The Luminous Lizard

Fans of Ivan T. Sanderson will be familiar with his experience with a small lizard found on the island of Trinidad, Proctoporus shrevei. Sanderson claimed that this lizard was bioluminescent, noting that the small pores on its sides "lit up for a few seconds like the portholes of a ship," when handled. An evaluation of dead specimens by Dr. H. W. Parker in 1939 led to the suggestion of three possible answers: 1) true bioluminescence, 2) phosphorescence, and 3) highly reflective scales. There have been attempts to answer the question once and for all over the years, and the lizard has made its appearance in BioFortean materials for some time.

I was recently informed of a paper that appears to have laid the controversy to rest. In the Caribbean Journal of Science, Vol. 40, No. 3, pp. 422-6 (2004), ("Shedding Light on the Luminous Lizard...") the authors (Knight, Gutzke, and Quesnel) collected live specimens and tested the three hypotheses. The final answer appears to be that the lizard's "port" scales are highly reflective. The authors note:

"We found that if P. shrevei is observed along the same plane from which light is directed, the normally obvious white ocelli cannot be seen against the reflection from all other scales. But, when viewed from an angle oblique to the light source, the ocelli appear brighter, while surrounding scales show no reflection. By varying the angle of reflected light, an illusion is created that the ocellar scales are intermittently emitting light, thus providing an explanation of Sanderson’s original account of the lizard 'switch[ing] on its portholes.'"

This illusion of luminescence is achieved not only through the reflectivity, but through changes in the skin tone:

"Further enhancing the illusory effect of the reflective scales, dark pigment surrounding each white ocellar scale varies in intensity, apparently depending on the stress level of the animal. We observed that pigmentation becomes darker when lizards are initially handled, but fades to a lower intensity after several minutes. The darker surrounding pigment heightens the reflective effect of the ocelli and makes them appear a brighter white. When viewed immediately after handling the lizards, the ocelli appear to pulse or fluctuate in brightness as the surrounding pigment changes intensity. After a quiescent period, the ocelli are still reflective but do not appear as bright as when the surrounding skin pigmentation is darker."

So, when the lizard is stressed, it's skin darkens and provides greater emphasis for the reflective scales. As it quiets, the skin lightens and the reflectivity is less noticeable.

You can read more about this in the paper (along with notes on the lizard's cold-adapted physiology, which is unusual in a tropical species), which can be downloaded here. (PDF)