Data from: Minimal variation in eutherian brain growth rates during fetal neurogenesis

A central question in the evolution of brain development is whether species differ in rates of brain growth during fetal neurogenesis. Studies of neonatal data have found allometric evidence for brain growth rate differences according to physiological variables such as relative metabolism and placental invasiveness, but these findings have not been tested against fetal data directly. Here, we measure rates of exponential brain growth acceleration in ten eutherian mammals, two marsupials, and two birds. Eutherian brain acceleration exhibits minimal variation relative to body and visceral organ growth, varies independently of correlated growth patterns in other organs, and is unrelated to proposed physiological constraints such as metabolic rate or placental invasiveness. Brain growth rates in two birds overlap with eutherian variation, while marsupial brain growth is exceptionally slow. Peak brain growth velocity is linked in time with forebrain myelination and eye opening, reliably separates altricial species born before it from precocial species born afterwards, and is an excellent predictor of adult brain size (r2 = 0.98). Species with faster body growth exhibit larger relative brain size in early ontogeny, while brain growth is unrelated to allometric measures. These findings indicate a surprising conservation of brain growth rates during fetal neurogenesis in eutherian mammals, clarify sources of variation in neonatal brain size, and suggest that slow body growth rates cause species to be more encephalized at birth.

脑发育演化领域的核心科学问题为：不同物种在胎儿神经发生阶段的脑生长速率是否存在差异。既往基于新生儿数据开展的研究，已发现脑生长速率差异存在异速生长（allometric）特征，且与相对代谢率、胎盘侵入性等生理学变量相关，但此类结论尚未借助胎儿数据进行直接验证。本研究对10种真兽类哺乳动物（eutherian mammals）、2种有袋类动物（marsupials）及2种鸟类的脑生长指数加速速率进行了测定。结果显示：真兽类动物的脑生长加速程度相较于躯体与内脏器官生长的变异度极低，且独立于其他器官的协同生长模式，同时与代谢率、胎盘侵入性等已提出的生理学约束因素无关联。2种鸟类的脑生长速率与真兽类动物的变异范围存在重叠，而有袋类动物的脑生长速率则异常缓慢。脑生长峰值速度在时间上与前脑髓鞘形成及睁眼事件紧密关联，可明确区分出生前达到该峰值的晚成物种与出生后达到该峰值的早成物种，同时可精准预测成体脑尺寸（r²=0.98）。躯体生长速率更快的物种，在个体发育早期具有更大的相对脑尺寸；而脑生长速率则与异速生长指标无关联。本研究结果揭示了真兽类哺乳动物胎儿神经发生阶段脑生长速率的惊人保守性，阐明了新生儿脑尺寸变异的潜在来源，并表明缓慢的躯体生长速率会使物种在出生时具有更高的脑化程度。