2. Endocrines and Populations 295 



but that this was frequently impossible because of destruction of the nests 

 and interference with the attempts of the females to maintain or reconstruct 

 them. Only 49 of the 72 attempts to build and maintain either of these two 

 types of nest were successful in these populations (Brown, 1953). South- 

 wick (1955b) noted a similar relationship between the type of nest and 

 litter survival, but he also found that litter survival was good in bowl or 

 covered nests regardless of the number or sex of the adults which were also 

 present. Litter survival was zero with no nests. These and Brown's (1953) 

 results imply that not all the females were prevented from maintaining 

 proper nests and nourishing then* litters irrespective of the conditions and 

 interference. These results are revealing, as they suggest strongly that more 

 is involved in litter mortality in these experiments than simple interference 

 or the type of nest maintained. Similar results were noted in other experi- 

 ments (Christian, 1956) except that in this case even those with no nests 

 sometimes survived. In these latter experiments it was also noted that if 

 litter mortality occurred when the populations were less than one-half of 

 maximum density, it occurred within the first four postnatal days and 

 might be attributed to interference which resulted in too much movement 

 of the litter by the mothers. Of additional importance is the fact that litter 

 survival declined regularly with increasing density and that this began to 

 occur well before interference could be significant. If the young survived 

 the first 4 days at these lower densities, they almost always were success- 

 fully weaned. However, at higher densities the increased litter mortality 

 usually occurred after the young were 10 days old. Until this age survival 

 equaled that seen at lower densities. These facts suggest that the defect was 

 primarily nutritional, even though there still were some deaths of young 

 less than 4 days old attributable to interference. The age of death of the 

 young is not known for most of the reported experiments, so that this 

 distinction was not possible. However, the fact that crowding and inter- 

 ference was not a factor in litters with proper nests suggests that perhaps 

 the behavioral changes which resulted in poor nest maintenance may be 

 based on an underlying alteration in maternal behavior, possibly due to 

 endocrine changes. Admittedly this is speculative, but the available evi- 

 dence suggests some such explanation. The problem certainly must be 

 investigated m greater detail. It is possible that the capacity to lactate 

 properly and the abilitj^ to maintain proper nests are closely related by a 

 common causal basis. However, until evidence for or against such a hy- 

 pothesis is available, it must be assumed that postnatal litter mortality at 

 high population densities may result from (1) inhibition of lactation, (2) 

 interference by other animals, or (3) changes in maternal behavior, which 

 apparently decreases with increasing population density. 



We are inclined to the view that interference of various sorts probably 



