Geckos, a diverse group of small lizards, are found across nearly every continent and represent one of the most successful reptile lineages, including approximately 2,000 species. The common perception is that these reptiles are primarily creatures of the night, silently hunting insects. This widespread belief, however, does not account for the full spectrum of activity patterns found within the Gekkota.
The Dominance of Nocturnal Geckos
The majority of geckos, estimated to be between 72% and 80% of all known species, are indeed active during the night or twilight hours. This preference for darkness is primarily an adaptation to the thermal environment of their warm habitats, allowing them to regulate their body temperature more effectively. By avoiding the intense midday sun, nocturnal geckos minimize the risk of overheating and desiccation, which is a major concern for small ectotherms.
Nighttime activity also provides access to a different array of insect prey that become active once the sun sets. Many familiar species, such as the common House Gecko (Hemidactylus species), exemplify this pattern as they emerge at dusk to forage around artificial light sources. Even in cooler environments, nocturnal species often utilize surfaces like rocks, which retain warmth from the day, to maintain a favorable body temperature during their active period.
Key Examples of Diurnal Species
While nocturnality is the norm, a significant number of geckos, comprising over 400 species, are active during the day, defying the general trend. The most prominent examples of this exception are the Day Geckos (Phelsuma), native mainly to Madagascar and the islands of the western Indian Ocean. Unlike their nocturnal relatives, these geckos are vibrantly colored, often displaying shades of emerald green, blue, and red.
This striking coloration serves a dual purpose, acting as camouflage against the lush tropical foliage and playing a role in intraspecies communication and territorial displays. Day Geckos are heliophilous, meaning they actively seek out strong sunlight to bask, which is necessary for their thermoregulation and absorption of ultraviolet radiation. Their daytime activity is also seen in other genera, such as Lygodactylus and Gonatodes, which have adapted their behavior to thrive where diurnal foraging is thermally feasible.
Biological Differences in Eye Structure
The shift between nocturnal and diurnal lifestyles required profound changes in the geckos’ visual systems. Nocturnal geckos possess specialized eyes that are significantly more light-sensitive than human eyes, enabling color vision even in dim moonlight. Their pupils are typically vertical slits which can constrict to a series of tiny pinholes during the day to protect the retina from excessive light. The retina itself is unusual, having evolved from an ancestral all-cone retina, with these cone cells becoming greatly enlarged and more light-sensitive to function effectively in low-light conditions.
The eyes of nocturnal species also feature a multifocal optical system, meaning the lens has concentric zones with different refractive powers. This specialization allows them to focus different wavelengths of light simultaneously, correcting for optical distortions and enhancing their ability to see in the dark. In contrast, diurnal geckos, such as the Phelsuma species, have eyes that are adapted for bright light, featuring smaller, round pupils and a monofocal optical system. These diurnal species have photoreceptors that function optimally under the high illumination of the day.