ADVERSE EFFECTS OF CROWDING ON LACTATION AND REPRODUCTION OF MICE AND TWO GENERATIONS OF THEIR PROGENY1

Abstract

Each of 7 populations of 20 male and 20 female mature mice (isolated since weaning) was placed in a 13 in. x 18 in. cage for 6 weeks which constituted experimental crowding. No young were born during this period. The mice were established as pairs after crowding, one pair per cage. High mortality of males during crowding restricted the number of pairs available to 88, of which 68 had litters and 56 pairs weaned litters after segregation into pairs. Weights of progeny were obtained at birth and each of the 56 litters were matched and half litters switched with one of equal size and age born to a female which had remained isolated. Weights of the young were obtained weekly for three weeks. The mean weights at birth of progeny of previously crowded and of isolated mice did not differ. Young nursed by crowded females were 15% lighter at weaning than those nursed by isolated mice (P<0.001). The weights of progeny did not differ significantly with respect to which mother bore them or to whether they were nursed by foster or by their own mothers. The mean weight of progeny nursed by crowded or by isolated mothers decreased with increasing litter size. Progeny from litters of 6 or more nursed by crowded mothers weighed significantly less (17%) than those nursed by isolated females, but the weights did not differ significantly for litters of 5 or less. Young nurtured by crowded mice were paired and their litters similarly matched and half litters exchanged with ones of similar age and size born to isolated mothers. Progeny nurtured by females in turn nurtured by crowded mothers weighed 8% less than their controls (P<0.06). They averaged 18% lighter than their controls for litters of 8, 9 and 10 (P<0.005) but were not significantly lighter in smaller litters. It was concluded that suppressed growth of progeny nurtured by crowded mothers, persisting for at least 2 generations, was due to quantitatively and/or qualitatively deficient lactation resulting from crowding. Such attenuation of the effects of crowding may explain the long-continued decline in natural populations following peak levels and a precipitous crash in numbers. An eighth population of 10 male and 10 female mice was sacrificed after 6 weeks of crowding and compared with 10 littermate segregated pairs for reproductive function. Uterine implantation scars were present in all females of both groups, but only 3 of the crowded females bore litters or ever appeared grossly pregnant, whereas all pf the segregated females bore litters. The number of implantations per mouse and number of young per litter were significantly less in the crowded females. It was concluded that crowding resulted in intrauterine mortality and probably diminihsed fertility, explaining why none of the mice in the preceding 7 populations ever appeared pregnant during crowding and why almost half never bore litters, even after segregation.