Activity: Gastroliths

Talia with the mock croc stomach

On SCOPE's Crocodiles episode, you saw Talia show us why crocs have stones in their stomachs. Here's how she did it:

What you need

• a lettuce
• a plastic container
• water
• some pebbles (bigger than marbles, smaller than golf balls)

What to do

1. Fill the plastic container about one third of the way with the pebbles.
2. Pour in water until it is level with the top layer of the pebbles.
3. Add some lettuce leaves, put the lid on and shake for 1 minute.
4. Strain out the water and take a look at what's left.

Ballast experiment

1. Use two identical plastic containers, one with pebbles in, one without.
2. Place both in water and observe the level they float in the water.

What's happening?

When a crocodile eats, it's a fairly no fuss affair, grab, tear, swallow - not a lot of chewing. As a result they need a pretty robust digestion process.

The crocodile's stomach is divided into two chambers; the first is powerful, muscular and contains gastroliths, which are literally stomach (gastro) stones (lith)!

A crocodile swallows stones, which it stores in its stomach to help grind up its food.

The pebbles in this experiment are going to be the gastroliths; the water plays the role of the crocodile's stomach juices!

Lettuce is hardly the usual meal for a croc - but friendlier to handle for at home scientists.

The pebbles crush and tear up the lettuce - just as gastroliths crush and tear up the food in a crocodile's stomach. A very handy digestion technique, but wait there's more!

There's another reason why crocodiles swallow stones: the stones also add weight to the crocodile and act as ballast. This helps the crocodile to sink when it exhales sufficient air from its lungs and allows it to lurk just below the surface of the water waiting for the opportunity to strike its prey. In the same way, the weighted container sits down low in a sink of water - just like a croc.