Physiological and morphological correlates of among-individual variation in standard metabolic rate in the leopard frog Rana pipiens

Abstract

SUMMARY Rates of standard metabolism (SMR) are highly variable among individuals within vertebrate populations. Because SMR contributes a substantial proportion of an individual's energy budget, among-individual variation in this trait may affect other energetic processes, and potentially fitness. Here, we examine three potential proximate correlates of variation in SMR:organ mass, serum T4 thyroxine and relative mitochondrial content, using flow cytometry. Body-mass-adjusted kidney mass correlated with SMR, but liver,heart, small intestine and gastrocnemius did not. Thyroxine correlated with SMR, as did mitochondrial content. These results suggest several novel proximate physiological and morphological mechanisms that may contribute to among-individual variation in SMR. Variation in SMR may be maintained by diverse environmental conditions. Some conditions, such as low resource availability, may favor individuals with a low SMR, through small organ size,or low thyroxine or mitochondrial content. Other conditions, such as high resource availability, may favor individuals with a high SMR, through large organ size, or high thyroxine or mitochondrial content.

Relationships between interspecific differences in the mass of internal organs, biochemical markers of metabolic activity, and the thermogenic properties of three small passerines

“…However, it is not clear whether large, energetically expensive organs are responsible for higher BMR, or whether they are necessary to support a higher BMR. Thus, the relationship between BMR and organ mass remains purely correlative, which is cause and which is effect remains unresolved (Steyermark et al 2005). What are the ecological and evolutionary implications of having larger visceral organs for higher BMR birds?…”

Relationships among running performance, aerobic physiology and organ mass in male Mongolian gerbils

“…Several early papers that examined intraspecific variation (e.g. Daan et al, 1990;Piersma et al, 1996) suggested that BMR is largely determined by the metabolic output of visceral organs, but subsequent studies of both mammals and birds have revealed little consistency in the specific organs that correlate with BMR [Table·7; for a related study on frogs see Steyermark et al (Steyermark et al, 2005)]. In one recent study of deer mice, individual organs were largely uncorrelated to BMR, but the combined mass of visceral organs was positively correlated to BMR while the opposite was true for musculoskeletal mass (Russell and Chappell, 2007).…”

Developmental and Immediate Thermal Environments Shape Energetic Trade-Offs, Growth Efficiency, and Metabolic Rate in Divergent Life-History Ecotypes of the Garter SnakeThamnophis elegans

“…This is surprising because it indicates that smaller snakes are more metabolically efficient than larger snakes at cool temperatures. Such high scaling exponents (b 1 1) have been reported in rapidly growing juvenile ectotherms (reviewed in Glazier 2015) and may be attributable to ontogenetic shifts in the relative mass of organs with differing oxygen demands or a size dependency of underlying mechanisms determining metabolic rate, such as T4 thyroxine or mitochondrial density (Steyermark et al 2005). The scaling exponent estimates did not differ between ecotypes or rearing treatment groups, implying that variation in the allometric scaling of metabolic rate did not drive the observed divergences in growth rate or metabolic rate.…”