Sea otters possess the densest pelage in the animal kingdom, packing up to one million hairs into a single square inch of skin. Because they live in chilly coastal oceans without a layer of insulating blubber, this ultra-dense two-layer coat is their primary defense against lethal heat loss.
- True of
- sea otters
- Numbers
- up to 1,000,000 hairs per square inch fur densityranges from about 170,000 to over 1,000,000 hairs per square inch depending on body location
- First fold
- Fold 1
2 sources
- National Park Service Sea Otter (Enhydra lutris) government
- Monterey Bay Aquarium Sea Otter aquarium
Unlike whales, seals, and sea lions, otters have zero blubber. To survive cold water without a thick layer of body fat, otters depend completely on their dense waterproof coats to trap an insulating cushion of air right against their skin.
- True of
- all otters
- Corrects
- Otters stay warm with blubber like seals.
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- U.S. Fish & Wildlife Service Southern Sea Otter (Enhydra lutris nereis) government
An otter's soft undercoat traps billions of tiny air bubbles against the skin. This captive blanket of air prevents cold water from ever touching the animal's skin, providing thermal insulation that keeps the otter warm even in sub-freezing ocean currents.
- True of
- all otters
- First fold
- Fold 1
2 sources
- SeaWorld All About Otters — Adaptations aquarium
- Monterey Bay Aquarium Sea Otter aquarium
Otters spend hours each day rigorously grooming their fur. They use their paws, claws, and teeth to comb out debris, spread natural waterproofing oils from skin glands, and blow air into their fur with their mouths to recharge the insulating air layer.
- True of
- all otters
- Corrects
- Grooming is merely cosmetic cleaning rather than life-saving insulation maintenance.
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- SeaWorld All About Otters — Adaptations aquarium
North American river otters have webbed digits on both their front and hind feet. Their non-retractable, sharp claws provide traction when scrambling across slippery river rocks, climbing muddy embankments, and grasping live fish or crayfish.
- True of
- North American river otters
- First fold
- Fold 1
2 sources
- National Park Service North American River Otters government
- SeaWorld All About Otters — Physical Characteristics aquarium
A sea otter's hind legs are modified into broad, paddle-like flippers with extensive webbing between the digits. Unlike terrestrial mustelids, the outer digit is the longest, creating an efficient hydrofoil surface that powers swimming through coastal waters.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Animal Diversity Web Enhydra lutris (sea otter) university
- SeaWorld All About Otters — Physical Characteristics aquarium
A North American river otter's tail accounts for roughly one-third of its total body length. Thick and muscular at the base, it tapers to a flattened point, acting as both an agile underwater rudder for sharp turns and a powerful thruster during high-speed swimming.
- True of
- North American river otters
- Numbers
- approximately one-third tail proportion of body length
- First fold
- Fold 1
2 sources
- National Wildlife Federation North American River Otter ngo
- SeaWorld All About Otters — Physical Characteristics aquarium
Both river otters and sea otters have valvular ears and nostrils equipped with specialized muscular flaps. When the animal submerges, these valves automatically clamp shut against water pressure, keeping water out of the respiratory tract and ear canals while diving.
- True of
- all otters
- First fold
- Fold 1
2 sources
- National Park Service North American River Otters government
- SeaWorld All About Otters — Physical Characteristics aquarium
Otters have an elongated, highly flexible vertebral column typical of mustelids. This vertebral flexibility allows otters to bend almost double while grooming every patch of fur and enables the wave-like dorsoventral undulations that generate efficient propulsion underwater.
- True of
- all otters
- First fold
- Fold 1
2 sources
- SeaWorld All About Otters — Physical Characteristics aquarium
- National Park Service North American River Otters government
Sea otters possess an extensible pouch of loose skin under each forearm (axilla). When diving to the ocean floor, an otter stashes collected clams, crabs, or urchins—along with its favorite anvil stone—into this pocket, leaving its forepaws unencumbered for further hunting.
- True of
- sea otters
- Corrects
- River otters also have pockets for storing stones.
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
The buoyant lift generated by a sea otter's outsized lungs, combined with the air trapped throughout its dense coat, produces high positive buoyancy. This buoyancy allows sea otters to effortlessly float, eat, nap, and nurse their pups while resting on their backs.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Library of Medicine Ontogeny of Oxygen Storage Capacity and Diving Ability in the Southern Sea Otter journal, 2015
When resting or grooming at the surface, sea otters routinely hold their forepaws and hind flippers up in the air above the surface. Because extremities lose heat quickly to cold water, keeping them exposed to the warmer air or sunlight drastically reduces overall thermal loss.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- SeaWorld All About Otters — Adaptations aquarium
Newborn sea otter pups possess a dense, woolly natal coat that retains an extraordinary volume of air. This coat makes the pup so buoyant that it cannot submerge or dive underwater, bobbing safely on the surface while its mother dives beneath the kelp canopy to forage.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Animal Diversity Web Enhydra lutris (sea otter) university
- Monterey Bay Aquarium Sea Otter aquarium
North American river otters have an exceptionally dense coat that insulates them in cold freshwater streams and lakes. Their fur contains up to 450,000 hairs per square inch, consisting of a short, wavy underfur shielded by longer, glossy guard hairs.
- True of
- North American river otters
- Numbers
- up to 370,000 to 450,000 hairs per square inch fur densityvaries across body regions with densest concentrations on the back and limbs
- First fold
- Fold 2
2 sources
- SeaWorld All About Otters — Physical Characteristics aquarium
- National Park Service North American River Otters government
A sea otter's front paws are small and dexterous, featuring tough, sensitive skin pads and semi-retractable claws. Sea otters do not use their forepaws for swimming; instead, they tuck them away or use them like hands to pry abalone from rocks, groom fur, and manipulate tools.
- True of
- sea otters
- Corrects
- Sea otters paddle with all four limbs when swimming.
