Abstract
Diel activity rhythms in mammals are regulated by an endogenous (circadian) timing system which is synchronized by environmental 24-hr periodicities called zeitgebers. Additional direct responses to stochastic environmental factors ensure the fine-tuning to the actual situation and may mask the circadian time course. Following an observational study on behavioral effects of visitor activities in a group of spider monkeys (Ateles geoffroyi) kept free-ranging on a small island of Lake Catemaco, Veracruz, Mexico, we analyzed the effect of weekly varying numbers of visiting tourist boats on the monkeys' diel activity rhythm. With small accelerometer-data logger devices we recorded the monkeys' locomotor activity continuously for several months each. Then we compared the data with those from spider monkeys living without tourist contact. Neither the duration of the monkeys' activity time (α) nor its phase relationship to the 24-hr solar day did change on different weekdays in either site. However, their activity level showed a clear 7-day rhythm. The monkeys of the tourist site showed highest activity on Saturday and Sunday, when the frequency of visiting tourist boats was highest, whereas those of the non-tourist site were least active on Sunday and Monday, when human activities were lowest there. While the monkeys of the non-tourist site usually displayed a distinct bimodal activity pattern peaking in the morning and late afternoon, the pattern in those of the tourist site mostly lacked a morning peak and varied more over time. Based on our results, we suggest that circadian entrainment is not involved in the differences between the diel activity rhythms of the spider monkeys from the two keeping sites and the differing 7-day variation in their activity level. Rather, these differences seemingly reflect direct responses to the differing human activities and thus may correspond to circadian masking effects.
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