Short answer: It’s not entirely understood! Some factors inducing green-to-brown color change include increased light exposure, cooler temperatures, increased stress, and social interactions.
I was recently asked whether or not green anoles (Anolis carolinensis) change colors to brown when they “hibernate.” Green anoles are a tree-dwelling species of lizard native to the southeastern United States. They are known for being able to change colors between green and brown (see above). Pet stores sometimes refer to the green anole as the “American chameleon,” but this is a misnomer as it is not a true chameleon (which are Old World lizards). Not being from the southeastern United States, I haven’t seen many green anoles, so didn’t have a good prior on this question. In any case, doing some more reading, it seems green anoles don’t actually hibernate – instead they are just relatively inactive during the fall and winter.
It’s not fully understood why green anoles change colors. Some popular hypotheses for green-to-brown color change included as a response to increased light exposure, cooler temperatures, and increased stress. Additionally, some work suggests that brown coloration corresponds to subordinance, while green coloration indicates social dominance. During male-male interactions, the “winner” will usually be green while the “loser” will usually be brown. Interestingly, green anoles most likely do not change color to match their background. Field studies have actually observed that green anoles are mismatched to the surface they are sitting on more often than would be expected by chance. Although it’s still not entirely clear, I think the best answer to the question on green anole coloration during “hibernation” is that periods of inactivity and lower stress correspond to cooler temperatures, which in turn likely correlate with brown coloration.
By contrast, the how of green anole coloration is more clear. The green-to-brown transition occurs due to the stimulation of cells that contain melanin pigments (melanophores). Above these melanophores rest xanthophores (cells containing yellow pigment) and iridophores (cells containing “plates” that appear blue-green). Stimulation of melanophores (likely via hormonal mechanisms) causes upward migration of melanin, which blocks out light from xanthophores and iridophores. This results in the visible change of color from green to brown. Reaggregation of melanin within melanophores restores the original green color (Taylor et al.).
The green anole has spread to islands in the Pacific and Caribbean, where it is considered an invasive species. The changes in coloration make the green anole extra difficult to detect, requiring specialized field observers. In a recent paper, a group working in the Ogasawara Islands in Japan tried to combine remote sensing and machine learning to more efficiently detect these lizards. For those interested in more in-depth anole information, I recommend the Anole Annals run by Jonathan Losos from WUSTL.
Not really 100% related, but in my readings, I came across this fun figure from Herpetology with regards to dewlap color and head bobbing displays for male Anolis to attract females. After nodding my head for 6 hours straight in every ophthalmology interview – I certainly sympathize…
- Vaughan GL. Photosensitivity in the skin of the lizard, Anolis carolinensis. Photochem Photobiol. 1987;46(1):109-114. doi:10.1111/j.1751-1097.1987.tb04743.x
- Yabuta S. and Suzuki-Watanabe A. 2011. Function of body coloration in green anoles (Anolis carolinensis) at the beginning of the breeding season: advertisement signaling and thermoregulation. Curr. Herpetol. 30(2): 155–158.
- Summers CH, Greenberg N. Somatic correlates of adrenergic activity during aggression in the lizard, Anolis carolinensis. Horm Behav. 1994;28(1):29-40. doi:10.1006/hbeh.1994.1003
- Jane F.F. Boyer and Lindsey Swierk. Rapid body color brightening is associated with exposure to a stressor in an Anolis lizard. Canadian Journal of Zoology. 95(3): 213-219. https://doi.org/10.1139/cjz-2016-0200
- Taylor, J.D., Hadley, M.E. Chromatophores and color change in the lizard, Anolis carolinensis . Z. Zellforsch. 104, 282–294 (1970). https://doi.org/10.1007/BF00309737
- Aota, T., Ashizawa, K., Mori, H. et al. Detection of Anolis carolinensis using drone images and a deep neural network: an effective tool for controlling invasive species. Biol Invasions 23, 1321–1327 (2021). https://doi-org.laneproxy.stanford.edu/10.1007/s10530-020-02434-y
- Vitt, L. J., & Caldwell, J. P. (2014). Herpetology: An introductory biology of amphibians and reptiles.