WEDDELL SEALS

Half or more of the Weddell seal diet is fish with the rest being squid, octopus, krill, mysids, isopods, amphipods, and decapods [3,4].

The Weddell seal commonly occurs on fast ice and nearshore pack ice along the Antarctic coast and in small populations in some subantarctic islands. The McMurdo diver will see them around sea ice cracks along islands, coastlines, and grounded icebergs. Weddell seals move around the sea ice, are not gregarious, and are spaced apart when seen hauled out on the sea ice. Due to the Weddell seal's preference for fast sea ice, the impact of predators such as leopard seals upon the Weddell seal population is minimal.

Here's a Weddell seal swimming up through a hole to haul out onto the ice.

The Weddell seal dives beneath stable contiguous sea ice and it keeps breathing and entry/exit holes open year round with its teeth. Its strong teeth project forward and are used to ream ice. These teeth are so critical to survival that they influence adult survival; when the teeth wear out, Weddell seals may lose their ability to maintain breathing holes and die at an earlier age than other seals.

Weddell seals can store a large amount of oxygen in their bodies mostly in their blood and muscles [5]. This enables them to stay underwater for a usual dive to 300-400 meters (980-1,310 feet) for fifteen minutes [5]. Weddell seals have been observed staying underwater for 82 minutes and diving down to 700 meters [5]. Weddell seals glide a lot in deep dives rather than swim continuously [2]. The lungs of the Weddell seal collapse from water pressure during a dive, thus decreasing the seal's buoyancy on descent [2]. The Weddell seals' limited oxygen storage is thus conserved by taking advantage of this physical change during a deep dive and reducing the amount of swimming during deep dives looking for fish [2]. After several dives, they can be observed coughing up a foamy white lung surfactant [6]. Their underwater swimming speed is estimated at 4-7 knots (4.5-8 mph or 7.4-13 kph) [5].

Weddell seals are restless when breathing at a hole when other seals are nearby. They peer down and if a hole fits only one seal, the seal will either dive as another seal comes up or reverse and face down to prevent the intruding seal from surfacing. Weddell seals adopt a head-down fighting posture with eyes looking forward, foreflippers extended and sometimes their jaw open. If a hole is large enough for two seals and the breathing seal refuses to leave, the arriving seal may surface with a fight usually ensuing. It has been suggested that a Weddell seal does not defend an area to the complete exclusion of other seals but to the discouragement of other seals. An intruding seal may be physiologically forced to take a breath which would override territorial aggression.

Breeding and pupping occur in the summer months. Breeding Weddell sea bulls set up under-ice territories of twenty meters diameter and tend to remain in the water where breeding takes place. Female Weddell seals move freely through the territories of the bulls. Subordinate males have their activity restricted by the dominant bull when moving through a territory. Females claim less well defined territories, individually or jointly with other females.

The mother gives birth to her newborn on the sea ice and stays with it for the first twelve days; after that, the mother will spend 30-40% of her time in the water while the pup remains on the sea ice. The pup is born at 29 kilograms and gains 10-15 kilograms per week.

Here a Weddell sea mother and pup float in shallow water under a sea ice crack; their entry/exit/breathing holes are visible as bright lights above them. By seven weeks of age, the pups can remain submerged for five minutes and dive down to 92 meters. The moulting of the pup's fur is complete in thirty days. The pup nurses for 45 days and, when weaned, the pup weighs 113 kilograms. Weddell seals have high juvenile survival due to a lack of predation in fast sea ice.

The Weddell seal vocalizes underwater and the diver is very aware of their presence even when they cannot be seen. Their calls make an eerie symphony for the diver. Weddell seals produce a wide range of calls: lengthy buzzes descending from a higher pitch to a lower pitch called "trills", whistles, and chirps. Certain vocalizations are associated with aggressive displays and have been characterized as a trill, a rapid chi-chi-chi, an eeeyo, and a chirrup. A teeth clacking sound was observed as seals passed one another entering and leaving breathing holes. A trill is used by mature males to establish and mark their underwater territory; it is associated with tense situations. When a trill isn't heeded, a fight may ensue.

Here's a mother and her pup. Weddell seals are highly vocal during the peak of breeding season at the Hutton Cliffs colony; researchers recorded almost twenty underwater calls per minute [1]. In mid-December when mating is almost over, the pups are being weaned, and adults dispersing, the underwater calls of Weddell seals at Hutton Cliffs decreased to two per minute [1]. Why? Their predators, leopard seals and killer whales, showed up at the fast ice edge about twenty kilometers away [1]. Weddell seals are no longer so isolated from their predators by distance from the fast ice edge since the edge shifts south as summer progresses [1]. Killer whales prowl the fast ice edge for prey and leopard seals can swim long distances under ice seeking out Weddell seals and their breathing holes [1]. Leopard seals and killer whales vocalize underwater and Weddell seals hear them [1]. Sounds are important for Weddell seals to communicate with their species but they also need to avoid detection by predators [1]. Absence of sound from Weddell seals is an anti-predation strategy when the risk of predation by leopard seals and killer whales is increased [1].

WAV sound file (886K) of Weddell seals underwater *** click on the ear to listen ***

AU sound file (443K) of Weddell seals underwater *** click on the ear to listen ***

The sonagram below corresponds to this sound file of Weddell seals

Text ©Peter Brueggeman. Photographs ©Norbert Wu.Photographs may not be used in any form without the express written permission of Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?