London's Pulse: Medical Officer of Health reports 1848-1972
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Hints from the Health Department. Leaflet from the archive of the Society of Medical Officers of Health. Credit: Wellcome Collection, London
City of London 1920
Report, Medical Officer of Health, on rat repression in the City 1920
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It is on facts somewhat similar to the above that it is suggested that if the sex
relationship of rats could be interfered with more permanent benefits would accrue than
from the capture and slaughter of all rats irrespective of sex.
Apparently rats exist in nature in the ratio of about seven males to ten females. I have, however, not been able to obtain any considerable series of numbers which would support this ratio. Mr. Jennison has favoured me with the sexes of the rats which have been raptured at the Zoological Gardens, Manchester, during recent years, and these are as follows:—
| Year. | Date. | Captures. | ||
|---|---|---|---|---|
| Males. | Females. | Total. | ||
| 6 | ||||
(Recent newspaper reports have stated that of 72 rats caught by the Sissinghurst
(Kent) Rat Club the proportion of females to males was seven to one.)
If these figures are calculated on the basis of seven males to ten females, it will be
seen that males are slightly fewer than they ought to be. For every 475 males there
ought to be caught only 680 females, as compared with 716 shown above. The increase
of females over males is possibly due to the fact that all females caught are killed
whilst the males are turned down, and the male is naturally reluctant at returning to
the trap. Comparatively few males appear twice in the traps, though occasionally odd
ones will return three or more times.
In general one would expect that proportionately more males would be caught
than females, since sexual instincts would cause the male to have a wider range of
0 .
journeying than the female, who is more frequently held up by care of young.
(32) The position is then that more females exist than males, and it is assumed, if
the males were turned down and the females slaughtered, that in the course of time the
sex relationship would be so disturbed that males would be sufficiently in excess to
harass females. This harassing would result in smaller litters, possible destruction of
young, possible high mortality amongst females and possibly the development of
cannibalism.
It appears to me quite clear that if the male rats were turned down, apart from the
damage done by such male rats, very little harm would result to the human
community, since it is on the number and fecundity of females that the increase in the
rat population depends, and the addition of a considerable number of males would
certainly not result in any additional increase as a consequence of their release. It is
further suggested that a rat once caught and turned down will only rarely return to
the trap and may leave the district, but even in such a case he would increase the
number of males in the adjoining area. It is a point of interest worthy of mention
that Mr. Jennison reports "that in one trap the captures average little more than half
those taken in 1915, this trap has been used 40 years and the captures at that time
were evidently the normal."