Question

What are whales and horses adapted for?

1. Illuminate (investigate, illustrate {draw, diagram, print}, describe, and explain) what scientists have discovered regarding the adaptations present in the cranial anatomy of baleen whales including: 1. the very large cranium with “telescoped” cranial and facial bones 2. Adaptations for aquatic breathing and sound generation (nasal opening (blowhole), lungs, air passages, phonic lips, etc.) 3. Adaptations for aquatic hearing (high frequency clicks involved in echolocation (toothed whales only), infrasound (ultra low frequency sound for long distance communication, etc.) Be sure to explain how infrasonic/very low frequency sound waves pass from the sound generating system into the water, passing through the water to other whales, entering their skull through the various bones of the skull, reach the auditory bullae with its involucrum, vibrate the fluid in the cochlea to stimulate tiny hairs to send nerve signals up the auditory nerves to the whale’s brain. 4. The baleen filter feeding system with the huge, curved jaws, pleated, distensible gullet supported by lower jaw, baleen curtains/sieves hanging from upper jaw bones, gigantic tongue, etc.

Answer 1

1) During their evolution from a terrestrial to an obligate aquatic lifestyle, the external bony nares of crown cetaceans (Neoceti, which includes odontocetes or toothed whales and mysticetes or baleen whales) has been reduced and migrated from the tip of the rostrum posteriorly to the top of the forehead. In addition to these positional changes in the external bony nares and presumably the nasal openings (blowholes), crown cetaceans differ from stem taxa (archaeocetes) in having a “telescoped” skull in which the facial bones, particularly the premaxilla and maxilla extend posteriorly to form most of the skull roof producing an elongate rostrum (beak) and dorsal nasal openings (Miller, 1923). The rearrangement of the cranial bones in mysticetes and odontocetes differs. In the mysticete skull the standard mammalian isthmus between the braincase and rostrum is reduced. The larger skulls and buccal cavities of mysticetes relative to body size permit the processing large volumes of water employed in filter feeding (Goldbogen et al., 2010). In mysticetes, this is accomplished by the posterior extension of the maxilla ventral to the frontals. In odontocetes, the maxilla (and premaxilla to a lesser degree e.g. extinct xenorophids) extend posteriorly and laterally so as to slide over the frontals and crowd the parietals laterally region of the skull by a distinct and different mechanism. It has been suggested that the restructuring of the odontocete skull facilitates adaptations for construction of a sound production and beam formation apparatus for odontocete biosonar (echolocation). During echolocation, odontocetes produce brief (normally submillisecond) high frequency sounds and form them into a forward transmission beam used to probe the environment for obstacles, prey, predators, and conspecifics by interpreting the returning echoes. These architectural changes of the cetacean skull via posterior movement of the nasal system in both odontocetes and mysticetes, have resulted in reduced olfactory capabilities of whales compared with their terrestrial relatives, although a rudimentary sense of smell is retained by extant mysticetes.

2)
Baleen whales (mysticetes) seem to use their larynx and vocal folds for sound production, but experimental evidence is lacking (Reidenberg and Laitman, 2007). Much more is known in toothed whales (odontocetes) such as dolphins (Madsen et al., 2004). Cranford et al. (1996) and Cranford (2000) hypothesized that echolocation clicks and whistles are produced in the nasal passage between the larynx (which is not used for sound production) and the single blowhole (Fig. 3B). The nasal passage possesses several nasal air sacs, which can be compressed by associated muscles, and phonic lips (also known as monkey lips or sonic lips) protruding into the lumen. Sounds are produced when air is pressed out of the nasal sacs into the nasal passage through the phonic lips, which are pressed together by muscles. Although the exact mechanism is not known, it is assumed that the sound is transmitted through specialized fatty tissue (anterior and posterior bursae) adjacent to the phonic lips. The anterior bursa (often referred to as the melon) radiates the sound energy forward and acts as an ‘acoustic lens’ (Zimmer et al., 2005). Toothed whales produce a wide variety of whistles and broadband sounds (clicks) with main energies at a few kilohertz (thus well detectable for humans) up to ultrasonic frequencies of >100 kHz, used for echolocating prey. Whales can also communicate acoustically in unspecialised ways; sounds produced by baleen whales when hitting the water surface during breaching are considered to be a communication signal (Dunlop et al., 2010).

