Generalist camouflage can be more successful than microhabitat specialisation in natural environments

Abstract

Background Crypsis by background-matching is a critical form of anti-predator defence for animals exposed to visual predators, but achieving effective camouflage in patchy and variable natural environments is not straightforward. To cope with heterogeneous backgrounds, animals could either specialise on particular microhabitat patches, appearing cryptic in some areas but mismatching others, or adopt a compromise strategy, providing partial matching across different patch types. Existing studies have tested the effectiveness of compromise strategies in only a limited set of circumstances, primarily with small targets varying in pattern, and usually in screen-based tasks. Here, we measured the detection risk associated with different background-matching strategies for relatively large targets, with human observers searching for them in natural scenes, and focusing on colour. Model prey were designed to either ‘specialise’ on the colour of common microhabitat patches, or ‘generalise’ by matching the average colour of the whole visual scenes. Results In both the field and an equivalent online computer-based search task, targets adopting the generalist strategy were more successful in evading detection than those matching microhabitat patches. This advantage occurred because, across all possible locations in these experiments, targets were typically viewed against a patchwork of different microhabitat areas; the putatively generalist targets were thus more similar on average to their various immediate surroundings than were the specialists. Conclusions Demonstrating close agreement between the results of field and online search experiments provides useful validation of online citizen science methods commonly used to test principles of camouflage, at least for human observers. In finding a survival benefit to matching the average colour of the visual scenes in our chosen environment, our results highlight the importance of relative scales in determining optimal camouflage strategies, and suggest how compromise coloration can succeed in nature.

The Eurasian Treecreeper (Certhia familiaris) has an effective camouflage against mammalian but not avian vision in boreal forests

“…Plausibly, adopting a generalist camouflage would provide flexibility in Treecreeper habitat use (Michalis et al 2017, Hughes et al 2019, Nokelainen et al 2020, Briolat et al 2021. Here, Treecreeper contrast to the background was similar across habitat scales (i.e.…”

“…2020, Briolat et al . 2021). Here, Treecreeper contrast to the background was similar across habitat scales (i.e.…”

“…2020, Briolat et al . 2021). The latter can be considered an imperfect compromise to match different backgrounds (Hughes et al .…”

“…This may either constrain habitat use to a narrow 'visual' niche (Merilaita et al 1999, 2001, Kjernsmo & Merilaita 2012, Michalis et al 2017 or, alternatively, animals may have adopted a more general resemblance to a preferred habitat type. In terms of camouflage, an animal may thus either be a specialist within a small visual niche or a generalist across a larger niche (Nokelainen et al 2020, Briolat et al 2021. The latter can be considered an imperfect compromise to match different backgrounds (Hughes et al 2019), which provides flexibility to animals' visual niche.…”

Varying benefits of generalist and specialist camouflage in two versus four background environments

“…The relative importance of luminance contrast between backgrounds in determining the effectiveness of imperfect strategies in this study echoes the results of a computer-based experiment where specialists and generalists performed similarly if the backgrounds differed in pattern, but specialists outperformed generalists when backgrounds varied in luminance ( Sherratt et al 2007 ). This suggests that human participants are more sensitive to contrasts in luminance than pattern, making it more difficult for a generalist target to evade detection if it is mismatched in luminance, at least when detecting targets on a computer screen (although recent work has shown good accordance between field- and computer-based results; Briolat et al 2021 ). Luminance and saturation contrast between targets and backgrounds were also shown to predict detection times when volunteers searched for conspicuous targets ( White et al 2017 ).…”