By Jennifer Greene
Geckos make up an extremely large group of species, the majority of which are nocturnal or at least crepuscular – that is, active at dawn and dusk. It is worth noting that geckos evolved from diurnal lizards, and initially had the full set of rods and cones that we discussed in last month’s issue of the Times. However, as time went on and these diurnal lizards were active only during bright daylight hours, their rod cells began to disappear, and eventually the ancestors of geckos lost their rod cells completely. When these lizards evolved into geckos, they began moving back into nocturnal niches in the environment, and needed to develop better nighttime vision once again. “In response to the demands of nocturnal vision without rods, the cones of nocturnal geckos have become much larger and more light-sensitive than those of their diurnal relatives” (Roth 2009).
Refresh your memory on the color spectrum and what wavelengths match which colors.
What does that mean? That means that geckos see at night, but they see in color. When we see at night, we are seeing in shades of grey, as rod cells simply pick up whether or not light is present, regardless of the color of that light. Geckos can see color at light levels that equate to dim moonlight – where we would hardly be able to see at all, much less determine color! There are studies, for example, that show that helmeted geckos can differentiate between the color blue and the color grey at extremely low light levels. Scientists were able to test this by dusting crickets in powder dyed either blue or grey. Crickets dyed blue were “tasty”, or had nothing extra added, while crickets dusted in grey powder were “distasteful”, and extra salt was added to the dust. Very, very quickly, the geckos learned the difference, and chose the blue crickets over grey crickets nearly every time. (Roth and Kelber, 2004)
|In the diagram above, you can see the test used in the study.
The crickets were offered to the geckos on forceps, and the ones coated in grey were always salted. The geckos almost always refused these crickets in favor of the tastier, non-salted blue crickets.
They made the choice of blue crickets independent of the intensity of the grey coloration on the crickets. These tests were performed at extremely low light levels, comparable to that of a night with no moonlight, and demonstrate that the geckos were capable of color vision.
An interesting point made in the study was that the scientists varied the shades of blue and grey to match in a black and white view (so if the geckos were not using color vision, the crickets would look identical), as well as grey colors that were brighter and darker to cover UV reflection. Why would they be concerned about UV reflection, you ask? Another study looking at crepuscular and nocturnal illumination in regards to a particular moth found that there is enough UV reflection at night for nocturnal animals to have UV sensitive vision. (Johnson, Kelber, et al 2005) Geckos in particular have eyes sensitive to blue and green, which makes sense when you consider that in most habitats, the wavelengths of light being reflected most fall into that color range. Most geckos have minimal red light sensing cones, which is what leads to the use of red light bulbs for heating nocturnal reptiles – they can, at best, see minimally when red light is used to illuminate their cage.
Instead of red, the cone cells in gecko eyes see into the UV range – UVA at least, if not into the UVB range. When testing spectral irradiance, or the radiation of various wavelengths of light off of surfaces, UV was found to be a substantial portion of light being reflected at night. This is due to the lower amount of visible light making it through our atmosphere, allowing for more UV radiation and non-visible light to make it through, relatively speaking. While UV is still being reflected, it is in much lower quantities (relative to overall light being reflected) during the day. I know, I know, it sounds confusing! During the day, because there is so much light coming through our atmosphere, it filters out most wavelengths, and what ends up making it through is mostly the visible spectrum, with smaller quantities of other wavelengths. At night, the light being reflected from the moon, as well as starlight, is less intense in visible light. This allows for a wider range of other wavelengths which may reflect better to make it through our atmosphere, so while there is a smaller amount of light being reflected, a larger portion of that is not visible light, but instead ranging into the infrared and ultraviolet (UV) range.
Comparing Honey Bees (Apis mellifera), Hawkmoths (M. sctellatarum, D. elpenor, H. lineata, H. gallii) and nocturnal geckos (Tarentola chazaliae).
