105
Chapter five: Feeding mechanisms
their post-canine teeth to filter feed (King 1983; Klages and Cockcroft 1990; Adam 2005;
Hocking et al. 2013). Hence, many species likely modulate their feeding mode, and use
multiple feeding depending upon the circumstances (Marshall et al. 2008, 2014, in press;
Hocking et al. 2013, 2014). However, biting and suction feeding modes are likely still the
most commonly used. Notable among phocids is the suction feeding specialty of bearded
seals (Erignathus barbatus). Their dental and orofacial morphology, feeding performance
(suction and hydraulic jetting), and trophic ecology converge with walruses. Bearded
seals consume infaunal invertebrates such as bivalves and tubeworms, as well as fish.
The upper palate of bearded seals is also vaulted, although not to the degree observed
in South American sea lions and walruses. They are capable of generating up to 91.2 kPa
of subambient pressure, very close to the capability of walruses (Marshall et al. 2008).
Orofacial muscles close the upper and lower lateral lips to prevent negative pressure loss,
whereas the broad lips at the anterior are pursed to create a circular aperture. These two
features create a pipette-like structure that enhances suction generation and direct those
subambient pressures in front of the animal to enhance subambient forces. Such behavior
has also been reported for walruses (Fay 1982) but also for harbor seals (Marshall et al.
2014) and Steller sea lions (Marshall et al., in press). Although not as impressive as
walruses, harbor seals and Steller sea lions can produce significant subambient pressures
(45 kPa) and hydraulic jetting (Marshall et al. 2014, in press).
The skull of walruses is distinctive in its size, fusion, and generally derived condition. The tusks dominate its morphology. The maxillary bones in the walrus are enlarged
to accommodate and anchor the tusks to the skull (Fay 1982; King 1983; Marshall 2002).
The frontal orientation of the premaxillary broadens and shortens the rostrum, which
is advantageous for benthic feeding. The tusks of walruses are not used for feeding.
Instead, tusks are used for male–male interactions and for hauling out of the water onto
ice. Walruses commonly use their tusks to pull and lift their bodies from the water,
hence the derivation of their Latin name Odobenus (tooth walker). The tusks are evergrowing upper canines that can grow up to a meter in length in males (Fay 1982; King
1983; Unger 2010). The upper and lower deciduous incisors of walruses are present
between the right and left canines (tusks); but these quickly fall out and are not replaced,
leaving a wide space between the two tusks on the upper jaw and canines on the lower
jaw. This loss creates a circular space that is part of the formation of a pipette and assists
in maintaining subambient pressures. Walruses can generate subambient pressures as
high as 108 kPa, which is just greater than 1 atmosphere of pressure (Kastelein et al.
1994). The movement of a piston-like tongue has been hypothesized to be responsible
for the substantial intraoral subambient pressures measured (Gordon 1984). The vaulted
palate enhances suction generation since there is a greater volume within the oral cavity to act on. Walruses are infaunal benthic specialists, consuming mostly bivalves and
other infaunal invertebrates, but with occasional exceptions such as marine birds and
other marine mammals. Functional studies have demonstrated that in addition to suction, walruses can perform the opposite behavior—hydraulic jetting. Large bivalves are
excavated from the seafloor by alternating suction with hydraulic jetting to remove the
sediment around their prey.
5.2.3.1 Suction and biting feeding modes
Unlike some odontocetes, it is noteworthy that extreme rostral elongation has not
evolved among pinnipeds. This suggests that biting and suction feeding modes are not
biomechanical trade-offs as in odontocetes but are perhaps synergistic in pinnipeds.
Short, wide jaws with high mechanical advantage for biting feeding modes should also
Chapter five: Feeding mechanisms
their post-canine teeth to filter feed (King 1983; Klages and Cockcroft 1990; Adam 2005;
Hocking et al. 2013). Hence, many species likely modulate their feeding mode, and use
multiple feeding depending upon the circumstances (Marshall et al. 2008, 2014, in press;
Hocking et al. 2013, 2014). However, biting and suction feeding modes are likely still the
most commonly used. Notable among phocids is the suction feeding specialty of bearded
seals (Erignathus barbatus). Their dental and orofacial morphology, feeding performance
(suction and hydraulic jetting), and trophic ecology converge with walruses. Bearded
seals consume infaunal invertebrates such as bivalves and tubeworms, as well as fish.
The upper palate of bearded seals is also vaulted, although not to the degree observed
in South American sea lions and walruses. They are capable of generating up to 91.2 kPa
of subambient pressure, very close to the capability of walruses (Marshall et al. 2008).
Orofacial muscles close the upper and lower lateral lips to prevent negative pressure loss,
whereas the broad lips at the anterior are pursed to create a circular aperture. These two
features create a pipette-like structure that enhances suction generation and direct those
subambient pressures in front of the animal to enhance subambient forces. Such behavior
has also been reported for walruses (Fay 1982) but also for harbor seals (Marshall et al.
2014) and Steller sea lions (Marshall et al., in press). Although not as impressive as
walruses, harbor seals and Steller sea lions can produce significant subambient pressures
(45 kPa) and hydraulic jetting (Marshall et al. 2014, in press).
The skull of walruses is distinctive in its size, fusion, and generally derived condition. The tusks dominate its morphology. The maxillary bones in the walrus are enlarged
to accommodate and anchor the tusks to the skull (Fay 1982; King 1983; Marshall 2002).
The frontal orientation of the premaxillary broadens and shortens the rostrum, which
is advantageous for benthic feeding. The tusks of walruses are not used for feeding.
Instead, tusks are used for male–male interactions and for hauling out of the water onto
ice. Walruses commonly use their tusks to pull and lift their bodies from the water,
hence the derivation of their Latin name Odobenus (tooth walker). The tusks are evergrowing upper canines that can grow up to a meter in length in males (Fay 1982; King
1983; Unger 2010). The upper and lower deciduous incisors of walruses are present
between the right and left canines (tusks); but these quickly fall out and are not replaced,
leaving a wide space between the two tusks on the upper jaw and canines on the lower
jaw. This loss creates a circular space that is part of the formation of a pipette and assists
in maintaining subambient pressures. Walruses can generate subambient pressures as
high as 108 kPa, which is just greater than 1 atmosphere of pressure (Kastelein et al.
1994). The movement of a piston-like tongue has been hypothesized to be responsible
for the substantial intraoral subambient pressures measured (Gordon 1984). The vaulted
palate enhances suction generation since there is a greater volume within the oral cavity to act on. Walruses are infaunal benthic specialists, consuming mostly bivalves and
other infaunal invertebrates, but with occasional exceptions such as marine birds and
other marine mammals. Functional studies have demonstrated that in addition to suction, walruses can perform the opposite behavior—hydraulic jetting. Large bivalves are
excavated from the seafloor by alternating suction with hydraulic jetting to remove the
sediment around their prey.
5.2.3.1 Suction and biting feeding modes
Unlike some odontocetes, it is noteworthy that extreme rostral elongation has not
evolved among pinnipeds. This suggests that biting and suction feeding modes are not
biomechanical trade-offs as in odontocetes but are perhaps synergistic in pinnipeds.
Short, wide jaws with high mechanical advantage for biting feeding modes should also