I. Biology and Natural History
Hawaiian monk seals are pinnipeds, which is the order of marine mammals including seals, sea lions and walruses. There are believed to be about 34 different species of pinnipeds. All are characterized by having large eyes, prominent snouts, streamlined shapes and four swimming flippers which typify the order. In fact, the word pinniped means “feather-” or “fin-footed” in Latin. Although they have successfully adapted to life in the sea, pinnipeds are thought to have evolved from terrestrial mammals about 20 million years ago and continue to retain strong ties to land. Their closest modern day terrestrial ancestors are the bear and the dog. Some biologists consider the pinnipeds to be so closely related to these species as to include them as members of order carnivora which includes the dogs, bears, wolves, raccoons and others. Whichever classification scheme you choose, the pinnipeds can be divided into three distinct families; 1) the Phocidae which are the “true,” or earless seals 2) the Otariidae, the eared seals and sea lions, and 3) the Odobenidae, which includes only the walrus.
The three families differ mainly with respect to the possession of external ears and their means of locomotion. The Otariidae or eared seals, have external ears as their name implies. In addition, they have long front flippers that measure up to one-third of their body length which they use to propel themselves through the water. In the water, their hind flippers serve mainly as a rudder or as an aid in steering. On land, the hind flippers are able to turn under the seal’s body and provide support, enabling them to make four-footed movements. Eared seals are fairly mobile on land as well as in the water.
In contrast, the Phocidae, or true seals, lack external ears and hear through small holes on either side of their head. In the water, their hind flippers propel them when swimming while their front flippers act as rudders or stabilizers . On land, the hind flippers are not able to turn under the animal and provide support. This makes travel on land rather difficult for the true seals, reducing it to somewhat of a wiggle.
The walrus, lone member of the Odobenidae, seems to combine features of both the eared and the true seals. Walruses essentially lack external ears. At sea, they paddle mainly with their front flippers, similarly to eared seals, although their front flippers are not nearly as large, and their rear flippers move in lateral movements similar to true seals. On land, walruses are able to turn their rear flippers under their bodies for support, and make four-footed movements.
The Hawaiian monk seal is considered to be a Phocid, or true seal, meaning it has no external ears and swims by using its hind flippers for propulsion and its front flippers as stabilizers. Below is the biological classification for the Hawaiian monk seal:
Kingdom Animalia Phylum Chordata (vertebrates) Class Mammalia (mammals) Order Pinniped (or, Carnivora; sub-order, pinniped) Family Phocidae ("true" seals- seals without ears) Genus Monachus Species schauinslandi
Like other species of marine mammals, pinnipeds have had to overcome a variety of factors to become well adapted to life in the ocean. By referring to the table below, we see that most groups of marine mammals evolved from terrestrial ancestors that were quite different from one another, yet in returning to the sea, all marine mammals developed a number of similar adaptations needed to cope with the aquatic environment in which they found themselves. These similar adaptations include the following:
- their bodies have become streamlined
- their front and hind limbs evolved into flipper-like appendages used for gliding through the water
- all possess a thick layer of blubber to insulate them from the cold, aid in buoyancy and serve as an energy reserve
- many of their external structures such as reproductive and sensory organs have become internalized in order to reduce drag while swimming
Marine Mammal Terrestrial Common Group Ancestors Traits Seals, Sea lions- Bear, dog (Pinnipeds) Streamlined bodies Dugong, Manatee- Elephant Flipper-like appendages (Sirenia) Thick layer of blubber Internalization of structures Whales, Dolphins- Camel, Cow (Cetaceans) Sea Otter- Weasel (Carnivora)
The development of similar traits among marine mammals is an example of convergent evolution. Convergent evolution refers to the evolution of similar characteristics in unrelated organisms due to similar demands posed by their environment, which in the case of marine mammals is the ocean.
As a group, the pinnipeds are mainly carnivorous, feeding on a wide variety of invertebrates and fishes. Occasionally they will feed upon birds or even other seals. They tend to eat the common seafood of their area, swallowing their catches head first to avoid being hurt by any fins or spines. In many regions pinnipeds are the most visible of the marine mammals because of their large size and their habit of frequenting coastal areas where they tend to congregate in large numbers to rest and sun themselves. This last characteristic does not hold true for Hawaiian monk seals, which seem to prefer a solitary life and rarely are found in groups.
II. A Closer Look at the Monk Seal: Hawaii’s Favorite Pinniped
The Hawaiian monk seal, Monachus schauinslandi, is Hawaii’s only pinniped. It is a species that is considered endemic to the islands. The word endemic means that the species is unique to a region (i.e., is found no where else) and is believed to have evolved there. The monk seal is one of Hawaii’s two endemic mammals. Hawaii’s only other endemic mammal is the Hoary bat.
Many other species found in Hawaii are considered indigenous, which means that they arrived in the islands under natural conditions such as by wind or by water, but can also be found in a number of different regions. Examples of animals indigenous to Hawaii are the humpback whale, the green sea turtle, and the Laysan albatross. Species that are either endemic or indigenous to a particular region are also considered native, or naturally occurring, to that region.
