# What's the smallest marine mammal? **URL:** \ **Category:** Factual Questions\ **Created:** [August 19, 2009, 5:00pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967 "2009-08-19T17:00:34Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Never\_Say\_Dice](https://avatars.discourse-cdn.com/v4/letter/n/ee59a6/32.png) [@Never\_Say\_Dice](https://boards.straightdope.com/u/Never_Say_Dice)\ **Post date:** [August 19, 2009, 5:00pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/1 "2009-08-19T17:00:34Z")
What’s the smallest marine mammal? ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [August 19, 2009, 5:08pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/2 "2009-08-19T17:08:20Z")
It is probably either the Commersons dolphin or the sea otter depending on your definition. Sea Otters don’t spend 100% of their time in the water. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 19, 2009, 5:44pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/3 "2009-08-19T17:44:16Z")
How large are the pinnipeds? I know that adult seals and sea lions are significantly larger than one might think, but are any of them down in the sea otter range? ---
**Author:** ![jayjay](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jayjay/32/6765_2.png) [@jayjay](https://boards.straightdope.com/u/jayjay)\ **Post date:** [August 19, 2009, 6:31pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/4 "2009-08-19T18:31:55Z")
Hmm…sea otters are bigger than I thought. Males can weigh 49 to 99 lbs. and be 4 to 5 feet long. The smallest pinniped is the Galapogos fur seal, which weighs about 65 pounds and is about 4 feet long as an adult. The vaquita, an endangered cetacean that lives in the Sea of Cortez, is about 5 feet long and weighs about 120 lbs., according to the World Wildlife Federation. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 19, 2009, 9:29pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/5 "2009-08-19T21:29:43Z")
You could even define the smaller river otters to be marine mammals. ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [August 19, 2009, 9:36pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/6 "2009-08-19T21:36:26Z")
Riber otters? I always thought that “marine” meant relating to the ocean or the sea. In my mind, that would exclude all fresh-water mammals. Speaking off the cuff, I would consider “marine mammals” to encompass cetaceans and maybe pinnipeds. The smallest cetacean and smallest pinniped are mentioned in jayjay’s post. ---
**Author:** ![Captain\_Amazing](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@Captain\_Amazing](https://boards.straightdope.com/u/Captain_Amazing)\ **Post date:** [August 19, 2009, 10:28pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/7 "2009-08-19T22:28:54Z")
The smallest cetacean would be, like Shagnasty said, the Commersons dolphin, which is smaller than the vaquita, with a length of between 3.6-5.6 feet, and a weight between 75-130 pounds. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [August 20, 2009, 1:28pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/8 "2009-08-20T13:28:14Z")
If river critters count, do muskrats count? So how come there aren’t ocean mice? Sea shrews? ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [August 20, 2009, 4:14pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/9 "2009-08-20T16:14:22Z")
> [@Sailboat](#): > > If river critters count, do muskrats count? > > So how come there aren’t ocean mice? Sea shrews? Probably because of heat loss. A small-bodied mammal would have to have some very innovative insulation or a ferocious metabolism to maintain its warm-blooded body temperature in the sea. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 20, 2009, 4:47pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/10 "2009-08-20T16:47:16Z")
> [@EdwardLost](#): > > Probably because of heat loss. A small-bodied mammal would have to have some very innovative insulation or a ferocious metabolism to maintain its warm-blooded body temperature in the sea. Not quite germane to the question of “marine mammals,” but there are aquatic insectivores: the [_desman_](http://en.wikipedia.org/wiki/Desman), a relative of the moles that lives in rivers in the Pyrenees and Russia, and the [\*otter shrews](http://en.wikipedia.org/wiki/Otter_shrew)\* of Africa. The duckbilled platypus is of course also aquatic. ---
