Can the circadian system of a diurnal and a nocturnal rodent entrain to ultraviolet light?

Spectral measurements of sunlight throughout the day show close corresponde nce between the timing of above ground activity of the European ground squi rrel and the presence of ultraviolet light in the solar spectrum. However, in a standard entrainment experiment ground squirrels show no entrainment t o ultraviolet light, while Syrian hamsters do entrain under the same protoc ol. Presented transmittance spectra for lenses, corneas, and vitreous bodie s may explain the different results of the entrainment experiment. We found ultraviolet light transmittance in the colourless hamster lens (50% cut-of f at 341 nm), but not in the yellow ground squirrel lens (50% cut-off aroun d 493 nm). Ultraviolet sensitivity in the ground squirrels based upon possi ble fluorescence mechanisms was not evident. Possible functions of ultravio let lens filters in diurnal mammals are discussed, and compared with noctur nal mammals and diurnal birds. Species of the latter two groups lack ultrav iolet filtering properties of their lenses and their circadian system is kn own to respond to ultraviolet light, a feature that does not necessarily ha s to depend on ultraviolet photoreceptors. Although the circadian system of several species responds to ultraviolet light, we argue that the role of u ltraviolet light as a natural Zeitgeber is probably limited.