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P. hydrobothynus is a sexually dimorphic species of otter. The male otters have very large and colorful mane made of orange, yellow, and red and a dark brown body with a white underside. They have a short stocky body, a short fat tail, and a much more robust pectoral girdle. Males are usually only 1 meter long, and 0.35 meters tall. The female otters are dark brown in color with a white underside and are camouflaged in the murky water and muddy banks. The female stellar river otters are built like a traditional river otter and are very streamline with a long wing-like tail, and a longer skinnier body. Females tend to be roughly 1.2 meters long, and are 0.2 m tall. The male and female both have very thick fur to stay dry and for insulation. The two sexes are so morphologically different they were first thought to be different species; however, a distinct red diamond pattern on the chest of this species was the first clue that the discoverers had in determining they were the same species. 

Like most species of otters when the pride is together they spend most of their time playing and frolicking in the water. This species is incredibly rambunctious and has been known to mess with boats traveling through the territory. There is one case of a few females boarding a small canoe and stealing the belongings of the person on board.

The females will typically be pregnant for 65-80 days. The young are born fully covered in fur and are able to open their eyes after just a week. The pups are able to walk around and swim just after they are able to open their eyes. This rapid development is supported by the constant support and protection of the pup by the pride. When they are young male and female pups look morphologically similar. It isn’t until they reach eight months old that the males begin to develop secondary sex characteristics. Their pectoral girdle starts to get larger and a small brown mane begins to form. The young males will begin to forage with the dominant male when they are a year old and are first taught to hunt for insects. It isn’t until they have been foraging for a month that the dominant males will allow the young males to begin hunting for A. galactonotus. After a few months of eating the frogs the color of the young males’ mane changes from dark brown to hues of bright red, orange and yellow. It has been observed that the primary color of frog that the males eat will determine the proportions of red, orange, and yellow in their mane. The amount of frogs that the male eats also determines how bright his mane will be. Males that are not able to eat many frogs may have a dull colored mane, or it may not be colored at all. The males that are the best at foraging and have the best territory will have the biggest and brightest manes. The female’s preference for the amount of each color they like in the male's mane has yet to be explored, however, it has been observed that they do prefer males with very large manes. 

Unlike the males, the female stellar river otters are much more docile and spend most of their days working together hunting and taking care of the young. They only rely on the male to construct and maintain the dens and to defend the territory. The females of this species have a very different social structure than the males of this species. Females of neighboring territories are capable of coexisting and leaving one another alone. This allows there to be many rafts of females throughout a single river system and makes it easy for young females to easily find a new raft if they so desire. 

Females will typically be won by males through battles over territory, however, males have also been seen to court young females by dancing for them in the water. They dance by doing flips, and spinning through the water and flashing their mane up and down above the water surface. There is also a behavior that males do that researchers have called the “wet noodle”. It involves the males sticking half of their body out of the water and shaking the upper half of their body back and forth making them look like a flailing noodle. Some males with very bright manes may be pretty enough to catch the attention of a female in the river system. If he can impress her with a brief dance in the water showing off his vibrant mane she may follow him back to his den. If the males den is high quality, then the female will stay with the male and he may be able to start his very own pride. 

Males have an intricate way of fighting one another. First, the males will assess each other’s fitness. The male with the biggest brightest mane and largest canines may be able to intimidate the other male and may not have to fight at all. However, when intimidation doesn’t work males use their large powerful forelimbs, and long canines to inflict devastating blows to their opponents.

The stellar river otter is an incredibly territorial and aggressive animal. The males will monitor their territory daily while foraging for frogs, and fights between neighboring, and young males happen frequently. When a young male is able to defeat a dominant male who is in charge of a pride he is able to take over the male’s pride. Many young males go around challenging other males but due to their lack of experience and size, they are very easily beaten. The male stellar river otter maybe six to eight years old before he is able to finally get a female of his own, but then he must spend the rest of his days defending his territory from rival males. The stellar river otter may live to be 20 years old and will spend its time defending its territory to the very end.

P. hydrobothynus lives in large social groups of three to twelve members. A group of otters is called a pride when they are on land, and a raft when they are in the water. There are never more than two males per pride of stellar river otters, and these males are always related. A male may have a number of females ranging from one to six. In a pride of otters, there is a dominant male who rules over the entire pride. If the second male is lucky he may have the opportunity to have a female of his own however this usually is not the case. Female stellar river otters may have one to three pups a year. The male will only have one pregnant female at a time and the entire group works together to raise the pups. This prevents overcrowding in the burrows and it gives the pups more attention and resources due to a group effort to raise them. Pups stay with the pride until they are two years old. Young males are kicked out of the pride by their father, and young females will either leave on their own or be taken by another prides male. 

The kidneys of the stellar river otter look very similar to the American river otter Lutra Canadensis. This is a multi-lobed kidney which is commonly found in many marine and aquatic mammals. The kidneys are multi-lobed and are fully equipped to remove toxins however they are not specialized for water conservation. Because the stellar river otter lives in a freshwater environment it does not need kidneys adapted for the conservation of water. Water is extremely prevalent in the habitat of the river otter and it has little need to conserve it when there is a river just outside its back door. The river otter also has a large multi-lobed liver, which supports the idea that its body is more adapted to purify toxins than it is to conserve water.

When looking at the male stellar river otter it possesses the morphology of a powerful predator. The two long pairs of canines and huge forelimbs make this mammal an intimidating predator. The mane of the stellar river otter can be erected and lowered to accommodate for the situation. When swimming the mane is lowered to make their body more streamline. However, there is evidence suggesting that the mane may be used when P. hydrobyothynus is turning and uses the mane like a sail. When walking around on land P. hydrobothynus keeps its mane erect at all times. P. hydrobothynus is able to erect and lower its mane by contracting and relaxing its arrectorus pilorum muscles. The arrectorus pilorum is the muscle responsible for causing goose bumps in humans. One of the other most distinguishing factors that the male’s skeleton has is its large olecranon process, while this does aid in swimming it is also very common in mammals that specialize in fossorial locomotion.