The Amazing Buoyancy of Sea Otters

 

While watching the Netflix documentary series Our Planet, I was struck by a scene that involved numerous sea otters floating in a group deep out into the ocean.  I wondered if there was something special about sea otters that might make it easy for them to float on top of the water.  Indeed, sea otters are incredibly buoyant animals thanks to several useful adaptations.  Though they are the smallest of all marine mammals, sea otters have the densest fur of any animal on Planet Earth.  Though increasing density would reduce their ability to float, their fur helps to trap a layer of air between the fur and the skin, which the otters maintain by blowing air into their fur.  This large volume of trapped air increases buoyancy and provides insulation for the otters, helping to keep them warm in the Northern Pacific Ocean, whose temperature can approach freezing.   Sea otters are also incredibly buoyant thanks to their improved lung capacity, which is over twice that of other animals their size.  In fact, they are so buoyant that they often struggle to dive down into the water, and they will often grab a rock to weigh them down during their dives.

Using the relationship between pressure and volume given when the otter is floating, it is possible to determine how much of the otter is submerged under water, and thus how buoyant it is.  By modeling the sea otter as a cylinder and using its average mass (29 kg for males), height (1.22 m for males), and skull width (0.1 m), its density can be calculated:

This density is less than that of seawater (~1025 kg/m³), allowing the otter to float; it is also less than that of the average human body (1010 kg/m³).  From these values, we can calculate what fraction of the otter is submerged when floating:

So, about 74% of the otter is submerged when floating.  We can compare this value to the fraction of the average human body that is submerged when floating:

In comparison, for the average human, about 99% of the body is submerged when floating!

Sea otters are incredibly buoyant animals, and their amazing physical properties can help us to learn more about the mechanisms behind buoyancy and about our own bodies.

References

1. https://oceantoday.noaa.gov/seaotteranatomy/#:~:text=The%20sea%20otter%20has%20a,up%20to%20five%20minutes%20underwater.

2. https://www.kqed.org/science/25908/the-fantastic-fur-of-sea-otters

3. https://seaworld.org/animals/all-about/otters/characteristics/#:~:text=The%20smallest%20river%20otter%20is,49%2D57%20lbs.)

4. https://ecos.fws.gov/ServCat/DownloadFile/106601

5. https://www.aqua-calc.com/page/density-table/substance/human-blank-body