In recent times, the major predators of hooded seals have been humans. The sealing industry began in the 18th century and these mammals were hunted for 150 years without any restrictive laws. More than 500,000 seals (hooded and harp seals) were caught per year between 1820 and 1860. At first, sealing was popular because there was a demand for oil and leather. After the 1940s, seals began to be hunted for their fur and one of the most prized species was the hooded seal, considered four times more valuable than other seals. A quota to limit hunting was introduced in 1971 and was set at 30,000.
Natural predators of hooded seals include sharks, polar bears, and killer whales. Polar bears mainly feed on harp and bearded seals but will hunt hooded seals when they are breeding on ice and are more visible, vulnerable targets.
Known Predators:
Hooded seals have blue-gray pelage with black spots over the body. The front of the face is black and this coloration extends posteriorly to just behind the eyes. Their limbs are rather small in proportion to their body, but are powerful nonetheless, making these seals excellent swimmers and divers. Hooded seals exhibit marked sexual dimorphism. Males are slightly longer than females, and reach 2.5 m in length; females average 2.2 m. The more significant difference between the sexes is weight. Males weigh up to 300 kg while females only weigh up to 160 kg. Also unique to males is the inflatable hood and nasal septum.
Hooded seals get their name from the inflatable “hood” on the top of the heads of males. The hood is not present until males are about 4 years old. When the hood is deflated, it hangs down over the upper lip. Males inflate this red, balloon-like nasal septum until it protrudes out of one nostril. Males use this nasal sac for aggressive display and also to get the attention of females.
Hooded seals have many characteristics that differentiate them from other phocids. They have the largest nostrils in the family. The skull is short with a wide snout. They also have a palate that projects posteriorly further than any other seal. One-third of the nasal bone extends beyond the edge of the maxilla. Their incisor formula is unique, with two upper and one lower incisor. The teeth are small and the tooth row is narrow. The dental formula is I 2/1, C 1/1, PC 5/5. Hooded seals have light and dark bands of cementum in the canines that can be used to determine age.
At birth the coloring of young is silver on the dorsal side, without spots, and blue-gray on the ventral side, which accounts for their nickname ”bluebacks.” Young are 90 to 105 cm in length when born and average 20 kg. Around age 1, differences between males and females can be observed; males begin growing larger in weight and length.
Range mass: 160 to 300 kg.
Range length: 2.2 to 2.5 m.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: male larger; ornamentation
The expected maximum lifespan of hooded seals is 35 years. In this sexually dimorphic species the differences in body size among males and females result in differences in longevity. In hooded seals males are larger and have shorter lifespans. The mortality rate of adults is 7 to 15% a year. One cause of death is known to be infections from the parasitic heartworm, Dipetalonema spirocauda. Before there were restrictions on hunting, humans were the main cause of death in hooded seals. Captive hooded seals have been reported to die from tuberculosis and cranial infections.
Typical lifespan
Status: wild: 35 (high) years.
Hooded seals do not have a very complete fossil record. One of the first few fossils found was from Anvers, Beligum in 1876, which dated to the Pliocene. In 1983 a paper was published claiming there were some fossils found in North America thought to be from Cystophora cristata. Of the three accounts, the most creditable discovery was from a sewer excavation in Maine. A scapula and humeri were found among other bones and thought to date to the post-Pleistocene. Of two other accounts, one was later reassigned to another species and the other left unsolved.
Hooded seals are able to dive for long periods of time due to their tolerance of hypoxia. Research discovered that oxygen delivery to the brain is boosted by an increase in density of brain capillaries. More importantly, these seals have neurons that are inherently hypoxia tolerant. Studies showed that their neurons were able to discharge four times longer than those of mice in hypoxic conditions.
Hooded seals are able to make vocalizations such as roars that can be heard easily on land. However, their most important form of communication is produced from the hood and septum. They are able to produce pulses ranging from 500 to 6 Hz, these sounds can be heard on land and in the water. They are often seen moving their inflated hood and nasal septum up and down, which can create sounds described as “pings” and “whooshes”. This method of communication can serve as male display for females but also serve as threats.
Communication Channels: visual ; acoustic
Perception Channels: visual ; tactile ; acoustic ; chemical
Hooded seals were hunted in high numbers starting in the 18th century. The popularity of their pelts, especially of the “bluebacks”, which are pup pelts, resulted in a rapid decline of populations. After World War II the hunt for hoodeds seals increased, resulting in concern that they would become endangered. In 1958 laws were introduced, followed by quotas in 1971. Recent efforts include treaties and agreements, banning of hunting in areas such as the Gulf of St. Lawrence, and bans on importation of seal products. Despite these measures hooded seal populations are still on the decline for unknown reasons.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
IUCN Red List of Threatened Species: vulnerable
There are no negative effects of hooded seals on humans.
