Tree file S2 from Nocturnal giants: evolution of the sensory ecology in elephant birds and other palaeognaths inferred from digital brain reconstructions

The recently extinct Malagasy elephant birds (Palaeognathae, Aepyornithiformes) included the largest birds that ever lived. Elephant bird neuroanatomy is understudied but can shed light on the lifestyle of these enigmatic birds. Paleoneurological studies can provide clues to the ecologies and behaviours of extinct birds because avian brain shape is correlated with neurological function. We digitally reconstruct endocasts of two elephant bird species, Aepyornis maximus and A. hildebrandti, and compare them with representatives of all major extant and recently extinct palaeognath lineages. Among palaeognaths, we find large olfactory bulbs in taxa generally occupying forested environments where visual cues used in foraging are likely to be limited. We detected variation in olfactory bulb size among elephant bird species, possibly indicating interspecific variation in habitat. Elephant birds exhibited extremely reduced optic lobes, a condition also observed in the nocturnal kiwi. Kiwi, the sister taxon of elephant birds, have effectively replaced their visual systems with hyperdeveloped olfactory, somatosensory and auditory systems useful for foraging. We interpret these results as evidence for nocturnality among elephant birds. Vision was likely deemphasized in the ancestor of elephant birds and kiwi. These results show a previously unreported trend towards decreased visual capacity apparently exclusive to flightless, nocturnal taxa endemic to predator-depauperate islands.

新近灭绝的马达加斯加象鸟（古颚总目Palaeognathae，象鸟目Aepyornithiformes）囊括了地球历史上体型最为庞大的鸟类类群。目前针对象鸟神经解剖学的研究仍较为匮乏，但其相关成果可为揭示这类神秘鸟类的生活方式提供关键线索。古神经学研究可为灭绝鸟类的生态与行为特征提供重要参考，因为鸟类的脑形态与神经功能存在显著相关性。本研究对两种象鸟——大象鸟（Aepyornis maximus）与希氏象鸟（A. hildebrandti）——进行了数字化颅内模重建，并将其与所有现存及新近灭绝的古颚总目主要支系的代表类群开展对比分析。在古颚总目类群中，栖息于森林生境的类群往往拥有更大的嗅球，而这类生境中用于觅食的视觉线索通常较为有限。本研究还发现不同象鸟物种的嗅球尺寸存在差异，这或许反映了它们在栖息生境上的种间分化。象鸟的视叶极度退化，这一特征同样见于夜行性的几维鸟（kiwi）。作为象鸟的姊妹分类单元，几维鸟已将自身的视觉系统替换为高度发达的嗅觉、躯体感觉与听觉系统，以此辅助觅食活动。基于上述结果，我们推断象鸟为夜行性动物。在象鸟与几维鸟的共同祖先类群中，视觉功能或许已被弱化。本研究结果揭示了一种此前未被报道的演化趋势：在捕食者匮乏的岛屿上特化的夜行性、无飞行能力的类群中，视觉能力呈现逐步退化的特征。