|A diurnal Phelsuma and a nocturnal Cyrtodactylus.|
Geckos are the only clade of lizards that are mostly nocturnal; 72% of the 1552 described species are active at night. Geckos possess numerous adaptations to low light and low temperatures, suggesting nocturnal activity evolved early in their evolution. These adaptations include the evolution of vocalization and acoustic communication, olfactory specialization, enhanced capability for sustained locomotion at low temperatures, shifts in diet and foraging mode, and the absence of the parietal foramen and pineal eye. Geckos have acute vision and many adaptations for seeing in low light including: large eyes, pupils capable of an extreme degree of constriction and dilation, retinas without foveae, short visual focal length, multifocal color vision, and rod-like photoreceptor cells in the retina that lack oil droplets. However, not all gecko species are nocturnal; more than 430 are diurnal. Many of these diurnal lineages have their own adaptations to living in warm, photopic environments including round pupils, UV-filtering crystallin lens proteins, smaller eyes, partial to complete foveae, cone-like photoreceptor cells in the retina and a return to higher energetic costs of locomotion. Geckos are thought to be ancestrally nocturnal and diurnality evolved multiple times. However, this hypothesis has never been tested in a phylogenetic framework.
Now, in a new paper Gamble et al. (2015) performed comparative analyses using a newly generated gecko phylogeny and examined the evolution of temporal activity patterns to: test the hypothesis of an early origin of nocturnality in geckos; verify repeated subsequent transitions to diurnality; and determine whether the evolution of temporal activity patterns has influenced diversification rates. The results provide the first phylogenetic analysis of temporal activity patterns in geckos and confirm an ancient origin of nocturnality at the root of the gecko tree. Gamble et al. identify multiple transitions to diurnality at a variety of evolutionary time scales and transitions back to nocturnality occur in several predominantly diurnal clades.
The authors found several transitions occurred deep in the phylogeny, including ancestors to the Pygopodidae, the New World sphaerodactyl geckos and the Phelsuma plus Lygodactylus clade. More recent transitions occurred in Rhoptropus, within New Zealand and New Caledonian diplodactylids (Naultinus and Eurydactylodes), and within Gymnodactylus, Ptyodactylus and Mediodactylus. Both Asian Cnemaspis clades seem to include multiple transitions, although additional taxonomic sampling is needed to confirm this. They also identified several well-supported eversions to nocturnality within otherwise diurnal clades, including Sphaerodactylus, Gonatodes, Phelsuma and the Pygopodidae. Their results indicate frequent shifts in temporal activity patterns in geckos at a variety of evolutionary timescales. Determining what factors initiate shifts in individual clades was beyond the scope of the paper, but they suggest three possible causes: climate, predators and competition.
Some shifts in activity pattern may be related to thermoregulation and evading extreme temperatures and desiccation. For example, geckos in the genus Sphaerodactylus appear to overheat easily and several species that inhabit hot, xeric habitats are nocturnal, including: S. leucaster, S. thompsoni and S. ladae in southern Hispaniola; S. roosevelti in south-west Puerto Rico; and S. inaguae from the Bahamas. Similarly, some gecko species living at high altitudes, such as Mediodactylus amictopholis, are thought to have shifted to diurnal activity to facilitate thermoregulation in colder climates. However, there are numerous counter inhabiting extreme environments. Pristurus and Rhoptropus, for instance, are diurnal genera that can be active at extremely high temperatures in arid environments while Homonota darwnii and Alsophylax pipiens live in cold climates at extreme latitudes and remain nocturnal. Furthermore, nocturnal geckos seem quite capable of regulating body temperature while hidden in retreats during the day and thus switching to diurnality solely for thermoregulatory purposes may be uncommon overall.
Predation could also instigate changes in temporal activity patterns in geckos and such shifts are well documented in other vertebrate species. Most predator-induced niche shifts in geckos involve the alteration of the spatial niche. However, the hypothesis that geckos may transition to a more conspicuous, diurnal lifestyle in environments where predators are less abundant or absent, such as on islands. Lack of predators is thought to be responsible for dramatic changes in phenotype and behavior in many island species, such as the evolution of flightlessness in birds. Thus, it is reasonable that similar selective pressures could alter temporal activity in geckos.
Shifts in temporal activity patterns may also be related to competition avoidance and the exploitation of underutilized resources. Temporal resource partitioning helps competitors coexist by avoiding direct confrontation or reducing resource overlap. For example, the early shift to nocturnality in ancient geckos has been attributed to avoiding competition with diurnal lizards and exploiting the relatively open nocturnal niche. The lack of competition with other diurnal lizards, mostly iguanians, is frequently cited as promoting transitions back to diurnality in geckos. Indeed, many diurnal geckos occur in regions with a paucity of iguanian species. The success of Phelsuma and Lygodactylus in Madagascar has been attributed to the lack of arboreal iguanians, with the exception of the extremely specialized chameleons.
The scenario presented here will be useful in reinterpreting existing hypotheses of how geckos have adapted to varying thermal and light environments. These results can also inform future research of gecko ecology, physiology, morphology and vision as it relates to changes in temporal activity patterns.
Gamble T, Greenbaum E, Jackman TR, Bauer AM (2015), Into the light: diurnality has evolved multiple times in geckos. Biological Journal of the Linnean Society. doi: 10.1111/bij.12536