Abstract
IntroductionArtificial light-at-night is known to affect a broad array of behaviours and physiological processes. In urbanized bird species, light-at-night advances important biological rhythms such as daily cycles of activity/rest and timing of reproduction, but our knowledge of the underlying physiological mechanisms is limited. Given its role as chronobiological signal, melatonin is a strong candidate for mediating the effects of light-at-night.ResultsWe exposed urban and rural European blackbirds (Turdus merula) to two light treatments equal in photoperiod but with different light intensities at night. The control group was exposed to 0.0001 lux (almost darkness), while the experimental group was exposed to 0.3 lux at night, simulating conditions recorded previously on free-living urban blackbirds. We obtained diel profiles of plasma melatonin for all birds in summer (July) and winter (January), while simultaneously recording locomotor activity. Daily patterns of melatonin concentrations were clearly affected by light-at-night in both seasons. In winter, melatonin concentrations of light-at-night birds were lower in the early and late night than in those of birds kept in darkness. In summer, melatonin concentrations of the light-at-night birds were lower through all night compared to birds kept in darkness. Locomotor activity in light-at-night birds was overall higher than in control individuals, both during the day and at night, and it increased sharply before dawn. In winter, the amount of activity before dawn in the light-at-night group correlated with changes in melatonin from midnight to late night: the greater the decrease in melatonin, the greater the amount of pre-dawn activity. Urban and rural birds responded similarly to light-at-night with respect to melatonin, but differed in their behaviour, with rural birds showing more locomotor activity than urban counterparts.ConclusionsThis study points to reduced melatonin release at night as a potential physiological mechanism underlying the advanced onset of morning activity of urbanized birds. Based on the pattern of melatonin secretion, we suggest that birds responded to light-at-night as if they were exposed to a longer day than birds kept under dark nights.
Highlights
Artificial light-at-night is known to affect a broad array of behaviours and physiological processes
During the long winter nights, plasma melatonin concentrations were significantly lower in the light-at-night group than in the dark-night group in the evening and in the morning, whereas both groups did not differ from each other at midnight (LMM, treatment*time of day interaction, t = −3.2, p-value calculated based on MCMC (pMCMC) < 0.001; Figure 1a, Table 1 and Additional file 1: S2)
Birds showed a 14.5% reduction in melatonin amplitude in winter compared to summer, and this difference was significant (LMM, t = −4.63, pMCMC < 0.001, Additional file 1: Table S1)
Summary
Artificial light-at-night is known to affect a broad array of behaviours and physiological processes. More than a century later, the use of artificial light is widespread and has lead to a dramatic change in the lifestyle of billions of people This surely brought economic and social benefits, but in the last decades the negative effects of the exposure to light-at-night have started to become apparent, especially because of the potential implications for humans [2,3]. Two of the most frequently reported effects of light-at-night are an advanced onset of activity in the morning [10,11,12] and an advanced seasonal onset of reproduction [10,12,13] These effects are not surprising given the importance of light as crucial information for the regulation of both daily and seasonal processes in avian species [14,15]. Two major characteristics of the melatonin rhythm have been studied extensively, in mammals and birds: the duration of nocturnal melatonin release and its diel amplitude
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