Differences in the reproductive hormone rhythm of tree sparrows (Passer montanus) from urban and rural sites in Beijing: the effect of anthropogenic light sources.

The pervasiveness of anthropogenic light in urban environments has increased the exposure to light of many animals. Since photoperiod is a regulator of the timing of reproduction in most temperate region birds, such light sources could potentially change the timing of reproduction. We compared the luteinizing hormone (LH), testosterone (T), and estradiol (E2) levels of tree sparrow (Passer montanus) populations sampled at two urban and two rural sites in China, and also performed a controlled photoperiod experiment to determine the influence of artificial light on the endocrine rhythm of these populations. LH levels of urban tree sparrows increased earlier than those of rural ones, but rural populations had higher LH peaks. A linear mixed model (LMM) indicates that increased exposure to light at night (LAN) significantly influenced the LH, T and E2 concentrations of free-living tree sparrows in urban environments compared to their rural counterparts. The results of the controlled photoperiod experiment showed that tree sparrows that were exposed to 6lux of light during the dark phase of the artificial photoperiod began to secrete LH earlier, and had lower peak LH levels, than control birds. A LMM indicates that LAN had a significant effect on LH levels in this experiment. Although urban tree sparrows began to secrete LH earlier than their rural counterparts, we found no corresponding advance in T or E2 secretion. On the contrary, peak T and E2 levels of urban birds were lower than those of rural birds. These results suggest that although anthropogenic light sources appear to advance the onset of LH secretion in urban tree sparrow populations, they also lower peak LH, and consequently levels of T and E2. A possible explanation for these observations is that greater exposure to anthropogenic light in urban environments stimulates LH secretion and may influence photosensitivity, but further experimental work is required to test this hypothesis.