- First fold
- Fold 2
2 sources
- Animal Diversity Web Enhydra lutris (sea otter) university
- SeaWorld All About Otters — Physical Characteristics aquarium
Compared to river otters, a sea otter has a shorter, less muscular, and horizontally flattened tail. While floating on its back at the sea surface, the otter gently sculls with its tail from side to side to change direction and maintain position among kelp fronds.
- True of
- sea otters
- First fold
- Fold 2
2 sources
- SeaWorld All About Otters — Adaptations aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
Rather than the sharp, slicing carnassial teeth typical of land carnivores, sea otters possess broad, flat, bunodont molars with thickened enamel. These specialized tooth crowns act like pestles to crush the thick armor of sea urchins, crabs, clams, and snails.
- True of
- sea otters
- Corrects
- Sea otters tear flesh with sharp wolf-like fangs.
- First fold
- Fold 2
2 sources
- SeaWorld All About Otters — Physical Characteristics aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
A North American river otter's dentition features pointed canine teeth for grasping slippery prey and sharp premolars that act like shears. This dental layout is optimized for piercing struggling fish and crunching through the chitinous carapaces of freshwater crayfish.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Park Service North American River Otters government
- Animal Diversity Web Lontra canadensis (northern river otter) university
Unlike sea otters, North American river otters have tight-fitting axillary skin without pockets or pouches. They do not store rocks or transport food in skin flaps, instead seizing fish and aquatic organisms directly in their jaws and eating smaller prey immediately in the water.
- True of
- North American river otters
- Corrects
- All otter species carry favorite rocks in skin pouches.
- First fold
- Fold 2
2 sources
- Animal Diversity Web Lontra canadensis (northern river otter) university
- SeaWorld All About Otters — Senses aquarium
Because water conducts heat away from the body roughly 25 times faster than air, otters maintain an exceptionally high resting metabolic rate—two to three times that of terrestrial mammals. To fuel this internal furnace, sea otters must consume 20% to 25% of their body weight in food each day.
- True of
- all otters
- Numbers
- 20% to 25% of body weight daily food consumption in sea otters
- First fold
- Fold 2
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
Despite their semi-aquatic specializations, North American river otters are remarkably swift on dry land. They can sprint at speeds up to 15 miles per hour and travel across snow and ice by alternating several bounding strides with long, frictionless belly slides.
- True of
- North American river otters
- Numbers
- up to 15 miles per hour top running speed on landcan move even faster by sliding on snow and mud
- Corrects
- River otters are slow and awkward when out of the water.
- First fold
- Fold 2
2 sources
- National Wildlife Federation North American River Otter ngo
- Animal Diversity Web Lontra canadensis (northern river otter) university
While otters paddle with their feet when swimming slowly, high-speed aquatic propulsion is powered by vertical (dorsoventral) undulations of the body and tail. This wave-like motion flexing through the elongated spine and webbed hind limbs mirrors the swimming biomechanics of dolphins and seals.
- True of
- all otters
- First fold
- Fold 2
2 sources
- SeaWorld All About Otters — Adaptations aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
Under a microscope, the outer cuticle of each otter guard hair features microscopic, ratchet-like scales. When wet, these opposing scales interlock with neighboring hairs, forming a tight, shingles-like shield that keeps water out and locks trapped air in.
- True of
- all otters
- First fold
- Fold 3
2 sources
- SeaWorld All About Otters — Adaptations aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
All other mammals in the order Carnivora have six lower incisors, but the sea otter has evolved to have only four. These lower incisors form a spade-like scoop used to pry and scrape edible meat out of cracked invertebrate shells.
- True of
- sea otters
- Numbers
- 4 lower incisor teethall other living carnivorans have 6 lower incisors
- First fold
- Fold 3
2 sources
- Animal Diversity Web Enhydra lutris (sea otter) university
- SeaWorld All About Otters — Physical Characteristics aquarium
Sea otters have exceptionally voluminous lungs, measuring approximately 2.5 times larger than expected for a terrestrial mammal of equivalent body size. This large respiratory volume acts as the otter's primary oxygen reservoir during benthic dives and provides vital buoyancy at the surface.
- True of
- sea otters
- Numbers
- roughly 2.5 times larger lung capacity relative to terrestrial mammals
- First fold
- Fold 3
2 sources
- National Library of Medicine Ontogeny of Oxygen Storage Capacity and Diving Ability in the Southern Sea Otter journal, 2015
- U.S. Department of the Interior 12 Facts About Otters for Sea Otter Awareness Week government, 2021
North American river otters possess paired subcutaneous anal scent glands that secrete a pungent, musky fluid. Otters deposit this scent on latrine mounds along riverbanks to communicate territory boundaries, identity, and reproductive status to other otters.
- True of
- North American river otters
- First fold
- Fold 3
2 sources
- Animal Diversity Web Lontra canadensis (northern river otter) university
- SeaWorld All About Otters — Physical Characteristics aquarium
The permanent dental formula of the North American river otter consists of 36 teeth: six upper and six lower incisors, two upper and two lower canines, eight upper and six lower premolars, and two upper and four lower molars. Biologists count annual cementum rings in these teeth to determine an otter's age.
- True of
- North American river otters
- First fold
- Fold 4
2 sources
- National Library of Medicine Clinical anatomy of the North American river otter (Lontra canadensis) journal, 2000
- Animal Diversity Web Lontra canadensis (northern river otter) university
An adult sea otter possesses 32 teeth, having lost two lower incisors and two premolars over evolutionary time compared to ancestral mustelids. This dental reduction leaves room for enlarged crushing molars capable of withstanding the immense pressures required to fracture shellfish.
- True of
- sea otters
- First fold
- Fold 4
2 sources
- SeaWorld All About Otters — Physical Characteristics aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
Sea otters utilize a countercurrent vascular heat exchange system in their flippers and limbs. Arteries carrying warm oxygenated blood from the heart run alongside returning veins, transferring heat to cooler venous blood before it can be lost across exposed foot surfaces.
