Shrews are highly sophisticated predators

Shrews are small mammals that are characterized by simplicity and primacy. A new study of the hunting method of water shrews has broken this traditional view. The study showed that remarkably sophisticated methods of detecting prey allow water shrews to catch small fish and insects that live in the water easily both at night and during the day.

This is a skill that water shrews really need. At about half the size of a normal mouse, water shrews have such high metabolism that they have to eat more than their weight each day and they can starve within half a day if they don't find something to eat. So water shrews are very formidable carnivores .

Ken Catania, Vanderbilt's professor of biological sciences, who led the study, said: "Water shrews often hunt at night, so I started to wonder how they could identify them. Their bait in the darkness is almost complete. '

Water shrews are tiny mammals, about half the size of a normal mouse.(Photo: Kenneth Catania)

Catania has partnered with James Hare and Kevin Campbell of the University of Manitoba to answer this question using a high-speed infrared camcorder. The results of their study are published in an article called ' Water Shrews that detect movements, shapes and smells to find underwater prey ' on February 9 in the Proceedings of the National Academy. of Sciences (Minutes of the American Academy of Sciences).

'Our research confirms that shrews in general and water shrews in particular are unusually adaptive creatures with the skills and behaviors that make other mammals jealous,' Catania said. .

Researchers need high-speed camcorders because water shrews have lightning-quick reflexes: It can strike about 50 times a second when it detects the appearance of prey and opens its mouth. prepare to bite in 20 times of a second. To observe how shrews hunt in the dark, scientists must also monitor their behavior in the infrared part of the spectrum, this spectral image is out of sight for shrews.

They observed that this small mammal can catch prey quickly and accurately at night as they do during the day and conclude that shrews have used three basic methods to hunt. To hunt in the dark, water shrews:

- Detect water movements due to prey trying to swim;
- Identify the appearance of prey by using their whiskers;
- Use the sense of smell underwater by blowing air bubbles from the nose and then inhaling again.

Catania discovered the third method in the 2006 study that was published in Science - that is, the ability to track the underwater odor of shrews through the way they breathe out bubbles and later that breathed in the air.

In a recent study, researchers have discovered that water shrews use up two good input methods. They use sensitive mustaches to determine the shape of the object they encounter. And then they are very sensitive to rippling currents like when a fish or an insect tries to escape.

Catania said: 'The combination of these methods poses an extremely difficult question for prey. If they stand still, they will be detected through the sense of smell. But if they try to swim away, they will create currents that reveal their location. '

After observing the hunting behavior of water rats in almost complete darkness, the researchers set out a series of experiments to identify specific prey detection methods that this small predator use and eliminate other methods.

By recording ultrasound cries as well as clear sounds, researchers can rule out the possibility of shrews using ultrasound, echolocation or electromagnetic induction to transmit appear prey.

To test the shrews' reaction to water currents, the researchers equipped a small fish tank with a glass bottom with some small faucets. They put water shrews into the aquarium and record their reactions as they turn on and turn off the tap. They found that shrews constantly attacked disrupted water movements created to cause disturbances when the prey escaped.

To test the ability to identify prey through the shape of water shrews, Catania and his colleagues created silicon objects with the shape of fish, then mixed them into animals made in silicon with rectangular and cylindrical shapes, and put them all in fish tanks with shrews. They then observed when the shrews ignored objects with geometric shapes but immediately snapped on fish-shaped objects after they touched the mustache on these objects.

The researchers also determined that movement also caused water shrews to attack , even when moving object targets do not take the form of a real fish. They created moving targets by attaching a small piece of iron to silicon-based objects and using a magnet placed at the bottom of the aquarium to make these objects move.

'One of the difficulties in conducting these experiments is that the time when shrews recognize the objects we offer as fake fish is also fast. You can only fool them a few times, ' Catania said.

Research supported by NSF Career Award and MacArthur award for Catania, Canada's Natural Science Research Council also supported Campbell and Hare.