In contrast, species that do not occur naturally in a region, but have been brought there by man or though human activity are called alien or introduced species. The introduction of alien species can often be detrimental to a native ecosystem if the species are not carefully managed. Examples of alien species found in Hawaii include the mongoose, the cockroach, ants and pigs.
Close up of a young seal’s head
Throughout the world, there have been three known species of monk seals: Hawaiian, Caribbean, and Mediterranean. Caribbean monk seals were last sighted in 1952. The species is now thought to be extinct. Mediterranean monk seals continue to survive in small numbers in isolated caves and beaches rarely visited by humans in the Mediterranean. The present population of Mediterranean monk seals is believed to be between 500 and 1,000 individuals and is thought to be declining. The population of the Hawaiian monk seal is currently estimated to be between 1,500 and 1,200 individuals. They are considered an endangered species.
Female seal with playful young pup in Northwestern Hawaiian Islands
Adult monk seals measure about seven feet in length and weigh between 400 and 600 pounds, with females often being larger than males. While at the breeding islands, monk seals feed on bottom and reef fishes, octopuses, eels, and spiny lobsters found in relatively shallow waters close to shore. Although they do not undergo a seasonal migration, scientists believe that monk seals travel many miles at sea for periods of a month or more. Therefore, monk seals must be able to find food in the open sea as well as in the shallow lagoon surrounding their breeding beaches. So much time may be spent at sea that monk seals, when seen on beaches, appear to have green fur. This green color is actually a type of marine algae which has grown in their fur.
While at sea, Hawaiian monk seals have been known to dive as deep as 600 feet to feed and can stay underwater for up to 20 minutes. The reason why monk seals are such expert divers is because they, like all other species of pinnipeds, have developed a very efficient means of using oxygen, allowing them to remain submerged for long periods of time without suffering brain damage or “the bends,” two common consequences of oxygen deprivation. Adult seals have the ability to slow their heart rate to a rate of 4 to 15 beats per minute while diving, as compared to a rate of 55 to 120 beats per minute under normal surface conditions. This phenomenon, known asbradycardia, reduces the seal’s need for oxygen and conserves it for the vital functions of the heart and brain. Bradycardia develops more fully in smaller pinniped species and more rapidly as the seal gets older.
Female Hawaiian monk seals become sexually mature around six years of age. Mating has only been observed in the waters off the Northwestern Islands and usually in the spring and summer months. Each year about 60% of all adult females give birth. The species is polygamous, meaning that the males will attempt to mate with more than one female at a time. This is the case even though adult male monk seals tend to outnumber adult females by ratios as high as three to one, and indicates that each adult female monk seal is disturbed several times. Pups are born about a year after mating takes place, usually between March and June, but births have been recorded in every month of the year. The female bears one pup at a time, and most give birth every two years, although some individuals give birth each year. Compared to other species of pinnipeds, the reproductive rate for Hawaiian monk seals is rather low. Scientists believe this may be due in part to the uneven sex ratio of the species and the low number of adult females in the population.
Female seal with young pup
Monk seal pup with wooly black coat prior to molting.
An important way in which Hawaiian monk seals differ from other species of earless seals (phocids) is the fact that they evolved entirely free of terrestrial enemies due to their living on remote oceanic islands. Because of this, they did not develop the need or the instinct to flee from predators. Hawaiian monk seals are genetically tame and easily approached by humans. Unfortunately, this trait has proven to be one of the major factors leading to the population decline of the species.
III. Factors Affecting the Population of Hawaiian Monk Seals:
A. Hunting and Human Disturbance
Human interaction with monk seals, as well as all other pinnipeds, is largely due to their reproductive need to inhabit coastlines. Because Hawaiian monk seals evolved in the absence of land predators and did not develop the need to flee, they often fell victim to the sealer’s clubs. In the early nineteenth century, Hawaiian monk seals, which were taken for their oil and pelts, suffered the same mass hunting as many of the other fur seals of the world. They were easy targets as they lay quietly basking on the beaches, and it wasn’t long until sealers came in great numbers to the islands to get rich on this new source of quick profits. During this period, thousands of Hawaii’s monk seals were slaughtered. Within only a few years, the population had been reduced so drastically that the seal grounds were deserted as the population was not large enough to make hunting the seals commercially worth while .
From the early 1900’s until the beginning of World War II, the few monk seals that remained were given a respite from human activity as the United States Government declared the Northwestern Hawaiian Islands a protected area- the Hawaiian Bird Reservation. During this time the monk seal population rebounded somewhat. However, military activity during World War II and afterwards disturbed the breeding grounds of the monk seals again, and the population began to decline steadily. Hawaiian monk seals are extremely sensitive to human activity. Mothers often abandon preferred pupping and haul out areas and even their pups prior to weaning, when disturbed by human visitors. Therefore, in order to help protect the species, it is important to enjoy monk seals from a distance, and give them the solitude they need to survive.
