Sirenians

Introduction to Sirenians

Manatees and dugongs are some of the most interesting animals on the planet and have been around for the past 50 million years.  The origin of the order’s name is a fascinating case of mistaken identity. Apparently, Carl Illiger (1811) thought that manatees looked just like mermaids and decided to call them Sirenia (after the Sirens of Greek myth).

There are currently four extant species of Sirenia: three manatee species (Trichechidae family) and their close cousin, the dugong (Dugong dugon).  Earlier in Sirenian history, however, there were dozens of species. It is not completely understood why so many earlier species have disappeared, although it has been suggested that the changing quality or quantity of sea grass was a contributing factor.

Manatees and dugongs are unique among marine mammals: they are herbivores, feeding primarily on high fiber seagrasses but have observed opportunistically eating barnacles, tunicates, fish and at times even their own feces.

All sirenian species are listed as either endangered, threatened or vulnerable by the IUCN – World Conservation Union.

Comparison Chart

Feature Florida Antillean Amazonian African Dugong
Fluke Type Paddle Paddle Paddle Paddle Forked
Nails Present 3-4 Present 3-4 Absent Present 3-4 Absent
Teeth Marching molars Marching molars Marching molars Marching molars Incisors (tusks) canines, premolars, molars
Skin type Pebbly Pebbly Smooth Pebbly Smooth
Color (color can vary due to algae coverage) Gray-brown, darker at birth Gray-brown,

darker at birth

Black or dark gray, often with white or pinkish belly Gray-brown Gray-brown on top, lighter below. Lighter at birth
Habitat Salt, Brackish, Fresh Salt, Brackish, Fresh Fresh Salt, Brackish, Fresh Marine
Snout Deflection Intermediate Snout Deflection Intermediate Snout Deflection Less Snout Deflection Less Snout Deflection Most snout deflection
Threats Loss of habitat,

boat strikes, red tide, pollution

Loss of habitat, disease, pollution, incidental mortality Loss of habitat, hunting, incidental mortality Loss of habitat, hunting, incidental mortality Loss of habitat, hunting, incidental mortality, boat strikes, pollution

West Indian Manatee (T. manatus manatus and T. manatus latirostris)

Photo: Bob Bonde
Photo: United States Geological Service

Introduction

The West Indian manatee is currently divided into two subspecies, the Antillean manatee (T. manatus manatus) and the Florida manatee (T. manatus latirostris). It has been suggested that there is a third subspecies based on genetics, differences in skull and body size, and geographic distribution.


Florida manatee (Trichechus manatus latirostris)

Photo: Doug Perrine
Photo: ©Doug Perrine

(Subspecies of West Indian manatee)

Habitat and Range

Animals are divided into four regional subpopulations (Atlantic, Upper St. Johns, Northwest, and Southwest) based on their preference for warm water refuges.

In warm weather manatees have been seen as far east as the Bahamas and Cuba, around the coastlines of Texas and Louisiana, and have been tracked as far north as Massachusetts and Rhode Island.

During the fall (November – March) manatees travel to their winter refuges due to lack of a long tolerance for temperatures less than 68oC. These refuges come in many flavors:  warm water discharges from powerplants, natural springs, and passive thermal basins.

Manatees are classified as marine mammals, although they also enjoy fresh and brackish environments (the scientific name for this adaptation is euryhaline). They have specialized kidneys that help maintain an internal water and salt balance; however, it appears they still need fresh water to drink.

Manatees tend to stay in depths less than 5 meters; as their food source, seagrass, grows in shallow waters.

Anatomy

  • The Florida manatee regularly exceeds ten feet in length and more than a ton in weight, with the record being some 3600 pounds.
  • They are noted for their extremely thick, tough skin, which can approach three inches in thickness.
  • Female manatees are often larger than their male counterparts. While manatees may appear to be chubby, the fact is that, like all extant Sirenians, they possess only a thin fat layer which highlights their need to remain within tropical and subtropical regions. Their rotund appearance is a combination of swollen ribs (pachyostotis and osteosclerosis), thick sheets of axial musculature, and the large intestinal tract they need to process the vegetation they consume.
  • Lungs of manatees stretch nearly two-thirds the length of their bodies and are oriented in the same horizontal plane as the manatee, enabling them to float easily.

Diet

When a manatee approaches food, muscles around the snout flatten and expand into what has been coined a “flare response” by biologist Chris Marshall (1998, 2003).

