In Hawaiian monk seals, females grow slightly longer, and often much heavier, than males. The long, fusiform body is robust, with short flippers. The relatively small head is wide and somewhat flat, and the eyes are spaced fairly widely apart. The muzzle is broad and compressed from top to bottom. The mystacial pads are large and fleshy, extending beyond the nostrils. The nostrils are in a sub-terminal position, pointed up, rather than in a terminal position facing forward, as is typical of most northern phocids. The vibrissae are smooth and not beaded, like in other monk seals, Antarctic seals, and both elephant seals. Hawaiian monk seal vibrissae vary from short to moderately long, and are black at the base, often having lighter yellowish-white tips. There can be a scattering of all-light vibrissae throughout. There are four retractable mammary nipples.
Just following the annual molt, most females and subadults are silvery to slate gray above, fading to cream or light silver gray below. During the course of the 11-12 months between molts the coat fades to dull brownish above and yellowish below. Males and some females become completely dark brown to blackish as they age. There can be a variable amount of light highlighting on the mystacial area and on both the upper and lower lips. Adults and juveniles can have a greenish cast from algal growth. Pups are born in a black woolly coat, which is molted completely by about the sixth week. The first molt from the lanugo coat is a shedding of individual hairs, but each successive annual molt is a more dramatic epidermal molt of hair and skin, which detaches in patches. Many animals have pale birthmarks that take the form of smallish irregular patches, which look like pale bleached areas on the darker pelage, and can occur anywhere on the body. Marks on the flippers that occur on digits result in whitish claws, in place of the normally black claws in the marked area.
Adults and some subadults have varying amounts of scars, particularly on the back and neck. Males can be heavily scarred on the lower jaw and neck, and some adult females become severely injured when mobbed by males attempting to mate with them. These injuries can include massive gashes and skin on the back torn away, that when healed develop into long, jagged, irregular scars. Injuries and scars from shark attacks are also regularly seen and include oval and semi-circular scars from tearing wounds and wounds formed by the penetration of individual teeth in rows.
The dental formula is I 2/2, C 1/1, PC 5/5.
Adult male Hawaiian monk seals reach lengths of about 2.1 m, females 2.4 m. Males weigh an average of 172 kg, females up to 272 kg. Pups are about 1 m and 16-18 kg at birth.
Can be Confused With
No other pinnipeds regularly occur within the tropical and subtropical central North Pacific habitat of this seal. However, in recent years northern elephant seals have been recorded at Midway Island. Northern elephant seals are much larger than Hawaiian monk seals, and only juveniles and small subadults could be confused with monk seals. The large size and more rounded shape of the head and muzzle of northern elephant seals are diagnostic. Additionally, elephant seal nostrils are terminal and forward facing. Also, female northern elephant seals have only two abdominal nipples.
Hawaiian monk seals are distributed throughout the northwestern chain of Hawaiian Islands from Nihoa Island to Kure Atoll. They are also now regularly seen on all of the main Hawaiian Islands, but particularly on Kauai, and births have been recorded on Kauai, Oahu, Maui and Molokai. Adult males involved in mobbing incidents with females have been translocated to the main Hawaiian Islands and Johnston Atoll, about 1,000 km south of the Hawaiian Archipelago. Hawaiian monk seals are considered non-migratory, and are typically faithful to their atoll or island of birth. However small numbers do relocate temporarily or permanently to nearby sites in the island chain. Long distance wanderers have been recorded at Palmyra Atoll and Wake Island.
Ecology and Behavior
On land, Hawaiian monk seals haul-out and breed on beaches of sand and coral rubble, and on rocky terraces. They sometimes leave the beach if vegetation is available for shade. The long breeding season lasts from late December to mid-August, although most pups are born between March and June. Males in this polygynous species patrol the water adjacent to rookeries, or haul-out beside females with pups. There are up to three times more breeding-age males than females at some colonies; this contributes to mobbing of estrus females, which are often severely injured and killed in these events.
When approached by another seal or human on land, Hawaiian monk seals often roll to present the underside to the intruder, arch the back, raise a flipper in the air, and open the mouth. They are generally solitary, both on land and at sea. Even when seals gather together on land, they are not generally gregarious, and only mothers and pups regularly make physical contact.
Hawaiian monk seals regularly forage in the shallow waters surrounding islands and within atolls. Their movements and habitat at sea are not well known, but recent work with satellite tags, dive recorders, and video camera tags on adult and subadult males has shown that theses animals frequently venture far offshore to deeper outer slopes of islands, reefs, and atolls, and to neighboring seamounts and banks, where they appear to feed on the bottom. They also venture to deep ocean areas, also presumably to forage. Most dives that have been recorded have been relatively shallow ones of 100 meters or less, with a record of one subadult male diving to at least 500 meters, which was the maximum the device could record.
Feeding and Prey
Hawaiian monk seals are thought to feed primarily at night on shallow-water reef fishes, eels, and cephalopods.
Threats and Status
Unlike the situation for the Mediterranean monk seal, the vast majority of Hawaiian monk seals live in protected areas, isolated from most direct human contact. Despite this, and as a direct result of 19th and early 20th century exploitation, the species is critically endangered, and numbers only about 1,400 animals. Recovery of the species has been affected to an unknown degree by military activities such as development and occupation of bases on several islands, and dumping of waste that started before World War II. Development of several fisheries in Northwestern Hawaiian Island waters has led to conflicts from entanglements and animals returning to haul-outs with long-line hooks in their mouths. The effect of these fisheries on the ecosystem and prey of monk seals is unknown.
Marine debris, particularly lost and discarded fishing gear has been shown to be a significant source of mortality for monk seals. Efforts have been underway to remove this debris from breeding beaches for many years, and more recently from waters surrounding these sites. Large predatory shark species occur, and congregate at sites in the Northwestern Hawaiian Islands, to prey upon fledgling albatrosses as well as seals. While the role of shark predation is not completely understood, actions have been taken to reduce shark numbers around certain monk seal breeding beaches in order to reduce mortality on newborn and juvenile seals.
Gilmartin, W.G. and J. Forcada. 2002. Monk seals Monachus monachus, M. tropicalis, and M. schauinslandi. pp. 756-759 in W.F. Perrin, B. Würsig and J.G.M. Thiewissen, eds. Encyclopedia of Marine Mammals. Academic Press.
Gilmartin, W.G., T.C. Johanos-Kam and L.L. Eberhardt. 1993. Survival rates for the Hawaiian monk seal (Monachus schauinslandi). Marine Mammal Science 9:407-420.
Kenyon, K.W. 1981. Monk seals - Monachus Flemming, 1822. pp. 195-220 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Vol. 2: Seals. Academic Press.
Lavigne, D.M. 1999. The Hawaiian monk seal: management of an endangered species. pp. 246-266 in J. Twiss and R Reeves, eds. Conservation and Management of marine mammals. Smithsonian Institution Press.
Ragen, T.J. and D.M. Lavigne. 1999. The Hawaiian monk seal: Biology of an endangered species. pp. 224-245 in J. Twiss and R Reeves, eds. Conservation and Management of marine mammals. Smithsonian Institution Press.