The Hawaiian name for the monk seal is "ilio-holo-i-ka-uaua," which means “dog running in the rough water.” They may look slightly doglike because they are somewhat closely related to canines. The seal’s common name comes from the thick fold of skin around the neck that resembles the hood of a monk’s robe. In addition, the seal lives a solitary lifestyle, unlike other seals that live in colonies. The seals are born with a black lanugo—a fur coat found on some infant mammals. They shed this as they grow, and as adults, they have dark gray backs and light-colored bellies. Hawaiian monk seals are 7 to 7.5 feet (2.1 to 2.2 meters) in length, with females larger than males. Pups weigh only 25 to 35 pounds (11 to 16 kilograms) when born, but grow up into 400- to 600-pound (180- to 270-kilogram) adults.
Hawaiian monk seals are endemic to Hawaii and are the only marine mammal found solely in U.S. waters. The majority of these animals live in the northwestern Hawaiian Islands and about 200 are found on the main islands. They are primarily marine animals but haul out on land to rest and give birth.
Fish, cephalopods (such as octopuses), and crustaceans make up their diet. While they usually hunt in shallow reefs, they’re known to dive over 900 feet (270 meters) to capture prey. In order to accomplish this, Hawaiian monk seals exhibit bradycardia—their heart rate slows down to about eight times less than the rate on the surface. This reduces the need for oxygen, so the seal can stay down longer.
Breeding occurs offshore. Females give birth to one pup on land in the spring or summer. The pups stay with their mothers for five to seven weeks, during which time they gain over 175 pounds (80 kilograms). The mother seal doesn’t eat while nursing and loses up to a third of her body weight. Hawaiian monk seals are one of the few seal species that will foster and nurse another female’s pups. Male monk seals are known to be aggressive enough to kill females of their own species. Hawaiian monk seals live up to 25 to 30 years in the wild, but their lives are too often cut short by human-induced disturbances.
Hawaiian monk seals are listed as endangered on the U.S. endangered species list and the State of Hawaii’s endangered species list, and are also protected by the Marine Mammal Protection Act. There are only about 1,100 Hawaiian monk seals left in the wild, and the population of monk seals in the northwestern Hawaiian Islands is currently declining at a rate of 4 percent per year. Tiger shark predation, particularly of young pups, contributes significantly to the dwindling number of Hawaiian monk seals. The bigger threat, though, comes from humans. They are at risk from entanglement in fishing gear, beach disturbance, overfishing, inadequate marine protected areas and no-take zones, invasive species, coral bleaching, canine diseases, ocean acidification, sea level rise, ineffective enforcement of marine resource regulations, and sometimes intentional killing.
There are only two mammal species native to Hawaii—the Hawaiian hoary bat and the Hawaiian monk seal.
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