Artificial light at night alters activity, body mass and corticosterone level in a tropical anuran

Photoperiod is a major factor regulating biological rhythms in animals and
plants. At low latitudes, annual variation in daylength is low and species
are expected to strongly rely on photic cues to reset their circadian
clocks. A corollary is that individuals should be strongly affected by
sudden changes in the photic regime as those generated by artificial light
at night (ALAN). We tested this hypothesis in an anuran in Costa Rica
(10°N). Using an outdoor experimental design, we exposed adult cane toads
Rhinella marina, a broadly distributed tropical anuran species to two ALAN
intensities (0.04 and 5 lx). Locomotor activity was reduced at the lowest
intensity, and the activity pattern shifted from crepuscular to nocturnal.
Contrary to humans and mice in which ALAN favour obesity, toads from the
two exposed groups did not gain mass whereas controls did. Corticosterone
was reduced at the highest intensity, a possible consequence of the
reduced activity of toads or the altered regulation of their circadian
pattern. Thus, the behavioural and physiological disruption that we
observed supports the hypothesis of the strong reliance on photic cues to
regulate circadian rhythms and control homeostasis in this intertropical
anuran. Furthermore, our results suggest that the negative effects of ALAN
on physiology, in particular body mass regulation, may differ between
vertebrate groups, thus preventing anticipated generalization before more
comparative studies have been carried out. We stress the importance of
considering the impact of the changing nocturnal environment in the
intertropical zone which host the largest fraction of biodiversity.