101
Chapter five: Feeding mechanisms
the skull is dominated by a broad narial basin and a rostrum comprised of enlarged premaxillary bones. The variation of the degree of rostral deflection of these premaxillary
bones relative to the palatal plane reflects the location within the water column where
feeding occurs most efficiently (Domning 1980, 1982; Domning and Hayek 1986). On one
extreme of rostral deflection are Amazonian manatees. They feed primarily at the surface
upon natant floating meadows of the Amazonian rivers, lakes and floodplains, inundated
vegetation of the vàrzea, igapó, and emergent grasses (Poaceae formally Gramineae;
Best 1981; Rosas 1994). West African manatees also primarily consume natant vegetation. Both species inhabit turbid aquatic habitats where submerged aquatic plants are not
widely supported (Best 1981). Corresponding these trichechids possess the least deflected
snouts (~25°–42° and 15°–40° respectively). On the other extreme of rostral deflection are
dugongs. Dugongs are benthic-feeding specialists that consume primarily sea grasses.
They consume both the above- and belowground biomass. Belowground biomass contains
rhizomes, which dugongs often target and are a rich source of carbohydrates. Dugongs
possess the greatest rostral deflection (70°). West Indian manatees are the ultimate generalists among sirenians. They inhabit a variety of habitats that span coastal marine,
estuarine, and freshwater ecosystems. They consume more than 60 species of aquatic
vegetation that are distributed throughout these habitats (Hartman 1979). Much of their
diet is comprised of sea grasses, brackish and freshwater submerged aquatic vegetation,
but many terrestrial grasses and other types of vegetation are also consumed (Hartman
1979). The locations of these food resources span the water column from benthic, midwater, natant, emergent, and terrestrial. Their intermediate snout deflection (29°–52°)
allows them to consume vegetation throughout these various locations (Domning 1980).
More recently, it has been demonstrated that such correlations are much more complex
and interesting. The tusk size, body size, and degree of snout deflection are among several suites of important ecomorphological traits that determined resource partitioning in
extinct dugongid assemblages (Velez-Juarbe et al. 2012).
Unlike many marine mammals, sirenians masticate and process their food. Plant matter is processed using teeth and/or cornified palatal pads. This processing aids in reducing
particle size, increasing surface area, and ruptures the tough plant cell walls. Sirenians
use hindgut fermentation to digest the cellulose and cell contents of the plants they consume (Reynolds and Rommel 1996). Plant matter, particularly grasses (which contain high
levels of silica) can be highly abrasive. This selection pressure has resulted in numerous
adaptations by herbivores, terrestrial and aquatic, to resist tooth wear, since the life of
an herbivore only lasts as long as their teeth. Classic examples include the hypsodont
teeth of grazers such as horses, the open rooted ever-growing incisors of rodents, and the
serial replacement of multi-rooted molars of elephants (Unger 2010). Trichechids exhibit a
novel mechanism of cheek-tooth replacement, which differs significantly from dugongs
(and other herbivores). At any one time 6–8 cheek teeth are erupted and functional. As
they wear, these teeth migrate horizontally from the posterior region of the tooth-row
to the anterior region (Domning and Hayek 1984). Teeth at the anterior locations have littleto-no crown remaining. Once non-functional, the roots are reabsorbed and the tooth falls
out. New molars erupt at the posterior tooth-row and migrate anteriorly as replacements.
Manatees apparently have an indeterminate number of teeth that can be replaced in this
manner. This is an evolutionary novel solution to coping with an abrasive diet. Dugongs
do not possess such a conveyor-belt mechanism to resist an abrasive diet and are thought
to be at a disadvantage compared to trichechids when consuming grasses. Instead, dugong
teeth are open-rooted, simple peg-like molars that consist of dentin covered by cementum. These molars erupt slowly over their lifetime, through anterior drift, also known
Chapter five: Feeding mechanisms
the skull is dominated by a broad narial basin and a rostrum comprised of enlarged premaxillary bones. The variation of the degree of rostral deflection of these premaxillary
bones relative to the palatal plane reflects the location within the water column where
feeding occurs most efficiently (Domning 1980, 1982; Domning and Hayek 1986). On one
extreme of rostral deflection are Amazonian manatees. They feed primarily at the surface
upon natant floating meadows of the Amazonian rivers, lakes and floodplains, inundated
vegetation of the vàrzea, igapó, and emergent grasses (Poaceae formally Gramineae;
Best 1981; Rosas 1994). West African manatees also primarily consume natant vegetation. Both species inhabit turbid aquatic habitats where submerged aquatic plants are not
widely supported (Best 1981). Corresponding these trichechids possess the least deflected
snouts (~25°–42° and 15°–40° respectively). On the other extreme of rostral deflection are
dugongs. Dugongs are benthic-feeding specialists that consume primarily sea grasses.
They consume both the above- and belowground biomass. Belowground biomass contains
rhizomes, which dugongs often target and are a rich source of carbohydrates. Dugongs
possess the greatest rostral deflection (70°). West Indian manatees are the ultimate generalists among sirenians. They inhabit a variety of habitats that span coastal marine,
estuarine, and freshwater ecosystems. They consume more than 60 species of aquatic
vegetation that are distributed throughout these habitats (Hartman 1979). Much of their
diet is comprised of sea grasses, brackish and freshwater submerged aquatic vegetation,
but many terrestrial grasses and other types of vegetation are also consumed (Hartman
1979). The locations of these food resources span the water column from benthic, midwater, natant, emergent, and terrestrial. Their intermediate snout deflection (29°–52°)
allows them to consume vegetation throughout these various locations (Domning 1980).
More recently, it has been demonstrated that such correlations are much more complex
and interesting. The tusk size, body size, and degree of snout deflection are among several suites of important ecomorphological traits that determined resource partitioning in
extinct dugongid assemblages (Velez-Juarbe et al. 2012).
Unlike many marine mammals, sirenians masticate and process their food. Plant matter is processed using teeth and/or cornified palatal pads. This processing aids in reducing
particle size, increasing surface area, and ruptures the tough plant cell walls. Sirenians
use hindgut fermentation to digest the cellulose and cell contents of the plants they consume (Reynolds and Rommel 1996). Plant matter, particularly grasses (which contain high
levels of silica) can be highly abrasive. This selection pressure has resulted in numerous
adaptations by herbivores, terrestrial and aquatic, to resist tooth wear, since the life of
an herbivore only lasts as long as their teeth. Classic examples include the hypsodont
teeth of grazers such as horses, the open rooted ever-growing incisors of rodents, and the
serial replacement of multi-rooted molars of elephants (Unger 2010). Trichechids exhibit a
novel mechanism of cheek-tooth replacement, which differs significantly from dugongs
(and other herbivores). At any one time 6–8 cheek teeth are erupted and functional. As
they wear, these teeth migrate horizontally from the posterior region of the tooth-row
to the anterior region (Domning and Hayek 1984). Teeth at the anterior locations have littleto-no crown remaining. Once non-functional, the roots are reabsorbed and the tooth falls
out. New molars erupt at the posterior tooth-row and migrate anteriorly as replacements.
Manatees apparently have an indeterminate number of teeth that can be replaced in this
manner. This is an evolutionary novel solution to coping with an abrasive diet. Dugongs
do not possess such a conveyor-belt mechanism to resist an abrasive diet and are thought
to be at a disadvantage compared to trichechids when consuming grasses. Instead, dugong
teeth are open-rooted, simple peg-like molars that consist of dentin covered by cementum. These molars erupt slowly over their lifetime, through anterior drift, also known