Maddin Hillary C, Sherratt Emma
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA; Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA.
J Anat. 2014 Jul;225(1):83-93. doi: 10.1111/joa.12190. Epub 2014 Apr 25.
It is widely accepted that a relationship exists between inner ear morphology and functional aspects of an animal's biology, such as locomotor behaviour. Animals that engage in agile and spatially complex behaviours possess semicircular canals that morphologically maximise sensitivity to correspondingly complex physical stimuli. Stemming from the prediction that fossorial tetrapods require a well-developed sense of spatial awareness, we investigate the hypothesis that fossoriality leads to inner ear morphology that is convergent with other spatially adept tetrapods. We apply morphometrics to otic capsule endocasts of 26 caecilian species to quantify aspects of inner ear shape, and compare these with a sample of frog and salamander species. Our results reveal caecilians (and also frogs) possess strongly curved canals, a feature in common with spatially adept species. However, significantly shorter canals in caecilians suggest reduced sensitivity, possibly associated with reduced reliance on vestibulo-ocular reflexes in this group of visually degenerate tetrapods. An elaboration of the sacculus of caecilians is interpreted as a unique adaptation among amphibians to increase sensitivity to substrate-borne vibrations transmitted through the head. This study represents the first quantitative analyses of inner ear morphology of limbless fossorial tetrapods, and identifies features within a new behavioural context that will contribute to our understanding of the biological consequences of physical stimuli on sensory function and associated morphological evolution.
内耳形态与动物生物学的功能方面（如运动行为）之间存在关联，这一点已被广泛接受。进行敏捷且空间复杂行为的动物拥有形态上能使对相应复杂物理刺激的敏感度最大化的半规管。基于穴居四足动物需要发达的空间意识这一预测，我们研究了穴居习性会导致内耳形态与其他空间能力强的四足动物趋同这一假说。我们将形态测量学应用于26种蚓螈物种的耳囊内铸型，以量化内耳形状的各个方面，并将这些与青蛙和蝾螈物种的样本进行比较。我们的结果显示，蚓螈（以及青蛙）拥有强烈弯曲的管道，这是与空间能力强的物种共有的特征。然而，蚓螈的管道明显更短，这表明其敏感度降低，可能与这组视觉退化的四足动物对前庭眼反射的依赖减少有关。蚓螈球囊的细化被解释为两栖动物中一种独特的适应性变化，以提高对通过头部传播的地面振动的敏感度。这项研究是对无肢穴居四足动物内耳形态的首次定量分析，并在新的行为背景下识别出相关特征，这将有助于我们理解物理刺激对感官功能及相关形态进化的生物学影响。
