Countergradient Variation in Locomotor Performance of Two Sympatric Polynesian Skinks (Emoia impar,Emoia cyanura)

Abstract

Physiological function in ectotherms is tightly linked to body temperature. As a result, the thermal sensitivity of physiological function may evolve to optimize fitness across different thermal environments. One hypothesis for the evolution of thermal sensitivity, coadaptation, predicts that optimal temperatures for performance should evolve to match the temperatures that an organism experiences in nature. Another hypothesis, countergradient variation, posits that genetic variation can compensate for decreased performance in cool environments, leading to physiological phenotypes that do not track environmental temperatures. On Mo'orea, French Polynesia, thermal ecology and physiology were studied in two morphologically similar skinks that differ in habitat use. Previous studies show that Emoia impar tends to inhabit closed-canopy and interior habitats that are cooler compared to those inhabited by Emoia cyanura, but these differences had not been quantified on Mo'orea. The goal of this study was to determine whether this pattern of habitat partitioning exists on Mo'orea and relates to interspecific differences in thermal physiology and to evaluate whether the evolution of thermal sensitivity supports coadaptation or countergradient variation. I found that E. impar inhabits closed-canopy habitats with cooler substrates and with higher altitudes compared to habitats of E. cyanura. Although the two species do not differ significantly in critical thermal minimum, E. impar has a significantly lower preferred body temperature and critical thermal maximum than does E. cyanura. Despite a preference for cooler habitats and temperatures, E. impar has a warmer optimal temperature for sprint speed and sprints faster than E. cyanura at all temperatures, which supports the countergradient model of thermal adaptation. These results are robust to three different curve-fitting functions and support the view that generalist/specialist trade-offs do not universally constrain the evolution of performance curves.

Countergradient Variation in Reptiles: Thermal Sensitivity of Developmental and Metabolic Rates Across Locally Adapted Populations

“…Adaptation of developmental physiological rates is an important, yet underutilized avenue of research for understanding population persistence under changing and novel environments. Countergradient variation for traits expressed later in the life history have been well documented in reptiles, including growth (Sears and Angilletta, 2003;Uller and Olsson, 2003;Li et al, 2011;Snover et al, 2015;Ortega et al, 2017), body size (Oufiero et al, 2011;Iraeta et al, 2013), scale size (Oufiero et al, 2011), preferred body temperature (Hodgson and Schwanz, 2019), nest date (Knapp et al, 2006;Edge et al, 2017), reproductive output (Knapp et al, 2006;Li et al, 2011;Fetters and McGlothlin, 2017), critical thermal limits and water loss (Kolbe et al, 2014) and locomotor performance (Niewiarowski, 2001;McElroy, 2014). It is possible that CnGV for traits observed later in life are also a consequence of developmental environment, such as food availability, temperature and stress (DuRant et al, 2013;Noble et al, 2018).…”

Size-Based Dominance Hierarchy In One Of Two Sympatric Cryptic Pacific Skinks (Emoia imparAndEmoia Cyanura)

“…This may be attributed to the fact that E. cyanura prefers open canopy habitats (Bruna et. al 1996;McElroy 2014) and therefore has much more available sunlight in basking habitats. There is less incentive for E. cyanura to return to the site of a disturbance if equal amounts of sunlight and similar substrate are available elsewhere, whereas E. impar must compete for the sparse sunlight patches coming through the canopy.…”

“…It has been proven that E. impar moves faster than E. cyanura at all temperatures, making E. impar a thermal generalist and E. cyanura a thermal specialist. (McElroy 2014). Emoia cyanura may have to utilize size to out compete the faster E. impar when sharing closed canopy habitat.…”

“…Map, Mo'orea, French Polynesia showing location of study sites (Table 1A in Appendix) preferring open canopy cover, a higher body temperature, and warmer substrates than E. impar (McElroy 2014).…”

Biogeographical history and coalescent species delimitation of Pacific island skinks (Squamata: Scincidae: Emoia cyanura species group)

“…These two species, despite their morphological and distributional similarities, are ecologically and physiologically divergent (Bruna et al, 1996b;McElroy, 2014) and have been resolved as evolutionarily independent lineages (Guillame et al, 1994;Bruna et al, 1995), with E. impar found to represent the sister taxon to all other ECSG species (Bruna et al, 1996a). Bruna et al (1996a) found E. cyanura as paraphyletic with respect to E. pseudocyanura from the island of Guadalcanal in the Solomon Islands, with very low genetic divergence between cytochrome-b mitochondrial DNA (mtDNA) lineages.…”