248 ECONOMIC MAMMALOGY 



of showing that rodents are a constant menace, a single year of very 

 favorable conditions of weather, shelter and food being sufficient to 

 start a disastrous plague. If fully aware of this danger, constant watch- 

 fulness and prompt measures for their suppression as soon as an 

 increase in numbers begins, might often prevent such plagues. Prairie- 

 dogs, ground squirrels and various other rodents are also very prolific. 

 From 86 female ground squirrels (Otospermophilus gramnmrus bee- 

 cheyi) were obtained 846 embryos, an average of nearly ten each. 5 

 Another (Citellus mollis) contained 13 embryos. 6 



Because of their fecundity and their ready response to varying 

 local conditions, various species of rodents, particularly mice and rats, 

 are subject to very great fluctuations in numbers. Serious mouse plagues 

 have occurred at intervals in various parts of the world, especially 

 in Europe and Asia. 7 "Lemmings have an average periodic fluctua- 

 tion of three and one-half years, the maxima occurring synchronously in 

 North America and Europe, and probably around the Arctic region." 8 

 In Scandinavia they indulge in somewhat periodic wholesale migra- 

 tions. On high plateaus they multiply until they exhaust their food 

 supply, then swarm out over the valleys in vast hordes, swimming 

 streams, turning aside for nothing, devouring the crops and other 

 vegetation in their paths, remnants of the army finally reaching the 

 sea, plunging in and perishing. In 1868 a steamer is said to have 

 steamed for a quarter hour through a swarm of lemmings extending as 

 far as the eye could see. The voles of Siberia perform similar migra- 

 tions. 9 Hawks, owls and other enemies, follow the migrating hordes 

 and prey upon them. Often disease breaks out and assists in reduc- 

 ing their numbers. Perhaps the steady annual drain upon crops 

 through the depredations of the small rodents is greater in the long 

 run, but certainly much less spectacular than the devastation wrought 

 by these great local outbreaks. 



In 1822, in 14 days, 1,500,000 field mice were caught in the district 

 of Zabern, Germany, 590,427 in the district of Nidda and 271,941 

 in the district of Putzbach. In 1856, 12,000 acres of land had to be 

 replowed and replanted because of the destruction of the first crop 



5 Jacobson, Rate of reproduction in Citellus beecheyi, Journ. Mammalogy, iv, 58, 

 1923. 



"Jewett, A breeding record of Citellus mollis, Journ. Mammalogy, iv, 191, 1923. 



7 Lantz, Meadow mice in relation to agriculture and horticulture, Yearbook U. S. 

 Dept. Agric. for 1905, pp. 363-373; An economic study of field mice (genus Mi- 

 crotus), U. S. Biol. Surv. Bull. No. 31, 1907. Bailey, Journ. Agric. Research, xxvn, 



523-524, 1924- 



8 Innis, The fur trade of Canada, p. 90, 1927. 



9 Lantz, U. S. Biol. Surv. Bull. No. 31. pp. 6-7, 1907. 



