Supplementary Material for: Sleep-Related Electrophysiology and Behavior of Tinamous (Eudromia elegans): Tinamous Do Not Sleep Like Ostriches

The functions of slow wave sleep (SWS) and rapid eye movement (REM) sleep, distinct sleep substates present in both mammals and birds, remain unresolved. One approach to gaining insight into their function is to trace the evolution of these states through examining sleep in as many taxonomic groups as possible. The mammalian and avian clades are each composed of two extant groups, i.e., the monotremes (echidna and platypus) and therian (marsupial and eutherian [or placental]) mammals, and Palaeognaths (cassowaries, emus, kiwi, ostriches, rheas, and tinamous) and Neognaths (all other birds) among birds. Previous electrophysiological studies of monotremes and ostriches have identified a unique “mixed” sleep state combining features of SWS and REM sleep unlike the well-delineated sleep states observed in all therian mammals and Neognath birds. In the platypus this state is characterized by periods of REM sleep-related myoclonic twitching, relaxed skeletal musculature, and rapid eye movements, occurring in conjunction with SWS-related slow waves in the forebrain electroencephalogram (EEG). A similar mixed state was also observed in ostriches; although in addition to occurring during periods with EEG slow waves, reduced muscle tone and rapid eye movements also occurred in conjunction with EEG activation, a pattern typical of REM sleep in Neognath birds. Collectively, these studies suggested that REM sleep occurring exclusively as an integrated state with forebrain activation might have evolved independently in the therian and Neognath lineages. To test this hypothesis, we examined sleep in the elegant crested tinamou (<i>Eudromia elegans</i>), a small Palaeognath bird that more closely resembles Neognath birds in size and their ability to fly. A 24-h period was scored for sleep state based on electrophysiology and behavior. Unlike ostriches, but like all of the Neognath birds examined, all indicators of REM sleep usually occurred in conjunction with forebrain activation in tinamous. The absence of a mixed REM sleep state in tinamous calls into question the idea that this state is primitive among Palaeognath birds and therefore birds in general.

慢波睡眠（slow wave sleep, SWS）与快速眼动睡眠（rapid eye movement, REM sleep）是哺乳类与鸟类共有的两种独立睡眠亚状态，其具体功能至今尚未阐明。探究这两种睡眠状态功能的一种可行思路，是通过在尽可能多的分类群中开展睡眠研究，追溯二者的演化历程。哺乳类与鸟类演化支各自包含两个现存类群：哺乳类涵盖单孔目动物（针鼹与鸭嘴兽）以及兽亚纲哺乳动物（有袋类与真兽亚纲[或称胎盘类]哺乳动物）；鸟类则包括古颚总目（鹤鸵、鸸鹋、几维鸟、鸵鸟、美洲鸵与䳍形目鸟类）与今颚总目（其余所有鸟类）。此前针对单孔目动物与鸵鸟的电生理学研究，发现了一种独特的“混合”睡眠状态——该状态兼具慢波睡眠与快速眼动睡眠的特征，与兽亚纲哺乳动物和今颚总目鸟类中已被清晰界定的睡眠状态均不相同。在鸭嘴兽中，该混合状态表现为：与快速眼动睡眠相关的肌阵挛抽动、骨骼肌放松以及快速眼球运动，同时伴随前脑脑电图（electroencephalogram, EEG）中慢波睡眠相关的慢波活动。鸵鸟中也观测到了类似的混合状态；不过除了伴随脑电图慢波活动之外，肌肉紧张度降低与快速眼球运动还会伴随脑电图激活现象——这一模式是今颚总目鸟类快速眼动睡眠的典型特征。综合来看，这些研究提示：仅以与前脑激活整合状态存在的快速眼动睡眠，可能分别在兽亚纲与今颚总目演化支中独立演化而来。为验证这一假说，我们对优雅凤头䳍（*Eudromia elegans*）开展了睡眠研究：这是一种小型古颚总目鸟类，在体型与飞行能力上更接近今颚总目鸟类。研究人员基于电生理与行为学指标，对24小时周期内的睡眠状态进行了评分。与鸵鸟不同，但与所有已被研究的今颚总目鸟类一致，该鸟类的所有快速眼动睡眠相关指标通常均伴随前脑激活现象。优雅凤头䳍中未观测到混合式快速眼动睡眠状态，这对“混合状态是古颚总目鸟类乃至所有鸟类的原始睡眠状态”这一观点提出了质疑。