Infrared camouflage in leaf-sitting frogs: a cautionary tale on adaptive convergence

TL;DR

Green frogs that match leaves in infrared light may not be using this for camouflage or temperature control, as experiments in natural settings showed no significant difference.

Contribution

The study experimentally tests and challenges the hypotheses of infrared camouflage and thermoregulation in leaf-sitting frogs.

Findings

Frogs with low NIR reflectance heated faster under a solar simulator.

No significant temperature differences were found between frog models in natural rainforest conditions.

NIR matching may be due to mechanisms of green coloration and translucence rather than adaptive convergence.

Abstract

Many cryptic green animals match leaves in invisible near-infrared (NIR) wavelengths. This observation is an enduring puzzle because animals do not see NIR light, so NIR background matching is unlikely to contribute to visual camouflage. Two alternative explanations have been proposed—infrared camouflage (i.e. matching the temperature of the background) and thermoregulation—but neither hypothesis has been experimentally tested. To test these hypotheses, we developed bilayer coatings that mimicked the reflectivity of green leaf-sitting frogs with high NIR (HNIR) or low NIR (LNIR) reflectance. Under a solar simulator in the laboratory, agar model frogs with LNIR reflectance heated up more quickly and reached higher temperatures than those with HNIR reflectance. However, when placed in a tropical rainforest (natural habitat of leaf-sitting frogs), HNIR and LNIR models did not significantly differ in the similarity of surface temperature to the adjacent leaves or in core temperature, thus failing to support the infrared camouflage and thermoregulation hypotheses, respectively. The lack of difference between treatments is probably due to the limited exposure of frogs to direct solar radiation in their natural habitats. We propose an explanation for NIR background matching based on specific mechanisms underlying green coloration and translucence in frogs and caution against assuming adaptive convergence.

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