Science
Photo by Reyer Boxem

ThebreakthroughJan van Gils

Delighted by diarrhoea

Photo by Reyer Boxem
Academic research seldom has swift results. This series is about that moment the puzzle pieces do fall into place. Part 6: Biologist Jan van Gils discovered why great knots avoided a particular type of clam.
7 December om 11:17 uur.
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om 11:17 uur.
December 7 at 11:17 AM.
Last modified on December 7, 2023
at 11:17 AM.
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Door Marit Bonne

7 December om 11:17 uur.
Laatst gewijzigd op 7 December 2023
om 11:17 uur.
Avatar photo

By Marit Bonne

December 7 at 11:17 AM.
Last modified on December 7, 2023
at 11:17 AM.

He was overjoyed when he saw the diarrhoea. Jan van Gils, professor of global change ecology of migrant birds, had spent four years trying to solve a mystery: Why did great knot birds eschew a seemingly perfect meal?

He remembers it like it was yesterday. ‘We were in Mauritania, where the birds go for the winter, and everyone was so happy.’ Ideally, he would’ve celebrated the discovery with a bottle of champagne. ‘But we were in an Islamic country, so we had to settle for a cup of local tea.’

Canary

The great knots that Van Gils studies can tell us a lot about climate change. The migratory bird is a true cosmopolitan, travelling to various places: it breeds in Siberia and winters in West Africa. ‘Everywhere it goes, the red knot is exposed to global warming. It can warn us of its effects, like a canary in a coal mine.’

For example, great knot chicks don’t hatch until the food availability peak has already passed, Van Gils says. Since they miss out on nutritious food, the birds are getting smaller. ‘Their beaks are also shrinking. This means they’re getting worse at accessing clams on the mudflats, one of their biggest food sources.’ 

Loripes

Interestingly enough, the great knots appeared to even be turned off by one particular clam, the Loripes orbiculatus. That was strange, since Loripes have a very thin shell, which makes it easy to digest. It’s pretty much a Michelin-star meal for birds. On top of that, Banc d’Arguin in Mauritania, where the birds live in the winter, is rife with Loripes. ‘Some seven thousand to a square metre’, says Van Gils.

And yet, the researchers rarely found the little white clam’s shells in the knots’ faeces. Why weren’t the birds eating them? 

Van Gils and his team tried to solve the mystery by putting their captive knots on ration: all they were allowed to eat were Loripes. It turned out this made the birds really sick. ‘Twenty minutes after eating the clams, they started getting diarrhoea and feeling weak.’

Rotten eggs 

Once the euphoria of the discovery had abated, the researchers went back to work, wondering why the clams were so bad for the great knots. They decided to smell the faeces ‘that just wouldn’t stop coming out of the birds’, says Van Gils. ‘It smelled like rotten eggs. That same smell was everywhere on the mudflats.’ 

The scientists suspected it had to do with the amount of sulphur in the clams. The clams’ gills are home to sulphur bacteria, that work in concert with the clams to produce their own food.

Tim Oortwijn, one of Van Gils’ PhD students, decided to test the hypothesis. Out in the field, he starved Loripes by putting the clams in a special bag, cutting them off from their food source. This means Loripes had to use its own stored sulphur for food. Ultimately, this resulted in a sulphur-free and very tasty clam. It also solved the great knots’ diarrhoea woes.

Long beak

In the end, though, the great knots need Loripes, says Van Gils. ‘The birds need the clams to supplement their diet of big, difficult to digest shells. But they clearly shouldn’t have too many of them.’ 

He’s already thinking about the next research question. Loripes reside deep down in the sands of the mudflats, which means knots need a long beak to get to them. ‘We’ve also noticed that the knots with longer beaks eat more Loripes’, says Van Gils. ‘Does that mean these birds are able to better withstand the sulphur in the clams? If so, why? There’s so much we don’t know yet. Quite a few PhD students could get their degrees here.’

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