Characterization of lipids in adipose depots associated with minke and fin whale ears: Comparison with “acoustic fats” of toothed whales

Abstract

18 2In an underwater environment where light attenuates much faster than in air, cetaceans 19 have evolved to rely on sound and their sense of hearing for vital functions. Odontocetes 20 (toothed whales) have developed a sophisticated biosonar system called echolocation, allowing 21 them to perceive their environment using their sense of hearing (Schevill and McBride 1956, 22 Kellogg 1958, Norris et al. 1961). Echolocation has not been demonstrated in mysticetes (baleen 23 whales). However, mysticetes rely on low frequency sounds, which can propagate very long 24 distances under water, to communicate with potential mates and other conspecifics (Cummings 25 and Thompson 1971). 26The mechanism of sound reception in cetaceans has been debated for centuries. 27Cetaceans have lost the external pinna and the ear canal has also been reduced to a narrow, increased separation between the skull and ears is thought to reduce bone conduction, aiding 32 directional hearing under water (Claudius 1858, in Yamada 1953, van Heel 1962. 33In the 1960's, the "jaw hearing" hypothesis was proposed for odontocete cetaceans 34 (Norris 1964). Odontocetes possess unusual mandibles, which have enlarged mandibular hiatuses 35 filled with discrete fat bodies that are in direct contact with the tympano-periotic complex. These 36 fats also cover the outer parts of the mandible in most species. It had been noted earlier that 37 physical properties of sound in water are similar to those in most body tissues (Reysenbach de 38 Haan 1957), so the ear canal is not well-suited for underwater sound reception. However, Norris 39 suggested that the fat bodies associated with the mandibles act as a preferential pathway for 40 sound to get from the aquatic environment to the ears because "fat especially is closely 41 3 impedance-matched to sea water" (Norris 1968). While the detailed mechanisms are still unclear, 42Norris's theory has been subsequently validated by behavioral, physiological, and anatomical 43 studies (e.g., Bullock et al. 1968, Brill et al. 1988, Ketten 2000. 44The "acoustic fats" involved with odontocete sound reception are an example of a 45 structural fatty tissue, as opposed to a storage tissue. Whereas the volume and lipid composition 46 of storage fat, such as human abdominal fat and marine mammal blubber, generally change with 47 body condition and diet, structural fats, such as those found in the feet, joints, and eye sockets, 48are metabolically inert and do not expand during obesity or shrink during fasting (Pond 1998). 49These structural fats contain fewer dietary components than storage tissues. The fatty melon in 50 the odontocete forehead, which is part of the high frequency sound transmission pathway during 51 echolocation, is another structural "acoustic fat" in odontocetes. Cranford et al. (1996) noted that 52 the melon remains intact even in emaciated animals, and Koopman et al. (2003) showed that the 53 lipid content and fatty acid (FA) composition of the melon is stable across body conditions, 54wh...

Nitrogen solubility in odontocete blubber and mandibular fats in relation to lipid composition

“…Finally, pig back fat lacks the short-chain components and WEs that elevated N 2 solubility in some odontocete tissues. Mysticete (baleen whale) fats also lack these components and WEs (Ackman et al, 1975b;Yamato et al, 2014), so perhaps they have relatively low N 2 solubility and experience lower N 2 loads as well. To date, no data exist on N 2 solubility in mysticete or river dolphin fats.…”

“…The observed global R statistic ranges from 0 to 1, with higher values indicating greater deviation from the null hypothesis. When the null hypothesis was rejected, a SIMPER (Similarity Percentages) analysis was used to reveal which individual FAs or FAlcs were the most important in terms of differences observed in the MDS plot and calculated by the ANOSIM test (see Clarke and Gorley, 2006;Lane et al, 2011;Yamato et al, 2014).…”

Sound Production for Communication, Echolocation, and Prey Capture

“…Differences in the posterior portion of the lower jaw (i.e., mandibular foramen) were shown to be significant in the sound-reception apparatus (Cranford et al 2008a(Cranford et al , 2008b. In a related study, the lipid composition of fatty tissues associated with mysticete ear fats (minke and fin whales, Balaenoptera acutorostrata, B. physalus,) was shown to differ from that of odontocete ear fats by being composed of lipids typically found in mammalian adipose tissues (Yamato et al 2014). The large size of mysticetes, and the fact that they are rarely kept in captivity and have never been trained, make them less amenable for hearing studies.…”

Variation in the endogenous intact waxes of odontocetes: There is more than one way to build an acoustic receiver

“…It has been hypothesized that this arrangement of long, straight-chain FA surrounding an inner core of shorter and branched FA and WE serves to focus the outgoing echolocation beams from the melon and the incoming sounds through the MFBs to the ear, because sound travels more slowly through the WE and shorter, branched FA than through TAG and the longer, straight chain FA (Koopman et al, 2006;Litchfield et al, 1973;Varanasi et al, 1975). While this hypothesis has been supported by measurements of sound through different regions of the melon (Goold & Clarke, 2000;Yamato et al, 2014), other studies have concluded that a large portion of sound refraction and collimation happens at the interface between the water and outer melon tissue rather than within the acoustic fat (Litchfield & Greenberg, 1979), and very little is known about how sound is received by the jaw fats and subsequently transferred to the ears. It is generally assumed that sound enters the fat bodies lying superficial to the mandible (the extra-mandibular fat bodies or EMFB), and then transferred across the thin portion of the mandible (the "pan bone," known as Norris' acoustic window) to the fat bodies lying within the mandibular fossa (the intramandibular fats bodies or IMFB) for conduction to the ear bones, which have a direct connection to the posterior portion of the IMFB (Koopman et al, 2006;Norris, 1968).…”