Sunday, February 19, 2017

Siren Songs

Last week we looked at one of the most productive ecosystems on earth, sea grass prairies. Sea grass prairies house many animals that fill very similar ecological roles to terrestrial counterparts. The most notable, and tragically least discussed in the previous post, is the manatee. Thankfully manatees and their cousins are such a rich topic that they've earned their own post.

 Try not to look so excited, geez.
Courtesy Tracy Colson via Flickr

Almost everyone has heard that manatees are likely the source of mermaid legends, or at least were mistaken for mermaids after the legend had been around for a while. In fact the name of the manatee's order, Sirenia (pronounced: sih-ree-nee-uh), actually comes from the sirens that tempted Odysseus while he sailed from home after the Trojan War. Many people find it a bit surprising that such large, grey, and cowish looking animals could be mistaken for half women, half fish; but beyond sailor's missing home there's actually a decent reason sirens could have been mistaken for their namesake. Female sirenians have breasts in the exact same position that human females do. Add the fact that they nurse their young while floating vertically with their heads above water, and you can see how people could have made the mistake that there were aquatic humans.

Though manatees are often called sea cows; it's believed that sometime around 50 million years ago a close relative of elephants slipped into lakes and streams to feed on the rich grasses growing underwater. Over time these four-limbed sirens became more and more adapted to the life aquatic and their descendants spread out across the globe. Then as the earth cooled to today's more familiar state many sirenians went extinct, leaving us with only four species.

Unfortunately there isn't anywhere in the world where you can see all 
four species at once. This manatee quartet (dibs on the band name) is made 
up entirely of one species.
Courtesy: David Hinkel via Flickr 

All sirenians scour the river, lake, or sea floor searching for underwater grasses to eat. Other marine mammals eat meat, and so have access to copious amounts of fat and protein to keep their weights up and their bodies warm. Sirenians are the only herbivorous marine mammals so they have to eat about 10% of their weight every day. Quite a lot when you consider they can weigh between 500 and 800 lbs. One study found that a single sirenian needed about an acre of constantly regenerating sea grass per year in order to get the food they need.

Sirenian mouths are well adapted to consuming large quantities of plants.  On every species the mouth is turned downwards to the seabed so the siren can look ahead as it grazes. Their teeth are specialized for grinding up veggies and are constantly replaced with new ones as they wear down, just like a shark's!

Modern sirenians are all found in the fresh and nearshore salt waters of the tropics and sub-tropics. They're extremely intolerant of cold and will move out of waters that fall below about 68 degrees Fahrenheit (20C). This need for environmental warmth may be a consequence of having evolved during a much warmer period in Earth's history. Only one sirenian, the Steller's Sea Cow (Hydrodamalis gigas), managed to adapt to frigid waters and survive to the modern era, but believe it or not, we ate them all. 

In case you ever thought your drawing skills aren't good enough for natural history; 
this is thought to be the most accurate drawing of a Steller's sea cow because 
it's the only one we know was drawn from a live specimen
Courtesy: Public Domain via Wikimedia Commons

Steller's sea cows were discovered by westerners looking to hunt sea otters along the Aleutian Islands where food could be scarce. The scientist who first described them tells us that they ate more algae than sea grass, and that their forelimbs were curved instead of paddle shaped to allow them to pull themselves along in the shallows while grazing. What's particularly fascinating about these animals was just how big they were, some individuals were as long as an adult orca! That's a lot of manatee. Steller's sea cows were so slow, easily reached, and edible that they were gone about 17 years after their discovery. Sadly we'll never know if we did major detriment to this species while it was thriving in an isolated population, or if they were already nearing extinction naturally when we dealt the final blow. Fortunately there may still be time to save the sea cow's closest relative, the dugong (Dugong dugon).

Om Nom Nom Nom
Courtesy: Corrie Barklimore via Flickr

Dugongs (no, not the pokemon) are the most marine of the sirenia; meaning they spend all their time in salt or brackish (mixed salt and fresh) water. Their most noticeable distinction from manatees is their forked dolphin-like tail. Manatees have big, round, paddle-like tails that are useful for holding position in still water. While dugongs are wide spread across the tropical Indian and Western Pacific Ocean their populations are spotty, probably due to human alterations in their habitat. The largest herds live along the coast of Northern Australia and in the Arabian Gulf where sea grass beds are still large and healthy. The challenge for these sirens has been human development of beaches where sea grasses grow off shore. Unfortunately sea grasses grow off of exactly the kind of sandy relaxing beaches humans like to put hotels and beach communities. When land plants are taken away by construction; dirt and sand run into the nearshore smothering the prairies. Fertilizer runoff from agriculture and golf courses also causes quick-growing algae to out compete sea grass. So when you choose a place to stay during your tropical vacation, keep in mind the resort's potential impact on dugongs before you book a room.

