Human speech has many complex spectral and temporal features traditionally thought to be absent in the vocalizations of other primates. Recent explorations of the vocal capabilities of non-human primates are challenging this view. Here, we continue this trend by exploring the spectro-temporal properties of gelada (Theropithecus gelada) vocalizations. First, we made cross-species comparisons of geladas, chacma baboons, and human vowel space area. We found that adult male and female gelada exhaled grunts-a call type shared with baboons-have formant profiles that overlap more with human vowel space than do baboon grunts. These gelada grunts also contained more modulation of fundamental and formant frequencies than did baboon grunts. Second, we compared formant profiles and modulation of exhaled grunts to the derived call types (those not shared with baboons) produced by gelada males. These derived calls contained divergent formant profiles, and a subset of them, notably wobbles and vocalized yawns, were more modulated than grunts. Third, we investigated the rhythmic patterns of wobbles, a call type shown previously to contain cycles that match the 3-8 Hz tempo of speech. We use a larger dataset to show that the wobble rhythm overlaps more with speech rhythm than previously thought. We also found that variation in cycle duration depends on the production modality; specifically, exhaled wobbles were produced at a slower tempo than inhaled wobbles. Moreover, the variability in cycle duration within wobbles aligns with a linguistic property known as 'Menzerath's law' in that there was a negative association between cycle duration and wobble size (i.e. the number of cycles). Taken together, our results add to growing evidence that non-human primates are anatomically capable of producing modulated sounds. Our results also support and expand on current hypotheses of speech evolution, including the 'neural hypothesis' and the 'bimodal speech rhythm hypothesis'.

中文翻译：

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狒狒和狒狒发声的不同声学特性及其对人类语言进化的影响

人类的语音具有许多复杂的光谱和时间特征，传统上认为在其他灵长类动物的发声中不存在。最近对非人类灵长类动物发声能力的探索正在挑战这一观点。在这里，我们通过探索gelada（Theropithecus gelada）发声的光谱时间特性来延续这一趋势。首先，我们对geladas、chacma狒狒和人类元音空间区域进行了跨物种比较。我们发现成年雄性和雌性狒狒呼出咕噜声 - 一种与狒狒共享的呼叫类型 - 具有与人类元音空间重叠的共振峰轮廓，而不是狒狒咕噜声。这些gelada咕噜声还包含比狒狒咕噜声更多的基频和共振峰频率调制。第二，我们将共振峰分布和呼出咕噜声的调制与由gelada雄性产生的派生呼叫类型（与狒狒不共享的类型）进行了比较。这些派生的呼叫包含不同的共振峰轮廓，其中一个子集，特别是摆动和发声的哈欠，比咕噜声更受调制。第三，我们研究了摆动的节奏模式，之前显示的一种呼叫类型包含与语音的 3-8 Hz 节奏相匹配的周期。我们使用更大的数据集来表明摆动节奏与语音节奏的重叠比以前认为的要多。我们还发现，周期持续时间的变化取决于生产方式；具体来说，呼出的摆动产生的速度比吸入的摆动慢。此外，摆动内循环持续时间的可变性与称为“门泽拉斯定律”的语言特性一致 因为周期持续时间和摆动大小（即周期数）之间存在负相关。总之，我们的结果增加了越来越多的证据，表明非人类灵长类动物在解剖学上能够产生调制声音。我们的研究结果还支持和扩展了当前的语音进化假设，包括“神经假设”和“双峰语音节奏假设”。