In recent years, commercial fishing has been promoted around the Northwestern Hawaiian Islands. Entanglement of monk seals in fishing nets and lines has been reported to have occurred somewhat frequently. Some researchers claim that because monk seals are curious and playful by nature that they may be “attracted” to the fishing gear and unfortunately become entangled in the process. There have also been several reports of fishermen, who when are unable to untangle the seals from their nets or longlines, harm or kill them as they eat the bait on their hooks. Therefore, it seems that regulating the types and quantity of fishing that are permitted in the Northwestern Hawaiian Islands could be important to the seal’s survival. Not only do non-selective types of fishing gear such as driftnets, gill nets and longlines pose a direct threat to the seals, but, the indirect effects of removing too much of the seal’s food organisms from the reefs and waters of the Northwest Hawaiian Islands is not known. Overfishing is thought to have been the primary cause for the extinction of the Caribbean monk seal. Let us hope that the future of the Hawaiian monk seal will be better secured through better fisheries management.
Shark attack is thought to be a major cause of death for Hawaiian monk seals. This is particularly true for younger seals or those that are injured. Most adults bear scars believed to have been inflicted by sharks. Tiger sharks, which frequent the Northwest Hawaiian Islands, are believed to be the main predator.
Ciguatera, a disease which results from ingesting fish which have a high concentration of a naturally occurring toxin stored in their flesh has claimed the lives of many Hawaiian monk seals. The toxin, ciguatoxin, is produced in the tissues of planktonic marine organisms called dinoflagellates, which occur at the bottom of the marine food chain. The toxin accumulates in fishes and other animals that feed on the dinoflaggelates, and tends to accumulates in higher concentrations in animals that occur higher on the marine food chain. Ciguatera is a disease also known to affect humans, and may prove to be fatal if not treated properly.
E. Mobbing by Adult Male Seals
While attempting to mate, adult male monk seals have been observed to violently attack pups, juveniles and sub-adult females, supposedly mistaking them for breeding females. When the attacks involve several adult males, the incident is referred to as mobbing. Mobbing proves lethal to its victims in many cases and is thought to have contributed to the population decline of monk seals on some of the Northwestern Hawaiian Islands. In many areas, male monk seals outnumber females by ratios as high as three to one. Studies are currently underway to determine if it may benefit the monk seal populations to remove adult males who appear to be particularly aggressive.
IV. Protective Measures
A. Federal Laws and Regulations
In addition, protection is provided by the Hawaiian Islands National Wildlife Refuge, which includes most of the seal’s current breeding islands. Protection is provided for those monk seal breeding islands that are not included as part of the National Wildlife Refuge (Kure Atoll and Midway Islands) through the State Seabird Sanctuary on Kure Atoll and by the United States Navy on Midway Islands. Hawaiian monk seals are also listed as endangered under Hawaiian State law. Violations are similar to those of the federal Endangered Species Act. Fines for violations of both these laws can be as high as $20,000.
In satisfying conditions of the federal Endangered Species Act, the National Marine Fisheries Service, who is the responsible federal agency for protecting the monk seals, established a recovery team consisting of government representatives and scientists to study Hawaiian monk seals and the habitat in which they live. It is the job of the recovery team to review past and present research, to eliminate causes of population decline where possible and to make recommendations as how to best manage the population of Hawaiian monk seals so that their numbers increase or stabilize.
In addition to the measures listed above, a set of guidelines has also been developed for those that come in contact with Hawaiian monk seals. If you come across monk seals in the course of your activities it is recommended that you abide by the following:
- Do not approach monk seals while on land or in the water. They may approach swimmers, divers or vessels out of curiosity, but in no instance should you approach them. This may be considered a form of harassment.
- When it is necessary to pass by a monk seal that has hauled itself on to a beach, it is recommended that you remain a minimum distance of 100 feet away from the seal, so as not to disturb them.
- Do your best to avoid female monk seals with pups, and never come between a mother and a pup.
- Report sightings of injured or sick seals or of harassment by others to the National Marine Fisheries Service in Honolulu at your earliest convenience. The seals will thank you! The number there is: (808) 955-8831.
B. International Protection
Hawaiian monk seals are protected outside the United States through provisions in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). This treaty protects the seals against trade in their parts or products (see Appendix II for more information).
Suggested Activities for Teachers
– Hawaiian monk seals and humpback whales are both marine mammals. As a class, list all the ways the two animals are similar, then, make a second list including all the ways the animals are different.
– Ask students to explain how adaptive radiation differs from convergent evolution. They should be able to give examples of each that exist in nature.
– Ask students to make a drawing that includes their favorite endemic, indigenous and alien species. Each species in the drawing should be labeled and its native status noted.
– Ask students the following questions: Are all native species endemic, why or why not? And, are all endemic species native, why or why not?
– As a class, pretend that you are the newly appointed Hawaiian Monk Seal Recovery Team. What steps would you take to protect and conserve Hawaii’s monk seal population? What would your recovery plan entail?