Sea Otters are marine mammals too

This month we crack into the life of sea otters as our “Whale of the Month.” Though they have a charming disposition, looks are deceiving for these feisty creatures!

Sea Otter vs River Otters

Before we get started, what is the difference between sea otters and river otters? Well, let’s start with size: sea otters are MUCH larger than river otters. Sea Otters can range from 60-100lbs depending on location! River otters, on the other hand, only reach about 10-30lbs. Their swimming behavior is a bit different too. River otters have four webbed feet and can be found in salt and fresh water swimming belly down. They spend time both on land and in the water. Sea otters, on the other hand, utilize their hind webbed feet and tail for swimming through the water (like a paddle) and rarely go on land. They are iconic for floating on their backs and holding hands while they sleep to stay close to their families.

The most significant difference is their fur. While they both use their fur coats to trap air to create an insulating layer, the make-up of their hair is vastly different. River otters have two layers of short, coarse fur. One layer is responsible for keeping warm, while the other is waterproof, to keep the animal dry. In contrast, sea otters have the densest fur in the entire animal world. They have up to a million hairs covering each square inch of their body! This is necessary to create a very flawless insulation layer to protect them from the cold water, which can reach freezing temperatures! That’s why you can see them grooming themselves a lot, to keep the air pockets intact. You can read more about how they and other marine mammals stay warm in our last post.

Cute but aggressive!

With such a cute disposition, one would think they are relatively docile. Beware though: looks are deceiving! These little otters are very aggressive when it comes to mating. Male sea otters will not hesitate to bite down on the female’s nose while mating underwater, causing deep cuts and pieces of flesh to be torn off! Once copulation has ended, the male releases the female. The mating ritual can be so violent it may result in the female’s death from drowning or the severe wounds inflicted. Sea otters will also attack and sexually harass other species, such as baby harbor seals. Finally, males have been seen to “kidnap” sea otter pups and hold them underwater to get their mother’s food.

Work smarter not harder!

Sea otters eat hard-shelled invertebrates such as sea urchins and various clams, mussels, and crabs. Their paws and teeth alone could not crack the thick shells. To overcome this hurdle, sea otters will dive down and collect large rocks or other hard objects. When back at the surface, they will float on their backs, place the stone on their bellies, and smash their shelled meal. Using this technique, they eat about 25% of their body weight in sea urchins and other hard-shelled invertebrates everyday.

Maintaining the ecosystem’s balance

Sea otters are a keystone species. A keystone species is heavily relied on by other species and the ecosystem, so much so that if it were removed, the ecosystem would change drastically. As sea urchins are a large part of the sea otter diet, they maintain the status of the sea urchin population and the health and abundance of kelp forests that sea urchins graze on. It is essential, because kelp forest are home to dozen species of fish and invertebrates that rely of them to grow and stay protected from predators.

sea otters are a keystone species
The sea otter is a keystone species. Credit: Anaïs Remili

With climate change, more apex predators, such as orcas and great white sharks, are becoming more prevalent in the otters’ habitats, which can affect/reduce sea otter populations. It could result in an unbalanced ecosystem. So let’s protect sea otters, and make sure the ecosystems stay healthy!


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Naomi Mathew is a PhD student at University of Louisiana at Lafayette. She works on bioacoustics in marine mammals from the Gulf of Mexico. She is the co-founder of Whale Scientists. You can read more about her here

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