• 1
    Records of fertility (sperm in male, pregnancy in female) have been made on a large number of small mammals collected for examination by the Scrub Typhus Research Unit stationed at Kuala Lumpur. Those of rats of the subgenus Rattus, which includes the commensal forms, are summarized here.
  • 2
    The rats concerned are Rattus rattus diardii, a house rat; R. r. jarak, an island form related to it; R. r. jalorensis, a forest and plantation rat which is a pest of oil palms; R. r. argentiventer a grassland rat, which is also a pest of ricefields and is important in the dissemmination of scrub typhus; R. exulans, a rat of very wide range, which can be a house-, a field-, or a forest rat; and R. annandalei, a rare forest rat.
  • 3
    The sex ratio of each was found to approximate to equality. The significant departures from equality observed were explicable for R. r. diardii by a higher death rate among males than females, for R. r. argentiventer by a greater range and hence a higher trapping rate among males.
  • 4
    The weight of the testes of mature males of R. r. jalorensis, jarak, and R. exulans was found to be 5 or 6 per cent, of the body weight, that for R. r. diardii, argentiventer, and R. annandalei about 3 per cent.
  • 5
    Mean weights at which fertility commenced were estimated from the regression line of fertility rate on body weight.
  • 6
    Mean weights during the first and last pregnancy were similarly estimated from the regression line of pregnancy rate on body weight.
  • 7
    Mean embryo numbers, and the distributions of litters of different sizes are given. For only two rats, R. r. diardii, and R. exulans was there any observed tendency for the embryo number to increase with the body weight. All but R. r. argentiventer showed signs of an intra-uterine mortality.
  • 8
    The distribution of embryos between right and left horns of the uterus was noted. In most rats they were entirely independent, but in R. r. argentiventer there was a marked tendency for all the embryos to be on one side or the other. R. r. jarak showed a tendency for embryos to be borne on the right side.
  • 9
    The embryo rate is used as a measure of the reproductivity and hence of the death rate. It is concluded that the rate is probably correlated with the incidence of predation and not with the form of rat.
  • 10
    The unusually low reproductivity of R. r. jarak is due to an unusually low embryo number. It is suggested that this is an effect of malnutrition. The unusually low reproductivity of R. r. argentiventer is due to an unusually low pregnancy rate. It is suggested that this may be explained by the low population density of this rat.