Animal colour change: proximate mechanisms, evolutionary ecology and response to anthropogenic impacts

Animal colour has multiple functions in nature, from sexual signalling to protecting individuals against visual predators. However, instead of being a fixed trait, colour and patterning may be changed over different time scales according to variable selective pressures. Early research on animal colour change focused on the pigments driving rapid and slow changes in a few animal groups, shedding light on cellular and some physiological mechanisms. Conversely, over the past 20 years of research, advances in colour quantification and increased emphasis on the adaptive function of colour change has prompted investigations within ecological and evolutionary frameworks. These include more recent research on how human impacts, such as pollution and habitat modification, may disrupt colour change by affecting animal physiology or exposing organisms to altered environmental conditions. Here, we comprehensively review what is known regarding the physiological and ecological mechanisms behind dynamic (i.e. reversible) colour change in animals. We begin with an overview of colour change mechanisms, covering classic cellular processes while highlighting new molecular and physiological insights. Next, we discuss the evolutionary ecology of colour change, reviewing evidence of its adaptive value and links to behaviour. Finally, we examine anthropogenic impacts on animal colour change, addressing how factors such as pollution and habitat modification influence colour plasticity. We conclude by identifying key unresolved questions and proposing directions for future research.