Are leopard seals dangerous?

When you think of seals, a moon-faced, snow white harp seal pup may come to mind.

With a reptilian-looking head, a taste for adorable penguins and a rather violent method for breaking apart their prey, leopard seals don't exactly fit this fluffy seal ideal. But are they actually as bad as some movies and documentaries would have you believe?

What do leopard seals eat?

Leopard seals are named for their spotty coats. Like their big cat namesakes, these Antarctic mammals are carnivores. They have one of the most varied diets of any seal. A large part of this is made up of tiny crustaceans called krill, but they also eat squids, octopuses, penguins and other seabirds. One leopard seal even regurgitated a sea snake. 

Leopard seals are the only seal species known to actively prey on other seals, taking pups of crabeater, Weddell, Ross, southern elephant and Antarctic fur seals.

Extensive predation by leopard seals is thought to play a substantial role in preventing the growth of some fur seal populations.  Experts also estimate that up to 78% of crabeater seals over the age of one have injuries or scars from leopard seal attacks.

The leopard seal's teeth reflect their varied diet. Their sharp canines and incisors are used to grab and tear large prey, whereas the molars are a trident shape and are used like a sieve to filter out water when they catch mouthfuls of smaller prey like krill.

Leopard seals are agile swimmers but less graceful on land. As true seals (in the family Phocidae) their small front flippers can't hold up their body, so on land they have to flop along on their bellies. Despite this, they are thought to spend up to 31% of their time hauled out. 

Leopard seals and penguins

Leopard seals are mainly found around Antarctica and the sub-Antarctic islands. Some are seen further north, such as on the coasts of New Zealand and Australia, though these are vagrants that have ended up there by accident.

In the Antarctic, leopard seal diets seem to vary through the year. In summer they primarily feed on fish and krill, and in winter when the sea ice expands, they are mostly sustained by bountiful colonies of seals and penguins in the shallower waters.

Leopard seals have a well-documented taste for penguins and at up to 3.5 metres long and 500 kilogrammes, they outmatch any species of these aquatic birds. The seals patrol shorelines, often stationing themselves at colonies, waiting to ambush birds as they transit between land and sea. A 2009 report found that at one colony, 12-16% of the gentoo penguins were consumed by leopard seals. 

These long and muscular marine predators are aided by excellent sight and smell underwater but face a couple of challenges once they have caught a meal. Leopard seals' small forelimbs and small claws aren't suited to holding their floating catches still. The seals also lack shearing teeth, which carnivores like lions and hyenas use to cut prey into swallowable pieces.

Instead, leopard seals must thrash and shake their prey to break it down. Holding one end, the seals rapidly sling their catch in an arc over their heads, smashing it on the water's surface until it breaks open. A clever solution for the seal, although one that might appear rather brutal to an outside observer. 

Is sharing really caring?

Leopard seals are solitary and often aggressive to one another, particularly around food. There have, however, been rare sightings of leopard seals appearing to work together to break down prey. Cooperative feeding is more commonly seen in social predators such as killer whales and wolves.

Witnesses report pairs of leopard seals tearing king penguins between them in acts of occasional kleptoparasitism - a behaviour where an animal steals food caught by another individual.

Hunting at a penguin colony can mean lots of competing predators. In the case of the kleptoparasitic leopard seals, up to 36 individuals were seen feeding together. It may be more energy efficient for the seal to share its meal and catch another from the ample supply of prey than to defend a kill or stash it somewhere safe.

About 50% of the time in the feeding pairs, one leopard seal would tear at the prey, while the other held on, anchoring it. Since the seal doesn't need to thrash the prey about to break it up alone, it likely uses less energy - although it's impossible to say whether the seal considers this.

Human interactions with leopard seals

When humans and leopard seals come face-to-face, our interactions don't always go well. Fatalities have occurred on both sides. 

On Ernest Shackleton's Antarctic expedition in the early 1900s, Major Thomas Orde-Lees was chased across the ice by a 'sea leopard', described by the Ohinemuri Gazette in 1917 as 'a blood-thirty [sic] monster'. The seal was shot by another member of the expedition party. In 1985, Scottish explorer Gareth Wood was almost dragged off the ice by a leopard seal. Wood's companions reportedly kicked the seal's head with their spiked crampons until it let go.

Like other seal species, leopard seals will chase or try to bite intruders. When they haul out near human populations, authorities tend to warn inquisitive people to keep their distance.

In 2003, researcher Kirsty Brown was dragged deep underwater by a leopard seal in Antarctica. Following her death, an investigation based on 30 years of recorded interactions found that the seals are generally more likely to attack humans at the ice edge. Humans are not typical prey for leopard seals, but our shape is probably fairly similar to that of a penguin as we waddle along on the ice. 

The inquiry also found that when interacting with humans in the water, leopard seals were typically curious rather than aggressive. Photographer Paul Nicklen encountered this in 2006 - a female leopard seal brought him increasingly worn out and dead penguins, which he interprets as the animal perhaps trying to teach him to hunt.

But contact isn't always initiated by the seals. A variety of seal species have been commercially hunted for their skins, flesh and fat. In 1918, former newspaper The Colonist described how leopard seal oil was 'good quality and utilisable for the same purposes as commercial seal and whale oils.'

Leopard seals are now protected by The Antarctic Treaty, which designates the Antarctic as a 'natural reserve, devoted to peace and science'. The Convention for the Conservation of Antarctic Seals prevents leopard seals and other species from being over-exploited by humans.

A secretive species

The leopard seal's Antarctic habitat usually keeps this species far away from prying human eyes. Together with their preferred solitude, this makes them notoriously tricky to study.

Population estimates of leopard seals range from as few as 18,000 to 440,000.

We also don't know about their breeding habits in great detail. Pups are born on the ice between November and December, though in South Georgia pupping occurs a little earlier. The breeding season is thought to begin soon after pups are weaned. Leopard seals are known to be quite vocal, possibly to attract a mate or for territorial signalling. Experts think that the seals mate in water in the wild. 

We don't know how much of an effect climate change is having on leopard seals either. The loss of ice used for pupping grounds, resting areas or as their mammal and bird prey's habitat would have a negative effect on this species. A decline in the supply of krill would also impact leopard seals and some of their other key prey species. Changes in leopard seal feeding behaviour and numbers could alert us to problems elsewhere in the food chain.

There are fundamental gaps in our knowledge of these incredible apex predators, making them something of an enigma. This may be part of the reason why leopard seals are occasionally reduced in some media to a supporting role as the mysterious villains.

As a predator of larger Antarctic animals such as penguins and other seals, leopard seals keep these animals' numbers in check. By maintaining balance in the ecosystem, leopard seals play an important role on our planet, regardless of any public image issues.