Abstract
REFERRING to Prof. P. Chavigny's report on rats in the trenches (NATURE, September 19, p. 53), Mr. C. B. Moffat, Enniscorthy, points out that the descendants of a pair of rats must in three years far exceed the twenty millions stated. At the end of the first year there should be 50 offspring, 500 grand offspring, 1000 great-grand offspring, 1250 great-great-grand offspring—2800 in all. Half of this number, supposing females equal males, multiplied by 2800, gives 3,920,000 at the end of the second year. At the end of the third year the number should be far more than five thousand millions. It has to be borne in mind, however, that female rats probably reach their limit or menopause long before three years. Thetmost secure data known to us are those of Helen Dean King (Anat. Record, vol. xi., 1916, pp. 269–87) on 76 females derived from a cross: between the wild Norway rat and the domesticated white rat. The average number in a litter was 6.7 (Prof. Chavigny speaks of 10); the average total number of litters for a female was 7.7; there is a sharp decline in fertility after the female is a year old, and the menopause appears at eighteen months. The sex ratio for 3955 individuals was 106.1 males to 100 females. We do not know how Prof. Chavigny reached the figure twenty millions, but. as Mr. Moffat recognises, there are various biological considerations which make the computation not so simple as it seems at first.
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The Rat Pest . Nature 102, 176 (1918). https://doi.org/10.1038/102176a0
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DOI: https://doi.org/10.1038/102176a0