Tree file S1 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, <i>Aepyornis maximus</i> and <i>A. hildebrandti</i>, 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隶属于古颚总目（Palaeognathae）、象鸟目（Aepyornithiformes），是有史以来体型最为庞大的鸟类。象鸟的神经解剖学研究尚待深入，但其相关数据可为揭示这类神秘鸟类的生活习性提供关键线索。古神经学研究可为灭绝鸟类的生态与行为学研究提供参考依据，因为鸟类的脑形态与其神经功能紧密相关。本研究通过数字化手段重建了大象鸟（Aepyornis maximus）与希尔德布兰象鸟（A. hildebrandti）两种象鸟的颅内模（endocasts），并将其与所有现存及新近灭绝的主要古颚类支系代表类群进行对比分析。在古颚类群中，我们发现栖息于森林环境的类群拥有较大的嗅球（olfactory bulbs）——这类生境中觅食依赖的视觉线索通常较为有限。我们还检测到不同象鸟物种的嗅球大小存在差异，这可能暗示了它们的生境存在种间分化。象鸟的视叶（optic lobes）极度退化，这一特征同样见于夜行性几维鸟（kiwi）。作为象鸟的姊妹群，几维鸟已将自身的视觉系统替换为高度发达的嗅感、体感与听觉系统，以辅助觅食活动。据此我们推断，象鸟具备夜行性特征；而象鸟与几维鸟的共同祖先很可能已弱化了视觉功能。本研究结果揭示了一种此前未见报道的视觉能力退化趋势，该趋势似乎仅局限于栖息于缺乏捕食者的岛屿、无法飞行的夜行性特有类群。