Differences in the thermal physiology of adult Yarrow's spiny lizards (Sceloporus jarrovii) in relation to sex and body size

Abstract

Sexual size dimorphism (SSD) is often assumed to reflect the phenotypic consequences of differential selection operating on each sex. Species that exhibit SSD may also show intersexual differences in other traits, including field-active body temperatures, preferred temperatures, and locomotor performance. For these traits, differences may be correlated with differences in body size or reflect sex-specific trait optima. Male and female Yarrow's spiny lizards, Sceloporus jarrovii, in a population in southeastern Arizona exhibit a difference in body temperature that is unrelated to variation in body size. The observed sexual variation in body temperature may reflect divergence in thermal physiology between the sexes. To test this hypothesis, we measured the preferred body temperatures of male and female lizards when recently fed and fasted. We also estimated the thermal sensitivity of stamina at seven body temperatures. Variation in these traits provided an opportunity to determine whether body size or sex-specific variation unrelated to size shaped their thermal physiology. Female lizards, but not males, preferred a lower body temperature when fasted, and this pattern was unrelated to body size. Larger individuals exhibited greater stamina, but we detected no significant effect of sex on the shape or height of the thermal performance curves. The thermal preference of males and females in a thermal gradient exceeded the optimal temperature for performance in both sexes. Our findings suggest that differences in thermal physiology are both sex- and size-based and that peak performance at low body temperatures may be adaptive given the reproductive cycles of this viviparous species. We consider the implications of our findings for the persistence of S. jarrovii and other montane ectotherms in the face of climate warming.

Sprint speed capacity of two alpine skink species, Eulamprus kosciuskoi and Pseudemoia entrecasteauxii

“…E. kosciuskoi lizards were significantly faster than P. entrecasteauxii lizards (Figure 1c). This is most likely due to their smaller size (Beal et al 2014). Between the sprint capacities of the male, female and gravid individuals, no significant difference was discerned for either species, which is also consistent with the Beal study (Figures 2a and 2b), though it suggests the higher thermal preference of gravid females (Schwarzkopf and Shine 1991) does not result in a significant increase in speed.…”

“…As temperatures increase, the thermal tolerances of the region's biota become increasingly important (Root et al 2003;Pickering et al 2004). The fitness of reptile species is closely correlated with temperature, and sprint speed capacity may be used as an indicator for lizard fitness (Beal et al 2014). For the two lizard species studied fitness as measured through sprint speed was shown to increase at an elevated temperature.…”

“…Faster individuals are more likely to avoid predation by fleeing and have an advantage as predators themselves. Greater speed has also been linked to social dominance and mating success (Beal et al 2014). Maximal sprint performance has been linked to optimal temperature, and it has been shown in a number of lizard species that 'hotter is better' (i.e.…”

“…The size and gravid status may all influence the lizard's speed and response to increased temperatures. Smaller and gravid lizards expected to be slower (Beal et al 2014), though gravid individuals may have a higher sprint capacity at elevated temperatures due to having a higher thermal preference that benefits embryogenesis (Schwarzkopf and Shine 1991;Clusella-Trullas and Chown 2014).…”

Researching functional ecology in Kosciuszko National Park

“…Beal et al (2014) found that body temperatures of Sceloporous jarrovii varied between sexes: it is possible that the data are only representative of female lizards.…”