Hooded seals have played an important role in subsistence for the natives of Greenland and Canada who hunt these seals for a source of food. They have also provided valuable goods including leather, oil, and fur. However excessive demand of these goods have negatively impacted populations of hooded seals.
Positive Impacts: food ; body parts are source of valuable material
Hooded seals often find themselves the host of parasitic worms, such as heartworms, Dipetalonema spirocauda. Often these parasites result in shortened lifespans. Hooded seals are predators of many fishes, such as polar cod, squid, and various crustaceans. They are preyed on by sharks, orcas, and polar bears.
Commensal/Parasitic Species:
Hooded seals eat a variety of marine prey, especially fish, such as redfish, herring, polar cod, and flounder. They also feed on octopus and shrimp. Some research indicates that during the winter and autumn hooded seals feed more on squid and switch to primarily fish in the summer, especially polar cod. Pups first begin feeding near the shore and eat mainly squid and crustaceans. When Arctic algae and phytoplankton bloom, their energy is transferred to fatty acids. These food sources are eaten by herbivores and make their way up the food chain to the top predators like the hooded seal. The fatty acids that begin at the bottom of the food chain are then stored in the blubber of the seals. This blubber is sustained throughout the autumn and winter and used as an energy resource in the summer during the molting and breeding season when fasting occurs.
Animal Foods: fish; mollusks; aquatic crustaceans
Primary Diet: carnivore (Piscivore , Eats non-insect arthropods, Molluscivore )
Hooded seals are generally found from 47° to 80° N latitude. They occur along the eastern coast of North America north of Maine. Hooded seals also reach the western tip of Europe, along the coast of Norway. They are mainly concentrated around Bear Island, Norway, Iceland, and northeast Greenland. In rare cases they have been found in Siberia.
Biogeographic Regions: nearctic (Native ); arctic ocean (Native ); atlantic ocean (Native )
Other Geographic Terms: holarctic
Hooded seals are found in coastal areas of Atlantic and Arctic Oceans. They are successful divers that spend much of their time in the water. They usually dive to a depth of 600 m but can go as deep as 1000 m. When they are on land they usually occur in areas with significant ice cover or made up of ice packs. They migrate annually in order to stay in areas where there is drifting pack ice.
Range depth: 100 to 1000 m.
Average depth: 600 m.
Habitat Regions: polar ; terrestrial ; saltwater or marine
Terrestrial Biomes: icecap
Aquatic Biomes: coastal
During the short time that the mother is giving birth and nursing her pup, several males will be in close proximity to her in order to obtain mating rights. At this time many males will aggressively threaten each other using their inflated nasal sac and even push each other out of the breeding area. Males do not typically defend personal territories; they only defend the area where there is a receptive female. A successful male will then mate with the female in the water. Once returning to land he will search for another female. Mating occurs typically through April and June.
Mating System: polygynous
Females reach the age of sexual maturity between 2 and 9 years old and it is estimated that most females give birth to their first young at around 5 years of age. Males reach sexual maturity a little later around 4 to 6 years old but often do not mate until much later. Females give birth to one young at a time through March and April. The gestation period is 240 to 250 days. During this time the fetus - unlike those of other seals - sheds its lanugo (a covering of fine soft hair that is replaced thicker pelage) in the uterus. These young are precocial and at birth are able to move about and swim with ease. They are independent and left to fend for themselves immediately after they have been weaned.
Breeding interval: Hooded seals breed once a year.
Breeding season: Hooded seals breed from April to June.
Range number of offspring: 1 to 1.
Range gestation period: 240 to 250 days.
Range weaning age: 5 to 12 days.
Range age at sexual or reproductive maturity (female): 2 to 9 years.
Average age at sexual or reproductive maturity (female): 3.2 years.
Range age at sexual or reproductive maturity (male): 4 to 6 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
Average birth mass: 18000 g.
Average number of offspring: 1.
Hooded seals have the shortest nursing period of any mammal, from 5 to 12 days. The milk of the female is rich in fat, which makes up about 60 to 70% of its content and allows the pup to double in size during its short nursing period. During this same period, the mother loses 7 to 10 kg each day. Females are protective of their pup during their short weaning interval. They fight potential predators, including other seals and humans. Males do not invest energy in defending their young. Since young are precocial at birth and already able to crawl and swim, little is done to help raise them.
Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)
The hooded seal (Cystophora cristata) is a large phocid found only in the central and western North Atlantic, ranging from Svalbard in the east to the Gulf of St. Lawrence in the west. The seals are typically silver-grey or white in color, with black spots that vary in size covering most of the body.[3] Hooded seal pups are known as "blue-backs" because their coats are blue-grey on the back with whitish bellies. This coat is shed after 14 months of age when the pups molt.[4] It is the only species in the genus Cystophora.
