两栖动物脑大小的生态适应与进化权衡

脑大小进化在达尔文进化论中处于重要地位，它能够对环境选择压力等重要的生态和进化过程产生深远影响，并在物种的保护中发挥了重要的作用，至今依然是进化生物学研究的热点课题。我们前期研究发现，两栖动物脑大小的进化与环境因素和肠长度密切相关，为此，我们假设环境变化引起繁殖点集群大小和操控性比的变化是导致脑大小生态适应以及食物变化是导致脑和肠权衡关系发生变化的主要因素。为检验该假设，本项目拟以横断山区七个自然保护区常见40余种两栖动物为模式动物，利用骨龄学、食物鉴定以及分子系统分析等方法，研究两栖动物脑大小：（1）是否存在物种间的变化；（2）如果存在，影响其变化的因素有哪些？以及（3）不同环境状况下繁殖点集群大小和操控性比的变化是否是导致脑大小变化的重要因素以及食物类型是否是导致脑和肠权衡关系发生变化的重要因素，从而揭示两栖动物脑大小对环境变化产生的生态适应以及食物变化对脑和肠权衡关系变化的影响机理。

Brain size varies dramatically between species and understanding the causes of this variation has deservedly received much attention in evolutionary biological research. From those preceding studies, it is evident that the evolution of brain size involves a trade-off between the cognitive benefits conferred by having a larger brain and the energetic limitations of increased neural investment. The ‘cognitive buffer hypothesis’ (CBH) states that increased cognitive abilities provided by a larger brain can facilitate appropriate behavioral responses towards uncommon, novel or complex socioecological alterations. However, cognitive advantages come at a cost, specifically, large brains are metabolically expensive as brain tissue consumes more energy per unit weight then most other somatic tissues. Brain size variation is fundamental trait and of central importance to Darwin’ evolutionary theory which are affected by environmental and genetic factors such as temperature, rainfall, and genetic structure. It is important to study the brain size variation for species conservation because increased brain size indirectly increases vulnerability to extinction in animals. Hence, the study on brain size evolution can produce important influences in the evolutionary and ecological and evolutionary process resulting from natural selection which benefits to breeding behavior and speciation in nature. Previous results from our studies have been shown that the impact of seasonality on brain evolution across species of anurans (frogs) experiencing a wide range of temperature and precipitation support the Expensive Brain Framework because relative brain size and the size of the optic tectum were negatively correlated with variability in temperature. Moreover, brain size exhibits a negative correlation with the length of the digestive tract. Therefore, we propose two hypotheses: (1) operational sex ratio (OSR), spawning-site density from environmental change results in ecological adaptation in frogs; (2) variation in food type at different age-group results in variation in brain-gut trade-off among species. In order to test two hypotheses, we plan to use skeletochronology, food analysis and phylogenetic methods to study brain size evolution among 40 species of anurans distributed in different latitude and altitude in seven National Nature Reserves at the Hengduan Mountains in western China. Our aims are: (1) to test whether variations in brain size occur among species, and how do vary; (2) what are the proximate factors affecting variations in brain size in anurans; (3) to test whether OSR, spawning-site density are mainly factors affecting brain size, as well as a markedly association between variation in food type at different age-group and brain size. According to results of testing two hypotheses, we will gain insight into the mechanisms of ecological adaptation in brain size facing different environments and the effect of food variation on brain-gut trade-off among species of anurans in the Hengduan Mountains.