RESUMO
A number of hypotheses about compensatory mechanisms that allow ectothermic animals to cope with the latitudinal decrease in ambient temperature ( TA) 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 ( Q10-SMR) at the colder end of the range of TAs 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 TA during the warmer parts of the day. Here, we created a dataset that includes Q10-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) Q10-SMR values tended to be positively correlated with latitude when measured at lower TAs, but negative correlated with latitude when measured at higher TAs, (ii) Q10-SMR measured at lower TAs were higher in temperate species, whereas Q10-SMR measured at higher TAs were higher in tropical species, and (iii) the experimental TA at which Q10-SMR was maximal for each species decreased with latitude. This is the first study to our knowledge showing that the relationship between Q10-SMR and latitude in ectotherms changes with the TA at which Q10-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'.
Assuntos
Anfíbios/metabolismo , Anfíbios/classificação , Anfíbios/genética , Animais , Mudança Climática , Ecossistema , Homeostase , Oxigênio/metabolismo , Filogenia , TemperaturaRESUMO
Intertidal organisms have evolved physiological mechanisms that enable them to maintain performance and survive during periods of severe environmental stress with temperatures close to their tolerance limits. The level of these adaptive responses in thermal physiology can vary among populations of broadly distributed species depending on their particular environmental context and genetic backgrounds. Here we examined thermal performances and reaction norms for metabolic rate (MR) and heart rate (HR) of seven populations of the porcelanid crab Petrolisthes violaceus from markedly different thermal environments across the latitudinal gradient of ~3000km. Physiological responses of this intertidal crab under common-garden conditions suggest the absence of local thermal adaptation along the geographic gradient (i.e., lack of latitudinal compensation). Moreover, thermal physiological sensitivities and performances in response to increased temperatures evidenced the existence of some level of: i) metabolic rate control or depression during warm temperature exposures; and ii) homeostasis/canalization (i.e., absence or low levels of plasticity) in physiological traits that may reflect some sort of buffering mechanism in most of the populations. Nevertheless, our results indicate that elevated temperatures can reduce cardiac function but not metabolic rate in high latitude crabs. The lack of congruence between HR and MR supports the idea that energy metabolism in marine invertebrates cannot be inferred from HR and different conclusions regarding geographic differentiation in energy metabolism can be obtained from both physiological traits. Integrating thermal physiology and species range extent can contribute to a better understanding of the likely effects of climate change on natural populations of marine ectotherms.
Assuntos
Adaptação Fisiológica/fisiologia , Braquiúros/fisiologia , Temperatura , Distribuição Animal , Animais , Mudança Climática , Metabolismo Energético/fisiologia , Meio AmbienteRESUMO
Digestive flexibility is important because it allows an animal to maximize energy and nutrient return from the diet consumed, and also to reduce the maintenance costs associated with one of the body's most expensive systems in terms of energy and protein requirements. Two different patterns of digestive flexibility have been described for vertebrates, one for species in which metabolic costs of homeostasis are relatively high and the gut is rarely empty (e.g., mammals and birds), and one for species in which metabolic costs of homeostasis are relatively low and the gut usually spends long periods of time empty (e.g., amphibians and reptiles). In this review we analyze the information on digestive tract down-regulation during fasting in fish, in order to evaluate the extent to which digestive flexibility in fish conforms to that in other species. We found that: (1) gut size decay during long-term fasting in fish appears to be almost linear with time, even for very long fasting periods. Thus, gut size temporal dynamics in fish during long-term fasting resemble those observed in some mammals species; (2) by contrast, histological changes during fasting in fish are more similar to those described for amphibians and reptiles; and (3) data on enterocyte turnover rates indicate that cell turnover times in fish are relatively short, and although longer than those observed in mammals, they are not very different from those reported for birds. In conclusion, current data suggest that both mechanisms, cell turnover rates and change in epithelial configuration, probably are involved in digestive tract regulation in fish.