School of Biological & Chemical Sciences, Queen Mary University of London, Mile End Road, E1 4NS, London, United Kingdom.
Front Zool. 2013 Jan 30;10(1):2. doi: 10.1186/1742-9994-10-2.
Many mammals have evolved highly adapted hearing associated with ecological specialisation. Of these, bats possess the widest frequency range of vocalisations and associated hearing sensitivities, with frequencies of above 200 kHz in some lineages that use laryngeal echolocation. High frequency hearing in bats appears to have evolved via structural modifications of the inner ear, however, studying these minute features presents considerable challenges and hitherto few such attempts have been made. To understand these adaptations more fully, as well as gain insights into the evolutionary origins of ultrasonic hearing and echolocation in bats, we undertook micro-computed tomography (μCT) scans of the cochleae of representative bat species from 16 families, encompassing their broad range of ecological diversity. To characterise cochlear gross morphology, we measured the relative basilar membrane length and number of turns, and compared these values between echolocating and non-echolocating bats, as well as other mammals.
We found that hearing and echolocation call frequencies in bats correlated with both measures of cochlear morphology. In particular, relative basilar membrane length was typically longer in echolocating species, and also correlated positively with the number of cochlear turns. Ancestral reconstructions of these parameters suggested that the common ancestor of all extant bats was probably capable of ultrasonic hearing; however, we also found evidence of a significant decrease in the rate of morphological evolution of the basilar membrane in multiple ancestral branches within the Yangochiroptera suborder. Within the echolocating Yinpterochiroptera, there was some evidence of an increase in the rate of basilar membrane evolution in some tips of the tree, possibly associated with reported shifts in call frequency associated with recent speciation events.
The two main groups of echolocating bat were found to display highly variable inner ear morphologies. Ancestral reconstructions and rate shift analyses of ear morphology point to a complex evolutionary history, with the former supporting ultrasonic hearing in the common bat ancestor but the latter suggesting that morphological changes associated with echolocation might have occurred later. These findings are consistent with theories that sophisticated laryngeal echolocation, as seen in modern lineages, evolved following the divergence of the two main suborders.
许多哺乳动物已经进化出高度适应生态特化的听觉。在这些动物中，蝙蝠拥有最宽的发声频率范围和相关的听觉敏感度，一些使用喉内回波定位的谱系的频率超过 200 kHz。蝙蝠的高频听觉似乎是通过内耳结构的改变而进化的，然而，研究这些微小的特征带来了相当大的挑战，迄今为止，很少有这样的尝试。为了更全面地了解这些适应，以及深入了解蝙蝠超声听觉和回声定位的进化起源，我们对来自 16 个科的代表性蝙蝠物种的耳蜗进行了微计算机断层扫描（μCT）扫描，涵盖了它们广泛的生态多样性。为了描述耳蜗的大体形态，我们测量了相对基底膜长度和匝数，并比较了回声定位和非回声定位蝙蝠以及其他哺乳动物之间的这些值。
我们发现蝙蝠的听觉和回声定位呼叫频率与耳蜗形态的两个度量值都相关。特别是，回声定位物种的相对基底膜长度通常较长，并且与耳蜗匝数呈正相关。这些参数的祖先重建表明，所有现存蝙蝠的共同祖先可能具有超声听觉能力；然而，我们也发现了多个杨翼手目亚目内的祖先分支中基底膜形态演化率显著下降的证据。在回声定位的阴翼手目内，树的某些分支的基底膜演化率有所增加，这可能与最近的物种形成事件相关的叫声频率变化有关。
两个主要的回声定位蝙蝠群体被发现具有高度可变的内耳形态。内耳形态的祖先重建和速率变化分析表明了一个复杂的进化历史，前者支持共同蝙蝠祖先的超声听觉，但后者表明与回声定位相关的形态变化可能发生在后来。这些发现与这样的理论一致，即现代谱系中所见的复杂喉内回声定位是在两个主要亚目分化后进化而来的。
