Abstract
Light pollution is known to affect important biological functions of wild animals, including daily and annual cycles. However, knowledge about long-term effects of chronic exposure to artificial light at night is still very limited. Here we present data on reproductive physiology, molt and locomotor activity during two-year cycles of European blackbirds (Turdus merula) exposed to either dark nights or 0.3 lux at night. As expected, control birds kept under dark nights exhibited two regular testicular and testosterone cycles during the two-year experiment. Control urban birds developed testes faster than their control rural conspecifics. Conversely, while in the first year blackbirds exposed to light at night showed a normal but earlier gonadal cycle compared to control birds, during the second year the reproductive system did not develop at all: both testicular size and testosterone concentration were at baseline levels in all birds. In addition, molt sequence in light-treated birds was more irregular than in control birds in both years. Analysis of locomotor activity showed that birds were still synchronized to the underlying light-dark cycle. We suggest that the lack of reproductive activity and irregular molt progression were possibly the results of i) birds being stuck in a photorefractory state and/or ii) chronic stress. Our data show that chronic low intensities of light at night can dramatically affect the reproductive system. Future studies are needed in order to investigate if and how urban animals avoid such negative impact and to elucidate the physiological mechanisms behind these profound long-term effects of artificial light at night. Finally we call for collaboration between scientists and policy makers to limit the impact of light pollution on animals and ecosystems.
Highlights
The study of the ecological consequences of artificial light at night has received great interest in the last decade, in the context of the effects on wildlife [1]
We showed that European blackbirds (Turdus merula) exposed to a light intensity at night of 0.3 lux, representative of the intensity measured with light loggers on individual blackbirds in an urban area, developed the reproductive system almost a month in advance, and moulted earlier, than conspecifics exposed to dark nights [10]
In linear mixed models (LMMs) we first assessed which was the best model by comparing AIC values, and we evaluated the significance of model parameters using a Monte Carlo Markov Chain (MCMC) approach through the function pvals.fnc in the R package languageR [20]
Summary
The study of the ecological consequences of artificial light at night has received great interest in the last decade, in the context of the effects on wildlife [1]. During the testicular regression phase blackbirds originating from urban areas responded differently than blackbirds from the forest when exposed to the light at night treatment, in that urban birds ended the reproductive cycle sooner than rural birds. These results already indicated pronounced effects of low light intensities at night on the timing of reproductive physiology. Our knowledge about long-lasting effects of such low light intensities at night on the seasonal organization of urban living animals is still limited In this context it is important to consider how photoperiodic information is integrated in avian species living in temperate areas.
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