Bergmann's Rule rules body size in an ectotherm: heat conservation in a lizard along a 2200‐metre elevational gradient

Abstract

Bergmann's Rule predicts larger body sizes in colder habitats, increasing organisms' ability to conserve heat. Originally formulated for endotherms, it is controversial whether Bergmann's Rule may be applicable to ectotherms, given that larger ectotherms show diminished capacity for heating up. We predict that Bergmann's Rule will be applicable to ectotherms when the benefits of a higher conservation of heat due to a larger body size overcompensate for decreased capacity to heating up. We test this hypothesis in the lizard Psammodromus algirus, which shows increased body size with elevation in Sierra Nevada (SE Spain). We measured heating and cooling rates of lizards from different elevations (from 300 to 2500 m above sea level) under controlled conditions. We found no significant differences in the heating rate along an elevational gradient. However, the cooling rate diminished with elevation and body size: highland lizards, with larger masses, have a higher thermal inertia for cooling, which allows them to maintain heat for more time and keep a high body temperature despite the lower thermal availability. Consequently, the net gaining of heat increased with elevation and body size. This study highlights that the heat conservation mechanism for explaining Bergmann's Rule works and is applicable to ectotherms, depending on the thermal benefits and costs associated with larger body sizes.

Effect of body mass and melanism on heat balance inLiolaemuslizards of thegoetschiclade

“…As such, Bergmann's rule is consistent here as a result of the role of thermal inertia due to body size (Gaston and Blackburn, 2000) and the thermoregulatory ability of these ectotherms. Also, our findings are in agreement with those of Zamora-Camacho et al (2014), who studied different populations of Psammodromus algirus inhabiting an altitudinal gradient. These authors found that thermal inertia is relevant for cooling rather than heating rates.…”

“…The differences between the ways of measuring heating and cooling rates (constants versus net gain or loss) may be related to the scale of the results obtained, being larger in the case of the thermal time constants and therefore more notable. Another possible reason for the differences in the results between the present study and that of Zamora-Camacho et al (2014) is the type of function for obtaining heating and cooling rates used, as they used a linear increment (or decrement) whereas we used a exponential curve. To some extent, heating and cooling rates of L. goetschi lizards reflect the solar radiation availability in the environments they inhabit, being slower at heating and/or cooling at higher latitudes, in relation to the cloudiness in the area (data not shown, see Moreno Azócar et al, 2015).…”

“…As previous studies have noted, the influence of thermal inertia (the resistance of a body to changing its temperature) is higher on cooling than on heating rates (Smith, 1976;Claussen and Art, 1981;Carothers et al, 1997;Zamora-Camacho et al, 2014) for live animals. This difference can be explained by behavioral or physiological adjustments to increase heat gain (Lillywhite, 1980;Blouin-Demers and Weatherhead, 2001) that buffer the effect of body size (thermal inertia), commonly observed in lizards (Porter and Tracy, 1983;Carrascal et al, 1992;Carothers et al, 1997;Heatwole and Taylor, 1987;Labra et al, 2009).…”

“…We obtained three different thermal time constants for live individuals -radiation heating (τ HR ), conduction heating (τ HC ) and cooling (τ C ) rates -while for euthanized individuals we obtained constants for radiation heating (τ He ) and cooling (τ Ce ). We also estimated net heat gain (NG) species average values following Zamora-Camacho et al (2014). However, as heating and cooling curves of our measurements were not linear, we used the slope as measured above [ln(T i −T a )/t], which differs from that used by Zamora-Camacho et al (which was T final −T initial /t, where T final and T initial are the final and initial temperatures, respectively, and t is the time elapsed during the experiment).…”

Thermal dependence of sprint performance in the lizard Psammodromus algirus along a 2200-meter elevational gradient: Cold-habitat lizards do not perform better at low temperatures

“…In fact, lizards from this system selected similar body temperatures in a laboratory thermal gradient, regardless of their elevational provenance (Zamora-Camacho et al, submitted). Therefore, lizards in the highest elevations seem to have adaptations such as darker dorsal color (Reguera et al, 2014) and larger body size (Zamora-Camacho et al, 2014b) that may allow them to cope with low environment temperature. Consequently, given that lizards from different elevations show similar body temperature, it is not surprising that they also show similar maximal sprint speed in experiments under controlled body temperature (32°C; Zamora-Camacho et al, 2014a).…”