Signal detectability and boldness are not the same: the function of defensive coloration in nudibranchs is distance-dependent

van den Berg, Cedric P., Endler, John A., and Cheney, Karen L. (2023) Signal detectability and boldness are not the same: the function of defensive coloration in nudibranchs is distance-dependent. Proceedings of the Royal Society of London Series B, Biological Sciences, 290 (2003). 20231160.

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Aposematic signals visually advertise underlying anti-predatory defences in many species. They should be detectable (e.g. contrasting against the background) and bold (e.g. using internal pattern contrast) to enhance predator recognition, learning and memorization. However, the signalling function of aposematic colour patterns may be distance-dependent: signals may be undetectable from a distance to reduce increased attacks from naïve predators but bold when viewed up close. Using quantitative colour pattern analysis, we quantified the chromatic and achromatic detectability and boldness of colour patterns in 13 nudibranch species with variable strength of chemical defences in terms of unpalatability and toxicity, approximating the visual perception of a triggerfish (Rhinecanthus aculeatus) across a predation sequence (detection to subjugation). When viewed from an ecologically relevant distance of 30 cm, there were no differences in detectability and boldness between well-defended and undefended species. However, when viewed at closer distances (less than 30 cm), well-defended species were more detectable and bolder than undefended species. As distance increased, detectability decreased more significantly than boldness for defended species. For undefended species, boldness and detectability remained comparatively consistent, regardless of viewing distance. We provide evidence for distance-dependent signalling in aposematic nudibranchs and highlight the importance of distinguishing signal detectability from boldness in studies of aposematism.

Item ID: 80343
Item Type: Article (Research - C1)
ISSN: 1471-2954
Keywords: aposematism, crypsis, predator psychology, secondary defences, signalling honesty, visual modelling
Copyright Information: © 2023 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License, which permits unrestricted use, provided the original author and source are credited.
Funders: Australian Research Council (ARC)
Projects and Grants: ARC FT190199313, ARC DP180102363
Date Deposited: 05 Feb 2024 22:23
FoR Codes: 31 BIOLOGICAL SCIENCES > 3103 Ecology > 310301 Behavioural ecology @ 50%
31 BIOLOGICAL SCIENCES > 3104 Evolutionary biology > 310499 Evolutionary biology not elsewhere classified @ 50%
SEO Codes: 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280102 Expanding knowledge in the biological sciences @ 100%
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