Nonhuman Primates Mask Signs of Pain

s/esr/v3/n2/p239-247>. This study describes the activity pattern of the blue-eyed black lemur Eulemur macaco flavifrons for the firsttime and investigates the parameters, such as season orhabitat, that may influence the distribution of activity overthe 24 h cycle. Four groups of E. m. flavifrons in 2 forestfragments with different degrees of degradation were fol-lowed for 24 h each month over a 7-month periodbetween July 2004 and July 2005. Blue-eyed blacklemurs exhibited a bimodal activity pattern which peakedduring the morning and evening twilight. The groupsconsistently showed activity bouts both during the dayand at night, a behavior that corresponds to Tattersall’s(1987) definition of cathemerality. The proportion of il-luminated lunar disc and the nocturnal illumination indexwere positively associated with the amount of nocturnalactivity. Total activity, both diurnal and nocturnal, wassignificantly higher in the secondary than in the primaryforest. In view of our results, the cathemeral behavior ofE. m. flavifrons may best be explained as flexible re-sponses to a framework of varying environmental factors,each of which may enhance or inhibit activity within thelemurs’ range of adaptability. This temporal behavioralplasticity may be an adaptation to an erratic and severeclimate with frequent droughts and cyclones and unpre-dictable resource availability.