Hawaiian Monk Seal
Monk seals are earless seals of the tribe Monachini. They are the only earless seals found in tropical climates. There are two genera of monk seals, Monachus and Neomonachus, comprising three species: the Mediterranean monk seal, Monachus monachus; the Hawaiian monk seal, Neomonachus schauinslandi; and the Caribbean monk seal, Neomonachus tropicalis, which became extinct in the 20th century. The two surviving species are now extremely rare and in imminent danger of extinction. All three monk seal species were classified in genus Monachus until 2014, when the Caribbean and Hawaiian species were placed into a new genus, Neomonachus.
Monk seals have a slender body and are agile. They have a broad, flat snout with nostrils on the top. Monk seals are polygynous, and group together in harems. They feed mainly on bony fish and cephalopods, but they are opportunistic. The skin is covered in small hair, which are generally black in males and brown or dark gray in females. Monk seals are found in the Hawaiian archipelago, certain areas in the Mediterranean sea (such as Cabo Blanco and Gyaros island), and formerly in the tropical areas of the west Atlantic Ocean.
All species experienced over-hunting by sealers. The Hawaiian monk seal experienced population drops in the 19th century and during World War II, and the Caribbean monk seal was exploited since the 1500s until the 1850s, when populations were too low to hunt commercially. The Mediterranean monk seal has experienced commercial hunting since the Middle Ages and eradication by fishermen. Monk seals have developed a fear of humans, and may even abandon beaches due to human presence. There are currently around 1,700 monk seals in total.
Taxonomy and Evolution[]
Monk seals are earless seals (true seals) of the tribe Monachini. The tribe was first conceived by Victor Blanchard Scheffer in his 1958 book Seals, Sea Lions, and Walruses: A Review of the Pinnipedia. There are two genera of monk seals, Monachus and Neomonachus, comprising three species: the Mediterranean monk seal (Monachus monachus), the Hawaiian monk seal(Neomonachus schauinslandi), and the Caribbean monk seal (Neomonachus tropicalis), which became extinct in the 20th century. All three monk seal species were classified in genus Monachus until 2014, when comparison of the species' mitochondrial cytochrome b DNAsequences led biologists to place the Caribbean and Hawaiian species in a new genus, Neomonachus.
Fossils of the Mediterranean and Caribbean species are known from the Pleistocene. The time of divergence between the Hawaiian and Caribbean species, 3.7 million years (Ma) ago, corresponds to the closing of the Central American Seaway by the formation of the Isthmus of Panama. The divergence between Mediterranean seals and the New World clade was dated to 6.3 Ma ago.
Habitat[]
The Hawaiian monk seal, as the name suggests, lives solely in the Hawaiian archipelago. Monk seals migrated to Hawaii between 4–11 million years ago (mya) through an open water passage between North and South America called the Central American Seaway. The Isthmus of Panama closed the Seaway approximately 3 million years ago. The species may have evolved in the Pacific or Atlantic, but in either case, came to Hawaii long before the first Polynesians. When monk seals are not hunting or eating, they generally bask on the beaches; Hawaiian monk seals tend to bask on sandy beaches and volcanic rock of the Northwest Hawaiian Islands.
The habitat of the Mediterranean monk seal has changed over the years. Prior to the 20th century, Mediterranean monk seals had been known to congregate, give birth, and seek refuge on open beaches. Since sealing had ended, they have left their former habitat and now only use sea caves for such behavior. More often than not, these caves are rather inaccessible to humans due to underwater entries, and because the caves are often along remote or rugged coastlines. Scientists have confirmed this is a recent adaptation, most likely due to the rapid increase in human population, tourism, and industry, which have caused increased disturbance by humans and the destruction of the species' natural habitat. Because of these seals' shy nature and sensitivity to human disturbance, they have slowly adapted to try to avoid contact with humans completely within the last century, and, perhaps, even earlier. The coastal caves are, however, dangerous for newborns, and are causes of major mortality among pups when sea storms hit the caves.
Caribbean monk seals were found in warm temperate, subtropical and tropical waters of the Caribbean Sea, Gulf of Mexico, and the west Atlantic Ocean. They probably preferred to haul out at sites (low sandy beaches above high tide) on isolated and secluded atolls and islands, but occasionally would visit the mainland coasts and deeper waters offshore. This species may have fed in shallow lagoons and reefs.
Description[]
Caribbean Monk Seal Skeleton
Monk seals are part of the family Phocidae (earless seals), the members of which are characterized by their lack of external ears, the inability to rotate the hind flippers under the body, and shed their hair and the outer layer of their skin in an annual molt. Monk seals as a whole vary minutely in size, with all adults measuring on average 8 feet (2.4 m) and 500 pounds (230 kg). They exhibit sexual dimorphism, in that the males are slightly larger than females, with the exception of the Hawaiian monk seal where females are larger. Its white belly, gray coat, and slender physique distinguish it from the harbor seal(Phoca vitulina), another earless seal. Much like elephant seals, they shed their hair and the outer layer of their skin in an annual molt.
