I have been requested to do a post on sirenians by my friend Nicole. Out of all the marine mammals, sirenians (manatees and dugongs) seem to get the least media time:
West Indian manatee gesturing for emphasis.
-whales show up because they're charismatic megafauna and people still hunt them, contravening the moratorium on whaling
-pinnipeds (seals, walrus & co.) get airtime for the same reason, see "clubbing baby harp seals" for details.
-dolphins (which are a subset of whales) have recently gotten attention due to The Cove and the movement for dolphin-safe tuna that I remember being a semi-big deal as a child during the early 90s. As an addendum to this, I suggest reading this post at Southern Fried Science. Dolphins have been popular in the public consciousness as long as I've been alive, possibly thanks to Flipper (the first movie came out in 1963) and Sea World.
-some types of otters are also considered marine mammals, though I think they don't seem so in most people's minds because they don't live in the open ocean.
-Wikipedia (always the most scholarly source) claims that polar bears can be considered marine mammals because they live on sea ice; WE REPORT YOU DECIDE
Just a dugong munching.
Sozzani, R. (Photographer). (2005). The Dugong, or sea cow [Web]. Retrieved from http://www.greenpeace.org/international/photosvideos/photos/the-dugong-or-sea-cow-2
But yeah, pretty much the only attention sirenians get in the media is when they're getting hit by boats and personal watercraft in Florida. However, there is more to these creatures! To conclude that manatees only exist as obstacles to watercraft is equivalent to dismissing my local raccoon population as simply obstacles to automobiles, which is what you would probably conclude if you lived here (we saw 10-11 dead raccoons on the road on an 11-12 mile stretch recently).
Do sirenians only consist of manatees and dugongs?
Yes. There are three species of manatee (West Indian manatees, Amazonian manatees and African manatees) and a single species of dugong. Up until 1768, there was a third type of sirenian, the massive Steller's sea cow, which was hunted to extinction within 27 years of its discovery, though other factors were involved in its demise.
Steller's sea cow?
Steller's sea cow was discovered on the Commander Islands east of Russia's Kamchatka Peninsula in the Bering Sea in 1741. Manatees (~4 m) and dugongs (~2.7 m) are much smaller than Steller's sea cow was; these animals clocked in at around 8 m (26 ft).
Self-Sullivan, C. (2009, August 1). Call of the siren - manatee & dugong research, education and conservation.
Retrieved from http://www.sirenian.org/caryn.html
It's suggested that the population at the Commander Islands was a remnant group, having been hunted to extinction elsewhere. It's not terribly surprising, given that they are large, slow, defenseless herbivores (they ate kelp) who lived near to the shore. As a result, they were hunted for their meat (according to anecdotes in Callum's An Unnatural History of the Sea, sailors claimed it was delicious), skin and blubber, which could be refined into an oil that burned smokelessly, which is incidentally a property of whale oil. Reports by Steller (the naturalist on the Bering expedition that found them) indicated an initial population of 1,500-2,000 animals. Clearly this did not last long after information about the population
It's been suggested by some (Anderson, 1995) that hunting was entirely directly responsible for their extinction; Anderson argues that reduction of the sea otter population (which lives on both the Commander Islands and Kamchatka) resulted in an explosion in the growth of sea urchins. Sea urchins eat kelp, which was incidentally the main (only?) food consumed by the sea cows. Thus, Anderson claims that part of this extinction event can be attributed to Steller's sea cows losing their source of food.
And that's pretty much it for Steller's sea cows. It's a small consolation, but as a result of being hunted, there are skeletons on display in museums, such as this one at the Muséum national d'histoire naturelle, in Paris.
Alright, so let's talk about manatees and dugongs. How are they different?
Size: Manatees are overall larger animals than dugongs.
Tails: Manatees have very wonderful paddle-shaped tails; dugongs' are split and fluked like a dolphin's.
Flippers: Manatees have vestigial nails on their flippers (scroll down); dugongs do not.
Heads: Manatees and dugongs differ in their dentition: how their teeth grow, what kind of teeth they have, etc. Manatees also tend to have a shorter snout than dugongs. It's more complicated than that, but this is one of those cases where if you're interested in the nitty-gritty of comparative sirenian cranial anatomy, you'll have to look elsewhere.
HERE HAVE A HELPFUL CHART:
sirenian. (2009). In Encyclopædia Britannica. Retrieved November 28, 2009, from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/546552/sirenian
These things are weird looking. What're their phylogenic relationships with other species?
As you may have noticed from the image of the manatee's toenails and references to snouts, modern sirenians somewhat resemble fusiform aquatic elephants; elephants are their closest living relatives (as far as common ancestors go), along with hyraxes. Yes, these little fat snuffly things:
Shang, N., and Carrick, G. (2006, July). Africa.
I like Wikipedia's description of the hyrax: "From a distance, a hyrax could be mistaken for a very well-fed rabbit or guinea pig."
