Compulsory Regime and Control of Environment in Animal Behaviour Iii. Light Level Preferences of Small Noctural Mammals

[Twenty individuals belonging to the rodent species Glaucomys volans, Microtus californicus, M. pennsylvanicus, Neotoma lepida, Peromyscus californicus, P. crinitus, P. floridanus, and Thomomys bottae, and the didelphid species Marmosa mitis were studied in activity-wheel enclosures in which they could control ambient illumination in 10 steps (darkness to 200 lux) by instrumental means. At each illuminance level we determined the time spent running (active time) and not running (inactive time), running speed, number of visits, and visit lengths. During activity, most individuals were highly selective for illuminance. Behaviour not only was generally species specific, but conspecifics often gave identical results. Since the illuminance preferences generally correlated well with habits and habitat preferences in the wild, volitional control of illumination tests probably give a reliable indication of the overall state of adaptation of the visual system of small nocturnal mammals; such tests could serve as Independent guides for Interpreting retinal histology. Both the mouse opossums and gopher spent most of their active time in total darkness and much of the remaining time at level 2 (0.7% of starlight). For the flying squirrels, active time generally peaked at level 2 or 3 (2.1 or 7.5 × 10-5 lux), for the Florida mice it peaked at level 5 (50% of the light on a clear moonless night), and for the California mice at level 7 (about 3 times full moon). lndividual woodrats and canyon mice had the most spread between the positions of their peaks - levels 5 to 7 for the former and 5 to 8 for the latter. Meadow voles were the only animals to have dual illuminance preferences (darkness and level 4). These dual preferences may be related to the fact that meadow voles need to feed at regular intervals and are active around-the-clock in the wild. Components of persisting physiological cycles, particularly digestion, may have influenced the degree to which visual and non-visual factors determined to voles' illuminance preferences; thus, non-visual factors may have predominated during primarily digestive phases and visual ones during primarily non-digestive ones. Eight of the animals showed preference shifts to dimmer levels during inactivity; 3 of these 8 showed marked shifts to darkness. For 13 of the 20 animals, running was fastest in the illuminance range preferred for activity. The results for the gopher and flying squirrels accord with our hypothesis that the proportionality of the running speed of certain rodents to illuminance is the result of selection for individuals that run as fast as navigational safety permits during twilights. Thus, factors that lead to such selection would not be expected to play a significant role for gophers and flying squirrels, and these two species did not exhibit the speed-illuminance dependence., Twenty individuals belonging to the rodent species Glaucomys volans, Microtus californicus, M. pennsylvanicus, Neotoma lepida, Peromyscus californicus, P. crinitus, P. floridanus, and Thomomys bottae, and the didelphid species Marmosa mitis were studied in activity-wheel enclosures in which they could control ambient illumination in 10 steps (darkness to 200 lux) by instrumental means. At each illuminance level we determined the time spent running (active time) and not running (inactive time), running speed, number of visits, and visit lengths. During activity, most individuals were highly selective for illuminance. Behaviour not only was generally species specific, but conspecifics often gave identical results. Since the illuminance preferences generally correlated well with habits and habitat preferences in the wild, volitional control of illumination tests probably give a reliable indication of the overall state of adaptation of the visual system of small nocturnal mammals; such tests could serve as Independent guides for Interpreting retinal histology. Both the mouse opossums and gopher spent most of their active time in total darkness and much of the remaining time at level 2 (0.7% of starlight). For the flying squirrels, active time generally peaked at level 2 or 3 (2.1 or 7.5 × 10-5 lux), for the Florida mice it peaked at level 5 (50% of the light on a clear moonless night), and for the California mice at level 7 (about 3 times full moon). lndividual woodrats and canyon mice had the most spread between the positions of their peaks - levels 5 to 7 for the former and 5 to 8 for the latter. Meadow voles were the only animals to have dual illuminance preferences (darkness and level 4). These dual preferences may be related to the fact that meadow voles need to feed at regular intervals and are active around-the-clock in the wild. Components of persisting physiological cycles, particularly digestion, may have influenced the degree to which visual and non-visual factors determined to voles' illuminance preferences; thus, non-visual factors may have predominated during primarily digestive phases and visual ones during primarily non-digestive ones. Eight of the animals showed preference shifts to dimmer levels during inactivity; 3 of these 8 showed marked shifts to darkness. For 13 of the 20 animals, running was fastest in the illuminance range preferred for activity. The results for the gopher and flying squirrels accord with our hypothesis that the proportionality of the running speed of certain rodents to illuminance is the result of selection for individuals that run as fast as navigational safety permits during twilights. Thus, factors that lead to such selection would not be expected to play a significant role for gophers and flying squirrels, and these two species did not exhibit the speed-illuminance dependence.]