Evolution of diel activity patterns in skinks (Squamata: Scincidae), the world's second‐largest family of terrestrial vertebrates

Abstract

Many animals have strict diel activity patterns, with unique adaptations for either diurnal or nocturnal activity. Diel activity is phylogenetically conserved, yet evolutionary shifts in diel activity occur and lead to important changes in an organism's morphology, physiology, and behavior. We use phylogenetic comparative methods to examine the evolutionary history of diel activity in skinks, one of the largest families of terrestrial vertebrates. We examine how diel patterns are associated with microhabitat, ambient temperatures, and morphology. We found support for a nondiurnal ancestral skink. Strict diurnality in crown group skinks only evolved during the Paleogene. Nocturnal habits are associated with fossorial activity, limb reduction and loss, and warm temperatures. Our results shed light on the evolution of diel activity patterns in a large radiation of terrestrial ectotherms and reveal how both intrinsic biotic and extrinsic abiotic factors can shape the evolution of animal activity patterns.

Cathemerality: a key temporal niche

“…During the Cenozoic, following the K-Pg event 66 Ma and then the Paleocene-Eocene Thermal Maximum event 55 Ma, the move to the daytime began in earnest, with many mammal [as well as gecko (Gamble et al, 2015) and skink (Slavenko et al, 2022)] species transitioning to specialising in daytime activity via cathemerality (Maor et al, 2017;Baker & Venditti, 2019). Despite the persistence of cathemerality in mammals since the K-Pg event, transition rates from cathemerality to either nocturnal or diurnal diel niches have been found to be about three times higher than transition rates from either nocturnal or diurnal to cathemeral (Maor et al, 2017; by contrast geckos are thought to have shifted primarily in only one direction, from nocturnality to diurnality via cathemerality; Slavenko et al, 2022). Maor et al (2017) suggest that diurnal or nocturnal niches may be more favourable for mammals, except where equally strong selection forces are acting in opposite directions (e.g.…”

Built for success: Distribution, morphology, ecology and life history of the world's skinks

“…Although the diet of around half of skink species is unknown, given the strong relationship between SVL and diet (Chapple, 2003), body size could be used to predict the likely dietary mode of skink species currently lacking data. Similarly, smaller lizard species are more likely to be diurnal (Meiri, 2008), a result that is also evident to some extent in skinks (Slavenko et al, 2022). For instance, although a quarter of lizard species worldwide are nocturnal, which is the predominate activity mode for gekkotans (Meiri, 2020), only 8% of skink species are nocturnal.…”

“…However, nocturnal lizards are largely absent from high elevations and cold climates (Vidan et al, 2017), and skinks exhibit relatively high diversity in these regions. Indeed, nocturnality in skinks in associated with fossoriality, limb reduction and loss and inhabiting warmer (Slavenko et al, 2022).…”

Activity patterns and reproductive behavior of the Critically Endangered Bermuda skink (Plestiodon longirostris)

“…However, the skinks still choose to retreat during the midday hours even though they could safely continue to bask and be active. This suggests that these activity patterns are at least partly controlled by an internal biological clock that has persisted in captivity (Slavenko et al, 2022). Although the captive environment has facilitated the Bermuda skinks to exhibit a similar bimodal pattern of activity and basking to those observed in the wild, to more closely replicate the Bermudan environment a gradual thermostatic heating and lighting system could be introduced to better replicate natural conditions.…”