The Cathemeral Activity Cycle

In 1999, Rasmussen proposed a model splitting cathemerality into three modes A, B, and C (Rasmussen, 1999). All three modes exhibit peaks of activity at dawn and dusk and involve some form of mixture of diurnal and nocturnal activity across the year. Curtis and Rasmussen (2002) linked these modes to habitat types, in the light of apparent associations of modes A and B with seasonal habitats in Madagascar and mode C occurring in rainforest habitat and lake-side reed beds. However, data have recently been published which call into question any strict connection between habitat types and cathemeral modes of activity (Donati and Borgognini-Tarli, 2006).

The approach I take here describes the different modes on the basis of annual photoperiodic changes. Periods of long daylengths (austral summer) and short daylengths (austral winter) coincide approximately with the wet and dry seasons, respectively, in western Madagascar and this approach permits comparison across all habitat types, as many cannot be described by two seasons. Furthermore, there is some indication that austral spring and autumnal equinoxes (daylength equal to nightlength) might serve as the triggering mechanism for changes in the ratios of diurnal to nocturnal activity (Figure 1) (Kappeler and Erkert, 2003; Donati and Borgognini-Tarli, 2006).

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