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 ) 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 ) at the colder end of the range of T s 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 during the warmer parts of the day. Here, we created a dataset that includes Q 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 values tended to be positively correlated with latitude when measured at lower T s, but negative correlated with latitude when measured at higher T s, (ii) Q measured at lower T s were higher in temperate species, whereas Q measured at higher T s were higher in tropical species, and (iii) the experimental T at which Q was maximal for each species decreased with latitude. This is the first study to our knowledge showing that the relationship between Q and latitude in ectotherms changes with the T at which Q 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'.