104
Marine Mammal Physiology: Requisites for Ocean Living
The skulls and mandibles of pinnipeds are the least derived among marine mammals
(with a few notable exceptions), and are more similar to those of canids. The modifications of craniodental morphology, particularly shape, have long been thought to be associated with feeding adaptations (King 1972). In general, pinniped crania are rounded and
sharply delineation from the facial region of the skull; they do not possess sinuses with
air chambers. The skulls of otariids are less variable than those of phocids, and many are
sexually dimorphic. Compared to closely related terrestrial carnivores, pinniped dentition
is reduced (~22–38 versus 44) (Berta et al. 2006). Although their dentition is heterodont, the
premolars and molars are uniform in cusp number, size, and often shape, resulting in a
virtually homodonty of these teeth. Hence, they are often collectively referred to as cheek
teeth or post-canine teeth. As a consequence, pinnipeds have lost the shearing carnassial
phenotype of terrestrial carnivores (Unger 2010).
Otariid skulls tend to be more dolichocephalic and with less variation of the postcanine teeth. Otariids usually possess deep transverse grooves on their incisors. The
canines are large relative to phocids and are thought to be used for grip-and-tear feeding.
The cheek teeth are uniformly homodont teeth but possess the addition of a large cingulum (shelf-like cusp) on the lingual surface, as well as a small cusp on the rostral surface of
each cheek tooth. The degree of cusps varies greatly (King 1983; Hillson 2005; Unger 2010).
Although distinctive phenotypes such as those found in crabeater and leopard seals are
not found among otariids, feeding specializations still exist. Many otariids possess interesting feeding mechanisms for capturing prey. Antarctic fur seals feed heavily upon zooplankton, including krill. Their post-canine teeth are among the smallest of any fur seal,
and hypothesized to be modified for straining krill (Bonner 1968; Repenning et al. 1971;
Riedman 1990). The skulls of South American sea lions (Otaria byronia) are perhaps the
most divergent. These skulls are large, robust, the upper palate is vaulted and elongated,
the jaws are short and broad, and the orofacial muscles are well developed, broadening
the snout further. This suite of characteristics is commonly associated with suction feeding specialization (King 1983) and indirect data strongly suggest that Otaria is capable of
powerful subambient pressures. These traits are exemplified in the skulls of walruses, a
well-known suction specialist.
The dentition of phocids is reduced relative to otariids (22–36 versus 34–38, respectively) (Berta et al. 2006). Among the phocids, there is a diversity of dental phenotypes most
of which are likely adaptations for piscivory and teuthophagy, but outstanding exceptions for filter feeding exists. At least two species, the leopard seal (Hydrurga leptonyx) and
the crabeater seal (Lobodon carcinophaga), possess intricate lophs of the cheek teeth that are
hypothesized to assist in filter feeding. The lophs of the distinctive post-canine teeth of
crabeater seals possess three long shearing cusps. Observations of captive crabeater seals
suggest they have the capability to ingest krill using suction, then employing a filtering
feeding mode that utilizes their elaborately lophed post-canine teeth (Ross et al. 1976;
Klages and Cockcroft 1990). Leopard seals are known to feed upon large vertebrates, such
as penguins and other seals, using a grip-and-tear feeding mode. The enlarged canines
and strong jaw and neck muscles are useful traits for raptorial biting and grip-and-tear
feeding modes. However, leopard seals also possess pronounced lophs on the post-canine
teeth and are thought to be as effective for filtering as crabeater seals (Øritsland 1977;
Hocking et al. 2013). Consumption of krill by leopard seals was once thought to be minimal but is now known to be important seasonally. The behavioral data demonstrate that
they are capable of suction feeding when consuming krill (Hocking et al. 2013). Although
the magnitude of this pressure remains unknown, a reinvestigation of the leopard teeth
morphology in conjunction with a feeding study supports the hypothesis that they do use
Marine Mammal Physiology: Requisites for Ocean Living
The skulls and mandibles of pinnipeds are the least derived among marine mammals
(with a few notable exceptions), and are more similar to those of canids. The modifications of craniodental morphology, particularly shape, have long been thought to be associated with feeding adaptations (King 1972). In general, pinniped crania are rounded and
sharply delineation from the facial region of the skull; they do not possess sinuses with
air chambers. The skulls of otariids are less variable than those of phocids, and many are
sexually dimorphic. Compared to closely related terrestrial carnivores, pinniped dentition
is reduced (~22–38 versus 44) (Berta et al. 2006). Although their dentition is heterodont, the
premolars and molars are uniform in cusp number, size, and often shape, resulting in a
virtually homodonty of these teeth. Hence, they are often collectively referred to as cheek
teeth or post-canine teeth. As a consequence, pinnipeds have lost the shearing carnassial
phenotype of terrestrial carnivores (Unger 2010).
Otariid skulls tend to be more dolichocephalic and with less variation of the postcanine teeth. Otariids usually possess deep transverse grooves on their incisors. The
canines are large relative to phocids and are thought to be used for grip-and-tear feeding.
The cheek teeth are uniformly homodont teeth but possess the addition of a large cingulum (shelf-like cusp) on the lingual surface, as well as a small cusp on the rostral surface of
each cheek tooth. The degree of cusps varies greatly (King 1983; Hillson 2005; Unger 2010).
Although distinctive phenotypes such as those found in crabeater and leopard seals are
not found among otariids, feeding specializations still exist. Many otariids possess interesting feeding mechanisms for capturing prey. Antarctic fur seals feed heavily upon zooplankton, including krill. Their post-canine teeth are among the smallest of any fur seal,
and hypothesized to be modified for straining krill (Bonner 1968; Repenning et al. 1971;
Riedman 1990). The skulls of South American sea lions (Otaria byronia) are perhaps the
most divergent. These skulls are large, robust, the upper palate is vaulted and elongated,
the jaws are short and broad, and the orofacial muscles are well developed, broadening
the snout further. This suite of characteristics is commonly associated with suction feeding specialization (King 1983) and indirect data strongly suggest that Otaria is capable of
powerful subambient pressures. These traits are exemplified in the skulls of walruses, a
well-known suction specialist.
The dentition of phocids is reduced relative to otariids (22–36 versus 34–38, respectively) (Berta et al. 2006). Among the phocids, there is a diversity of dental phenotypes most
of which are likely adaptations for piscivory and teuthophagy, but outstanding exceptions for filter feeding exists. At least two species, the leopard seal (Hydrurga leptonyx) and
the crabeater seal (Lobodon carcinophaga), possess intricate lophs of the cheek teeth that are
hypothesized to assist in filter feeding. The lophs of the distinctive post-canine teeth of
crabeater seals possess three long shearing cusps. Observations of captive crabeater seals
suggest they have the capability to ingest krill using suction, then employing a filtering
feeding mode that utilizes their elaborately lophed post-canine teeth (Ross et al. 1976;
Klages and Cockcroft 1990). Leopard seals are known to feed upon large vertebrates, such
as penguins and other seals, using a grip-and-tear feeding mode. The enlarged canines
and strong jaw and neck muscles are useful traits for raptorial biting and grip-and-tear
feeding modes. However, leopard seals also possess pronounced lophs on the post-canine
teeth and are thought to be as effective for filtering as crabeater seals (Øritsland 1977;
Hocking et al. 2013). Consumption of krill by leopard seals was once thought to be minimal but is now known to be important seasonally. The behavioral data demonstrate that
they are capable of suction feeding when consuming krill (Hocking et al. 2013). Although
the magnitude of this pressure remains unknown, a reinvestigation of the leopard teeth
morphology in conjunction with a feeding study supports the hypothesis that they do use