Data from: Within species support for the expensive tissue hypothesis: a negative association between brain size and visceral fat storage in females of the Pacific seaweed pipefish

The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the high cost of brain development and maintenance is predicted to constrain adaptive brain size evolution (the expensive tissue hypothesis, ETH). Here, we test the ETH in a teleost fish with predominant female mating competition (reversed sex roles) and male pregnancy, the pacific seaweed pipefish Syngnathus schlegeli. The relative size of the brain and other energetically expensive organs (kidney, liver, heart, gut, visceral fat, and ovary/testis) was compared among three groups: pregnant males, nonpregnant males and egg producing females. Brood size in pregnant males was unrelated to brain size or the size of any other organ, whereas positive relationships were found between ovary size, kidney size, and liver size in females. Moreover, we found that the size of energetically expensive organs (brain, heart, gut, kidney, and liver) as well as the amount of visceral fat did not differ between pregnant and nonpregnant males. However, we found marked differences in relative size of the expensive organs between sexes. Females had larger liver and kidney than males, whereas males stored more visceral fat than females. Furthermore, in females we found a negative correlation between brain size and the amount of visceral fat, whereas in males, a positive trend between brain size and both liver and heart size was found. These results suggest that, while the majority of variation in the size of various expensive organs in this species likely reflects that individuals in good condition can afford to allocate resources to several organs, the cost of the expensive brain was visible in the visceral fat content of females, possibly due to the high costs associated with female egg production.

大脑是脊椎动物体内能量消耗最高的器官之一。据此，大脑发育与维持的高成本被预测会限制适应性脑尺寸演化（昂贵组织假说（expensive tissue hypothesis, ETH））。本研究以雌性主导交配竞争（性别角色反转）且雄性妊娠的硬骨鱼——太平洋海藻海龙（Syngnathus schlegeli）为研究对象，对该假说展开检验。我们比较了怀孕雄性、非怀孕雄性与产卵雌性三组个体的大脑及其他高能耗器官（肾脏、肝脏、心脏、肠道、内脏脂肪以及卵巢/精巢）的相对尺寸。怀孕雄性的育幼幼崽数量与大脑尺寸及其他任一器官尺寸均无关联，而雌性的卵巢尺寸、肾脏尺寸与肝脏尺寸之间存在正相关关系。此外，我们发现怀孕雄性与非怀孕雄性的高能耗器官（大脑、心脏、肠道、肾脏及肝脏）尺寸以及内脏脂肪含量均无显著差异。然而，不同性别间的高能耗器官相对尺寸存在显著差异：雌性的肝脏与肾脏尺寸大于雄性，而雄性的内脏脂肪储存量多于雌性。进一步分析显示，雌性个体的大脑尺寸与内脏脂肪含量呈负相关，而雄性个体的大脑尺寸与肝脏及心脏尺寸则呈现正相关趋势。本研究结果表明，尽管该物种各类高能耗器官的尺寸差异大多反映出身体状态良好的个体能够为多个器官分配资源，但大脑的高成本在雌性的内脏脂肪含量中有所体现，这可能与雌性产卵所关联的高成本有关。