Remaining in the water would also have given Placerias some protection against land-based predators such as Postosuchus.

Maybe the smell had a defensive purpose, designed to keep away land-based predators when the alien slept.

Late Cretaceous genera became much larger, including some of the largest land-based predators ever to exist, but most of these later genera had proportionately small forelimbs with only two digits.

Ducks' nests are raided by land-based predators, and brooding females may be caught unaware on the nest by mammals, such as foxes, or large birds, such as hawks or eagles.

Heard Island has vast colonies of penguins and petrels, and large harems of land-based marine predators such as elephant seals and fur seals.

Deep guttural sounds are used to indicate land-based predators while short "band whistle" chatter indicates danger from the air.

The lack of land-based predators on the island means that the bracken habitat is an ideal place for the vole, with the population reaching around 20,000 during the summer months.

"The key areas to consider at the moment are those in the range of land-based predators," like penguins or fur seals, said Mr. Everson.

Neither land-based predators nor birds of prey are hunted.

One of the main Mediterranean nesting sites for the spectacular pink flamingo is a secluded man-made island in a salt marsh where water levels are kept constant and chicks are safe from land-based predators.