

 constituent cells (Wilson, 1907), provided 

 the isolated elements can wander together 

 and form clumps of sufficient size. Under 

 one set of experimental conditions, com- 

 plete reconstitution of a sponge occurred 

 in agjrree;ates of about 2000 cells. Those 

 with onlv fortv to 500 cells did not regen- 

 erate CGaltsoff, 1925). 



The failure of the smaller agg^regates to 

 develop appears to result from their lack of 

 enough food reserve to tide over the period 

 of reorg-anization when feeding is impos 

 siM-^. The smaller ratio of surface to total 

 bulk in these larger, successful aggregates 

 is also a factor, since it is associated with 

 reduced exposure of anv given cell to bac- 

 terial and protozoan attack. Aggregates of 

 good size freouentlv wthstand conditions 

 that destroy the smaller ones, yet still 



