Weddell seal

A Weddell seal at a breathing hole

Weddell seals are commonly found on fast ice, or ice fastened to land, and gather in small groups around cracks and holes within the ice.[11] In the winter, they stay in the water to avoid blizzards, with only their heads poking through breathing holes in the ice.[8] These seals are often observed lying on their sides when on land. Weddell seals are non-migratory phocids that move regionally to follow the distribution of breathing holes and exit cracks within the ice changes between seasons.[12] The species is primarily restricted to Antarctic waters: physical factors, such as glacial movement and tidal action, may increase fluctuations in distributions.

Weddell seals dive to forage for food, maintain breathing holes in fast ice, and explore to find more ice holes.[13] They have been observed to dive as deep as 600 m for up to an hour.[12] These seals exhibit a diel dive pattern, diving deeper and longer during the day than at night.[14] After dropping away from a breathing hole in the ice, the seals become negatively buoyant in the first 30 to 50 m, allowing them to dive with little effort.[15] Weddell seals are top predators in the Antarctic. They eat an array of fish, bottom-feeding prawns, cephalopods and crustaceans.[16] A sedentary adult eats around 10 kg (22 lb) a day, while an active adult eats over 50 kg (110 lb) a day.[8] Antarctic cod and silverfish constitute the majority of their diet. Cephalopods are common prey, and crustacean remains are sometimes found in Weddell seal scat, but at much lower rates than other prey species. They are opportunistic feeders that hunt in different parts of the water column depending on prey availability. Weddell seals hunt in both pelagic and benthic-demersal habitats.[17] Although seabirds are not usually part of their diet, there have been several sightings of them chasing and killing penguins in the wild. Other Antarctic phocids are known to be seabird predators, resulting in implications that penguin hunting is a learned behavior. There are recordings of four different penguin species being attacked by Weddell seals: gentoo penguins, an emperor penguin, an Adélie penguin, and a chinstrap penguin. It has not been confirmed, however, if the penguins were consumed after being killed.[18] Scientists believe Weddell seals rely mainly on eyesight to hunt for food when there is light. However, during the Antarctic winter darkness, when there is no light under the ice where the seals forage, they rely on other senses, primarily the sense of touch from their vibrissae or whiskers, which are not just hairs, but very complicated sense organs with more than 500 nerve endings that attach to the animal's snout. The hairs allow the seals to detect the wake of swimming fish and use that to capture prey.[19] Weddell seals have no natural predators when on fast ice. At sea or on pack ice, they are prey for killer whales and leopard seals, which prey primarily on juveniles and pups.[8]

Baby Weddell seal, Adélie Land

Weddell seals return to fast ice colonies during the spring for birthing and breeding.[20] Weddell seal populations will often return to the same breeding sites over consecutive breeding seasons.[21] Depending on the latitude it inhabits, this marine mammal gives birth from early September through November, with those living at lower latitudes giving birth earlier.[12] Weddell seals usually give birth to one pup per year,[12] however the Weddell seal is one of the only species of seals that can give birth to twin pups.[3] Birth of the pup only takes around one to four minutes. Newborn pups weigh about 25–30 kg and grow to two times their weight within their first week of life.[12] The pups take their first swim around one to two weeks of age.[12] During the first two weeks mother Weddell seals distinguish their pups through olfactory smells, specialized vocalizations, and stay in the same spatial area.[22] After six to seven weeks, pups are weaned and begin to hunt independently.[8]

Weddell seal pup with its grey natal coat, Deception Island

Weddell seals show moderate polygyny and genetic analyses of mating success have suggested how factors such as size, diving ability, site-specific experience enhances success in male Weddell seals.[23] Additionally, fast ice breeding grounds causes females to cluster in large aggregations, making it easier for males to take control over his own harem.[21] The mating season occurs during austral spring between late November and December after pups are weaned and females begin ovulating.[12] During the mating season, Weddell seals make noises loud enough to be felt through the ice.[3] Males defend underwater territories during the breeding season and have been observed to fight.[24] Copulation has only been observed to occur under water, where the female submits to the male as he approaches her dorsal side. The female is often bitten on the neck by her partner if she tries to escape or terminate copulation.[25] The seals are normally around six to eight years old when they first breed, but this can be much earlier for some females.[9] Weddell seals undergo delayed implantation. The embryo is not embedded into the uterus until the beginning of austral summer, between mid-January and mid-February, allowing for birth under more favorable environmental conditions.[12] Juvenile Weddell seals (0–2 years old) have a high mortality probability, whereas no differences can be found between the mortality rate of animals older than 2 years and that of adults (Hastings et al., 1999). Maximum lifespan of the species is estimated at least 25 years.

Male and female Weddell seals communicate through a variety of sounds, specifically males sometimes use “trills” to communicate. Weddell seals are also able to communicate to each other through different mediums. Weddell seals on ice are able to hear the calls of Weddell seals in the water as long as noise level on land is low and they are in close proximity of one another. Sound waves can be transmitted either through the ice itself or from water to breathing holes where female Weddell seals are usually breeding.[26] There have been recordings of Weddell seal vocalizations that are described as songs. Their songs consist of repetitive sequences of the same vocal elements, and they only vary slightly over time. Individual Weddell seals can each produce their own unique song, but singing behavior is not common when observed.[27] Vocalizations are also important in mother-pup Weddell seal interactions. Mother Weddell seals use vocalizations to call their pups from further distances when smell can no longer be used efficiently (Opzeeland et al., 2011). Pups also use higher, more urgent vocalizations when hungry to alert their mothers to feed.[28]