The generic name Cystophora means "bladder-bearer" in Greek, from the species' unusual sexual ornament – a peculiar inflatable bladder septum on the head of the adult male. This bladder hangs between the eyes and down over the upper lip in the deflated state. In addition, the hooded seal can inflate a large balloon-like sac from one of its nostrils. This is done by shutting one nostril valve and inflating a membrane, which then protrudes from the other nostril.[5]
Adult males are 2.6 meters (8 ft 6 in) long on average, can grow to 3.5 m, and weigh 300–410 kg (660–900 lb). Sexual dimorphism is obvious from birth and females are much smaller: 2.03 meters (6 ft 8 in) long and weighing 145–300 kg (320–661 lb).[6][7] The color is silvery; the body is scattered with dark, irregular marks. The head is darker than the rest of the body, and without marks.
Hooded seals live primarily on drifting pack ice and in deep water in the Arctic Ocean and North Atlantic. Although some drift away to warmer regions during the year, their best survival rate is in colder climates. They can be found on four distinct areas with pack ice: near Jan Mayen Island (northeast of Iceland); off Labrador and northeastern Newfoundland; the Gulf of St. Lawrence; and the Davis Strait (off midwestern Greenland).[4][6] Males appear to be localized around areas of complex seabed, such as Baffin Bay, Davis Strait, and the Flemish cap, while females concentrate their habitat efforts primarily on shelf areas, such as the Labrador Shelf.[8] Hooded seals are known to be a highly migratory species that often wander long distances, as far west as Alaska and as far south as the Canary Islands and Guadeloupe.[6] Prior to the mid 1990s, hooded seal sightings in Maine and the east Atlantic were rare, but began increasing in the mid 1990s. From January 1997 to December 1999, a total of 84 recorded sightings of hooded seals occurred in the Gulf of Maine, one in France and one in Portugal. From 1996 to 2006, five strandings and sightings were noted near the Spanish coasts in the Mediterranean Sea. There is no scientific explanation for the increase in sightings and range of the hooded seal.[9][10]
The diet of the hooded seal is composed primarily of various amphipods (crustaceans), euphausiids (krill), and fish, including Atlantic Argentine, capelin, Greenland halibut, cod, herring, and redfish.[4][11] They also are known to eat squid, sea stars, and mussels.[4] Relative to the other species, hooded seals consume 3 times the proportion of redfish; percentages of capelin were similar in relation to closely related species.[11] Capelin is considered a more common choice of sustenance during the winter season. Their diet is considered to be rich in lipids and fatty acids.[12]
Hooded seals tend to feed in relatively deep waters ranging from 100–600 m (330–1,970 ft), and dive from 5 to 25 minute durations. However, some dives can go deeper than 1,016 m (3,333 ft) and as long, or longer, than 52 minutes. Diving is rather continuous, with approximately 90% of their time spent submerged during the day and night, although dives during the day are generally deeper and longer. Dives during the winter are also deeper and longer than those in the summer. It is known that the hooded seal is generally a solitary species, except during breeding and molting seasons. During these two periods, they tend to fast as well. The seals mass annually near the Denmark strait around July, at the time of their molting periods, to mate.[13][14] Hooded seals are a relatively unsocial species compared to other seals, and they are typically more aggressive and territorial. They demonstrate aggression by inflating the "hood" (which is explained in the "Nasal Cavity" section below). They frequently migrate and remain alone for most of the year, except during mating season.[4][6]
The hooded seal is known for its uniquely elastic nasal cavity located at the top of its head, also known as the hood.[4] Only males possess this nasal sac, which they begin to develop around the age of four.[15] The hood begins to inflate as the seal makes its initial breath prior to going underwater. It then begins to repetitively deflate and inflate as the seal is swimming. The purpose of this happening is for acoustic signaling, meaning that it occurs when the seal feels threatened and attempts to ward off hostile species when competing for resources such as food and shelter.[16] It also serves to communicate their health and superior status to both other males and females they are attempting to attract.[15] In sexually mature males, a pinkish balloon-like nasal membrane comes out of the left nostril to further aid it in attracting a mate. This membrane, when shaken, is able to produce various sounds and calls depending on whether the seal is underwater or on land. Most of these acoustic signals are used in acoustic situation (about 79%), while about 12% of the signals are used for sexual purposes.[17]
There are four major breeding areas for the hooded seal: the Gulf of St. Lawrence; the "Front" east of Newfoundland; Davis Strait (between Greenland and northern Canada); and the West Ice near Jan Mayen. Male hooded seals are known to have several mates in a single mating season, following the hypothesis that they are polygynous. While some males will defend and mate with just one female for long periods of time, others will be more mobile and tend to mate with multiple females for shorter periods of time, generating maximum offspring within the population.[18] Most males reach sexual maturity by 5 years of age.[19]
