Animal transparency: How should we define form and function?

Abstract Animals use colour for a wide range of adaptive functions, ranging from cryptic colours that blend into their environments to bright, conspicuous signals that convey information, either to attract mates or to ward off predators and rivals. However, perhaps one of the most intriguing adaptations is how animals can make use of the absence of colour through transparency. Animal transparency has long been understood as a form of camouflage, allowing predators to see straight through their prey as if it were not there. However, transparency can take many different forms, both in terms of the degree of transparency, ranging from opaque through translucent to transparent and in the extent of coverage, with different combinations of transparent and opaque regions. Despite this variation, transparency has often either been regarded as a unique form of concealment or synonymised with background‐matching camouflage. Yet, empirical evidence is increasingly demonstrating how different forms of transparency may facilitate different defensive and communicative strategies. Here, we contextualise the potential functions of transparency into the wider framework of visual ecology, review the evidence for different strategies and highlight areas in need of continued research. We find that, despite its seemingly intuitive role in camouflage, transparency can fulfil many different functions, including facilitating several conceptually distinct forms of camouflage (e.g. background matching, disruption, masquerade and edge diffusion), mimicry (both Batesian and Müllerian) and enhancing communicative signals (such as aposematism, mate choice and territory defence). Yet, many ecological and behavioural questions remain to be addressed, and caution is needed when assessing or interpreting the function of transparency. Read the free Plain Language Summary for this article on the Journal blog.