Testing the metabolic homeostasis hypothesis in amphibians

Abstract

A number of hypotheses about compensatory mechanisms that allow ectothermic animals to cope with the latitudinal decrease in ambient temperature ( T_A) have been proposed during the last century. One of these hypotheses, the 'metabolic homeostasis' hypothesis (MHH), states that species should show the highest thermal sensitivity of the metabolic rate ( Q_10-SMR) at the colder end of the range of T_As they usually experience in nature. This way, species should be able to minimize maintenance costs during the colder hours of the day, but quickly take advantage of increases in T_A during the warmer parts of the day. Here, we created a dataset that includes Q_10-SMR values for 58 amphibian species, assessed at four thermal ranges, to evaluate three predictions derived from the MHH. In line with this hypothesis, we found that: (i) Q_10-SMR values tended to be positively correlated with latitude when measured at lower T_As, but negative correlated with latitude when measured at higher T_As, (ii) Q_10-SMR measured at lower T_As were higher in temperate species, whereas Q_10-SMR measured at higher T_As were higher in tropical species, and (iii) the experimental T_A at which Q_10-SMR was maximal for each species decreased with latitude. This is the first study to our knowledge showing that the relationship between Q_10-SMR and latitude in ectotherms changes with the T_A at which Q_10-SMR is assessed, as predicted from an adaptive hypothesis. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.

Keywords: Q10; energetic metabolism; physiological flexibility; standard metabolic rate; ‘metabolic homeostasis’ hypothesis.

Figure 1.

(a) Relationship between standard metabolic rate (SMR) and ambient temperature (T_A) for a hypothetical temperate species (bold blue dashed line) and for a hypothetical tropical species (bold red dashed line). (b) Thermal sensitivity (Q₁₀) values as a function of the experimental thermal range for each hypothetical species; these results were derived from the SMR curves depicted in (a). The period of activity/inactivity for each species is marked with a horizontal continuous/dashed line. In both panels a hypothetical distribution of T_A for a temperate (blue bell) and a tropical (red bell) habitat is represented by a Gaussian curve.

Figure 2.

Phylogenetic trees used in phylogenetic analysis. The particular species that were included in each comparison (i.e. thermal range) are indicated with letters: (a) 5–15°C, (b) 10–20°C, (c) 15–25°C, (d) 20–30°C. Ma = million years ago.

Figure 3.

Relationship between Q₁₀ values and absolute latitude for different thermal ranges: (a) 5–15°C, (b) 10–20°C, (c) 15–25°C and (d) 20–30°C.

Figure 4.

Comparisons of thermal sensitivity values (Q₁₀) between temperate and tropical species, estimated at different thermal ranges: (a) and (c) 5–15°C and 10–20°C, (b) and (d) 15–25°C and 20–30°C. Sample size for each group is given above or below the error bars (which represent ± 1 s.e.) in the left panels.

Figure 5.

Distribution of activation energies (E_t) for (a) tropical and (b) temperate species. (Online version in colour.)