298 J- J- Christian 



growth by inhibiting reproduction, but the growth curve becomes sharply 

 truncated; (2) if egress from a population is permitted, food limitation 

 has no effect on reproduction; (3) feeding must be random and food cannot 

 have been a factor increasing competition, otherwise all the mice would not 

 have been affected equally. Extrapolating from these results to natural 

 populations, it is evident that a limited supply of food will not affect repro- 

 duction or otherwise affect the population so long as emigration is possible, 

 but if emigration becomes impossible, owing to a complete saturation of 

 the available habitat, reproduction will cease without any other apparent 

 effect. Whether or not there is competition for food in species other than 

 house mice must be determined, as increased competition would increase 

 pituitary-adrenocortical activity with all its sequelae. There may be in- 

 creased competition for food in voles (Frank, 1953), although it seems un- 

 likely that in general there will be an increase in competition over that 

 already present due to social factors (cf. above) . However, the usual result 

 of emigration from a population is increased mortality: the animals which 

 leave enter strange territories and very rapidly become mortality statistics 

 (Errington, 1943, 1954b; Calhoun, 1948; Davis, 1953). Physiologic adap- 

 tive mechanisms apparently are always operative in relation to population 

 density and therefore would reflect the relative density whether or not the 

 growth of a population was limited by food, and a distinction between food- 

 limited and self-limited populations should be possible by comparing the 

 reproductive activity of the populations and the general conditions of the 

 individual animals. Adrenal weights and other parameters of increased 

 pituitary-adrenocortical activity should also differentiate between the two 

 unless a population was food-limited at a point close to its maximum self- 

 limited size. 



These statements and conclusions are based on the results of experiments 

 with house mice and seem fairly conclusive for this species, possibly even 

 for voles, but it is difficult to believe that food is not a competitive factor 

 for all species and for all populations. The results of experiments to date 

 with populations of fixed size or freely growing indicate that food is not 

 even a competitive factor within the existing social structure, as all anmials 

 are apparently equally affected irrespective of their social rank (Strecker 

 and Emlen, 1953; Christian and LeMunyan, 1958; Christian, 1959b). It 

 should be remembered that if ever and when ever food becomes an object 

 of competition, shortages then necessarily will act through the generalized 

 physiologic adaptive responses and limit population growth in a density- 

 dependent fashion just as does purely social competition. The net conclu- 

 sion from these results is that a shortage of food per se probably rarely 

 limits population growth as the peculiar combination of effects which 

 result from a food shortage is seldom seen. However, this statement is not 