3) The cetacean ear differs from that of terrestrial mammals in several ways. The outer ear has neither a pinna nor a functional airfilled auditory canal. The ear canal is narrow, filled with cellular debris and most likely non-functional. Middle and inner ears are encased in a bony structure (the tympanic bulla), which is connected only by cartilage and connective tissue to the skull (Au and Hastings, 2008; Mooney et al., 2012). It is currently assumed that in toothed whales, acoustic energy is conducted through the fatty canal of the lower jaw directly to the tympanic bulla. The malleus is not connected directly to the tympanic membrane, but instead is connected via a ligament to the tympanic bulla. It remains unclear how ossicles are acoustically coupled to the bulla. Removal experiments have revealed that the malleus is less important for hearing than the incus and stapes (Ketten, 1997; Au and Hastings,2008) The odontocete sound conduction pathway is not applicable to mysticetes because their lower jaw is not connected to the temporal (ear) bones. The auditory sensitivities of both suborders of whales differ considerably because of their different lifestyles. Physiological and behavioural experiments have shown that toothed whales can hear up to 200 kHz, while no such data exist for baleen whales (Richardson et al., 1995; Au, 2000b). Ketten (1997, 2000) concluded that the ear of baleen whales is adapted to lowfrequency hearing, based on comparative cochlear morphometry.
Comparison between aquatic and terrestrial hearing : The main difference between hearing in air and water relates to the mismatch in acoustic impedance (see Glossary) between the sound receptor and the medium. In air, sound pressure fluctuations directly oscillate a thin membrane (tympanum) on the outside of the body, whereas in water, such a membrane could not pick up sound directly because the animal moves in phase with the medium (see above). On land, sound is transmitted from this tympanum to the auditory ossicles, and from the perilymphatic labyrinth to the auditory end organs of the inner ear. Fishes, cetaceans and amphibious tetrapods have to rely on different pathways for conducting the sound underwater to the inner ear (Hetherington and Lombard, 1982).
Mysticete sound reception is enabled by the vibration of the relatively stiff and dense skull in response to the sound waves passing through the body of the whale. The advantage to mysticetes of using low-frequency (long-wavelength) sounds becomes evident when considering the motion or displacement of the scatterer (i.e. the skull), instead of the scattered pressure, as described by Rayleigh. The scattered pressure from low-frequency acoustic waves becomes ineffective as an excitation mechanism, because the amplification of the scattered pressure on the surface of the TPC is negligible for waves longer than the body of the animal. Consider, for example, that the wave length for a 20 Hz sound in water is 75 m, which is at least three times longer than the bodies of largest fin whales. At the same time, the amplitude of the oscillations (displacement) of the scatterer (skull) grows with the wavelength of the incident sound. The air spaces associated with the TPCs play a minor role for the pressure forcing mechanism, but only for high frequencies (above 5 kHz). At those frequencies, the air spaces helped to establish a “resonant cavity” for the sound waves propagating through the soft tissues towards the ears. The waves in the soft tissues are much too long below 5 kHz for the air spaces to be significant contributors to the pressure-distribution calculation. The most important function for these interconnected air spaces may be to maintain sufficient air volume in the tympanic cavity around the ossicular chain to allow the ossicles to vibrate free of damping or interference by nearby soft tissues. A similar mechanism has also been proposed for the enlarged pterygoid sinuses in Ziphius cavirostris.

4) The baleen of baleen whales are keratinous plates. They are made of a calcified, hard α-keratin material, a fiber-reinforced structure made of intermediate filaments (proteins). The degree of calcification varies between species, with the sei whale having 14.5% hydroxyapatite, a mineral that coats teeth and bones, whereas minke whales have 1–4% hydroxyapatite. In most mammals, keratin structures, such as wool, air-dry, but aquatic whales rely on calcium salts to form on the plates to stiffen them.[60] Baleen plates are attached to the upper jaw and are absent in the mid-jaw, forming two separate combs of baleen. The plates decrease in size as they go further back into the jaw; the largest ones are called the "main baleen plates" and the smallest ones are called the "accessory plates". Accessory plates taper off into small hairs. The baleen, a sieve-like structure in the upper jaw made of keratin, is used to filter plankton, among other food sources, from the water. Ventral grooves are expandable concave furrows that line a whale's throat and have the peculiarity of stretching like an accordion. They allow whales to open their mandible to 90 degrees to gulp water and prey. They expel saltwater through the baleen plates, leaving behind krill and plankton. The whale's tongue plays a vital role in filter feeding, as it is used to push the water out of its mouth while keeping the prey trapped in its baleen bristles. As humpback whales do not have teeth, their food is swallowed whole and travels down the esophagus into the preliminary stomach (known as the forestomach) to be ground.