Throughout all areas, the hooded seals whelp in late March and early April and molt from June to August.[9] The four recognized herds are generally sorted into two distinct populations: a Northeast (NE) Atlantic population and a Northwest (NW) Atlantic population. It is estimated that 90% of the total NW population give birth on the "Front". The NE herd whelping (giving birth) around Jan Mayen generally disperse into the sea after they breed in March. From April through June, after the breeding season, this species travels long distances to feed and then eventually gather together once again. Although some individuals return to the same area of ice in July to undergo moulting, the majority of the herd molt further North. After molting, the species disperses widely again to feed in the late summer and autumn before returning to the breeding areas again in late winter.[20][21][22]
Pups are about 1 metre (3 ft 3 in) long at birth and weigh about 24 kilograms (53 lb). They are born on the ice from mid-March to early April with a well-developed blubber layer and having shed their pre-natal coat. They are born with a slate blue-grey coat (giving them the name "blueback"), with a pale cream color on the belly, which they will molt after about 14 months. Nursing of the pup lasts for an average of only 4 days, the shortest lactation period of any mammal, during which the pup doubles in size, gaining around 7 kilograms (15 lb)/day. This is possible because the milk that they drink has a fat content of 60%.[23] The female pup will mature between ages 3 and 6, whereas the male pup will mature between ages 5 and 7.
Researchers find that due to a pup's differing needs in regards to sustaining work and foraging while under water compared to adults, the skeletal and cardiac muscles develop differently. Studies show that cardiac blood flow provides sufficient oxygen to sustain lipolytic pathways during dives, remedying their hypoxic challenge. Cardiac tissue is more developed than skeletal muscles at birth and during the weaning period, although neither tissue is fully developed by the end of the weaning period.[24] Pups are born with fully developed hemoglobin stores (found in blood), but their myoglobin levels (found in skeletal tissue) are only 25–30% of adult levels. These observations demonstrate that pup muscles are less able to sustain either aerobic or anaerobic ATP production during dives than adults. This is due to the large stores of oxygen, either bound to hemoglobin or myoglobin, which the seals rely on to dive for extended periods of time.[25] This could be a potential explanation for pups’ short weaning period as diving is essential to their living and survival.[24]
Hooded seals shed their lanugo fur in the womb and are born with a thin non lanugo fur coat, which is a less efficient thermoregulating fur coat compared to some to other seal species (e.g., leopard seals).[26] Studies have shown that newborn hooded seals undergo a stage after birth where the thickness of their blubber grows rapidly, which maximizes their ability to thermoregulate and compensate for their thin fur coat.[27] This process of fur removal in the womb and alternatively using blubber may be advantageous because water cannot penetrate blubber like it does with fur, therefore blubber can insulate the seals in both water and air fairly uniformly.[27]
The hooded seal can live to about age 30 to 35.[6]
Prior to the 1940s, adult hooded seals were primarily hunted for their leather and oil deposits. More recently, the main threats are hunting, including subsistence hunting, and bycatch. Seal strandings are not considered a large threat to hooded seal populations but are highly researched. Seal pups are hunted for their blue and black pelts and many mothers are killed in the process, attempting to protect their young. Hunting primarily occurs in areas of Greenland, Canada, Russia, and Norway.[4] Overall, northwest Atlantic hooded seal populations are stable or increasing whereas the northeast Atlantic populations have declined by 85–90% within the last 60 years.[2]
It was believed by the scientific community that sonar was leading to mass stranding of hooded seals. After multiple sonar tests on captive seals, ranging from 1 to 7 kHz, it became evident that it had little effect on the subjects. The first test on each subject yielded differing results, ranging from reduced diving activity and rapid exploratory swimming. A difference was only noted for all subjects on their initial exposure.[26]
Conservation practices, brought about by international cooperation and the formation of the Northwest Atlantic Fisheries Organization (NAFO) led to hooded seal population increases. It is now required to hold a license to hunt hooded seals in international waters and each license is set a quota. Total allowable catch of hooded seals are set at 10,000 annually.[4]
The hooded seal is protected under the Marine Mammal Protection Act of 1972.[28]
Mother with pup.
The hooded seal (Cystophora cristata) is a large phocid found only in the central and western North Atlantic, ranging from Svalbard in the east to the Gulf of St. Lawrence in the west. The seals are typically silver-grey or white in color, with black spots that vary in size covering most of the body. Hooded seal pups are known as "blue-backs" because their coats are blue-grey on the back with whitish bellies. This coat is shed after 14 months of age when the pups molt. It is the only species in the genus Cystophora.
