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
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.