So, back to our friends, the nocturnal geckos. Aside from the study on helmeted geckos and their ability to differentiate between grey and blue colored crickets, there really isn’t much in the way of studies on their sight. They are capable of multifocality, or the ability to have multiple focal zones, while interestingly, the day gecko (top row) had only one focal zone. The varying colors in the diagram below show how much light was passing through different parts of the pupil. The study noted large variation between individual animals in sight, which raises an interesting question for keepers – do different geckos have varying ability to see? Do some geckos need glasses? That’s rhetorical, of course, but it’s an interesting thought that not all geckos see as well as others. See the study in works cited for more details on this particular study on gecko vision. (Roth, Lundstrom, et al 2009)
The limited other studies on nocturnal vision and non-mammalian animals active at night show that with a full moon, the available light spectrum is nearly identical to that of daytime colors. Naturally, it is not as bright or as intense as midday light levels, but the range of light is similar. However, on moonless nights, the color range shifts towards the red or infrared end of the spectrum, meaning that things visible during moonless nights would seem to be redder in tone than they would during daylight or a full moon. But geckos can’t see red, right? So are they blind on moonless nights? Not quite – there are other sources of light, such as star light, as well as other reflective surfaces bouncing light off of each other, leaving enough light for the blue and green seeing geckos to still be active.
The diagram above illustrates the relative levels of different wavelengths of light at different times of day – showing that while there are lower amounts of light, the wavelengths available are still similar to that of daytime illumination. Note the impact that light pollution has on the colors of available light – interesting to consider what our captive geckos may be experiencing with the ranges of light available to them indoors, entirely surrounded by artificial light sources.
In addition, geckos seek out light to thermoregulate, which seems counter-intuitive to what many keepers have observed with their own animals. Yes, we can keep geckos without visible light – but one study performed on Tokay Geckos demonstrated that using visible light in addition to heat enabled them to more precisely control their body temperatures both during the day as well as night. (Sievert and Hutchinson, 1988) The conclusion the researchers came to was that “it appears that G. gecko is using the position of the light source as well as time of day in establishing diel (24 hour period of time) cycles of temperature selection.” So while geckos may not actively bask out in the open under bright, white lights, they do utilize the light source as a reference point for seeking out basking areas to reach their preferred body temperature.
Nothing here is intended to drastically change established husbandry practices of reptiles we have been keeping in captivity and breeding successfully for many years. I do, however, hope that it encourages some thought for naturalistic enclosures, or helps those with difficult species try new things to help their geckos become established. I feel it also highlights how little we still know about these incredible animals and their natural habitat, especially when compared to other species commonly kept in captivity. When setting up naturalistic displays, I hope you find the information here helpful in setting up basking areas, full spectrum lighting, or even whether you feel those things are needed. There is still a lot to learn, and next month, we will be examining diurnal basking lizards.
Works Cited/ References
Lina S.V. Roth, Linda Lunstrom, Almut Kelber, Ronald H.H. Kroger, Peter Unsbo (March 30th, 2009). The pupils and optical systems of gecko eyes. Journal of Vision, Vol. 9 no. 3, article 27 .
Almut Kelber and Lina S.V. Roth (March 1st, 2006). Nocturnal colour vision – not as rare as we might think, The Journal of Experimental Biology, Vol. 209
Beate Roll (July 2001), Gecko vision – retinal organization, foveae, and implications for binocular vision, Vision Research, Volume 41 Issue 16
Lynnette M. Sievert, Victor H. Hutchinson (Sept. 1988. Light versus Heat: Thermoregulatory Behavior in a Nocturnal Gecko Lizard (Gekko gecko), Herpetologica, Vol 44 No. 3
Lina S.V. Roth, Almut Kelber (December 2004). Nocturnal color vision in geckos, Proceedings of the Royal Society of Biological Sciences, Volume 271
Carrie C. Veilleux, Molly E. Cummings (July 30th, 2012). Nocturnal light environments and species ecology: implications for nocturnal color vision in forests, The Journal of Experimental Biology, Volume 215
Sonke Johnsen, Almut Kelber, Eric Warrant, Alison M. Sweeney, Edith A. Widder, Raymond L. Lee Jr., Javier Hernandez-Andres (December 20th, 2005). Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor, The Journal of Experimental Biology, Volume 209