- True of
- sea otters
- First fold
- Fold 4
2 sources
- SeaWorld All About Otters — Adaptations aquarium
- National Library of Medicine Clinical anatomy of the North American river otter (Lontra canadensis) journal, 2000
Sea otters have large reniculate (multilobed) kidneys made of numerous distinct filtering units called reniculi. These specialized organs can produce hyper-concentrated urine that excretes excess sodium and chloride at concentrations matching seawater, allowing otters to drink ocean water without dehydrating.
- True of
- sea otters
- Corrects
- Marine mammals get all their water from food and cannot drink ocean water.
- First fold
- Fold 4
2 sources
- National Library of Medicine Osmoregulation in marine mammals journal, 2001
- Animal Diversity Web Enhydra lutris (sea otter) university
Unlike river otters and most terrestrial members of the mustelid family, sea otters do not have anal scent glands. Living entirely in open marine waters where water currents quickly wash away scent marks, they rely on vocalizations, visual displays, and tactile contact rather than scent trails.
- True of
- sea otters
- Corrects
- All mustelids use musky scent glands to mark territory.
- First fold
- Fold 4
2 sources
- SeaWorld All About Otters — Physical Characteristics aquarium
- Animal Diversity Web Enhydra lutris (sea otter) university
Otters have prominent arrays of thick, stiff whiskers called vibrissae around their snouts. These tactile sensors detect minute water movements and pressure waves caused by swimming fish, crabs, or mollusks, allowing otters to hunt effectively in pitch-black or murky waters.
- True of
- all otters
- First fold
- Fold 1
2 sources
- SeaWorld All About Otters — Senses aquarium
- National Wildlife Federation North American River Otter ngo
Water neutralizes the focusing power of the cornea because the refractive index of water matches corneal tissue. To compensate, otters use powerful ciliary and iris sphincter muscles to squeeze and deform their flexible crystalline lens into a rounded shape underwater, maintaining sharp visual acuity.
- True of
- all otters
- Corrects
- Otters are blind or severely blurry underwater.
- First fold
- Fold 2
2 sources
- SeaWorld All About Otters — Senses aquarium
- National Library of Medicine Adaptations for amphibious vision in sea otters (Enhydra lutris) journal, 2020
Each vibrissa follicle is embedded in a blood sinus capsule richly innervated with sensory nerve endings. In river otters that forage with their mouths, the corresponding somatosensory areas of the brain dedicated to processing facial tactile information are markedly enlarged.
- True of
- all otters
- First fold
- Fold 3
2 sources
- SeaWorld All About Otters — Senses aquarium
- Animal Diversity Web Lontra canadensis (northern river otter) university
Otters have a tapetum lucidum, a mirror-like layer of cells positioned directly behind the retina. Unabsorbed light that passes through photoreceptors bounces off the tapetum and strikes the retina a second time, significantly boosting visual sensitivity during twilight and deep, murky dives.
- True of
- all otters
- First fold
- Fold 3
2 sources
- National Library of Medicine Adaptations for amphibious vision in sea otters (Enhydra lutris) journal, 2020
- SeaWorld All About Otters — Senses aquarium
Unlike most other marine mammals that chase fish, sea otters specialize in hunting slow-moving bottom invertebrates. Their diet includes sea urchins, abalone, rock crabs, Dungeness crabs, clams, mussels, sea snails, and octopuses, which they pry from rocky reefs or excavate from sandy sea bottoms.
- True of
- sea otters
- Must keep
mostly
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- Marine Mammal Commission Southern Sea Otter government
North American river otters are opportunistic predators in freshwater environments. They predominantly catch abundant, slower-swimming fish like minnows, suckers, sculpins, and carp, alongside seasonal abundance of crayfish, freshwater crabs, frogs, salamanders, and large aquatic beetle larvae.
- True of
- North American river otters
- First fold
- Fold 1
2 sources
- National Park Service North American River Otter (Olympic National Park) government
- National Wildlife Federation North American River Otter ngo
While floating on its back, a sea otter places a flat rock on its chest and holds a clam, crab, or abalone with both front paws, pounding the shell against the stone until it shatters. This makes sea otters one of the very few non-human marine mammals known to regularly use stone tools in the wild.
- True of
- sea otters
- Corrects
- River otters also use rocks on their chests to crack open clams.
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Park Service Sea Otters government
Because sea otters have no blubber to keep them warm in freezing ocean water, they maintain a high body temperature through an intensely rapid metabolism. To produce enough heat, an adult sea otter must consume 20 to 25 percent of its entire body weight in seafood each day.
- True of
- sea otters
- Numbers
- 20% to 25% of body weight daily daily food consumption
- First fold
- Fold 3
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- U.S. Geological Survey Sea otter survey encouraging, but comes up short of perfect story government
Genetic and behavioral research across sea otter populations demonstrates that tool use is passed down primarily along maternal lineages. Pups spend their first six months watching their mothers forage, adopting their mother's specific preferred prey types and stone-tool hammering techniques.
- True of
- sea otters
- Must keep
mostly
- First fold
- Fold 4
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- Science The evolutionary antiquity and ecological context of tool use in sea otters journal, 2019
North American river otters are semiaquatic mammals that thrive across diverse freshwater environments. They establish home ranges around rivers, streams, lakes, ponds, and wetland estuaries, requiring permanent water bodies bordered by healthy riparian vegetation and unpolluted shorelines.
