How Are Biological Rhythms Induced

In many hibernators circannual cycles of reproductivity, body mass, and hibernation are known to be generated by an endogenous program either independently or synchronized by environmental cues such as photoperiod or ambient temperature (Spermophilus lateralis: Kenagy, 1980; Eutamias ssp.: Kenagy, 1981; Marmota: Davis and Finnie, 1975). Least chipmunks (Eutamias minimus), for example, have strong internal programming and emerge from hibernation with year-to-year precision, irrespective of environmental conditions (Kenagy, 1981). In other species, emergence dates correlate with changes in temperature of the air or soil, and snow cover (Michener, 1977; Bronson, 1980; Murie and Harris, 1982; Kenagy, 1985; French and Forand, 2000). In the tropical mouse lemurs (Microcebus murinus), the prehibernation fattening phase was found to be dependent on photoperiod (Genin and Perret, 2000). The occurrence of daily torpor, on the other hand, seems to be a rapid response to food restriction, whatever the photoperiod, but enhanced by short photoperiod and low ambient temperature (Genin and Perret, 2003).

In C. medius, circannual cycles of reproduction, body mass, and body temperature depend on the variations of the photoperiod (Pages and Petter-Rousseaux, 1980; Petter-Rousseaux, 1980). Day length is a very reliable cue for such predictable seasonal changes as occurring in Kirindy forest. Indeed, the mean dates of entrance into hibernation do not change greatly between the years, despite differences in the progression of the vegetation period and thus the availability of food resources, due to differences in amount and timing of precipitation during the rainy season (Table 1, Figure 1). Taking the last change of tree hole before hibernation as an indicator of when hibernation starts, the mean date varied only little more than a week between the years 1999 and 2001. Within one year, however, the range is much greater, with the first animals occupying their hibernation tree holes at the end of March, and the last at the end of May (Figure 2). The opposite pattern is true for birth dates. The population is highly synchronized within one year, with all females giving birth within a tight time frame of about 2 weeks. Between the years, however, variation is high, and birth may occur any time between December and February (Figure 2). The cause for such flexibility remains unknown, but climatic conditions or food availability after the emergence from hibernation when mating takes place seem plausible. Thus, these seasonal patterns seem to be flexible up to a certain degree within the individual, as well as on the population level.

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