ARCTIC INSTITUTE OF NORTH AMERICA TECHNICAL PAPER NO. 15 THE LEMMING CYCLE AT BAKER LAKE, NORTHWEST TERRITORIES, DURING 1959-62 By CHARLES J. KREBS PUBLISHED SEPTEMBER 1964 CONTENTS Frrmtispiece: General sketch-map 6 Abstract 7 Acknowledgments 7 Introduction 8 Area and climate . 9 Vegetation and habitats 10 Population density 11 Live trapping . 11 Results 14 Snap trapping 15 Results 16 Other census methods 18 Results 18 Summary and conclusions 18 Reproduction . 20 Length of breeding season 22 Litter size . 23 Pregnancy rates . 25 Age at reproductive maturity 26 Mortality . 27 Prenatal mortality 27 Post-natal mortality 28 Adults 28 Juveniles . 30 Movements and migrations 32 Local movements . 32 Migrations 33 Changes in extrinsic factors 34 Weather • 35 Predators . 35 Disease and parasites . 36 Food . 37 Changes in intrinsic factors 39 Age determination 39 Body weight distributions 40 Mean body weights 43 Organ weights 44 Adrenal gland 46 Testis 46 Spleen 47 Fat changes 47 Social relationships 48 Discussion . 50 Historical approaches and background 50 Reproduction . 51 Mortality 54 Migrations 55 Weather and synchrony . 57 Predators . 59 Disease and parasites . 59 Body weight changes . 59 Three current hypotheses 61 Food supply hypothesis. 61 Christian's stress hypothesis • 63 Chitty's polymorphic behaviour hypothesis. 65 Conclusions 67 Summary . 67 References . 100 Tables 1. Temperature and precipitation data during 1959--62 and the mean values for 195CHSO as recorded by the Baker Lake meteorological station. 69 2. Area covered by the principal habitat types an the Main Study Area. 70 3. Dominant plant species in the habitats of the Main Study Area, "New Lake", and Prince River. 70 4. Dominant plant species in the habitats of the Aberdeen Lake area . 71 5. Dominant plant species in the moss habitats on the islands of Baker Lake and the south bank of the Thelon mouth 71 6. Numbers of Lemm:us on Quadrat 1 during 1959--62 . 72 7. Numbers of Dicrostonyx on Quadrat 3 in 1960-2 72 8. Lemm:us snap-trapping indices, 1959--62 . 73 9. Dicrostonyx snap-trapping indices, 1959--62 . 74 10. Timing of summer breeding periods in Lemm:us females, 1959--62 . 75 11. Timing of summer breeding periods in Dicrostonyx females, 1959--62 . 75 12. Length of the summer breeding seasons of Lemm:us and Dicro'Stonyx, Main Study Area, 1959--62 . 75 13. Numbers of corpora lutea in Lemmas females, Main Study Area, summers 1959--62 76 14. Numbers of corpora lutea in Dicro'Stonyx females, Main Study Area, summers 1959--62 76 15. Number of embryos in Lemm:us females, Main Study Area, 1959-62 77 16. Number of embryos in Dicro'Stonyx females, Main Study Area, summers 1959-62 77 17. Number of corpora lutea and embryos in Lemm:us females at Aberdeen Lake, summers 1960-2 . 78 18. Number of corpora lutea and embryos in Dicrostonyx females at Aberdeen Lake, summers 1960-2 78 19. Crude pregnancy rates, Lern:mus, summers 1959-62 . 79 20. Crude pregnancy rates, Dicrostonyx, summers 1959--62 • 80 21. Median body weights at maturity for Lemm:us and Dicrostonyx males and females, 1959-62 . 81 22. Partial prenatal mortality data for Lemm:us females, summers 1959--62, Main Study Area 81 23. Partial prenatal mortality data for Dicro'Stonyx females, summers 1959--62, Main Study Area . 81 24. Minimum survival factor estimates for Lemm:us, summer 1960 . 82 25. Minimum survival factor estimates for Lemm:us, summer 1961, Quadrat 1 . 82 26. Minimum survival factor estimates for Dicro'Stonyx, summer 1960, Quadrat 2 82 27. Minimum survival factor estimates for Dicro'Swnyx, summer 1961, Quadrat 3 83 28. Minimum survival factor estimates for Dicrostonyx, summer 1962, Quadrat 3 83 29. Minimum survival rates for Lemm:us oonverted to a 28-day period. 83 30. Minimum survival rates for Dicro'Stonyx converted to a 28--day period. 84 31. Survival estimates for juvenile Lemm:us on Quadrat 1, summer 1960 . 84 32. Survival estimates for juvenile Lemm:us on Quadrat 1, summer 1961 • 84 33. Survival estimates for juvenile Dicrostonyx on Quadrat 2, summer 1960 85 34. Survival estimates for juvenile Dicrostonyx on Quadrat 3, summer 1961 85 35. Survival estimates for juvenile D~crostonyx on Quadrat 3, summer 1962 85 36. Length of every movement recorded within periods of live trapping for Lemmus, summers 1960-1 86 37. Length of every movement recorded within periods of live trapping for Dicrostonyx, summers 1960-2 86 38. Standing forage at the end of summer . 87 39. Estimate of percentage forage utilization in the spring of the decline, June 1961 87 40. Body weight distributions for Lemm:us males on the Main Study Area, 1959--62 88 41. Body weight distributions for Dicrostonyx males on the Main Study Area, 1959--62 90 42. Body weight distributions for Lemm:us males at Aberdeen Lake, 1960-2 . 