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| 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.
Citation
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