- True of
- North American river otters
- First fold
- Fold 1
2 sources
- National Park Service North American River Otter (Olympic National Park) government
- National Wildlife Federation North American River Otter ngo
Sea otters are the most aquatic of all otter species, spending almost their entire lives at sea. They inhabit nearshore marine waters of the North Pacific Ocean, usually remaining within a mile of the coast in shallow rocky reefs and giant kelp forests where they feed, rest, and rear their pups.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- U.S. Fish & Wildlife Service Southern Sea Otter (Enhydra lutris nereis) government
When resting, grooming, or sleeping on their backs at the ocean surface, sea otters wrap long strands of giant kelp fronds around their chests and bodies. Rooted to the sea floor by sturdy holdfasts, the kelp acts as a natural mooring line that keeps rafts of otters from drifting into open seas.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Park Service Sea Otters government
Sea otters are classic keystone predators in nearshore marine ecosystems. They feed heavily on herbivorous purple and red sea urchins that graze on giant kelp. By keeping sea urchin numbers in check, otters prevent the destruction of kelp holdfasts, allowing lush underwater kelp canopies to thrive.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Just Add Otter: How Sea Otters Save Kelp Forests aquarium
- U.S. Geological Survey Dynamic and context-dependent keystone species effects on kelp forests government
Because North American river otters occupy high positions in freshwater food webs, their presence serves as an indicator of watershed health. River otters require clean, unpolluted water with high dissolved oxygen and abundant fish and amphibian prey, vanishing quickly from polluted or degraded waterways.
- True of
- North American river otters
- First fold
- Fold 1
2 sources
- National Park Service Tracking the Return of River Otters government
- U.S. Fish & Wildlife Service River Otter Revival government
River otters shelter in dens known as holts, located along riverbanks, lakeshores, or marshes. Rather than excavating their own tunnels, otters frequently adapt abandoned beaver lodges, muskrat burrows, hollow logs, or undercut tree roots, often featuring underwater entrances for safety from land predators.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Wildlife Federation North American River Otter ngo
- National Park Service Yellowstone River Otters government
In northern regions where rivers freeze, river otters do not hibernate. As stream levels drop beneath winter surface ice, air gaps form that enable otters to swim and breathe while hunting beneath the ice sheet. They also travel across frozen ground by running and sliding on their bellies.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Park Service Yellowstone River Otters government
- National Wildlife Federation North American River Otter ngo
When sea otters are absent, herbivorous sea urchins multiply unchecked and aggressively graze down kelp holdfasts, severing the giant algae from the seafloor. This overgrazing produces barren underwater rockscapes known as urchin barrens, causing severe biodiversity loss for fish and invertebrates.
- True of
- sea otters
- Corrects
- Urchin barrens are natural ocean landscapes rather than damaged ecosystems.
- First fold
- Fold 3
2 sources
- U.S. Geological Survey Dynamic and context-dependent keystone species effects on kelp forests government
- U.S. Fish & Wildlife Service The Keystone Species Role of the Sea Otter government
Although physically capable of diving deeper, sea otters conduct the vast majority of their feeding dives in water depths under 40 meters (130 feet), frequently between 5 and 20 meters. This shallow bathymetric zone allows easy access to benthic invertebrates on rocky reefs and sandy ocean floors.
- True of
- sea otters
- Must keep
usually
- Numbers
- less than 40 meters (130 feet) typical foraging depth
- First fold
- Fold 3
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- Marine Mammal Commission Southern Sea Otter government
Along waterways, river otters establish communal latrine sites on prominent shoreline features like points of land, fallen logs, or rock outcrops. At these sites, otters deposit urine, anal gland secretions, and spraint (musk-scented feces) to share information about identity, sex, and breeding readiness.
- True of
- North American river otters
- First fold
- Fold 3
2 sources
- National Park Service North American River Otter (Olympic National Park) government
- National Wildlife Federation North American River Otter ngo
Along the coasts of Alaska, British Columbia, and Washington, North American river otters regularly swim and hunt marine fish and crabs in ocean bays and tidal estuaries. However, because salt crystals mat their dense underfur and compromise its insulation, they must regularly visit freshwater pools to rinse.
- True of
- North American river otters
- Must keep
can
- Corrects
- Any otter seen swimming in ocean water must be a sea otter.
- First fold
- Fold 3
2 sources
- National Park Service North American River Otter (Olympic National Park) government
- Monterey Bay Aquarium Sea Otter aquarium
Giant kelp grows up to two feet per day, absorbing large amounts of dissolved carbon dioxide through photosynthesis. By preventing urchins from deforesting kelp beds, sea otters indirectly increase coastal carbon storage, allowing healthy kelp forests to draw down millions of metric tons of carbon annually.
- True of
- sea otters
- Numbers
- 4.4 to 8.7 teragrams of carbon stored across range carbon drawdown enhancement
- First fold
- Fold 4
2 sources
- U.S. Geological Survey Dynamic and context-dependent keystone species effects on kelp forests government
- Frontiers in Ecology and the Environment Can trophic cascades affect the storage and flux of atmospheric carbon? An analysis of sea otters and kelp forests journal, 2012
Persistent aquatic pollutants like mercury, PCBs, and organochlorine pesticides concentrate in sediment and small organisms before biomagnifying up the food chain. Because river otters consume large quantities of predatory fish and crayfish, these contaminants accumulate in their livers and kidneys.
- True of
- North American river otters
- First fold
- Fold 4
2 sources
- U.S. Geological Survey Mercury and halogenated hydrocarbons in river otters government
- National Park Service Tracking the Return of River Otters government
In Elkhorn Slough, striped shore crabs burrow into muddy tidal banks and consume the root networks of marsh-stabilizing pickleweed. By eating large volumes of these burrowing crabs, sea otters dramatically curb crab populations, reducing creek bank erosion by 69 to 90 percent and helping marshes survive.
- True of
- southern sea otters
- Must keep
California
- Numbers
- 69% to 90% reduction in creek bank erosion bank erosion reduction
- First fold
- Fold 4
2 sources
- Nature Top-predator recovery re-establishes counter-stressor cascades that mitigate salt-marsh erosion journal, 2024
- National Wildlife Federation Otters Against Erosion ngo, 2025
By consuming aquatic prey like fish, crabs, and crayfish in rivers and repeatedly depositing nutrient-rich spraint on dry land, river otters transport marine and freshwater nitrogen and phosphorus into riparian zones, fertilizing shoreline trees, grasses, and shrubs along riverbanks.
