Experimental evidence suggests that specular reflectance and glossy appearance help amplify warning signals.
Specular reflection appears as a bright spot or highlight on any smooth glossy convex surface and is caused by a near mirror-like reflectance off the surface. Convex shapes always provide the ideal geometry for highlights, areas of very strong reflectance, regardless of the orientation of the surface or position of the receiver. Despite highlights and glossy appearance being common in chemically defended insects, their potential signalling function is unknown. We tested the role of highlights in warning colouration of a chemically defended, alpine leaf beetle, Oreina cacaliae. We reduced the beetles' glossiness, hence their highlights, by applying a clear matt finish varnish on their elytra. We used blue tits as predators to examine whether the manipulation affected their initial latency to attack, avoidance learning and generalization of warning colouration. The birds learned to avoid both dull and glossy beetles but they initially avoided glossy prey more than dull prey. Interestingly, avoidance learning was generalized asymmetrically: birds that initially learned to avoid dull beetles avoided glossy beetles equally strongly, but not vice versa. We conclude that specular reflectance and glossiness can amplify the warning signal of O. cacaliae, augmenting avoidance learning, even if it is not critical for it.
Centre of Excellence in Biological Interactions, University of Jyväskylä, Department of Biological and Environmental Sciences, PO Box 35, 40014 University of Jyväskylä, Jyväskylä, Finland.|Molecular Evolutionary Biology, Department of Biology, Universität Hamburg, Martin-Luther-King Pl. 3, 20146, Hamburg, Germany.
Samuel J
SJ
Waldron
Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Waurn Ponds, Victoria, 3216, Australia.
John A
JA
Endler
Centre of Excellence in Biological Interactions, University of Jyväskylä, Department of Biological and Environmental Sciences, PO Box 35, 40014 University of Jyväskylä, Jyväskylä, Finland. janne.valkonen@jyu.fi.
Janne K
JK
Valkonen
Department of Ecosystem Studies, School of Environmental Science, The University of Shiga Prefecture, 2500, Hassaka-cho, Hikone City, Shiga, 522-8533, Japan.
Atsushi
A
Honma
Molecular Evolutionary Biology, Department of Biology, Universität Hamburg, Martin-Luther-King Pl. 3, 20146, Hamburg, Germany.
Susanne
S
Dobler
Centre of Excellence in Biological Interactions, University of Jyväskylä, Department of Biological and Environmental Sciences, PO Box 35, 40014 University of Jyväskylä, Jyväskylä, Finland.
Johanna
J
Mappes