化石證據顯示已滅絕的古代鯨魚跟牠們的陸地親戚有類似的聽覺

原文網址：www.sciencedaily.com/releases/2017/06/170608123650.htm

化石證據顯示已滅絕的古代鯨魚跟牠們的陸地親戚有類似的聽覺

鯨魚仰賴牠們靈敏的聽力在水下生活，但是牠們不同種類之間的聽覺卻有著重大差異。鬚鯨將頻率調至次聲波以進行長距離通訊，其頻率太低使得人耳無法聽見。齒鯨則反其道而行，牠們用的超音波頻率遠遠高出人類能聽到的範圍。

6月8日發表在《當代生物學》(Current Biology)的論文中，研究人員根據已經滅亡的早期鯨魚留下的化石證據，提出在鯨魚演化成我們今日所知的完全水生動物之後，才開始產生前述的聽覺差異。他們的根據是發現原鯨(protocete)這種可以同時在水中和陸地生活的鯨類，聽覺似乎跟牠們陸上的偶蹄類親戚更為相似，包括豬、河馬和駱駝。

法國國家科學研究中心和法國蒙佩利爾大學的Maeva Orliac表示：「我們發現原鯨的耳蝸不同於現存的鯨魚和海豚，且牠們的聽覺能力更近似於牠們生活在陸上的親戚。」

原鯨缺乏高度專精的聽力顯示早期的鯨魚無法跟今日的鯨目(cetacean，包含鯨魚和海豚)一樣，進行回音定位或長距離溝通。

研究人員從西非多哥的海洋沉積物中找到了年代為4500萬年的原始鯨類遺骸，對其進行研究後得到了上述結論。他們的研究對象為兩種早期鯨魚骨骸中保留的骨性迷路(bony labyrinth)，此空腔是聽覺器官的所在位置。

Orliac和她的同事Mickaël Mourlam利用微電腦斷層掃描來透視岩石和化石的內部結構，大致上就跟利用X光掃描儀使得我們可以看見人體內部的骨頭一樣。這些影像讓他們可以分析岩骨(petrosal bone)內部的坑洞，其為安置平衡覺和聽覺器官的位置。

Orliac表示：「根據掃描儀提供的影像，我們可以製作出一個虛擬模型來顯示這些動物活著時，用來放置牠們聽覺器官的空腔型態為何。整個過程相當冗長且困難，因為這個空洞塞滿了沉積物，有些還有再結晶的現象。另一方面，原鯨的岩骨又特別厚且緻密，使得影像品質變低，有時還會讓分析受阻。」

然而，掃描結果還是呈現出早期鯨目的聽覺能力較接近於牠們活在陸上的表親。只有在鯨魚找到徹底回歸海洋的方法之後，牠們才特化出可以聽見超音波或次聲波的能力，如同我們今日所見的。

研究人員表示這項發現強調出要得到鯨魚演化史的精確圖像時，研究早期鯨目化石有其重要性。此外，研究也顯示鯨魚過去的演化歷程比之前描述的還要更加複雜。

Orliac表示他們會在12月重回多哥進行野外調查以尋找更多的原鯨化石樣本。在多哥經由牙齒遺骸辨識出來的三種樣本中，他們目前為止只描述了其中兩種。因此，他們希望可以找到一具化石樣品，能讓他們探討第三個種類的耳部構造。

Extinct early whales listened like their relatives on land, fossil evidence shows

Whales rely on a keen sense of hearing for their underwater existence. But whales show surprisingly vast differences in hearing ability. Baleen whales tune into infrasonic sounds -- at frequencies too low for humans to hear -- to communicate over long distances. Toothed whales do just the opposite, relying on ultrasonic frequencies too high for humans to hear.

Now researchers reporting in Current Biology on June 8 have fossil evidence from extinct early whale species to suggest that those differences in hearing arose only after whales evolved into the fully aquatic animals we know today. That's based on their findings that whales known as protocetes, which spent time both in water and on land, appear to have hearing more like their terrestrial, even-toed ungulate relatives, including pigs, hippos, and camels.

"We found that the cochlea of protocetes was distinct from that of extant whales and dolphins and that they had hearing capacities close to those of their terrestrial relatives," says Maeva Orliac of CNRS and Université de Montpellier in France.

Protocetes' lack of hearing specialization suggests that the early whales were unable to echolocate and communicate through long-distance calls in the way that modern-day cetaceans, the group including whales and dolphins, do.

The researchers came to those conclusions based on studies of 45-million-year-old protocetid whale remains found in marine deposits from Togo in West Africa. The researchers studied the bony labyrinth, a hollow cavity that would have housed the hearing organ, in two species of early whales.

Orliac and her colleague Mickaël Mourlam used micro-CT scanning to peer inside the internal structures of rocks and fossils, in much the same way that an X-ray scanner makes it possible to see bones inside a person's body. Those images allowed them to analyze the internal cavities of the petrosal bone, which shelters the organs of hearing and balance.

"Based on the scans provided by the scanner, we could extract a virtual mold of the hollow cavity that used to contain the hearing organ when the animal was alive," Orliac says. "This process was long and difficult because this cavity was filled with sediments and partly recrystallized and because the petrosal bone in cetaceans is particularly thick and dense, which lowers the quality of the images and sometimes impedes analyzing them."

Nevertheless, the scans suggest that early cetaceans had hearing closer to that of their terrestrial relatives. Specialization to infrasonic or ultrasonic hearing as seen in modern whales came only later, in whales that had already found their way back to the sea.

The findings highlight the importance of studying these early cetaceans to get an accurate picture of whales' evolutionary history. It also suggests that whales' evolutionary past is more complicated than had previously been described, the researchers say.

Orliac says they'll be back in field in Togo in December to search for additional protocetid whale specimens. They've so far described two of three species identified in Togo based on dental remains. They hope to find a specimen that will allow them to explore the ear of the third.

原始論文：Mourlam and Orliac. Infrasonic and Ultrasonic Hearing Evolved after the Emergence of Modern Whales. Current Biology, 2017 DOI: 10.1016/j.cub.2017.04.061

引用自：Cell Press. "Extinct early whales listened like their relatives on land, fossil evidence shows." ScienceDaily. ScienceDaily, 8 June 2017.