Plague and the Regulation of Numbers in Wild Mammals.

1. A short account is given of the occurrence of plague epidemics in wild mammals other than “domestic” rats and mice. Enough is now known to show that such epidemics are often of great practical importance.2. The method by which most rodents regulate their numbers is as follows: increase in numbers over several years up to a point at which an epidemic of some sort occurs, which kills off a large proportion of the population. Increase then takes place again, and is followed by another epidemic, and so on indefinitely.3. These periodic fluctuations are probably controlled by widespread climatic fluctuations, the best evidence for this being that in certain cases the former run synchronously in widely separated countries.4. There appears to be a dominant short period in fluctuation of three to four years, and a larger movement of period ten to eleven years, both in North America and Europe. The fact that the 11-year sunspot cycle roughly corresponds with the larger movement is significant.5. The data available from Eastern Asia suggest that there too, small mammals fluctuate periodically in numbers, and with similar periods to those of North America and Europe. There is also some evidence that the maxima of the 10–11-year fluctuations coincide in Eastern Asia and in North America, just as those of the 3–4-year period coincide in Northern Canada, Greenland and Norway.6. Evidence is given that the plague marmot (Arctomys bobac), and other rodents carrying plague, are liable to these fluctuations. If this proves to be true, it may be possible, when fuller data are available, to forecast with some accuracy the years of epidemics among these animals, and if this can be done we shall have some means of gauging the likelihood of the occurrence of plague outbreaks in the human population of those regions.7. The available data are admittedly fragmentary, but it is probable that between the years 1931 and 1934 epidemics among A. bobac in Transbaikalia and Mongolia will be severe, and that these events will lead to an increase of plague mortality in man.