109
Chapter five: Feeding mechanisms
Milne 2007). For example, in river otters (Lontra canadensis), the digastric muscles are
hypertrophied compared to terrestrial carnivores, which is thought to allow the rapid
jaw closure necessary for catching elusive fish with their mouths underwater (Goswami
2006). This is also reflected in their cranial morphology; river otters possess broad mastoid
processes where the enlarged digastric muscles originate (Goswami 2006). River otters
also possess sharp carnassials necessary for piercing and shearing fish (Popowics 2003). In
contrast, sea otters possess short, blunt skulls with bunodont dentition, used for crushing
hard, benthic prey (Kenyon 1969).
However, recent functional data (Timm 2013) suggest that sea otters exhibit further
specialized feeding methods. Sea otters forage on the bottom, in waters as deep as 40
meters (Kenyon 1969; Riedman and Estes 1990). Their diet is varied but shellfish and
urchins comprise a large portion. Otters use their forepaws to excavate clams, and to pry
shellfish and urchins from the rocky substrate, sometimes using tools. Food is usually
consumed at the surface, and the behavioral observations suggest that otters do not use
their teeth underwater, even when feeding on fish. Upon surfacing, fish are killed by a bite
to the head. A rock or some other tool is usually carried in a flap of skin in the axilla region
(under the arm) and is used to pound open shellfish. The spines of urchins are simply
b itten off, and the test (shell) of the urchin is crushed with the cheek teeth. Their cheek
teeth are broad, flat, and covered with thick enamel. The shearing cusps of the carnassial
teeth have been lost; sea otters are adapted for crushing their food (Kenyon 1969).
It is commonly understood that sea otters are durophagous and forage primarily upon
infaunal, but also epibenthic, invertebrates. However, Northern and Russian sea otters
(Enhydra lutris kenyoni and Enhydra lutris lutris, respectively) also prey on epibenthic fish and
occasionally other vertebrates (Riedman and Estes 1988). Sea otters differ from other otters
in that the morphological traits are correlated with durophagy but also that durophagy is
enhanced by an extreme blunt and wide skull and mandible (Timm 2013), and large occlusal
surface area relatively to body size. Sea otters were found to possess a relatively high masseteric mechanical advantage compared to other otter species, which is thought to increase
force at the most posterior molars as an adaptation for durophagy. This feature in other
mammals is known to provide additional control over mastication (Radinsky 1985). A morphometric analysis of sea otters demonstrated that they exhibited taller and wider mandibular rami, and shorter, blunter skulls, than other otters (Timm 2013), which is consistent with
increased bite force at the carnassials of other mammals (Sacco and Van Valkenburgh 2004;
Figueirido et al. 2009; Nogueira et al. 2009). Furthermore, the blunt, crushing carnassials, and
all molars in general, are known to possess relatively thicker enamel (Fisher 1941; Kenyon
1969) that is resilient in its structure (Chai et al. 2009) and is 2.5 times stronger than human
enamel (Ziscovici et al. 2014). In addition, both morphological and kinematic investigations
of sea otter biting demonstrates that they can display wide gapes (Timm 2013), while maintaining powerful bite forces, useful adaptations for feeding upon bivalves.
5.2.5 Order Carnivora: Polar bears
Polar bears (Ursus maritimus) are considered to be marine mammals since they spend a significant time in the marine environment and rely upon marine mammals to survive. This
particular species arose only <1 million years ago from brown bears, which are omnivorous (Slater et al. 2010). Although they tend to be completely carnivorous (Slater et al. 2010),
when required, polar bears will resort to omnivory and supplement its diet with fruits,
vegetation, and various other food material (Iversen et al. 2014). However, recent data demonstrate that the cranial morphology of polar bears does diverge from brown bears and
Chapter five: Feeding mechanisms
Milne 2007). For example, in river otters (Lontra canadensis), the digastric muscles are
hypertrophied compared to terrestrial carnivores, which is thought to allow the rapid
jaw closure necessary for catching elusive fish with their mouths underwater (Goswami
2006). This is also reflected in their cranial morphology; river otters possess broad mastoid
processes where the enlarged digastric muscles originate (Goswami 2006). River otters
also possess sharp carnassials necessary for piercing and shearing fish (Popowics 2003). In
contrast, sea otters possess short, blunt skulls with bunodont dentition, used for crushing
hard, benthic prey (Kenyon 1969).
However, recent functional data (Timm 2013) suggest that sea otters exhibit further
specialized feeding methods. Sea otters forage on the bottom, in waters as deep as 40
meters (Kenyon 1969; Riedman and Estes 1990). Their diet is varied but shellfish and
urchins comprise a large portion. Otters use their forepaws to excavate clams, and to pry
shellfish and urchins from the rocky substrate, sometimes using tools. Food is usually
consumed at the surface, and the behavioral observations suggest that otters do not use
their teeth underwater, even when feeding on fish. Upon surfacing, fish are killed by a bite
to the head. A rock or some other tool is usually carried in a flap of skin in the axilla region
(under the arm) and is used to pound open shellfish. The spines of urchins are simply
b itten off, and the test (shell) of the urchin is crushed with the cheek teeth. Their cheek
teeth are broad, flat, and covered with thick enamel. The shearing cusps of the carnassial
teeth have been lost; sea otters are adapted for crushing their food (Kenyon 1969).
It is commonly understood that sea otters are durophagous and forage primarily upon
infaunal, but also epibenthic, invertebrates. However, Northern and Russian sea otters
(Enhydra lutris kenyoni and Enhydra lutris lutris, respectively) also prey on epibenthic fish and
occasionally other vertebrates (Riedman and Estes 1988). Sea otters differ from other otters
in that the morphological traits are correlated with durophagy but also that durophagy is
enhanced by an extreme blunt and wide skull and mandible (Timm 2013), and large occlusal
surface area relatively to body size. Sea otters were found to possess a relatively high masseteric mechanical advantage compared to other otter species, which is thought to increase
force at the most posterior molars as an adaptation for durophagy. This feature in other
mammals is known to provide additional control over mastication (Radinsky 1985). A morphometric analysis of sea otters demonstrated that they exhibited taller and wider mandibular rami, and shorter, blunter skulls, than other otters (Timm 2013), which is consistent with
increased bite force at the carnassials of other mammals (Sacco and Van Valkenburgh 2004;
Figueirido et al. 2009; Nogueira et al. 2009). Furthermore, the blunt, crushing carnassials, and
all molars in general, are known to possess relatively thicker enamel (Fisher 1941; Kenyon
1969) that is resilient in its structure (Chai et al. 2009) and is 2.5 times stronger than human
enamel (Ziscovici et al. 2014). In addition, both morphological and kinematic investigations
of sea otter biting demonstrates that they can display wide gapes (Timm 2013), while maintaining powerful bite forces, useful adaptations for feeding upon bivalves.
5.2.5 Order Carnivora: Polar bears
Polar bears (Ursus maritimus) are considered to be marine mammals since they spend a significant time in the marine environment and rely upon marine mammals to survive. This
particular species arose only <1 million years ago from brown bears, which are omnivorous (Slater et al. 2010). Although they tend to be completely carnivorous (Slater et al. 2010),
when required, polar bears will resort to omnivory and supplement its diet with fruits,
vegetation, and various other food material (Iversen et al. 2014). However, recent data demonstrate that the cranial morphology of polar bears does diverge from brown bears and