Science, Tech, Math › Animals & Nature True Seals Scientific name: Phocidae Share Flipboard Email Print Harp seal - Pagophilus groenlandicus. Photo © Kevin Schafer / Getty Images. Animals & Nature Mammals Amphibians Birds Habitat Profiles Reptiles Insects Marine Life Forestry Dinosaurs Evolution View More By Laura Klappenbach Ecology Expert M.S., Applied Ecology, Indiana University Bloomington B.S., Biology and Chemistry, University of Illinois at Urbana-Champaign Laura Klappenbach, M.S., is a science writer specializing in ecology, biology, and wildlife. our editorial process Laura Klappenbach Updated March 08, 2017 True seals (Phocidae) are large marine mammals that have a rotund, fusiform shaped body with small fore flippers and larger rear flippers. True seals have a coat of short hair and a thick layer of blubber beneath their skin which provides them with superb insulation. They have webbing between their digits which they use while swimming by spreading their digits apart. This help to create thrust and control as they move through the water. When on land, true seals move by crawling on their stomach. In water, they use their rear flippers to propel themselves through the water. True seals have no external ear and consequently their head is more streamlined for movement in water. Most true seals live in the Northern Hemisphere, although some species occur south of the equator. Most species are circumpolar, but there are a number of species such as gray seals, harbor seals, and elephant seals, which inhabit temperate regions. Monk seals, of which there are three species, inhabit tropical or subtropical regions including the Caribbean Sea, Mediterranean Sea, and Pacific Ocean. In terms of habitat, true seals inhabit shallow and deep marine waters as well as open water with drifting ice floes, islands, and mainland beaches. The diet of true seals varies between species. It also varies seasonally in response to the availablity or scarcity of food resources. The diets of true seals includes crabs, krill, fish, squid, octopus, invertebrates, and even birds such as penguins. When feeding, many true seals must dive to considerable depths to obtain prey. Some species, such as the elephant seal, can stay underwater for long periods of time, between 20 and 60 minutes. True seals have an annual mating season. Males build up reserves of blubbler prior to the mating season so they have sufficient energy to compete for mates. Females also build up blubber reserves prior to breeding so they have sufficient energy to produce milk for their young. During the breeding season, true seals rely on their fat reserves because they do not feed as regularly as they do during the non breeding season. Females become sexually mature at four years of age, after which time they bear a single young each year. Males reach sexual maturity a few years later than females. Most true seals are gregarious animals that form colonies during their breeding season. Many species undergo migrations between breeding grounds and feeding areas and in some species these migrations are seasonal and depend on the formation or receding of ice cover. Of the 18 species of seals alive today, two are endangered, the Mediterranean monk seal and the Hawaiian monk seals. The Caribbean monk seal went extinct sometime during the past 100 years due to over hunting. The main factor contributing to the decline and extinction of true seal species has been hunting by humans. Additionally, disease has caused mass fatalities in some populations. True seals have been hunted by humans for several hundred years for their meet, oil, and fur. Species Diversity Approximately 18 living species Size and Weight About 3-15 feet long and 100-5,700 pounds Classification True seals are classified within the following taxonomic hierarchy: Animals > Chordates > Vertebrates > Tetrapods > Amniotes > Mammals > Pinnipeds > True Seals True seals are divided into the following taxonomic groups: Monk seals (Monachini) - There are two species of monk seals alive today. Members of this group include the Hawaiian monk seal and the Mediterranean monk seal.Elephant seals (Miroungini) - There are two species of elephant seals alive today. Members of this group include the northern elephant seal and the southern elephant seal.Leopard seals and relatives (Lobodontini) - There are three species of leopard seals and their relatives alive today. Members of this group include crab-eater seals, leopard seals, and Weddell seals.Bearded seals and relatives (Phocinae) - There are 9 species of bearded seals and their relatives alive today. The bearded seals and their relatives include harbor seals, ringed seals, harp seals, ribbon seals, hooded seals, and others.