**Author:** ![jayjay](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jayjay/32/6765_2.png) [@jayjay](https://boards.straightdope.com/u/jayjay)\ **Post date:** [August 20, 2009, 4:49pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/11 "2009-08-20T16:49:12Z")
> [@Sailboat](#): > > If river critters count, do muskrats count? > > So how come there aren’t ocean mice? Sea shrews? There are water shrews that spend a good deal of time hunting in the water. Their innovative insulation needs are met by trapping air under their fur underwater…they essentially carry a bubble of air around. ---
**Author:** ![TruCelt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/trucelt/32/523_2.png) [@TruCelt](https://boards.straightdope.com/u/TruCelt)\ **Post date:** [August 20, 2009, 4:52pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/12 "2009-08-20T16:52:20Z")
> [@jayjay](#): > > Hmm…sea otters are bigger than I thought. Males can weigh 49 to 99 lbs. and be 4 to 5 feet long. Wow. Ignorance fought! I always pictured them to be about 1.5 - 2 feet. ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [August 20, 2009, 6:20pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/13 "2009-08-20T18:20:56Z")
> [@Captain\_Amazing](#): > > The smallest cetacean would be, like Shagnasty said, the Commersons dolphin, which is smaller than the vaquita, with a length of between 3.6-5.6 feet, and a weight between 75-130 pounds. I stand corrected. I had always read that it was the vaquita aka gulf porpoise. ---
**Author:** ![Oukile](https://avatars.discourse-cdn.com/v4/letter/o/13edae/32.png) [@Oukile](https://boards.straightdope.com/u/Oukile)\ **Post date:** [August 20, 2009, 7:16pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/14 "2009-08-20T19:16:33Z")
> [@EdwardLost](#): > > Probably because of heat loss. A small-bodied mammal would have to have some very innovative insulation or a ferocious metabolism to maintain its warm-blooded body temperature in the sea. I’m not quite sure that body temperature itself is the issue, since small mammals can live in cold regions and small aquatic birds do very well. On the other hand, a small animal would have a relatively higher need of oxygen (because of higher metabolism and probably relatively higher energy expense for swimming). With less body mass to store oxygen, that would make a lower ability to dive. ---
**Author:** ![TheMadHun](https://avatars.discourse-cdn.com/v4/letter/t/53a042/32.png) [@TheMadHun](https://boards.straightdope.com/u/TheMadHun)\ **Post date:** [August 20, 2009, 8:37pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/15 "2009-08-20T20:37:03Z")
Lemmings, briefly. ---
**Author:** ![Tamerlane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tamerlane/32/7602_2.png) [@Tamerlane](https://boards.straightdope.com/u/Tamerlane)\ **Post date:** [August 21, 2009, 12:05am UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/16 "2009-08-21T00:05:38Z")
> [@jayjay](#): > > Hmm…sea otters are bigger than I thought. Males can weigh 49 to 99 lbs. and be 4 to 5 feet long. > [@TruCelt](#): > > Wow. Ignorance fought! I always pictured them to be about 1.5 - 2 feet. Y’all have to come visit CA. They’re still repulsively cute ;), but yeah, pretty good sized. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [August 21, 2009, 3:57pm UTC](https://boards.straightdope.com/t/whats-the-smallest-marine-mammal/506967/17 "2009-08-21T15:57:25Z")
> [@Oukile](#): > > I’m not quite sure that body temperature itself is the issue, since small mammals can live in cold regions and small aquatic birds do very well. On the other hand, a small animal would have a relatively higher need of oxygen (because of higher metabolism and probably relatively higher energy expense for swimming). With less body mass to store oxygen, that would make a lower ability to dive. Retaining body heat in air is a lot easier than it is in water, because water has much higher thermal mass than air. Hair and fur insulate by trapping air against the body and cutting down on heat loss by convection. As someone pointed out above, hairy (and feathery) animals in the water keep warm by keeping a “bubble” of air trapped around their bodies. But these animals have to spend a lot of time at the surface preening. The alternative to hair/fur is blubber insulation. But a layer of blubber makes the animal bigger. Most sea creatures, large and small, are cold blooded. The warm blooded ones, whales, dugongs, tuna, great-whites, etc., are mostly large bodied. I doubt a mammal the size of a mouse could spend a majority of its time in the water and still retain its warm-bloodedness. It could adapt by evolving a tolerance for a low core body temperature while in the water (and maybe warming back up at the surface).