Photonic crystals cause active colour change in chameleons

Abstract and Introduction

Article Submission Details

  • Received: 16 Jun 2014

  • Accepted: 22 Jan 2015

  • Published: 2 Mar 2015

  • DOI: 10.1038/ncomms7368

Key Research Focus

  • Photonic Crystals and Color Change in Chameleons
      Many chameleons, specifically panther chameleons, demonstrate dynamic color changes during social interactions, which are believed to be due to the dispersion or aggregation of pigments in their skin chromatophores.

  • Hypothesis
      The active tuning of guanine nanocrystals in a layer of dermal iridophores contributes significantly to color changes.

Research Methodologies Used

  1. Microscopy

  2. Photometric videography

  3. Photonic band-gap modeling

Findings Summary

  • Chameleons utilize a dual-layer structure of iridophores:
      - Superficial (S-) Iridophores: Contains guanine nanocrystals responsible for rapid color changes.
      - Deep (D-) Iridophores: Reflects near-infrared sunlight; provides thermal protection.

Historical Context

Early Observations

  • Chameleons have been noted for various remarkable features since their description by Aristotle, including:
      - Projectile tongues
      - Independently movable eyes
      - Zygodactylous feet
      - Distinctive slow movement and rapid color shifts.

Functions of Color Change in Other Species

  • Physiological Color Changes: Seen in many vertebrates, primarily for:
      - Camouflage
      - Communication
      - Thermoregulation

  • Mechanisms generally involve modulation in skin brightness via chromatophores contained within dermal layers.

Unique Mechanisms in Non-Chameleons

  • Squid and Structural Color: Squid utilize specialized iridophores with multilayer nano-reflectors for rapid color tuning through plasma membrane invaginations.

  • In Fish and Amphibians: Similar color change mechanisms involving structural components are noted.

Mechanism Behind Color Change

Chameleons’ Unique Structure

  • S-Iridophores:
      - Arranged in a triangular lattice of guanine nanocrystals.
      - Modifications in the proximity of these crystals can lead to noticeable color shifts.

  • D-Iridophores:
      - Contains larger, disorganized guanine crystals present in all panther chameleons and several non-chameleon species.
      - Not involved in color changes, but plays a role in reflecting sunlight and regulating heat absorption.

Panther Chameleon (Furcifer pardalis) Specific Findings

  • Skin Composition:
      - Two types of chromatophores:
        - Melanin-containing dark chromatophores.
        - An unidentified blue pigment.

  • Adult males show significant color variation and rapid changes when competing or courting.

Experimental Techniques Used

  • Raman Spectroscopy: To analyze pigment types in skin samples.

  • Photometry: Measures optical responses using high-resolution RGB analysis.

  • Histological and TEM Analysis: To examine iridophore structures providing insights into crystal arrangements and their quantum color properties.

Observations of Color Change in Male Competitors

  • Rapid color shifts noted (e.g., green to yellow/orange).

  • Blue patches turning whitish and red patches becoming brighter.

Structural Analysis

In-Vivo Photometry & Skin Structure

  • Analyzing male chameleons revealed:
      - Skin consists of two superimposed layers of S-iridophores with guanine crystals contributing to color change.
      - Color transitions occurred by adjusting lattice parameters, altering reflectivity and thus perceived color.

Findings from Osmotic Pressure Experiments

  • Subjecting the skin samples to hypertonic solutions leads to observable shifts in crystal arrangements, mimicking in-vivo behavior (blue shift in color).

Optical Modeling

Band-Gap Modeling

  • Simulation performed using a face-centered cubic lattice model for guanine crystals under various conditions to understand optical responses and color simulation.
      - Brillouin Zone Analysis: Computation of band structures to predict reflectivity based on different lattice parameters.

Higher Reflectivity in D-Iridophores

  • Noted high reflectance in the near-infrared, beneficial for thermal protection under direct sunlight, likely an evolutionary advantage.

Comparative Functionality and Evolution

Iridophore Variations

  • Notable differences between chameleon and non-chameleon species in terms of iridophore organization, with chameleons possessing a dual-layer system.

Evolutionary Implications

  1. Survival Advantage: D-Iridophores provide protection against intense sunlight, which is crucial for species in open environments.

  2. Thermal Regulation: Reflectivity may assist in moderating exposure to variable sunlight, a survival advantage for certain species.

Future Directions

  • Research required to explore potential evolutionary origins of D-Iridophores pertaining to habitat complexity and environmental adaptations.

Methods Overview

Animal Welfare and Experimental Approval

  • All animal maintenance and experimental procedures were ethically approved in accordance with Swiss law.

Skin Structure Examination Practices

  • Skin samples collected from live chameleons, preserved quickly to maintain structural integrity for analysis.

Measurement Techniques

  • Use of TEM and spectroscopic systems for precise measurement of crystal dimensions and photometry of color response under various experimental conditions.

Acknowledgements and References

Acknowledgements

  • Research dedicated to Jean-Pol Vigneron.

  • Acknowledgment of funding from the University of Geneva, Swiss National Science Foundation, and SystemsX.ch initiative.

Selected References

  1. Stuart-Fox, D. & Moussalli, A. (2008). Selection for social signalling drives the evolution of chameleon colour change.

  2. Nilsson Skold, H., Aspengren, S. & Wallin, M. (2013). Rapid color change in fish and amphibians-function, regulation, and emerging applications.

  3. Arsenault, A. C. et al. (2007). Photonic-crystal full-colour displays.

Additional Notes

  • The findings imply a significant breakthrough in understanding physiological and structural color change in chameleons, which may extend to exploring similar mechanisms in other species. Future work may consider the molecular foundations of these processes and their relevance in evolutionary biology.

Key takeaways:

  • Crystalline structures