Connect the Spots: Revealing Leopard Seal Secrets

Jennifer Vernon
for National Geographic News

February 19, 2004

This story is one of a series looking at National Geographic
Crittercam research. Crittercam is a research instrument worn by wild
animals and equipped with a video camera and other
information-gathering equipment. (Get the basics on underwater and terrestrial Crittercams.)

For more on this story, tune in to the Crittercam: Leopard Seals episode on the National Geographic Channel in the U.S. Watch video previews online.

Ask Tracey Rogers, director of the Australian Marine Mammal Research Centre at Sydney's Taronga Zoo, to name a special someone who influenced her career path and she might mention a leopard seal named Astrid.

Rogers said it was an encounter with Astrid while she worked as a seal trainer at the zoo that inspired her to pursue a Ph.D. The pivotal moment came when Rogers entered the leopard seal pool for the first time. An emergency forced her to leave the confines of the safety cage, and Astrid followed, quick to notice the fish still lodged under Rogers's arm.

Rogers said she froze, realizing that a very large predator was after the fish she carried. "I basically was panicking," Rogers recalled. "I was thinking, 'I'm going to get really bitten here.'" Rogers closed her eyes, only to open them moments later to a barrage of air bubbles. Astrid had indeed come closer but was gently nudging the fish with her nose out from underneath Rogers's arm. When it floated clear, Astrid ate it, then turned to look back into Rogers's mask.

"I thought, 'Oh God, you're a smart seal. You guys are great!'" Rogers said the encounter swayed her not only to pursue her Ph.D. but to study leopard seals in particular after she realized they were "really special."

Skilled Predators

Leopard seals (Hydrurga leptonyx) certainly live up to the "leopard" part of their name. These Antarctic marine hunters are stealthy and skilled predators. They are the only seal known to hunt other warm-blooded animals. Leopard seals typically feed on penguins and the pups of other seal species but will eat "just about everything they come across," Rogers said.

Their hunting tactics depend on the element of surprise. Attacking from behind, a leopard seal will use its powerful jaws to grab a seal pup by the head and crush its skull. To catch penguins, a leopard seal will lurk near the water's edge just under the surface to watch shadows on the ice above. Spotting a penguin, it will lung out of the water and onto the ice to snatch its prey and drag it back into the water. The seals will also back into alcoves along the ice or beach, coil up into an S-shape, and then spring forward "like a crocodile," Rogers said.

Leopard seals have highly specialized mouths that are well equipped to handle a wide variety of prey, including fish and krill. Their front canine teeth are large and feline, perfect for grasping prey and slashing it open. Each molar has three distinct cusps that interlock when their jaw is closed, forming a sieve for straining krill from the water.

Krill compose approximately 40 percent of a leopard seal's diet. However, in Prydz Bay, Antarctica, where Rogers has conducted her studies, leopard seals appear to bypass these tiny crustaceans in favor of abundant small mammals and birds.

Rogers has found that individual seals seem to develop a specialty for hunting particular species. So while one individual might be good at catching seal pups, another might be adept at catching penguins.

Unanswered Questions

Despite such insights, there is very little data on leopard seal behavior. Questions abound regarding the species's population dynamics and breeding habits. Scientists like Rogers still do not know whether leopard seals stay in the areas where they were born, if either sex establishes new territories, how long pups stay with their mothers, or why juveniles roam as far north as Australia.

Even the total population size is under debate. Rogers suspects that many males may be missed during visual surveys conducted during breeding season because males are underwater calling to and listening for females. Seeking a more accurate estimate, Rogers has deployed sonar buoys beneath the ice to record and count vocalizations.

Male and female vocalization patterns themselves are of great interest to Rogers and her colleagues, in particular because leopard seals are such solitary creatures. To find one another they must communicate clearly over large distances. Their songs, therefore, must be highly structured, Rogers noted, and can be ornamented with distinctive trills reminiscent of bird or cricket calls.

Female leopard seals have been found to sing as well as males during breeding season, a notable exception in the animal world. Such a skill is necessary, given that each solitary female reaches her three- to four-day receptive phase independently, a phenomenon known as asynchronous estrus. If females were unable to convey this important mating and location information, males would not know which females were receptive or where to find them.

Yet little is known about what happens when males and females meet, Rogers said. Adult females are noticeably larger than males, growing up to 3.6 meters (12 feet) long and weighing up to 450 kilograms (990 pounds). So, Rogers wonders, do the larger females select their mates if more than one male answers their call?

Another curious aspect to leopard seal vocalizations is their use of high-pitched clicks up to 165 kilohertz, a frequency far beyond the range of normal human hearing. Rogers speculates leopard seals might use these clicks to echolocate food and air holes during dark, ice-bound winters.

With "a myriad of questions" still to be answered, Rogers hopes future use of instruments such as Crittercam (see sidebar) might reveal more detailed information on the species's feeding patterns, hunting techniques, and mating behaviors. Such information will be crucial to a better understanding of these mysterious creatures and the fragile Antarctic habitat they call home.