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

Area of Science:

• * Zoology and evolutionary biology, focusing on animal coloration and camouflage mechanisms.
• * Biophysics, investigating thermal properties and radiative transfer in biological systems.

Background:

• * Many cryptic animals exhibit near-infrared (NIR) reflectance similar to foliage, a trait puzzling for visual camouflage as animals cannot perceive NIR light.
• * Proposed explanations include infrared camouflage (matching background temperature) and thermoregulation, but these have lacked experimental validation.
• * Understanding NIR reflectance in animals is crucial for deciphering evolutionary pressures and the function of coloration beyond visible light.

Purpose of the Study:

• * To experimentally test the hypotheses that NIR background matching in frogs serves as infrared camouflage or aids thermoregulation.
• * To investigate the role of NIR reflectance in the survival and physiology of leaf-sitting frogs.
• * To explore alternative explanations for NIR background matching in an ecological context.

Main Methods:

• * Development of artificial frog models with bilayer coatings exhibiting either high NIR (HNIR) or low NIR (LNIR) reflectance, mimicking leaf-sitting frogs.
• * Controlled laboratory experiments using a solar simulator to compare temperature changes in HNIR and LNIR models.
• * Field experiments in a tropical rainforest to assess surface and core temperatures of models relative to adjacent leaves in the natural habitat.

Main Results:

• * Laboratory tests showed LNIR models heated faster and reached higher temperatures than HNIR models under simulated sunlight.
• * In the natural rainforest habitat, no significant difference was found in the thermal similarity between HNIR/LNIR models and surrounding leaves, or in their core temperatures.
• * These findings did not support the infrared camouflage or thermoregulation hypotheses in the frogs' natural environment.

Conclusions:

• * The study failed to provide experimental support for infrared camouflage or thermoregulation as explanations for NIR background matching in frogs.
• * Limited exposure to direct solar radiation in the natural habitat may explain the lack of temperature differences between models.
• * Alternative explanations for NIR background matching, potentially linked to the specific mechanisms of green coloration and translucence in frogs, are proposed, cautioning against assumptions of adaptive convergence.