NO. 3591 RODENT ETHOLOGY — EISENBERG 27 



tively intolerant, solitary Dipodomys nitratoides ceased effective 

 reproduction with the greatest number of square inches per animal 

 and at approximately the same grams per unit area as did the less 

 tolerant, semisolitary Peromyscus crinitus. 



20| Number of 

 Individuals 



Figure 8. — Population growth for Peromyscus californicus; note the extreme stability. 



An examination of the causes of population curtailment under 

 confined conditions suggests four mechanisms: (1) During a plateau 

 period, the females are generally anestrous with no evidence of normal 

 estrous cycles; (2) the males born into the system show retarded 

 gonadal maturation or even slowed growth; (3) males and even 

 females may exhibit wounds as evidence of abnormal fighting, and 

 such individuals may even die; (4) newborn litters may be neglected 

 or destroyed. It would appear that different species have dift'erent 

 thresholds of sensitivity for each of the four suggested pathologies. 

 Thus, a solitary species, when forced to live as a pair in a small cage, 

 may not exhibit reproduction because of a failure on the part of the 

 female to cycle and, although they live compatibly, they are not 

 exhibiting a natural tolerance. In a similar manner, the recruilm.ent 

 failure may be a result of male gonadal failure; agonistic beluiAior 

 may be so intense that overt fighting and death result; and, if j^oung 

 are born, they may die from maternal neglect or cannibalism. 



Lidicker (1965) has published the results of similar population 

 studies utilizing Peromyscus maniculatiis, P. trvei, Mus mvscvhis, and 

 Oryzomys palustris. He concludes that species which normally do not 

 experience high densities (or are dispersed in their native habitat) 

 are more prone to exhibit overt fighting and cannibalism of young at 

 high densities. My data concur in suggesting that tolerant species 

 can form high densities with a minimum of agonistic behavior, but I 

 believe the evidence shows that all systems of density regulation are 