The Mediterranean monk seal has a short, broad, and flat snout, with very pronounced, long nostrils that face upwards. The flippers are relatively short, with small slender claws. The monk seal’s physique is ideally suited for hunting its prey: fish, octopus, lobster, and squid in deep water coral beds. The fur coats of males is generally black, and brown or dark gray in females. Pups are about 3.3 feet (1 m) long and weigh around 33–40 pounds (15–18 kg), their skin being covered by 0.4-to-0.6-inch (1 to 1.5 cm) fur, usually dark brown or black. On their bellies, there is a white stripe, which differs in color between the two sexes. This hair is replaced after six to eight weeks by the usual short hair adults carry.
The Hawaiian monk seal (whose Hawaiian name means "the dog that runs in rough waters")[16] has a short, broad, and flat snout, with long nostrils that face forward. The Hawaiian monk seal has a relatively small, flat head with large black eyes, eight pairs of teeth, and a short snout with the nostrils on top of the snout and vibrissae on each side. The nostrils are small vertical slits which close when the seal dives underwater. Additionally, their slender, torpedo-shaped body and hind flippers allow them to be very agile swimmers. Adult males are 300 to 400 pounds (140 to 180 kg) in weight and 7 feet (2.1 m) in length while adult females tend to be, on average, slightly larger, at 400 to 600 pounds (180 to 270 kg) and 8 feet (2.4 m) in length. When monk seal pups are born, they average 30 to 40 pounds (14 to 18 kg) and 40 inches (1.0 m) in length. As they nurse for approximately six weeks, the grow considerably, eventually weighing between 150 to 200 pounds (68 to 91 kg) by the time they are weaned, while the mother loses up to 300 pounds (140 kg).
Caribbean monk seals had a relatively large, long, robust body, could grow to nearly 8 feet (2.4 m) in length and weighed 375 to 600 pounds (170 to 272 kg). Males were probably slightly larger than females, which is similar to Mediterranean monk seals. Like other monk seals this species had a distinctive head and face. The head was rounded with an extended broad muzzle. The face had relatively large wide-spaced eyes, upward opening nostrils, and fairly big whisker pads with long light-colored and smooth whiskers. When compared to the body, the animal's foreflippers were relatively short with little claws and the hindflippers were slender. Their coloration was brownish and/or grayish, with the underside lighter than the dorsal area. Adults were darker than the more paler and yellowish younger seals. Caribbean monk seals were also known to have algae growing on their pelage, giving them a slightly greenish appearance, which is similar to Hawaiian monk seals.
Behavior[]
Diet and Predation[]
Hawaiian monk seals mainly prey on reef dwelling bony fish, but they also prey on cephalopods and crustaceans. Juveniles and sub-adults prey more on smaller octopus species, such as Octopus leteus and O. hawaiiensis, nocturnal octopi species, and eels than do adult Hawaiian monk seals. Adult seals feed mostly on larger octopus species such as O. cyanea. Hawaiian monk seals have a broad and diverse diet due to foraging plasticity which allows them to be opportunistic predators that feed on a wide variety of available prey. Tiger sharks and Galapagos sharks are both predators of the Hawaiian Monk Seal.
Mediterranean monk seals are diurnal and feed on a variety of fish and mollusks, primarily octopus, squid, and eels, up to 6.5 pounds (2.9 kg) per day. They are known to forage mostly at depths of 150 to 230 feet (46 to 70 m), but some have been observed by NOAA submersibles at a depth of 500 feet (150 m). Mediterranean monk seals prefer hunting in wide-open spaces, enabling them to use their speed more effectively. They are successful bottom-feeding hunters; some have even been observed lifting slabs of rock in search of prey. They have no natural predators.
Reproduction and Development[]
Very little is known of the Mediterranean monk seal's reproduction. It is suggested that they are polygynous, with males being very territorial where they mate with females. Although there is no breeding season since births take place year round, there is a peak in October and November. This is also the time when caves are prone to wash out due to high surf or storm surge, which causes high mortality rates among monk seal pups, especially at the key Cabo Blanco colony. Pups make first contact with the water two weeks after their birth and are weaned at around 18 weeks of age; females caring for pups will go off to feed for an average of nine hours. Most individuals are believed to reach maturity at four years of age. The gestation period lasts close to a year. However, it is believed to be common among monk seals of the Cabo Blanco colony to have a gestation period lasting slightly longer than a year. Mediterranean monk seals generally live to be 25 to 30 years old.
Hawaiian monk seals are polygynous. The breeding season takes place throughout the year, excluding the fall, but peaks during April and May. Shark attacks cause a high pup mortality, from 19% to 39%. It is thought that pups are weaned at about six weeks, and are thought to reach sexual maturity at three years. Their average lifespan is 25 to 30 years. Not much is known of the Caribbean monk seal's reproduction. It is thought that they bore a single pup every two years. Their gestation period, lactating period, and sexual maturity age are unknown.