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 10 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.脑发育演化中的核心问题之一，是不同物种在胎儿神经发生（fetal neurogenesis）阶段的脑生长速率是否存在差异。针对新生个体数据的研究已发现，脑生长速率差异存在异速生长（allometric）证据，且与相对代谢率、胎盘侵袭性（placental invasiveness）等生理变量相关，但上述结论尚未直接通过胎儿数据进行验证。本研究对10种真兽类哺乳动物（eutherian mammals）、2种有袋类动物（marsupials）以及2种鸟类的脑生长指数加速度速率进行了测算。真兽类动物的脑生长加速度，相较于躯体与内脏器官生长的变异程度极低，且与其他器官的协同生长模式相互独立，同时也与代谢率、胎盘侵袭性等已提出的生理约束因素无关。两种鸟类的脑生长速率落在真兽类的变异范围内，而有袋类动物的脑生长速率则异常缓慢。脑生长速度峰值在时间上与前脑髓鞘形成（forebrain myelination）及睁眼时间高度关联，能够可靠地区分在此峰值前出生的晚成物种（altricial species）与在此之后出生的早成物种（precocial species），同时可精准预测成体脑容量（决定系数r²=0.98）。躯体生长速率更快的物种，在个体发育（ontogeny）早期的相对脑容量更大；而脑生长速率则与异速生长指标无关。本研究结果表明，真兽类哺乳动物在胎儿神经发生阶段的脑生长速率存在令人意外的保守性，厘清了新生个体脑容量变异的来源，并提示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。