Feeding a Diurnal Rodent at Night Induces Weight Loss and Body Temperature Reduction Without Food-Anticipatory Activity.

An important component of behavioral plasticity is the ability of animals to shift the timing of daily or seasonal activities to accommodate changing physical and biotic conditions, a phenomenon known as temporal niche switching. Temporal niche switching in response to experimentally altered food availability in the laboratory is often accompanied by locomotor activity that anticipates the time of daily food access ("food-anticipatory activity" = FAA), which reflects the plasticity of the mammalian circadian system. Most FAA studies have been conducted in nocturnal rodents, reporting robust anticipatory increases in locomotor activity and physiological variables such as body temperature before food presentation, but this kind of plasticity has not been tested in strictly diurnal species. In the present study, we show that the diurnal antelope ground squirrel (AGS; Ammospermophilus leucurus) fails to exhibit FAA when subjected to temporally restricted food access. We centered 8 h of food restriction either in the middle of the active/light phase (daytime feeding; DF) or the rest/dark phase (nighttime feeding; NF). In contrast to findings in nocturnal rodents, NF AGSs did not show FAA, lost body mass, decreased overall wheel-running activity, and exhibited decreases in body temperature, with 2 animals becoming hypothermic. DF squirrels ate more than NF squirrels, had greater activity levels, and maintained normal body temperature. These results indicate that exclusive nighttime food availability was not sufficient to support normal energy balance in this strictly diurnal rodent. Thus, whereas negative energy balance can promote shifts toward daytime activity in some nocturnal mammals, AGS did not show the reciprocal shift toward nighttime activity when food was restricted to the dark phase. This suggests that, relative to nocturnal species, strictly diurnal mammals may have a limited capacity to compensate behaviorally and physiologically for food availability during the rest phase of the circadian cycle.