''They are famous for their ferocity,'' said David Hocking, a research student who has been studying the marine giants.

A diet of large animals helps the seals become enormous - some females weigh 500 kilograms. But this food is not available all year round, and in winter the seals must survive on tinier creatures, such as Antarctic krill.

''The question is, 'How does such a big carnivore feed on such small prey?' '' said Mr Hocking, of Monash University and Museum Victoria.

In a study of leopard seals at Taronga Zoo he has shown for the first time that they have the ability to suck small pieces of food into their mouth. They then appear to use their unique, interlocking teeth like a cage to trap food as they expel the water.

Mr Hocking said whales used their baleen to sieve krill from large volumes of water. ''What is remarkable is we think the leopard seal may be doing a similar thing and filter-feeding using their teeth.''

He said knowledge about the feeding habits of Antarctic leopard seals had mainly been gained by scientists observing their behaviour from shore, where the penguins lived.

For his project he studied nine seals in Sydney and at Melbourne Zoo, including Taronga's two leopard seals, Sabine and Casey, and two Australian fur seals, one sub-Antarctic fur seal and four Californian sea lions.

He placed small fish or squid into a specially constructed feeding box, as well as scattering them around the pool, and filmed the seals eating underwater. The heads of the fish were pointed outwards because ''seals prefer swallowing fish head-first to avoid the spines''.

He also checked the seal's teeth and examined museum specimens of seal skulls.

Given their aggressive reputation, Mr Hocking was impressed with how readily the well-trained leopard seals opened their mouths for inspection. ''They're adorable animals.''

The finding that leopard seals can suck and appear to filter feed is a possible key to their survival in winter and could help predict how they will adapt to climate change when krill may be less abundant.

The other seal types were also found to use suction, biting and snapping. One of the Australian fur seals also repeatedly blew bubbles through her nose to try to get food out of a feeding box.