Penguins change their minds on their favourite food

15 July 2007


Penguins living in the Antarctic have changed their minds about their favourite food, a change of diet that could have been triggered by the hunting of whales and seals over the last two hundred years.

Now, you might think that an obvious way to find out what a penguin has been eating would be to look inside its stomach but that's only useful for penguins that are still alive.

But luckily, Penguins live up to that old cliché "you are what you eat" because they leave behind traces of what they ate in their eggshells, feathers and bones.

Steven Emslie and William Patterson, from the United States, set about investigating the diets of Adelie Penguins from the ANtartic continent by collecting eggs and feather samples from modern day penguins and also the fossilised remains of penguins that lived 38 thousands years ago.

What they were looking for was the balance of two chemical compounds or isotopes inside the penguin remains known as carbon 13 and nitrogen 15 - both of them are naturally occurring, but they are found in different ratios in different types of animals.

So, for example in fish, the ratio of c13 to n15 tends to be higher than in crustaceans like krill, those shrimp like creatures that live in cold oceans.

By measuring the ratio of C13 to N15 the researchers could predict what the penguins had been eating.

And it turns out all the ancient fossil penguins had isotope ratios that hinted at a diet composed mainly of fish, but that in all the modern penguins the isotope ratios were much lower indicating a shift towards eating Krill.

And that fits neatly with what has been going on the Southern Ocean. Since the 19th century seal hunters and whalers have been doing a very effective job at getting rid of lots of the seals and whales that used to live in the seas around Antarctica and those are the creatures that used to feed on krill.

But now that the numbers of these big krill-eating mammals has gone right down, the number of krill, as we might predict, has gone up.

And it seems that penguins have begun to take advantage of these krill, possibly partly because they are so abundant but also because their other food, the fish, have been declining, with new fisheries opening up in the southern ocean.

And the bad news is that there are plans now being made for people to start fishing the abundant krill around the Antarctic so they can be made into fish meal to feed the fish we grow in farms to eat ourselves. And the chances are that we would do a very good job at getting rid of all those krill as well, which could endanger the penguins.


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