project has provided the first complete sectioning of whole brains
of the Florida manatee (Trichechus manatus laterostris).
Nine brains were cut in different planes of section and stained
for nerve cells and fibers. Systematic microscopic study of the
neuronal architecture of these specimens has provided us with a
comprehensive overview of how the manatee's internal brain structures
are organized. Alternate sections, stained for cell bodies and for
myelinated fibers (in transverse, sagittal and horizontal planes)
reveal details about the relative sizes, degree of complexity, and
spatial distribution of the many cellular groups and fiber tracts.
All of the brain structures found in manatees are common to most
mammals. But in the manatee many of these nuclei and tracts are
Macroscopic and microscopic study of our anatomical specimens
reveal several distinguishing features of the brain of manatees:
lobulated somatic sensory nuclei of the brainstem that process
information from the manatee's mobile browsing mouth parts (trigeminal
nuclei), forelimb (cuneate nucleus) and the fluke (Bischoff's
nucleus); Somatic sensory regions from the face are also relatively
enlarged in the thalamus as well as the cerebral cortex.
Facial motor nuclei in the medulla that produce the prominent
orofacial behaviors of the proboscis, lips and vibrissae are relatively
Particularly large are all the sensory brain structures of the
auditory system, which indicate that hearing is well developed
in manatees. This hypertrophy is seen in the massive auditory
nuclei of the brainstem and thalamus, especially the cochlear
nuclei, nuclei of the lateral lemniscus, and medial geniculate
nucleus. The region of cerebral cortex estimated to be auditory
in function is also relatively large;
Also, noticeable are the relatively small eye, optic nerve and
sensory visual regions of the lateral geniculate nucleus, superior
colliculus, and thalamus;
Relatively small, too, are the motor nuclei that innervate eye
muscles (oculomotor, trochlear and abducens nuclei);
The brainstem sensory and motor nuclei associated with the animal's
large tail and enlarged viscera are also more complex and enlarged;
Olfactory circuits are small, but taste circits are prominent.
Of special note is that the manatee's cerebral cortex is relatively
smooth and unconvoluted, suggesting that this cortex is less complexly
interconnected within and between cortical areas, as well as with
extracortical regions of the brain. The implied relative simplicity
of cortical circuits perhaps reflects the relative simplicity
of the manatee's sensory-motor life-history traits
Brain nuclei (amygdala,basal forebrain nuclei, hypothalamus, hippocampus)
associated with socio-emotional expression are not very elaborate
or enlarged in manatees. Again, such relative simplicity may reflect
the relatively calm social and emotional lifestyles often noted
of these mammals.
These various features indicate a minimal behavioral role of vision;
a perceptuo-motor emphasis on audition, and upon somato-sensation
from perioral structures (expected from a browsing animal with
large mobile proboscis and vibrissae) as well as from the forelimb
and fluke (not expected).
We are preparing a series of papers (electronic and hardcopy)
which, for the first time, will provide a detailed description
and portrayal of the important brain features (mentioned above)
of this unique endangered marine mammal.