Data from: The correlated evolution of antipredator defences and brain size in mammals

Mammals that possess elaborate antipredator defences such as body armour,
spines and quills are usually well protected, intermediate in size,
primarily insectivorous and live in simple open environments. The benefits
of such defences seem clear and may relax selection on maintaining
cognitive abilities that aid in vigilance and predator recognition, and
their bearers may accrue extensive production and maintenance costs. Here,
in this comparative phylogenetic analysis of measurements of
encephalization quotient and morphological defence scores of 647 mammal
species representing nearly every order, we found that as lineages evolve
stronger defences, they suffer a correlated reduction in encephalization.
The only exceptions were those that live in trees—a complex
three-dimensional world probably requiring greater cognitive abilities. At
the proximate level, because brain tissue is extremely energetically
expensive to build, mammals may be trading off spending more on elaborate
defences and saving by building less powerful brains. At the ultimate
level, having greater defences may also reduce the need for advanced
cognitive abilities for constant assessment of environmental predation
risk, especially in simple open environments.

拥有诸如躯体装甲、棘刺与尖刺等复杂反捕食防御机制的哺乳动物，通常具备完善的防护能力，体型中等，主要以昆虫为食，栖息于结构简单的开阔环境中。此类防御机制的优势显而易见，或可弱化对维持警戒与天敌识别相关认知能力的选择压力，而拥有此类防御的物种则需承担高昂的防御结构生产与维持成本。本研究针对涵盖几乎所有哺乳纲类群的647个哺乳动物物种的脑化商数（Encephalization Quotient）与形态防御评分开展比较系统发育分析，结果显示：随着演化出更强的防御机制，相应类群的脑化程度会出现相关下降。唯一的例外是树栖类群——这类复杂的三维环境往往需要更高水平的认知能力。从近因机制层面来看，由于脑组织的构建与维持需要极高的能量成本，哺乳动物或可在投入更多资源打造复杂防御结构与缩减大脑发育以节约能量之间形成权衡。从终极演化层面来看，在结构简单的开阔环境中，拥有更强防御能力的物种或许也降低了对持续评估环境捕食风险所需的高级认知能力的需求。