Unimodal daily distribution of activity in antelope ground squirrels under stable environmental conditions
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Unimodal daily distribution of activity in antelope ground squirrels under stable environmental conditions Roberto Refinetti 1 Received: 14 April 2020 / Accepted: 26 May 2020 # Mammal Research Institute, Polish Academy of Sciences, Bia?owie?a, Poland 2020
Abstract Like many other animals, white-tailed antelope squirrels exhibit a bimodal daily pattern of activity in the field during the summer. Because seasons in the field involve not only changes in ambient temperature but also changes in photoperiod and food availability, the cause of the bimodal pattern is not known. In this study, behavioral activity of white-tailed antelope squirrels was studied under controlled environmental conditions in the laboratory. Without the rise in ambient temperature around noontime, the activity pattern was unimodal under a summer-like photoperiod, and this was so when activity was monitored either with running wheels or with infrared motion detectors. Some of the animals exhibited several bouts of activity each day, but the bouts were not uniform in timing or duration, so that the activity pattern of the whole group was unmistakably unimodal with a peak or plateau at noontime. Thus, the bimodal pattern observed in the field in this species is not due to an intrinsic property of the circadian system or to the presence of a long photoperiod but likely to the heat of summer noontime. Keywords Ammospermophilus leucurus . Bimodal activity . Circadian rhythm . Locomotor activity . Photoperiod
Introduction It has long been known that animals are not equally active at all times of the day but instead arrange their activity in a pattern that is more or less regularly repeated day after day (Aschoff 1964). Some species are primarily diurnal, some are primarily nocturnal, and many species have idiosyncratic patterns of activity (Refinetti 2008). Many field studies have documented bimodal activity patterns, usually with a peak at dusk and another peak at dawn. In diurnal rodents, the bimodal pattern has been observed most frequently in the summer and not in the winter, for example, in red squirrels (Wauters et al. 1992), degus (Kenagy et al. 2002), Mexican fox squirrels (Koprowski and Corse 2005), and Mediterranean voles (Pita et al. 2011). Because ambient temperature is usually higher in the middle of the day and can reach dangerous levels during the
Communicated by: Karol Zub * Roberto Refinetti [email protected] 1
Department of Psychology, University of New Orleans, New Orleans, LA 70148, USA
summer in some locations, it has been inferred that the bimodal activity pattern of diurnal rodents is due to avoidance of extreme heat, with the animals staying inactive in cooler places around noontime. However, observations in field studies are very often confounded by multiple environmental variables interacting in a convoluted manner. Rather than being a result of heat avoidance, bimodality could well be an intrinsic characteristic of the circadian organization of behavior, as demonstrated by Aschoff in several spe
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