. Animal ecology. Animal ecology. 134 ANIMAL ECOLOGY shorter intervals of three or even two years. Now, by examin- ing the records of skins obtained by the Hudson's Bay Company in Canada, it is possible to find out when the lemming years have occurred in Arctic Canada. This is made possible by the fact that the arctic fox numbers depend mainly upon those of lemmings, since the latter are the staple food of the fox. The curve of fox skins, which shows violent and regular fluctuations with a period of three or four years, can therefore be used as an index of the state of the lemming population i
. Animal ecology. Animal ecology. 134 ANIMAL ECOLOGY shorter intervals of three or even two years. Now, by examin- ing the records of skins obtained by the Hudson's Bay Company in Canada, it is possible to find out when the lemming years have occurred in Arctic Canada. This is made possible by the fact that the arctic fox numbers depend mainly upon those of lemmings, since the latter are the staple food of the fox. The curve of fox skins, which shows violent and regular fluctuations with a period of three or four years, can therefore be used as an index of the state of the lemming population in different years. When we compare the lemming years in. Fig. II.—The fluctuations in numbers of lemmings are very violent and very regular, and synchronise in Scandinavia and Canada. Curve A shows the number of arctic fox skins taken annually by the Hudson's Bay Company in Canada. Curve B shows the number of skins taken each year in the whole of Canada, from 1920-1924. Diagram C shows the lemming years in Canada deduced from the fox curves A and B. Diagrams D and E show the known dates of maximum ("lemming years") of lemmings in Norway and Greenland (the latter being incomplete). (From Elton.^*) Canada and Norway the curious fact emerges that they synchronise almost exactly in the two countries, and there is little doubt that the numbers are controlled by some common factor, which can only be climate. The other records which exist show that Greenland and the islands of the Canadian Arctic Archipelago also have lemming maxima at the same time as the others. II. In the lemming we have therefore an animal which undergoes regular and violent fluctuations in numbers from year to year, which can be analysed into three main processes : first of all, the epidemic which kills them off when a certain. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations ma
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