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Figure 2. Schematic representations of the three cathemeral modes during austral summer and austral winter. (A, B) Mode A (n = 2) combines data from Ampijoroa (16°19'S; 46°49'E) and Anjamena (16°03'S; 45°55'E). (C, D) Mode B (n = 4) combines data from Ampijoroa, Kirindy (20°03'S; 45°55'E), Sainte Luce (24°45'S; 47°11'E), and Pointe Saziley, Mayotte (12°58'S; 45°11'E). (E, F) Mode C (n =1) shows data from Andreba (17°38'S; 48°31'E). Gray areas indicate time periods before sunrise and after sunset, the times of which were taken for each location for the summer and winter solstices on December 21 and June 21, respectively, and averaged (Curtis and Rasmussen, 2002).

forests of the Sambirano region and montane rainforests (Overdorff and Rasmussen, 1995; Andrews and Birkinshaw, 1998; Colquhoun, 1998) and in H. griseus alaotrensis in lake-side reed beds (Mutschler, 1998). Qualitative descriptions of the activity cycle indicate that E. coronatus and E. f. sanfordi also exhibit this type of cathemeral activity in Sambirano montane rainforest (Freed, 1996).

Tattersall's definition of cathemerality holds true in the light of data collected since its publication (Tattersall, 1987). The model proposed by Rasmussen (1999) also continues to hold true when more recent data are integrated, but additional data are required to test it further and we must bear in mind that substantial variation exists within the three modes (Curtis and Rasmussen, 2002).

ENVIRONMENTAL CUEING MECHANISMS Circadian Rhythms, "Zeitgeber," and Masking

The spontaneous period of the circadian rhythm (the internal biological clock) deviates slightly from the 24-hr day and must be synchronized each day by rhythmic environmental cues called "zeitgebers" (from German, meaning "time-giver"). Sunrise and sunset are the main "zeitgebers" that reset the biological clock to the daily light-dark cycle. Illumination, temperature, humidity, rainfall, food availability, and social factors can also entrain activity (Bartness and Albers, 2000). Only diurnal, nocturnal, and ultradian (period length significantly shorter than 24 hr) activity rhythms fulfill the criteria of "true" circadian rhythms that are controlled by one or more internal clocks (Bartness and Albers, 2000). So how can cathemerality be described in chronobiological terms?

Experiments carried out on E. f. albifrons indicate that cathemerality results from masking of a true nocturnal rhythm by external factors which override the endogenous clock, either stimulating or inhibiting activity (Erkert, 1989; Erkert and Cramer, 2006). The primary zeitgeber is sunset, controlling the onset of activity, and the secondary "zeitgeber" is sunrise, controlling cessation of activity. Figure 3a shows close synchronization between onset of activity and the primary zeitgeber in a nocturnal primate (Galago moholi), while cessation of activity and the secondary zeitgeber are less synchronized (Bearder et al., 2006). Figure 3b shows data for the cathemeral Eulemur mongoz, where the negative association between sunset and activity onset is greater, suggesting that sunset acts as the primary zeitgeber and confirming the inherent nocturnality of this species. Furthermore, negative associations between activity onset and cessation with sun-

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