Lemming Cycling and Climate Change.
Lemmings have been the source of numerous myths and legends as well as a unique system for biological studies (Stenseth and Ims 1993b). For most people, lemmings are known for two phenomena: population cycles and large-scale movements ending with mass suicide. Although the latter is a myth that has been perpetuated by Walt Disney’s 1958 film White Wilderness (Henttonen and Kaikusalo 1993), lemming population cycles are a well-known phenomenon with little understanding of the proximate and ultimate factors driving these cycles.
Charnov and Finerty (1980) proposed a hypothesis that incorporates physiologic factors for reproduction and behavior. During the low-density phase of the cycle, individuals are hypothesized to live near relatives, but can remain spatially separated. Resources are plentiful allowing high reproduction and dispersal to unoccupied patches. As the local population increases, the number of empty patches decreases forcing dispersing individuals to potentially move farther encountering non-familial individuals. The combination of aggressive non-kin interactions, high population density, and increased resource competition lead to decreased reproduction and survival. In addition, stress involved in aggressive interactions and reduced resources may affect body growth and maturation time or compromise individual’s immune system leading to increased morbidity or mortality. The Charnov-Finerty hypothesis may provide the most valuable starting point for understanding population cycles as it is based on a sound theoretical foundation that connects population cycles to physiologic factors that work at the individual level
The Charnov-Finerty (1980) hypothesis includes a number of untested assumptions. For example, it is not known if the proportion of related individuals is higher during low vs. peak phases of the cycle. In addition, although studies have shown that microtine rodents may recognize and behave differently to kin, no studies have looked at kin recognition in the Norwegian lemming. Low levels of lemming aggression have been observed in a decreasing population; however, aggression would be most likely during the high-density peak phase of the cycle. Dispersal by Norwegian lemmings during low- and high-density phases of the cycle is also unknown. Finally, physiologic parameters, such as body condition, hormone levels, and immune function – all of which represent the underlying physiologic mechanisms for the Charnov-Finerty hypothesis – are virtually unknown. As a result, the Charnov-Finerty hypothesis represents the most promising and least tested hypothesis for lemming cycles.
Norwegian lemmings are a good system for testing population cycle hypotheses due to their size, availability, and classic population cycling. Although many microtine rodents exhibit cycling, the Norwegian lemming is the only species that shows drastic population explosions, seasonal and periodic migrations, and is large enough to use for hematological and radio-telemetry studies. Unfortunately, the Norwegian lemming has been largely ignored as most literature on microtine population cycles has focused primarily on two genera: Microtus and Clethrionomys. Additionally, information on the basic ecology of the Norwegian lemming is lacking and scattered throughout regional Scandinavian journals.