Season dependent effects of urban environment on circadian clock of tree sparrow (Passer montanus)

Abstract

Great efforts have been made recently to understand the effect(s) of urban environments on the circadian and seasonal physiology of wild animals, but the mechanisms involved remain largely unknown. Most laboratory studies and a few studies on animals in the wild suggest alterations occur in the physiological functions of organisms in urban habitats. Here, we addressed the effects of the interaction of seasons and urban environments on clock gene expression in three tissues of tree sparrows (Passer montanus). Tree sparrows (N = 30 per site per time of year) were procured from rural and urban habitats during periods corresponding to their three physiological states, i.e., June (longest photoperiod; reproductive phase), September (equinox photoperiod; refractory phase), and December (shortest photoperiod; sensitive phase). Birds (N = 5 per time per site per month) were sampled at six time points; ZT1, ZT5, ZT9, ZT13, ZT17, and ZT21 (ZT0 = sunrise time) and clock gene expression in the hypothalamus, pineal gland, and retina was studied. Our results show that there is persistence of the circadian clock in both rural and urban birds throughout the year. In urban birds Bmal1, Npas2, Per2, and Cry1 acrophases were advanced, compared to rural birds, while Clock acrophase was delayed, depending on the tissue and time of year. This difference could be because of changes in the availability, duration, and intensity of sunlight during different times of the year and/or differential photoreceptor sensitivities, differential physiological states, or a combination of all these factors. These important results reveal, for the first time in any species, season-dependent effects of an urban environment on the molecular machinery of the circadian clock.