ANIMAL AGGREGATIONS 



401 



not reappear in that locality without a new 

 immigration. In practical insect control 

 measures the accepted practice is to reduce 

 the given population until it is highly vul- 

 nerable, but usually not to continue until 

 every individual has been killed (H. S. 

 Smith et al., 1933; Soper and Wilson, 

 1942). 



In some instances the minimal possible 

 population can establish itself and repro- 

 duce. One pair of Norway rats is said to 

 have successfully colonized small Deget 

 Island in the Kattegat (Hinton, 1931), and 

 a single pair of beavers, introduced into a 

 suitable locality, has estabhshed a con- 

 tinuing beaver colony (Cook, 1943). With 

 bacteria, despite the usual necessity for a 

 larger inoculum among bacteria in general, 

 a sohtary anthrax bacillus inoculated into 

 a guinea pig can become established, multi- 

 ply, and eventually produce the death of 

 the host animal (Theobald Smith, 1934). 

 Usually with bacteria, as with many other 

 organisms, certain viruses included, the 

 effect of the invasion of a host depends, 

 among other things, on the number of in- 

 vaders: the smaller the inoculum, the more 

 chance that the host will kill them all (Mc- 

 Coy, 1932). Such results are summarized 

 by the left limb of curve B in Figure 139 

 and have definite implications concerning 

 proto-social cooperation and disoperation. 



Reindeer herds that spend the summer 

 along the southern expanse of the Eurasian 

 tundra have a minimum number deter- 

 mined by the herd's relation to the charac- 

 teristic swarms of blood-sucking insects. 

 The minimum number that can be main- 

 tained with safety in such pasturing herds 

 is placed at 300 to 400 animals by Sdob- 

 nikov (1935). • A smaller herd is difficult 

 to tend and keep together. It cannot readilv 

 be put on "tandara," as the reindeer tend- 

 ers call their device of stopping a herd 

 on the morning of a hot day and making 

 the animals remain near one place until the 

 flight of the attacking warble flies ceases. 

 At the opposite extreme, an overlarge herd 

 also has disadvantages in the summer. It 

 tramps down the pasture and worsens 

 feeding; also a large herd collects more in- 

 sects around it. 



The colonies of Mennonites (p. 399) 



• The English translation was kindly fur- 

 nished by Mr. Charles Elton from the files of 

 the Bureau of Animal Population, Oxford Uni- 

 versity, England. 



offer another type of example of minimum 

 populations, this time at the human level. 

 It may be recalled that Mennonite leaders 

 regard twenty to fifty families, depending 

 on local conditions, as approximating the 

 minimum population that can be expected 

 to maintain a continuing Mennonite com- 

 munity. It is helpful to bring a human ag- 

 gregation into the discussion for several 

 reasons. We are encouraged to test some of 

 the simpler human social problems against 

 those presented by groups of other animals. 

 We are also reminded of the complexity of 

 the relations with which we are dealing. It 

 is obvious that in groups of people, such 

 factors as internal and external leadership, 

 single-mindedness, and many other human 

 traits tend to modify and obscure basic 

 biological relationships, yet similarities with 

 nonhuman populations do exist. Safety fac- 

 tors concerned merely with the size of a 

 Mennonite colony illustrate some of the 

 human variations of the problem of popu- 

 lation minimum, fust as the other instances 

 provide bacterial, sponge, insect, heath- 

 hen, elephant, and prongbuck variations of 

 the same problem. Recognition of the 

 known but unmentioned human complica- 

 tions should make us more cautious about 

 oversimplification of group relations among 

 other species, in which many of the com- 

 plicating factors are entirely unknown. 



The existence of minimal populations 

 implies that the optimal population size 

 will be somewhat larger. Certain well- 

 tested instances may illustrate the extent of 

 the phenomena and the type of situation 

 in which optimal populations exist. Eggs 

 of Arhacia, the common sea urchin of 

 southern New England, are shed into the 

 sea water, where they are fertilized if 

 freely swimming spermatozoa are present. 

 These matters can readily be manipulated 

 in the laboratory. The number of egfjs pres- 

 ent and the closeness with which they are 

 packed together can also be experimentally 

 controlled. Some fifty minutes after fertili- 

 zation, at usual temneratures, the eggs 

 divide into two cells. The second cleavage 

 takes place about thirty or forty minutes 

 later. One set of experimental results is 

 illustrated diagrammatically in Figure 140. 



The amount of acceleration at mid- 

 second cleavage among crowded eggs, as 

 compared with accompanying sparse ones, 

 may average as much as three minutes, and 

 the difference in one long series of tests 



