ANIMAL AGGREGATIONS 



403 



The primary effect produced by these ani- 

 mals on their habitat seems to be the pro- 

 duction of muskrat burrows. Old burrows 

 are reconditioned year after year and 

 hence aid in the development of a good 

 muskrat population. Young muskrats driven 

 from the home den by their mother, also 

 use unoccupied burrows. 



Minimum and optimum populations are 

 known, too, for marsh-dwelling muskrats. 

 In many Iowa marshes, seven pairs per 

 ninety to 100 acres of good marsh is a 

 minimum breeding population. In such 

 marshes two or three pairs per acre make 

 an optimum population when the muskrats 

 are in a favorable phase of what some- 

 times appears to be a long-time breeding 

 cycle. Although muskrats do not pair off 

 in a fixed breeding pattern, the breeding 

 pair is still a useful descriptive device. The 

 failure of oversmall populations to repro- 

 duce comes from the absence of enough 

 males to mate with the females during their 

 short period of sexual receptivity. When 

 too few males are present, they simply may 

 not be around at the proper time. 



It is worth continued emphasis that def- 

 inite figures concerning minimum or opti- 

 mum population densities often must be 

 advanced with caution, whether the ani- 

 mals are relatively simple with self-con- 

 tained food as in sea urchin eggs (Allee, 

 1938), or more complex forms as musk- 

 rats (Errington, 1945) or man. It is diffi- 

 cult to describe exactly these functional 

 population levels under controlled condi- 

 tions, and helpful statements regarding 

 what constitutes an optimal population 

 density in the field require skill, caution, 

 knowledge, and wisdom on the part of the 

 observer. The optimal population density 

 also varies with the function being meas- 

 ured and upon whether an immediate or 

 a long-time view is being considered. 



One of the more common examples of 

 optimal density is furnished by studies on 

 longevity. In addition to the lengthened 

 survival of aggregated animals in the pres- 

 ence of many toxic agents and other ad- 

 verse environmental conditions (p. 360), 

 increased density— up to some optimum— 

 often increases longevity when conditions 

 seem to be generally favorable. 



The life span of Drosophila, imder cer- 

 tain experimental conditions, is longest at 

 the population density of thirty-five to 

 fifty-five flies per 1 ounce culture bottle 



(Pearl, Miner, and Parker, 1927). Subop- 

 timal numbers of flies may be unable to 

 gain an adequate control of "wild" organ- 

 isms or prevent overgrovi^h of yeasts on 

 which Drosophila feeds. The increased 

 death rate with higher densities is prob- 

 ably related, as is frequently true, to local 

 food shortage or to an excess of excretory 

 products or to both combined. 



The situation is complex. Analogies ex- 

 ist between many aspects of the physiology 

 of aggregations and of individual animals. 

 For example, Shcherbakov (1935) re- 

 ported that Rubner's rule (p. 113) holds 

 with groups of Drosophila melanogaster. 

 The rate of oxygen consumption is higher 

 with 200 of these flies in 30 cc. of space 

 than when only two are present. Here, as 

 in many other instances, the effect of 

 crowding depends on the criteria being 

 used; metabolism, in this instance, pro- 

 ceeds faster in the crowded condition, and, 

 as might be expected, longevity decreases. 



Another well-tested instance of optimal 

 population density is furnished by num- 

 bers in the litters of inbred guinea pigs 

 (Wright, 1922; Wright and Eaton; see 

 Allee, 1945). Those bom in litters of two 

 or even three show better survival records 

 at weaning time at the age of thirty-three 

 days than do those born in smaller or larger 

 litters. The factors underlying this optimal 

 litter size are complex. Apparently, in a 

 vigorous strain of guinea pigs a litter of 

 one is in itself an indication of lowered 

 vitality. Then, too, the larger size at birth 

 of single guinea pigs may result in birth 

 injuries and so produce an immediate or 

 a deferred rise in death rate (cf. p. 656). 

 Here again the cooperation, such as it is, 

 between two or three litter mates as con- 

 trasted with those born singly, or in larger 

 litters, is inseparable from the general life 

 processes of the growing organisms. 



Tribolium Populations 



It is hard enough to ascertain the popu- 

 lation density that is optimal under gen- 

 eralized experimental conditions. The analy- 

 sis of the major causal relations is more 

 difficult. We give now one well-tested ex- 

 ample—that of the flour beetle, T. confti- 

 sum— in which the optimal population 

 density for rapid initial increase is clearly 

 above the minimal density at which repro- 

 duction occurs. The methods of culturing 

 Tribolium and of assaying population 