- True of
- North American river otters
- First fold
- Fold 4
2 sources
- National Park Service Tracking the Return of River Otters government
- Frontiers in Ecology and Evolution River otters as nutrient vectors linking aquatic and terrestrial ecosystems journal
Otters close their muscular ears and nostrils tightly when they plunge underwater to hunt. A sea otter typically dives for one to two minutes to harvest urchins, crabs, and clams from coastal kelp forests.
- True of
- all otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Park Service Yellowstone River Otters government
Sea otters float on their backs at the water's surface to rest and sleep. To prevent ocean swells, wind, and tidal currents from drifting them out into the open sea, they loosely drape fronds of giant kelp or bull kelp across their bodies like a living anchor.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Giant Kelp aquarium
- National Park Service Sea Otter (Enhydra lutris), Channel Islands National Park government
The sea otter is one of the few mammals known to use stone tools. While floating comfortably on its back, an otter places a flat rock on its chest as an anvil and hammers hard-shelled prey—such as clams, mussels, crabs, or sea urchins—against the stone until the shell breaks apart.
- True of
- sea otters
- Corrects
- Only sea otters use rocks as chest anvils; river otters do not balance rocks on their chests to open prey.
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Park Service Sea Otter (Enhydra lutris) government
Underneath each foreleg, a sea otter has an expansive pouch formed by loose, stretchy skin in the armpit area. This built-in pocket allows the otter to stash freshly harvested shellfish or a favorite anvil stone during a dive, leaving its sensitive front paws unencumbered while swimming.
- True of
- sea otters
- Corrects
- The armpit pouch is a loose flap of external skin, not an internal marsupial pouch.
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Park Service Sea Otter (Enhydra lutris) government
When sea otters finish foraging and need to rest, groom, or sleep, they gather on the ocean surface in social groups known as rafts. Rafts can range from just two or three individuals to gatherings of dozens or even hundreds of otters floating side by side in calm kelp canopies.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- U.S. Geological Survey A Raft of Sea Otters, Alaska government
- NOAA Office of National Marine Sanctuaries Sea Otter: A Floating Keystone government
Newborn sea otter pups possess an extremely dense, woolly coat known as natal fur that traps a thick volume of air bubbles. This trapped air provides such intense buoyancy that the pup floats high on the water like a cork, making it physically incapable of diving below the surface.
- True of
- sea otters
- Corrects
- Sea otter pups cannot dive at birth because their buoyant natal fur prevents them from submerging.
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter Program: Rescuing and Releasing Surrogates and Pups aquarium
- NOAA Office of National Marine Sanctuaries Sea Otter: A Floating Keystone government
While floating on her back at the ocean surface, a sea otter mother rests her newborn pup directly on her chest and belly. This cradling position keeps the pup warm, dry, and protected from frigid water temperatures and rolling ocean waves during its first weeks of life.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Park Service Sea Otter (Enhydra lutris) government
An otter's dense underfur stays warm by maintaining a trapped pocket of air next to the skin. During intensive grooming sessions, sea otters roll in the water, rub their coats with their paws, and deliberately blow warm air deep into their fur to replenish this vital insulating air bubble layer.
- True of
- sea otters
- First fold
- Fold 1
2 sources
- Monterey Bay Aquarium Life of a Rescued Sea Otter aquarium
- SeaWorld All About Sea Otters — Behavior aquarium
North American river otters fold their forelegs against their sides and slide belly-down down muddy banks, grassy slopes, and steep snowdrifts into the water. This tobogganing behavior serves as both an energy-efficient travel technique and an engaging form of social play.
- True of
- North American river otters
- First fold
- Fold 1
2 sources
- National Park Service Yellowstone River Otters government
- National Wildlife Federation North American River Otter ngo
Individual sea otters often develop strong preferences for particular stones that offer the ideal weight, flatness, and texture for cracking hard shells. An otter will retain its favored stone across multiple dives and keep it tucked under its arm between uses rather than hunting for a new rock on every trip.
- True of
- sea otters
- Must keep
some
- First fold
- Fold 2
2 sources
- Georgia Aquarium Southern Sea Otter aquarium
- National Park Service Sea Otter (Enhydra lutris) government
While floating on their backs in rafts, sea otters occasionally link their front paws together. This behavior is a practical anchoring mechanism that keeps resting pairs or mothers and pups from drifting away in open water currents when kelp fronds are sparse.
- True of
- sea otters
- Must keep
sometimes
- Corrects
- Hand-holding is a practical anti-drift behavior, not a romantic gesture or continuous habit.
- First fold
- Fold 2
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- NOAA Office of National Marine Sanctuaries Sea Otter: A Floating Keystone government
Because a young sea otter pup cannot dive or swim against ocean currents, the mother wraps long strands of giant kelp around the floating pup before submerging to hunt. The kelp anchors the buoyant pup in place, keeping it safe from drifting away while the mother forages on the seafloor.
- True of
- sea otters
- First fold
- Fold 2
2 sources
- Monterey Bay Aquarium Giant Kelp aquarium
- National Park Service Sea Otter (Enhydra lutris), Channel Islands National Park government
Lacking an insulating layer of blubber, sea otters rely on a soaring metabolic rate to generate internal body heat and maintain a core temperature of around 100 degrees Fahrenheit. To power this furnace, an adult sea otter must consume roughly 20 to 30 percent of its body weight in seafood each day.
