Endocrine–circadian interactions in birds: implications when nights are no longer dark

Abstract

Biological clocks are evolved time-keeping systems by which organisms rhythmically coordinate physiology within the body, and align it with rhythms in their environment. Clocks are highly sensitive to light and are at the interface of several major endocrine pathways. Worryingly, exposure to artificial-light-at-night (ALAN) is rapidly increasing in ever more extensive parts of the world, with likely impact on wild organisms mediated by endocrine–circadian pathways. In this overview, we first give a broad-brush introduction to biological rhythms. Then, we outline interactions between the avian clock, endocrine pathways, and environmental and internal modifiers. The main focus of this review is on the circadian hormone, melatonin. We summarize information from avian field and laboratory studies on melatonin and its relationships with behaviour and physiology, including often neglected developmental aspects. When exposed to ALAN, birds are highly vulnerable to disruption of behavioural rhythms and of physiological systems under rhythmic control. Several studies suggest that melatonin is likely a key mediator for a broad range of effects. We encourage further observational and experimental studies of ALAN impact on melatonin, across the full functional range of this versatile signalling molecule, as well as on other candidate compounds at the endocrine–circadian interface. This article is part of the theme issue ‘Endocrine responses to environmental variation: conceptual approaches and recent developments’.

Endocrine responses to environmental variation

“…ALAN disrupts biological clocks, which are evolved time-keeping systems that align physiology with daily or seasonal rhythms in environmental temperature and light [ 15 ]. Biological clocks are regulated by melatonin and thyroid hormone, and physiological responses are coordinated via glucocorticoid levels and receptors [ 15 , 46 – 48 ].…”

“…ALAN disrupts biological clocks, which are evolved time-keeping systems that align physiology with daily or seasonal rhythms in environmental temperature and light [ 15 ]. Biological clocks are regulated by melatonin and thyroid hormone, and physiological responses are coordinated via glucocorticoid levels and receptors [ 15 , 46 – 48 ]. ALAN is increasing globally as a result of increasing urbanisation and human activity [ 49 , 50 ] and its disruption of the endocrine-mediated time-keeping system can have pronounced effects on reproduction, growth and behaviour, and thereby reduce fitness and ecosystem function [ 51 – 53 ]…”

“…A range of other hormones regulate various biological functions to match animal responses to environmental conditions. For example, leptin and gonadotropins regulate feeding and reproduction, respectively [ 10 – 12 ], and melatonin elicits a cascade of molecular responses to entrain a circadian sleep–wake cycle in response to light stimuli, which can even influence central thyroid processing ([ 13 – 15 ]; figure 1 ).

Figure 1 Schematic summary of the Theme Issue.

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