The effect of ecologically relevant variations in light level on the performance of Mongolian gerbils on two visual...

Tests of rodent visual capacities are typically performed under standard laboratory illumination. However, light level can have subtle and complex effects on behavior in rodents. We tested Mongolian gerbils (Meriones unguiculatus) - a species that shows individual differences in activity pattern - on visual tasks at three ecologically relevant levels of ambient illuminance: approximating moonlight (1 lx), dawn or dusk (10 lx), and low daylight (100 lx). A jumping task and a grating discrimination quantified depth perception and grating acuity, respectively. Illuminance variations had important effects on behavior. Gerbils jumped faster in lower light on the depth discrimination task, but were also less accurate than in bright light. Daytime activity levels (possibly reflecting variations in activity pattern) mediated these effects, with animals that were awake during a lower proportion of daytime hours exhibiting a trend toward jumping faster with lower light level as compared to more day-active gerbils. Moreover, while illuminance did not affect acuity overall, it interacted with activity levels in a complex way: in both the darkest and lowest light levels, animals that were awake during a greater proportion of daytime hours showed higher acuity levels than animals that were less active during the day. These results indicate that gerbils show behavioral profiles in vision tasks that represent an interaction between visual ability and illuminance-sensitive motivational or emotional actors, perhaps including chronotype. Thus, the most ecologically relevant assessments of the visual performance of rodents will likely be achieved by testing at species-specific preferred light levels.