- True of
- sea otters
- Must keep
about
- Numbers
- 20–30% of body weight daily daily food consumptionrequired to maintain high metabolic rate
- approx. 100°F (37.8°C) core body temperaturemaintained through metabolic heat production
- First fold
- Fold 2
2 sources
- Monterey Bay Aquarium Threats Facing Sea Otters aquarium
- U.S. Geological Survey Slowly Swimming Towards Recovery: California's Sea Otter Numbers Holding Steady government
The primary social unit in North American river otters is a maternal family group consisting of an adult female and her offspring of the year. The mother constructs the natal den, nurses the pups, teaches them swimming and foraging techniques, and protects them from predators until they mature.
- True of
- North American river otters
- Corrects
- River otters do not live in nuclear packs with both parents; fathers do not help raise the litter.
- First fold
- Fold 2
2 sources
- National Park Service North American River Otter (Big Thicket National Preserve) government
- Smithsonian's National Zoo North American River Otter zoo
Following the breeding season, adult male North American river otters disperse and take no part in denning, provisioning, or rearing offspring. Males either wander alone across large home ranges or join loose bachelor groups that fish and travel together outside the nursery areas.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Park Service North American River Otter (Big Thicket National Preserve) government
- Animal Diversity Web Lontra canadensis (northern river otter) university
Although adult river otters are masters of aquatic locomotion, young pups are naturally reluctant to enter deep water. Around two months of age, the mother nudges, coaxes, or carries her reluctant pups into the water by the scruff of the neck, supporting them until they learn the swimming stroke.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- SeaWorld All About Otters — Care of Young aquarium
- Smithsonian's National Zoo North American River Otter zoo
North American river otters establish communal latrine sites on prominent riverbanks, point bars, and fallen logs. By depositing spraint (scat), urine, and pungent musk from paired anal glands, otters advertise territory ownership, social status, and reproductive readiness without direct fighting.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Park Service Yellowstone River Otters government
- Animal Diversity Web Lontra canadensis (northern river otter) university
Play behavior among river otters—such as mock wrestling, water tag, and tail chasing—serves a serious biological purpose. These dynamic social games hone agility, strengthen the muscular core, and develop the fast neuromuscular reflexes needed to capture evasive prey underwater.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Wildlife Federation North American River Otter ngo
- Animal Diversity Web Lontra canadensis (northern river otter) university
Because nursing and carrying a buoyant pup requires tremendous maternal investment, female sea otters normally give birth to only one pup per breeding cycle. While twin births occur in roughly two percent of pregnancies, a mother can rarely nourish or transport two pups, and only one survives.
- True of
- sea otters
- Must keep
almost always
- Numbers
- 1 pup typical litter sizesingle pup per reproductive cycle; twins rare
- First fold
- Fold 2
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- U.S. Fish & Wildlife Service Sea Otter (Enhydra lutris) government
River otters are known for solo object play, frequently balancing small stones on their paws, juggling pebbles across their forelimbs, and dropping shells to retrieve them from riverbeds. This object manipulation refines tactical dexterity in their sensitive paw pads.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Wildlife Federation North American River Otter ngo
- SeaWorld All About Otters — Behavior aquarium
When resting at the surface, sea otters fold their forepaws and hind flippers over their chests or faces to keep them dry. Because their paw pads and flipper soles have less hair than the rest of their body, elevating them above the cold surface prevents rapid heat loss.
- True of
- sea otters
- First fold
- Fold 2
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- National Park Service Sea Otter (Enhydra lutris) government
Although superbly adapted for swimming, North American river otters spend much of their time on dry land. They travel miles overland across ridges between watersheds, run across frozen river ice, and rest in dry shoreline shelters called couches.
- True of
- North American river otters
- First fold
- Fold 2
2 sources
- National Park Service Yellowstone River Otters government
- Alaska Department of Fish and Game River Otter Species Profile government
When opening stubborn or thick-shelled prey like abalones and mussels, sea otters hammer the item against their chest anvil with rapid percussive blows. Field researchers have clocked sea otters striking shells against rocks at rates of up to 45 blows in 15 seconds, equivalent to three blows per second.
- True of
- sea otters
- Must keep
up to
- Numbers
- up to 3 strikes per second strike frequencyrecorded at 45 blows in 15 seconds during foraging
- First fold
- Fold 3
2 sources
- Journal of Mammalogy Tool-Using Behavior of the California Sea Otter journal, 1964
- National Park Service Sea Otter (Enhydra lutris) government
Wild sea otter rafts are typically divided by gender. Females with nursing pups congregate in maternal resting rafts in protected coastal kelp forests, while adult and subadult males assemble in separate bachelor rafts along the outer boundaries of the population's territory.
- True of
- sea otters
- Must keep
usually
- First fold
- Fold 3
2 sources
- U.S. Geological Survey Sexual Segregation in Sea Otters and Its Role in Range Expansion government, 1993
- Animal Diversity Web Enhydra lutris (sea otter) university
Sea otter pups depend entirely on their mothers for six to eight months. The mother supplies rich milk, grooms the pup's dense fur constantly, and demonstrates how to crack shells until the pup develops adult diving capabilities and independent foraging skills.
- True of
- sea otters
- Numbers
- 6–8 months maternal dependency periodduration of nursing and foraging instruction
- First fold
- Fold 3
2 sources
- Monterey Bay Aquarium Sea Otter Program: Rescuing and Releasing Surrogates and Pups aquarium
- U.S. Geological Survey A Sea Otter Mom Nursing Her Pup government
Grooming is an essential survival activity for sea otters rather than casual hygiene. Because dirty or matted fur allows cold seawater to breach the air layer and touch the skin, sea otters spend between three and five hours every day combing, squeezing, and cleaning every section of their pelt.
- True of
- sea otters
- Numbers
- 3–5 hours daily daily grooming timetime dedicated to fur maintenance
- First fold
- Fold 3
2 sources
- Monterey Bay Aquarium Life of a Rescued Sea Otter aquarium
- National Park Service Sea Otter (Enhydra lutris) government
While sea otters prefer shallow nearshore kelp beds, they frequently dive to depths between 30 and 100 feet (9 to 30 meters) to harvest invertebrates from the benthic ocean floor. Submergence times usually span one to two minutes, though dives exceeding four minutes have been recorded.
