On the lancelets of West Africa.



  • 1During 1948–1952 a large number of small terrestial mammals were collected in connexion with Scrub-Typhus investigations.
  • 2Simple standard data of weight and state of fertility of these animals were kept, and the results are summarized in table 15.
  • 3With 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.
  • 4Males 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.
  • 5The 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.
  • 6Females 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.
  • 7The 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.