While most otters lack a pouch, the sea otter possesses a remarkable anatomical adaptation often referred to as a pocket. This specialized pouch is intrinsically linked to the sea otter’s distinctive foraging behavior and its status as one of the few mammals known to regularly use tools.
Anatomy and Location of the Otter Pouch
The sea otter’s pouch is not a true internal marsupial pouch but a highly developed fold of loose skin. This baggy pocket is located under each foreleg, specifically in the armpit area. The fold is substantial enough to extend across the chest, creating a secure, temporary storage area.
Unlike many marine mammals, sea otters lack blubber and rely on incredibly dense fur for insulation, trapping air next to the skin. This need for dense fur in cold ocean waters results in the flexible, loose skin required to form the pouch. The pocket’s capacity is significant, allowing the otter to secure multiple items at once. Otters preferentially use the left side to store tools, leaving their dominant right paw free for manipulation.
How the Pouch Facilitates Foraging and Tool Use
The primary function of the pouch is to serve as a temporary cargo net during a foraging dive. Since a typical dive lasts around 55 seconds, the otter must collect multiple prey items before returning to the surface. The otter secures clams, crabs, or sea urchins in the pouch, allowing it to continue hunting on the seabed without surfacing after every catch.
Once the otter surfaces, it rolls onto its back and uses its chest as a floating table, retrieving items from the pouch with its agile forepaws. The pouch is also central to the sea otter’s tool-using behavior. An otter selects a preferred rock to use as a hammer or an anvil for cracking open hard-shelled prey. This chosen tool, which can be 6 to 15 centimeters in size, is securely tucked into the armpit pocket when not in use. Storing and reusing a favorite rock allows the otter to access a high-calorie diet of mollusks and crustaceans that would otherwise be inaccessible.
Pouch Differences Across Otter Species
The specialized pouch seen in the sea otter is largely absent or greatly reduced in most other species. For instance, river otters, such as the North American river otter, do not possess this pronounced flap of skin. This anatomical difference is a direct result of their differing ecological needs.
River otters primarily inhabit freshwater environments, where their diet consists mainly of fish, crayfish, and smaller, softer prey. These food sources do not require a hard tool for access, nor do they necessitate bulk-storage capacity for deep dives. The sea otter’s unique pouch is an evolutionary response to its wholly marine life and diet of hard-shelled benthic invertebrates, a structure evolved specifically for tool and food storage.