Manatees exhibit numerous anatomical specializations which might be expected to have functional neuroanatomical correlates. Large, heavy, Pachystosis (thickening of bones, regardless of density); Osteosclerosis (replacement of cancellous with compact bone) Combination of these: pachyosteosclerosis. Fusiform Short, inflexible neck Relatively short, paddle-shaped forelimbs, nonflexible digits, Supination, and pronation limited Absence of hindlimbs. Tail modified into dorsoventrally flattened fluke Proboscis specialized into lateral lip portions Upper palate ribbed and hard Specialized migrating molars, front tooth replacements Bones are large and dense and therefore heavy, lacking marrow except in spine. Lungs are long (elongate, flattened ellipsoids, somewhat thicker in middle, but not much smaller at either end)protruding caudally beneath the spine and above the abdominal cavity throughout rib cage. Lungs not divided into lobes Long GI tract. Eyes small and weakly mobile

STRUCTURE, FUNCTION & ADAPTATION OF MANATEE EAR Ear canals closed, Tympano-periotic complex is complex and large Ear structures mature at birth. Intercochlear distance are closer to those of smaller phocoienids. IATDs (interaural time distances) will fall between a minim of 58 usec and 258 usec (calculated from intercochlear distances. Calculated IATD's imply T. Manatus needs an upper frequency limit of 50-90 kHz to use phase cues for sound location No evidence for acute ultrasonic hearing. They likely hear little above 20 kHz & thus unable to detect phase diffs. Intensity daffy from head shadow may provide some direction cues. However, intensity is generally most useful at higher frequencies. So sound localization is suspected to be poor.

MIDDLE EAR: Ovoid tympanic space (middle ear cavity, housing the ossicles) is defined by broad soft-tissue walls interiorly and laterally and by bony walls superiorly and medially. Middle ear cavity large and line with thick vascularized fibrous sheet. Bimorphic construction and complex shape of tympanic membrane make estimates of active area difficult, but expected to have a complex frequency dependent response pattern. Cochlear windows are large but conventionally structured. Ossicular chain is massive, nearly straight and loosely joined. Stapes is remarkable in being columnar. No conventional mammalian, stirrup like crura. Stapedial footplate is medially convex, surface area of 21.5 mm, it bulges into vestibule Chorda tympani (ant 2/3 tongue) is large (parasympathetic preganglionic fibers to submandibular and sublingual glands) Chorda tymp cross sec asrea is 10% of human facial nerve (19.6 mm sq) This implies it is nearly one third of manatee facial. Malleus is thick ovoid with ventrolateral manubrial flange Large tensor tympani muscle inserts on a small lat malleal pedicle.

INNER EAR: Vestisbular stem seem poorly developed and small.

COCHLEA: conventional orientation and morphology Cochlear duct structures are poorly developed, esp at basal end. There is no outer osseus spiral lamina. Little base to aspex differentiation. At thickest basal point, membrane is 200u wide and 7 u thick. Apically membrane is 600 u and 5 u thick. Manatees expected to have a center freq similar to humans but narrower in overall srange. At base of cochlear, basement membrane is slightly thicker at the lateral edge with small pop of mesothelial cells at limbal edge. These cells inc wustan apically and may inc basilar membrane reactive massd at apex: which would lower the minimal resonant freq of that region.

Why don't manatees avoid boats? Boat power spectra below 5 kHz.? Inner ear structures imply they lack sensitivity and directionality compared to most mammals. Manatee periotics are intracranial, closely spaced and at tached to bone. Ossicles are loosely articulated massive. Inner ear structures are poorly developed with little longitudinal variation. consistent with low-freq, non-acute ear. Highly derived zygomatic process raises important questions about novel sound conduction mechanisms in manatees. There is no obvious non-acoustic function consistent with extraordinary hypertrophgy of the zygomatic process, but there are acoustic processes: Zygomatic-squamosal-periotic relationship is reminiscent of mandible-tympano-periotic assoc of Odontocetiz; Tissues are sufficiently close to the density of sea water to be a low impedance channel to the ear. Probable that inflated zygomatic process has unique resonance characteristics compared to the rest of the skull, and it may function as a low freq channel.

Besst Chch potentials overly the zygomatic region) In terrestrials, the middle ear acts as a transformer which counteracts a 30 db loss from the impedance mismatch between air and fluid filled cochlear. Middle ears also tuned: each species has a characteristic middle ear resonance (based on mechanical properties of the middle ear components) This is generally the freq of best sensitivity for that species. Increasing stiffness of the system improves transmission of high freq, while large ossicular mass and voluminous middle ear cavities favor low frequencies. Middle ear system of manatus is large and mass dominated, implying low freq tuning, but extreme density of the oscines adds stiffness, Consequently, transmission of low freq will be less efficient and sharpness of tuning will be decreased. Few functional changes have occurred in the sirenian auditory periphery since the Eocene.

Lack of vomeronasal organ Hydrostasis (Domning & Buffronel) Hydrostasis (static postures) due to pachyosteosclerosis, Increases body to bone mass, Sirenians no longer have efficient control surfaces, hydrofoils. They have to regulate depth and maintain horizontal posture by hydrostatic rather than hydrodynamic means Compared with terrestrials or aquatic carnivores, manatees shifted Center of gravity (CG) forward and Center of balance (CB) backwards (to coincide at same locus). Expanded the lungs backward, shift skeletal weight forward, reductions of pelvic appendages, shortening neck or enlargement of tail shifts both centers backward to maximize stability CG should lie slightly below the CB. Maneuverability in yaw and pitch maximized by placing both CG and CB, as well as greatest concentration of mass, close to the middle of the animal's length. Skeletal ballast concentrated in thoracid region. Steering organs should be close to front and back ends to produce maximum turning moments. No need for deep diving (like seals and cet aceans), so no pulmonary specializations and skeletal weight promotes neutral bouyancy. Forward Trim angle greater in infants. Adults have horizontal trim.