- True of
- sea otters
- Must keep
usually
- Numbers
- 30–100 feet (9–30 m) typical foraging depthdepth range for harvesting benthic invertebrates
- 1–2 minutes typical dive durationduration of normal foraging dives
- First fold
- Fold 3
2 sources
- U.S. Geological Survey Foraging Depths of Sea Otters and Implications for Coastal Marine Communities government, 2007
- National Park Service Sea Otter (Enhydra lutris) government
River otters possess remarkable physiological diving adaptations, including a slowed heart rate and rich oxygen storage in their blood and muscles. While ordinary hunting dives last only one to two minutes, a river otter can remain submerged for up to eight minutes when evading danger.
- True of
- North American river otters
- Must keep
up to
- Numbers
- up to 8 minutes maximum dive durationmaximum breath-hold duration under water
- First fold
- Fold 3
2 sources
- Smithsonian's National Zoo North American River Otter zoo
- National Wildlife Federation North American River Otter ngo
Female river otters do not dig their own burrows from scratch; instead, they commandeer abandoned beaver lodges, muskrat burrows, hollow root systems, or undercut riverbanks. These dens usually feature underwater entrance tunnels that safeguard vulnerable pups from land predators.
- True of
- North American river otters
- First fold
- Fold 3
2 sources
- National Park Service North American River Otter (Big Thicket National Preserve) government
- Smithsonian's National Zoo North American River Otter zoo
Unlike river otters that give birth on dry land in concealed burrows, California sea otters deliver their young directly in the water. The mother scoops the newborn onto her chest immediately upon birth, licking and blowing warm air into its fur to fluff and dry its buoyant natal coat.
- True of
- southern sea otters
- Must keep
most
- First fold
- Fold 3
2 sources
- Monterey Bay Aquarium Sea Otter aquarium
- SeaWorld All About Sea Otters — Reproduction aquarium
Female North American river otters typically produce a litter of one to three pups in early spring, though litters of up to four or five occasionally occur. Newborn pups are born blind and helpless inside the natal den, opening their eyes after about a month of constant maternal care.
- True of
- North American river otters
- Must keep
usually
- Numbers
- 1–3 pups (rarely up to 5) typical litter sizeaverage litter born in spring
- First fold
- Fold 3
2 sources
- National Park Service North American River Otter (Big Thicket National Preserve) government
- National Wildlife Federation North American River Otter ngo
Both sea otters and river otters utilize embryonic diapause, or delayed implantation. Following fertilization, the developing blastocyst temporarily halts growth and floats free in the uterus for several months, ensuring that pups are born during spring or summer when conditions are optimal.
- True of
- all otters
- First fold
- Fold 4
2 sources
- SeaWorld All About Sea Otters — Reproduction aquarium
- Animal Diversity Web Lontra canadensis (northern river otter) university
In addition to balancing loose rocks on their chests, some sea otters transport tough mussels to intertidal boulders along the shore. By repeatedly smashing shells against the same rock formation over years, otters create distinct wear marks and shell middens studied by archaeologists.
- True of
- sea otters
- Must keep
some
- First fold
- Fold 4
2 sources
- National Park Service Sea Otter (Enhydra lutris) government
- U.S. Geological Survey Sea Otter Tool Use Data Catalog government, 2019
Within a single kelp forest ecosystem, individual sea otters develop specialized dietary preferences passed down from their mothers. One otter may specialize in hard-shelled crabs and clams, while another targets sea urchins or abalone, reducing direct foraging competition among neighbors.
- True of
- sea otters
- First fold
- Fold 4
2 sources
- Monterey Bay Aquarium Threats Facing Sea Otters aquarium
- U.S. Geological Survey Slowly Swimming Towards Recovery: California's Sea Otter Numbers Holding Steady government
All otters belong to the subfamily Lutrinae within the mustelid family. Thirteen living otter species are traditionally recognized by the IUCN SSC Otter Specialist Group, native to freshwater and marine habitats across North America, South America, Eurasia, and Africa, but naturally absent from Australia and Antarctica.
- True of
- all otters
- Must keep
recognized
- Numbers
- 13 species across 5 continents species count
- First fold
- Fold 2
2 sources
- IUCN SSC Otter Specialist Group Otter Species of the World ngo
- San Diego Zoo Wildlife Alliance Otter zoo
The giant otter (Pteronura brasiliensis) of the Amazon, Orinoco, and Pantanal basins is the longest member of the weasel family, reaching nearly six feet (1.8 meters) in length. Unlike solitary otter species, giant otters live in tight-knit, highly vocal family packs of up to a dozen individuals that cooperatively hunt fish.
- True of
- giant otters
- Must keep
up to
- Numbers
- up to 6 feet (1.8 meters) maximum length
- First fold
- Fold 3
2 sources
- San Diego Zoo Wildlife Alliance Giant Otter zoo
- World Wildlife Fund Giant Otter ngo
Weighing only 4 to 11 pounds, the Asian small-clawed otter (Aonyx cinereus) is the world's smallest otter. Native to wetlands and mangrove swamps across South and Southeast Asia, it possesses reduced, blunt claws that do not extend past its toe pads and incomplete webbing, providing finger dexterity to probe mud for crabs.
- True of
- Asian small-clawed otters
- Numbers
- 4 to 11 pounds (2 to 5 kg) body weight
- First fold
- Fold 3
2 sources
- Smithsonian's National Zoo Asian Small-Clawed Otter zoo
- San Diego Zoo Wildlife Alliance Asian Small-Clawed Otter zoo
Taxonomists recognize three living subspecies of sea otters: the southern sea otter (Enhydra lutris nereis) along the California coast; the northern sea otter (Enhydra lutris kenyoni) from Washington State through Alaska; and the Russian sea otter (Enhydra lutris lutris) across the Kuril and Commander Islands.
