Testing the metabolic homeostasis hypothesis in amphibians

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'.