Like all Sirenians, manatees have a horny “crushing plate” in front of the teeth that help crush the tough vegetation that makes up their diet.  Their chewing is aided by their famous “marching molars,” a feature they share with elephants and one species of kangaroo, but curiously not with dugongs.  As teeth become worn out from abrasion, they are replaced by new ones (with brand new grinding edges). Manatees can eat 10-15% of their body weight each day, and it takes approximately seven days to digest the food they eat.

Lifestyle and Behavior

Manatees spend approximately 6-8 hours feeding and two to twelve hours a day resting. They are quite capable of resting on the surface or the bottom and come up for air an average of every three to five minutes.

They are considered “semi-social,” and while they seem to get along well with each other, they do not form permanent social bonds like dolphins, and  in general are considered to have a less complex social life.

Manatees spend a portion of their time playing and socializing with other individuals. Manatees can be observed kissing (muzzle to muzzle) and mouthing one another.  Play can also be more active and include tail thrashing, spyhopping (vertically poking its head out of the water), and a lot of physical contact.

Sex and Reproduction

The only other way to distinguish males from females is by waiting to see if one rolls over in front of you. In males, the genital slit is close the navel; in females it is closer to the anus. Mating involves one focal female being courted by a group of males called an estrous or mating herd.  The scientific name for this mating activity is “scramble polygyny”. Manatees can mate at any time during the year, although a preponderance of calves are born in the spring and summer.

The gestation period is about 13 months, with a single calf the usual outcome (about one in a hundred births are of twins.) The newborn weighs about 60 pounds and is a little over a yard long. Manatee babies are often born dark and tend to lighten as they age. Babies may stay with their mothers for more than two years.

Nursing takes place from a teat located in the “armpit” (axilla) just behind the flipper.

Senses

  • Vision: Manatees are able to distinguish some colors but are thought to have poor visual acuity. In any case, one would not expect manatees, who often dwell in murky rivers to have the clear vision of an eagle soaring through empty space.
  • Hearing: Although manatees have barely visible external ear openings, they have excellent hearing and can hear frequencies up to 96000 Hz. Their internal ear bones are quite large and well developed. Scientists believe that in manatees sound is transmitted though the fat-filled jaw and is carried to the earbones.
  • Tactile: Manatees have hairs all over their bodies, although the face is 30 percent hairier than the rest of the body. The 3000 body whiskers, evenly spaced every two or three inches around the animal’s body, contain 20-50 nerve fibers each. The vibrissae on their backs, while sparse, are thought to be used to detect changes in current, temperature, and movement of other animals.
  • Taste: There is some limited evidence that manatees may use taste to determine hormones of receptive females in their environment. Their taste buds are located posteriorly on the tongue.

Antillean Manatee (Trichechus manatus manatus)

Photo: ©Doug Perrine

(Subspecies of West Indian Manatee) Much of the information is similar to the Florida manatee with some differences noted below

Habitat and Range

The Antilleans ranges over 21 countries in Central and South America and the Caribbean. Like their American brethren, Antillean manatees thrive in fresh, salt, and brackish water and feed on any available vegetation.

Anatomy

The Antillean manatee differs from its Florida cousin mainly through certain characteristics in the skull. It’s almost impossible to tell them apart by just looking.

They probably run a bit smaller on average and much lighter in weight, but that doesn’t help any individual identification. The major factor separating the subspecies is the deep water and strong currents in the Straits of Florida.


Amazonian Manatee (Trichechus manatus inunguis)

Photo: Doug Perrine
Photo: ©Doug Perrine

Introduction

The Amazonian manatee lives in a more specialized environment than other manatees, living solely in freshwater, and preferring blackwater lakes and oxbows. The species seems to have been separated from his manatee and dugong cousins by certain geological events which closed the Pacific entrance to the upper Amazon River.

Anatomy

The species name “inunguis” literally means “without nails,” and sure enough, the Amazonian manatee lacks the three or four nails present in the other varieties. It also has characteristic white markings on its belly that are not present in other manatees. The Amazonian manatee also has a smooth skin, which contrasts sharply with the wrinkled, pebbly skin of the other manatee species.

The Amazonian manatee has a less deflected snout than does the West Indian manatee. This is probably an adaptation for dining of emergent and natant seagrass. The same is true for the West African manatee.


West African Manatee (Trichechus manatus senegalensis)

The West African manatee is considered to be least studied large animal in Africa. It is assumed to be similar to the West Indian manatee (it is hypothesized that they arrived in African through transoceanic currents during the late Pliocene.).

