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This paper is based upon the study of 8207 wild mice (Mus musculus ). The bulk of the animals were obtained from 8 January 1942 to 7 January 1943, when 1067 mice were obtained from urban habitats, 1150 from flour buffer depots, 1218 from cold stores and 2033 from corn-ricks, totalling 5468. Trapping with break-back traps provides a biased sample of the population, larger animals being caught more readily than smaller ones. In collecting from a rick, methods can be employed which yield a virtually complete population. In the absence of information regarding age, weight is adopted as the basis of classification. The criterion of fecundity adopted for females is the presence of corpora lutea in the ovary and for males the presence of numerous sperms in the cauda epididymidis. 25% alcohol or methylated alcohol sufficiently preserved whole animals waiting for examination in hot weather, without making the recognition of sperm difficult, or effecting any significant alteration in weight when dried. The following information has been obtained for mice from the four habitats: (a ) They breed throughout the year. (b ) The evidence is against seasonal differences either in the percentage number of adult females pregnant, or in the number of embryos per litter. (c ) The number of nestlings per litter found in the ricks varied from 2 to 13, with an average of 5⋅83. Communal nests are quite common. (d ) The sex ratio for urban is 51⋅83% male, flour depots 50⋅43%, ricks 44⋅61% and cold stores 48⋅19%. (e ) Sex is correlated with weight, i. e. there is a preponderance of males in the lighter weight groups, and of females in the heavier weight groups. (f ) Fecundity in the female is reached in the 7⋅5 g. weight group in all four habitats, though the percentage number reaching it in the ricks and particularly in the cold stores is much lower than in the domestic and flour depot samples. In the male fecundity is attained at a somewhat heavier weight, (g ) The pregnancy rates increase with the weight of the mouse in all four habitats, (h ) The proportion of fecund females pregnant during the year is 0⋅2194 for urban, 0⋅3166 for flour depots, 0⋅4060 for ricks, 0⋅2653 for cold stores, (i) The annual litter productivity is as follows: urban 5⋅52, flour depots 7⋅97, ricks 10⋅22, cold stores 6⋅68 litters per annum , (j ) The average number of embryos per pregnant female is not significantly different between urban, flour depot and rick samples, giving an average of 5⋅60; that for the cold stores is significantly higher, namely, 6⋅37. There is no correlation with weight of the parent. (k ) On an average a larger number of embryos were present in the right than in the left horn of the uterus. (l ) The embryo productivity rates for the four environments are as follows: urban 30⋅91, flour depots 44⋅63, ricks 57⋅23 and cold stores 42⋅55 embryos per pregnant female per annum . The annual daughter productivity rates are: urban 16⋅22, flour depots 24⋅79, ricks 31⋅51 and cold stores 23⋅15. (m ) It is estimated that the nestling production rate is in the order of 3% less than the embryo rate. The most striking differences presented by the cold-store mice are their greater weight and somewhat larger number of embryos. The rick mice show the highest productivity rates. The urban rates are lowest. The rates for the mice from the flour depots and cold stores are average. From one depot in particular, the mice had very thin skin and pelage, and torn and crumpled ears. Special attention may be drawn to the mice in cold storages. They apparently live normally in an environment which has a temperature never more than 15° F and can breed in this cold environment all the year round. A brief account of a rick mouse population is given.

The reproduction of the house-mouse ( Mus musculus ) living in different environments