1. During 1948–1952 a large number of small terrestial mammals were collected in connexion with Scrub-Typhus investigations.
2. Simple standard data of weight and state of fertility of these animals were kept, and the results are summarized in table 15.
3. With a few exceptions, the sex ratio approximated to equality. Of the exceptions, most could be shown to be due to bias in collecting. In a few species particularly associated with man, however, there was an apparent loss of males in the higher weight groups, suggesting a higher death rate among males. In the house mouse, Mus musculus, however, there was an excess of males.
4. Males showed a distinct annual variation in the fertility and weight of testes. Forest rats showed an increase in fertility during the middle of the year, but the house shrew, Suncus murinus, showed a cycle of reverse phase. The variation did not appear to be great enough to affect breeding.
5. The mean weight of the males at the onset of fertility was calculated from the regression of percentage-fertile on body-weight, with scales suitably adjusted.
6. Females showed a slight annual variation in pregnancy rate too slight to delimit any breeding seasons. The mean weight at onset of pregnancy was similarly estimated.
7. The numbers of embryos were recorded and used to calculate a mean embryo number. Combined with the pregnancy rates this gave an estimate of the reproduction rate.