Habitat and Range

The West African Manatee is currently found from Senegal to Angola. They inhabit estuarine lagoons, large rivers, and freshwater lakes, traveling freely between salt and freshwater and may venture about 20 miles a day.  There appear to be two populations – one coastal and one inland, but there seems to be no significant differences between them.

Anatomy

  • Like the Amazonian manatee, the African manatee tends to be a bit more slender than the West Indian manatee, and according to some accounts, have more bulging eyes.
  • The West African manatee has a less deflected snout than does the West Indian manatee. This is probably an adaptation for dining of emergent and natant seagrass.

Diet

Like other manatees, they are primarily herbivores, but have been observed eating clams and mollusks or even fish caught in nets. They will also graze off low-hanging mangroves.

Lifestyle and Behavior

The West African manatee has been observed travelling and feeding both at night and during the day. Like other manatees, they are semi-social and have been observed in small groups of up to six animals. Like their cousins in the Americas, they can breed all year round. Their lifespan has been estimated to be thirty years; much shorter than that of the West Indian manatee.


Dugongs (Dugong dugon)

Photo: Doug Perrine
Photo:© Doug Perrine

Introduction

There is only one species of dugong presently extant (unlike the manatee of which there are three living species). This is the Dugong dugon.

Habitat and Range

Dugongs range from the western Pacific to the eastern coast of Africa; they are found in the coastal waters of 37 countries and territories. Unlike manatees, dugongs are exclusively marine and do not require access to fresh water.

Anatomy

  • Manatees and dugongs attain a similar size; as with manatees, female dugongs tend to be larger.
  • The skull of a dugong is quite different from that of the manatee, notably on account of its sharply down-turned premaxilla, which are stronger in males. The upper snout is divided as in manatees, with each side moving independently. However, the snout ends in a flattened “rostral disk.”  The sharply deflected snout indicates that they are exclusively benthic (bottom) feeders.
  • The oral disk is larger than that of manatees.
  • Dugongs also have smooth skin and a forked tail, unlike the paddle tail of the manatee.

Dentition

One of the most important differences between dugongs and manatee is the teeth. Early dugongs developed tusks (incisors) rather than the marching molars characteristic of manatees (although the cheek teeth move of the dugong more forward with age, they are not replaced). The continually growing tusks generally erupt only in male dugongs during puberty. While females have them, they generally remain unerupted until late in life.

Respiration

They are better divers than manatees and have been known to dive to a depth of 128 feet and have been observed feeding at nearly that depth (Sheppard et al. 2006).   However, they spend most of their time in water depth less than 3 meters (Chilvers et al 2004).

Diet

  • They will often yank up the entire plant leaving trails of where they have fed.
  • Their downturned snout confines them to benthic feeding, unlike manatees who can eat anywhere along the water column.

Reproduction

Different populations of dugongs have adopted different mating practices. In some areas, an amorous male establishes a court that is visited by females eager for his attentions, a very different practice from that of manatees. These are known as lekking areas and are used only during mating and probably not by all males. More than one male may occupy a lekking area, and unlike manatees, dugongs are somewhat territorial and will defend their areas. As with manatees, females can mate with several males (polyandry).


Steller’s Sea Cow (Extinct)

The most recently extinct sirenian is the Steller’s Sea Cow (Hydrodamamlis gigas), a northern species that went extinct around 1768 (Domning 1978). (The species was about 7.5- 10.6 meters long and weighed in at about eight- ten tons.). So far as we know, this is the only Sirenian that ever ventured from warm southern waters. They most likely lived on a diet of kelp.

Europeans discovered Steller Sea Cows in 1741, when a group of Russian sailors about the brig St. Peter were shipwrecked off the Commander Islands (the utmost western point of the Aleutian chain)..  All the men got sick from scurvy as the only food they could find were sea otters and another peculiar sea mammal no one had ever seen before. A biologist named Georg Steller (whom the cows were named after) was right on the money when he recognized the animal as a sirenian of some sort. He compared its skin texture to the bark of an oak, and said it was black, wrinkled, hairless, and tough as stone. He also mentioned that the lips were covered with bristles (like manatees) but that the animal had only horny plates, no teeth. The flippers bore no nails, and at least according to Steller, it didn’t dive. When Steller was shipwrecked there were an estimated 2000 animals.  They became extinct approximately 27 years after their discovery.