Artificial light at night disturbs the activity and energy allocation of the common toad during the breeding period

The presence of artificial light at night (ALAN) is currently a global phenomenon. By altering the photoperiod, ALAN may directly affect the physiology and behaviour of many organisms, such as the timing of daily rhythms, hormonal regulation, food intake, metabolism, migration and reproduction. Surprisingly while it is known that ALAN exposure strongly influences health of humans and laboratory animals, studies on wildlife remain scarce. Amphibians are one of the most nocturnal groups of vertebrates and exhibit an unfavourable conservation status in most parts of the world. In order to gain insight into the consequences of ALAN, we experimentally exposed 36 adult breeding male common toads, Bufo bufo, to a light intensity of 0.1, 5 or 20 lux for 20 days, to investigate the activity using infrared cameras and the whole-body oxygen consumption by respirometry, as well as body mass and food intake. ALAN reduced toad activity over 24 h by 56% at 5 lux and by 73% at 20 lux. It did not affect the total energy expenditure but altered energy allocation. Indeed, standard energy expenditure increased by 28% at 5 lux and by 58% at 20 lux, while activity energy expenditure decreased by 18% at 5 lux and 38% at 20 lux. Finally, body mass and food intake were not affected. This study suggests that ALAN plays a large role in the activity and energy metabolism of common toads, which may have a long-term negative effect on the fitness of common toad populations. Generalizing these results to other taxa is crucial for conservation of biodiversity in an increasingly light world.