- True of
- sea otters
- Must keep
recognized
- Numbers
- 3 geographic subspecies subspecies count
- First fold
- Fold 4
2 sources
- U.S. Fish & Wildlife Service Sea Otter History and Distribution government
- Marine Mammal Commission Southern Sea Otter government
According to the IUCN SSC Otter Specialist Group, 12 of the world's 13 otter species are in decline, and five are classified as Endangered or Vulnerable on the IUCN Red List. Across Asia, Africa, and South America, otters face escalating threats from habitat fragmentation, dams, agricultural runoff, and poaching.
- True of
- all otters
- Must keep
most
- Numbers
- 12 of 13 otter species in decline species in decline
- Corrects
- Otters are thriving globally and no longer need conservation protection.
- First fold
- Fold 4
2 sources
- IUCN SSC Otter Specialist Group Otter Species of the World ngo
- World Wildlife Fund Giant Otter ngo
Despite their whiskers and aquatic lifestyles, otters are not rodents. They are carnivorous mammals within the family Mustelidae. Their closest evolutionary cousins on land include weasels, badgers, wolverines, ferrets, and minks, with whom they share paired anal scent glands and flexible, elongated bodies.
- True of
- all otters
- Corrects
- Otters are related to beavers and muskrats.
- First fold
- Fold 1
2 sources
- Smithsonian's National Zoo North American River Otter zoo
- San Diego Zoo Wildlife Alliance Otter zoo
During the 18th and 19th centuries, international commercial fur traders relentlessly hunted sea otters across the North Pacific Rim for their luxurious pelts. An estimated pre-trade population of 150,000 to 300,000 otters was reduced to fewer than 2,000 individuals by 1911, surviving in roughly 13 isolated colonies.
- True of
- sea otters
- Must keep
fewer than
- Numbers
- ~300,000 down to fewer than 2,000 by 1911 population collapse
- First fold
- Fold 3
2 sources
- U.S. Fish & Wildlife Service Sea Otter History and Distribution government
- Marine Mammal Commission Southern Sea Otter government
After 20th-century pollution and unregulated trapping extirpated North American river otters from 11 states and reduced them in dozens more, state agencies launched restoration programs starting in 1976. Over 4,000 otters were translocated across more than 20 states, restoring self-sustaining breeding populations.
- True of
- North American river otters
- Must keep
more than
- Numbers
- over 4,000 otters released across 22+ states translocations
- First fold
- Fold 3
2 sources
- Association of Fish & Wildlife Agencies River Otter Management and Conservation ngo
- U.S. Fish & Wildlife Service River Otter Revival government
Networks of coastal Marine Protected Areas (MPAs) restrict commercial fishing gear, protect kelp forest canopies, and prevent disturbance. In protected reserves along central California, sea otters benefit from higher densities of crabs, urchins, and abalone, fostering stable and resilient coastal populations.
- True of
- sea otters
- First fold
- Fold 3
2 sources
- NOAA Office of National Marine Sanctuaries Sea Otter: A Floating Keystone government
- Monterey Bay Aquarium Sea Otter aquarium
In 1911, the United States, Great Britain (for Canada), Japan, and Russia signed the International Fur Seal Treaty, banning commercial pelagic hunting of sea otters. In 1972, the U.S. Marine Mammal Protection Act established comprehensive federal safeguards, enabling surviving colonies to gradually rebound.
- True of
- sea otters
- First fold
- Fold 4
2 sources
- U.S. Fish & Wildlife Service Sea Otter History and Distribution government
- Marine Mammal Commission Southern Sea Otter government
When the Exxon Valdez tanker spilled 11 million gallons of crude oil into Prince William Sound in March 1989, an estimated 2,800 sea otters died in the acute aftermath. Because sea otters have no blubber, oil soaked their underfur and destroyed its trapped air bubbles, leading to fatal hypothermia within hours.
- True of
- northern sea otters in Prince William Sound
- Must keep
Prince William Sound
- Numbers
- estimated ~2,800 sea otters killed acute mortality estimate
- Corrects
- Oil spills only harm otters if they swallow the oil.
- First fold
- Fold 4
2 sources
- U.S. Geological Survey 25 Years After Exxon Valdez, Sea Otter Population at Pre-Spill Levels government
- NOAA Office of Response and Restoration Oil Spill's Silver Lining: Over Three Decades After Exxon Valdez government
The parasite Toxoplasma gondii produces infectious oocysts shed in cat feces. Rain runoff flushes cat litter and feces down storm drains and rivers into coastal waters, where kelp-grazing snails and filter-feeding bivalves concentrate the parasite. When eaten by sea otters, it causes fatal brain inflammation.
- True of
- southern sea otters
- First fold
- Fold 4
2 sources
- U.S. Geological Survey Toxoplasmosis in Wildlife government
- Centers for Disease Control and Prevention Poisoned Sea Otters and One Health government
Great white sharks do not eat sea otters because otters lack insulating blubber. However, white sharks investigating surface silhouettes frequently take exploratory bites of floating otters. These non-consumptive bite injuries cause fatal trauma, creating a mortality barrier at the northern and southern edges of their range.
- True of
- southern sea otters
- Must keep
California
- First fold
- Fold 4
2 sources
- U.S. Geological Survey Sea otter survey encouraging, but comes up short of perfect story government
- Marine Mammal Commission Southern Sea Otter government
Microcystin is a powerful liver toxin produced by freshwater blue-green algae blooms in nutrient-polluted inland lakes. Rivers carry the toxin into ocean bays like Monterey Bay, where marine mussels and clams absorb it. Sea otters eating the tainted shellfish suffer severe, acute liver failure.
- True of
- southern sea otters
- First fold
- Fold 5
2 sources
- Centers for Disease Control and Prevention Poisoned Sea Otters and One Health government
- U.S. Geological Survey Sea Otter Diseases and Toxins government
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