However, a more accurate description of hyraxes are that they are the remaining examples of a group of animals that look like rodents (but aren't) that used to be much more widespread. They belong to the Afrotheria clade, which along with sirenians and elephants includes elephant shrews (which are not actually shrews but that's okay because they are still precious), tenrecs, aardvarks and others.
Hmmm, that doesn't actually help much, does it. Perhaps the most interesting features of hyraxes are that they display a number of early mammalian features, including:
-they have poorly developed internal temperature regulation (which they deal with by huddling together for warmth, and by basking in the sun like reptiles).
-Unlike other browsing and grazing animals, they do not use the incisors at the front of the jaw for slicing off leaves and grass. Instead, they use molar teeth at the side of the jaw.
-Unlike the even-toed ungulates (deer, antelope, pigs, giraffes, etc.) and some of the macropods (kangaroos, wallabies, etc.), hyraxes do not chew cud to help extract nutrients from coarse, low-grade leaves and grasses.
They do, however, have complex, multi-chambered stomachs that allow symbiotic bacteria to break down tough plant materials, and their overall ability to digest fiber is similar to that of the ungulates. (Thanks Wikipedia!)
Duke, D. (2008, January 1). The Chaircat's blog.
Retrieved from http://animalorphanagekenya.org/blog/page/2/
Hyraxes inhabit the Middle East and various parts of Africa. They have been doing this for a very long time, on the order of 40 million years. 40 mya places hyrax development in the Eocene epoch (55.8 ± 0.2 to 33.9 ± 0.1), known as the time in which modern mammals emerged (go here for more details about the Eocene).
Due to living the Middle East, there are a number of biblical references to hyraxes; however, they were generally translated as "rabbit" or "hare" because the European translators had no idea what hyrax was. This is disappointing because hyraxes are cooler than lagomorphs. Here is Rocky the Hyrax producing lovely vocalizations:
Moving away from hyraxes in particular though, the Eocene marks the appearance of the earliest sirenians, including such august figures as Prorastomus and Protosiren. Check this illustration of Prorastomus, plus some collectible figures.
Honestly though, to explain sirenian evolutionary relationships, I think this chart does it most succinctly. For reference, Trichechus refers to manatees, Hydrodamalis is Steller's sea cow and Dugong is self-referential. This chart even has a handy-dandy geologic time scale built in for your convenience!
That was a long tangent.
Yeah, pretty much.
So what do sirenians do?
Well...they eat a lot of sea grass? According to Gerstein (1994), manatees are capable of understanding discrimination tasks; Dierauf & Gulland (2001) state that their abilities in discrimination and task-learning behaviors are similar to that of dolphins and seals in acoustic and visual studies.
They're basically very chill animals, eating their delicious sea grass and swimming around in warm shallow water (about 3-7 ft./1-2m deep). However, manatees have problems, many due to humans. The main causes of manatee mortality include: watercraft collisions (40% for West Indian manatees), cold stress, hurricanes, red tide (harmful algal blooms), entanglement/swallowing fishing equipment, getting crushed in water control mechanisms, etc. If you'd like to read more, Stith, Slone and Reid (2006) have written a review and synthesis of manatee data in Everglades National Park detailing much about manatee mortality.
Photo by J. Marino at Crystal River Manatee Tours
Those of you who were around in the 80s will remember the media attention given to manatee casualties and gruesome injuries due to watercraft collisions. As a response to this, the U.S. Geological Survey has launched the Sirenia Project, which monitors the West Indian manatee and supporting their recovery.
In a more popular measure, in 1981, musician Jimmy Buffett and former U.S. Senator Bob Graham formed the renowned Save the Manatee Club, where you can adopt a Florida manatee.
While hunted in the past for their meat, skin and bones, by and large manatee and dugong populations are protected by laws in most countries. However, poaching does occur, specifically mentioned in the West Indian and African manatee populations. Dugongs are legally hunted by aboriginals in Australia for subsistence purposes; they are poached elsewhere due to a demand for dugong products. For more information on this, please consult Sirenian International.
Well, that was depressing.
Here's some information that's a bit more light-hearted:
This video shows yawning manatees and provides a closer look at their mouth anatomy, which is kind of weird.
-The oldest manatee in captivity is Snooty, who was born in captivity and resides at the South Florida Museum. Snooty was born in 1948, making him 61 years old on his last birthday.
-There are locations in the United States where it is legal to swim with manatees. However, if you're going to do so, don't be this guy. Manatees are easily stressed, they do not need people harassing them.
Anderson, P.K. (1995). Competition, predation, and the evolution and extinction of steller's sea cow, hydrodamalis gigas. Marine Mammal Science, 11(3), 391 - 394.
Myers, P. 2002. "Dugongidae" (On-line), Animal Diversity Web. AccessedNovember 21, 2009 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Dugongidae.html.