Bimodal Nocturnal Activity of the Western Toad (Bufo boreas) in Relation to Ambient Illumination

The western toad (Bufo boreas) showed a nocturnally bimodal activity pattern on a transect-road during July in the North Cascades Mountains of Washington. Toads were found between about 2030 and 2400 h, and then again between 0200 and 0400 h but not during the day or the middle of the night. Data on facing directions suggest that the toads seek the transect area rather than merely crossing the road twice during the night as part of some pattern of movement in the environment. Both activity periods occur when the ambient illumination is between 10-5 and 10-1 lux, a 4 log-unit range, whereas the daily range on the transect exceeds 9 log units of illumination. The results help relate vision and phototactic behavior to the foraging and general ecology of the species.