92 43. Body weight distributions for Dicrostonyx males at Aberdeen Lake, 1960-2 92 44. Body weight distributions fox Lemm:us males on the outlying areas, summer 1961 93 45. Mean body weights of Lem:mrus males of the winter and spring generations, summers 1959-62 . 94 46. Mean body weights for Dicrostonyx males of the winter and spring genera- . tions, summers 1959-62 . 95 47. Standardized mean organ weights and fat index for Lemmu.s males, 1959-62. 96 48. Standardized mean organ weights and fat index for Dicrostonyx males, 1959-62 97 49. Amount of wounding shown on skins of Lemmus males from the Baker Lake area, 1959-62 . 98 50. Amount of wounding shown on skins of Dicrostonyx males from the Baker Lake area, 1959-62 99 Figures 1. Map showing the location of Baker Lake, the Main Study Area, and the peripheral sampling areas 9 2. Location of the live-trapping quadrats on the Main Study Area . 12 3. Summer habitat distribution in Lemmus and Dicrostonyx on the Main Study Area . 17 4. Generalized density changes, 1959-62 . 19 5. Components of reproduction in polyoestrous mammals . 22 6. Body weight distributions for Lemmus males, July 16-31, 1960 and 1961 41 7. Generalized aunualchronology of generations and litters for Lemmus and Dicrostonyx . 42 8. Pitelka's food supply hypothesis . 62 9. Christian's stress hypothesis. 64 10. Chitty's polymorphic behaviour hypothesis . 66 65° Hadso7U 60° + + General sketch -map THE LEMMING CYCLE AT BAKER LAKE, NORTHWEST TERRITORIES, DURING 1959-62 Charles J. Krebel Abstract A four-year study covering one cycle in numbers of the brown and varying lemmings has been carried out at Baker Lake, Keewatin, N.W.T. An attempt was made to describe the events of the cycle in detail by snap-trapping and live-trapping techniques and by detailed autopsies on about 4,000 animals and from this to determine what explanations would fit the observations. Increase began from very low numbers in the summer of 1959 with tremendous popula­ tion growth occurring over the winter of 1959>--60. Little further increase occurred in the peak summer of 1960. A great decline occurred over the winter of 1960--1, and this decline continued through the summer of 1961 on the Main Study Area. Little change occurred over the winter of 1961-2 and the phase of increase began again in the summer of 1962. Two changes in reproduction were associated with this cyclc-changes in the length of the breeding seasoIi and in the weight at sexual maturity. Winter breeding occurred in the period of increase but not in the decline, and a shortened summer breeding season occurred in the peak and to some degree in the decline. Young male Lemmus matured in the summers of increase but not in the peak or in the decline; young females matured in the increase and decline summers but not in the peak. The major change in mortality was a very high juvenile mortality in the summer of the decline. Changes in the quality of the animals were manifested not only by these reproductive and mortality changes but also by a 20--30 per cent increase in mean body weights of the adults in the peak summer compared with the increase or decline summers. The role of the extrinsic factors is considered. There was no widespread destruction of the habitat, or any 'evidence of quantitative or qualitative food shortage in the animals of the decline. Neither predators, disease, nor parasites seemed to be the cause of the observed changes in mortality. The role of the intrinsic factors is also considered. Summer adrenal and spleen weights showed no clear relationship to the cycle. The amount of wounding on skins showed strong seasonal changes and was not a simple function of density. It was concluded from these observations that this lemming cycle could not be adequately explained by the conventional extrinsic factors such as food supply, but rather that it is essentially a self-regulatory phenomenon. The stress hypothesis proposed by Christian was also rejected as an adequate explanation of these events. The idea of Chitty that populations change in quality during changes in density was fully supported by these results. Behaviour­ al changes in the population may constitute the crux of the lemming cycle, and Chitty's sug­ gestion that these fluctuations may represent a genetic polymorphism involving aggressive behaviour is not refuted by these data. Acknowledgments I wish to express my gratitude to Dr. Dennis Chitty and Dr. Ian McT. Cowan for their stimulation and help throughout this study. I am also indebted to Mr. Andrew H. Mac­ pherson of the Canadian Wildlife Service for information and assistance in the field work. lDepartment of Zoology, Indiana University, Bloomington, Indiana. 7 r 8 THE LEMMING CYCLE AT BAKER LAKE DURING 1959-62 Support for the field work was provided by the Arctic Instimte of North America.