Physiological rhythms are influenced by photophase wavelength in a nocturnal and a diurnal rodent species from South Africa.

The quality and quantity of light changes significantly over the course of the day. The effect of light intensity on physiological and behavioural responses of animals has been well documented, particularly during the scotophase, but the effect of the wavelength of light, particularly during the photophase, less so. We assessed the daily responses in urine production, urinary 6-sulfatoxymelatonin (6-SMT) and glucocorticoid metabolite (uGCM) concentrations in the nocturnal Namaqua rock mouse (Micaelamys namaquensis) and diurnal four striped field mouse (Rhabdomys pumilio) under varying wavelengths of near monochromatic photophase (daytime) lighting. Animals were exposed to a short-wavelength light cycle (SWLC; ∼465-470 nm), a medium-wavelength light cycle (MWLC; ∼515-520 nm) and a long-wavelength light cycle (LWLC; ∼625-630 nm). The SWLC significantly attenuated mean daily urine production rates and the mean daily levels of urinary 6-SMT and of uGCM were inversely correlated with wavelength in both species. The presence of the SWLC greatly augmented overall daily 6-SMT levels, and simultaneously led to the highest uGCM concentrations in both species. In M. namaquensis, the urine production rate and urinary 6-SMT concentrations were significantly higher during the scotophase compared to the photophase under the SWLC and MWLC, whereas the uGCM concentrations were significantly higher during the scotophase under all WLCs. In R. pumilio, the urine production rate and uGCM were significantly higher during the scotophase of the SWLC, not the MWLC and LWLC. Our results illustrate that wavelength in the photophase plays a central role in the entrainment of rhythms in diurnal and nocturnal African rodent species.

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