Daytime aversive stimuli do not phase shift behavioral rhythms under light-dark cycles in a strictly diurnal rodent

Recent studies have shown that cyclic aversive stimuli (time-specific footshocks) act as a nonphotic zeitgeber, shifting circadian behaviors to the daytime in nocturnal rodents through entrainment. It has remained untested whether diurnal species exhibit similar plasticity in behavioral timing. This study investigated whether antelope ground squirrels (<i>Ammospermophilus leucurus</i>, AGS), naturally diurnal rodents, shift activity timing in response to cyclic aversive stimuli delivered at specific phases of the light-dark (LD) cycle. We conducted two experiments with 20 AGS housed in custom cages featuring a safe nesting area and a separate foraging area rendered potentially aversive by unsignaled time-specific footshocks. In Experiment 1, animals were subjected to a 12:12 LD cycle. One group was exposed to a foraging area that produced footshocks during the light phase, and a control group with footshocks during the dark phase. In Experiment 2, under a 16:8 LD cycle, animals were divided into three groups, with footshock exposure either during the first or second half of the light phase or during the dark phase. Following treatments, animals were released into constant darkness (DD) to assess free-running rhythms. Contrary to findings in nocturnal rodents, AGS did not exhibit consistent complementary shifts to nocturnal activity as an avoidance of footshocks received during daytime. Most animals maintained diurnal activity, showing minor, and inconsistent phase adjustments. In Experiment 2, animals exposed to footshocks during part of the light phase also failed to reliably shift activity to the "safe" portion of the light phase. These findings show AGS do not substantially shift activity patterns in response to cyclic aversive stimuli and that a 24-h cyclic fear stimulus fails to override the LD cycle as a zeitgeber. This suggests a lack of plasticity in circadian behavior and highlights the importance of species-specific differences in response to potential nonphotic zeitgebers.