Dugongs role as grazers is extremely important for sea grass prairies because they remove old grasses and fertilize new ones. As they browse, dugongs pull sea grass up by the roots creating noticeable  tracks of bare sand like someone set the lawnmower way too low. Like any herbivore dugongs make a lot of waste and as their poop falls into the furrows they've created in the grass it provides critical nutrients for the regeneration of the prairie.

Who else wishes it was possible to hire manatees to mow
your lawn instead of goats?
Courtesy: Ruth Hartnup via Flickr

The sirenian that is the next most comfortable in ocean is the West Indian Manatee (Trichechus manatus). West Indian Manatees are the most familiar and well-studied of all sirenians because they live in highly populated regions of North, Central, and South America. You may have heard of Florida and Antillean manatees in your travels, and those are legitimate, but they are names for distinct populations of West Indian Manatees. Antillean manatees are usually found in the Southern Caribbean, and Central and South America. You can probably guess where Florida manatees are most often found. For a deeper look at what makes a species, and how populations of one species can be distinct from one another without being totally different, check out the D&T posts "Celebrating Sculpin Speciation" and "Ghost Faced Killer (Whales)".

Most West Indian manatees split their time between fresh and salt water depending on the season. In Florida these manatees spend their winters huddled nears springs of warm water, and since the advent of modern electricity generation, the heated water outflows from power plants. As ocean temps warm up in the summer Florida manatees  spread out in search of lush prairies and can be found travelling as far north as Virginia! Interestingly, although as many as 500 manatees have been seen in one spot during the winter, they aren't particularly social. The mother-offspring bond is the strongest social connection in manatees. Young manatees will stay with their mothers for two years after they're born, learning where warm refuges and feeding grounds are located.

The green on his back is algae, kind of like the stuff that grows in sloth fur.
Courtesy: Keith Ramos via Flickr

The final two sirenians are the most river-adapted species, the Amazonian manatee (Trichechus inunguis) and the West African manatee (Trichchus senegalensis). Both can be found deep into large rivers in their respective continents. Amazonian manatees have stable populations as far up the Amazon water shed as Peru and Columbia, and West African manatees can be found living in lakes 200 miles inland from the sea. These manatees deal with particularly murky water compared to dugongs and West Indian manatees, so the sensitive bristles that cover all sirenians' bodies are especially important for finding food. It's kind of like if a cat had whiskers all over their body instead of just their face.

Yep, that's a baby Amazonian manatee. Commence squealing with delight
Courtesy: Harvey Barrison via Flickr

We know the least about African manatees because the humans of their homeland have struggled with colonialism and the unrest it often instills in the colonized. However, this chaos may actually benefit the manatees; as development in some countries where they're found has been slow. Without speed boats, big riverside properties, and erosion problems African manatees have fewer risks to their survival than sirens in more altered regions of the world.

Dugongs and manatees are one of the weirdest and most fascinating marine mammals on the planet. For every human that's encountered them it's as hard to resist the song of these sirens calling us to discover more about them. But unlike the sirens of Greek Myth, the lure of dugongs and manatees will bring every species to a healthier life and a richer planet.

References:

Mayaba, Theodore B., Kamla, Aristide T. & Self-Sullivan, Carlyn, "Using Pooled Local Expert Opinions (PLEO) to Discern Patterns in Sightings of Live and Dead Manatees (Trichechus senegalensis, Link 1785) in Lower Sanaga Basin, Cameroon", PLOS ONE, July 21st, 2015.   

Satizabal et Al., "Phylogeography and Sex-Biased Dispersal Across Riverine Manatee Populations (Trichechus inguinus and Trichechus manatus) in South America", PLOS ONE, December 20th, 2012.

Sulzner et Al., "Health Assessment and Seroepidemiologic Survey of Potential Pathogens in Wild Antillean Manatees (Trichechus manatus manatus)", PLOS ONE, September 12th, 2012.

Macdonald, Nicole, "Dugong dugon- Dugong" Animal Diversity Web. Accessed via: http://animaldiversity.org/accounts/Dugong_dugon/#food_habits

Weinstein, Brett & Patton, James, "Hydrodamalis gigas- Steller's Sea Cow", Animal Diversity Web, Accessed via: http://animaldiversity.org/accounts/Hydrodamalis_gigas/





    

















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