a statistical test of ribcage strength among terrestrial, semi-aquatic, and fully aquatic mammals, and found that Ambulocetus clustered with fully aquatic mammals, because they assigned a very high rib density on par with fully aquatic sirenians which use their heavy, osteosclerotic ribs as ballast. They then concluded Ambulocetus could not walk on land, but cautioned the study was limited by a lack of information on the exact density of the bone, the location of the centre of mass, and the reliance of false ribs for thoracic support. Hearing Modern cetaceans have highly specialised ear bones to hear underwater as well as to detect certain frequency ranges. Unlike most other mammals, cetacean ear bones are comparatively thick, and so preserve more reliably in the fossil record. Modern cetaceans have air sinuses surrounding the ear bones (peritympanic sinuses), which acoustically isolate the ear by reflecting sound moving through the head and interrupting both bony and fleshy connections of the ear to the skull. Like later archaeocetes, Ambulocetus had at least one such sinus between the tympanic bone and the skull base. The evolution of these sinuses also seems to have caused some restructuring of the skull base due to the development of bony walls surrounding the sinuses. The ectotympanic of all cetaceans, including Pakicetus and Ambulocetus, has a bony growth (involucrum) on the medial lip speculated to aid in the detection of low-frequency sounds. All cetaceans also have a vertical crest ("sigmoid process") right in front of the ear canal, which is speculated to be related to the increasing size of the malleus bone in the middle ear. As for the outer ear, terrestrial mammals channel sound in via an ear canal, but those of modern cetaceans are either narrowed or completely plugged, the sound being picked up (at least for toothed whales) by a fat pad in the lower jaw running to the ectotympanic bone. The mandibular foramen size can determine the size of the fat pad, and that of Ambulocetus is larger than that of Pakicetus and terrestrial mammals, but is smaller than later archaeocetes and toothed whales. Nonetheless, a lot of the change to the external auditory apparatus occurred between Pakicetus and Ambulocetus. These early archaeocetes may have developed such an external ear to either: better hear underwater; facilitate bone conduction of vibrations on dry land as some low-lying terrestrial creatures do (namely turtles and subterranean mole rats); or