January 16 2024, 18:59

I continue reading Ed Yong’s book Immense World about how animals interpret signals from the surrounding world. Very interesting stuff about seals.

What I learned. Seals are known for their ability to use their extremely sensitive whiskers to precisely follow the hydrodynamic trail left by their prey. To us, it looks like this: a fish swims through a complex curve, then a minute later a seal appears and follows the same curve (although a straight line might have been shorter to the fish). Research has shown that a seal can track a swimming herring from up to 180 meters away — which is comparable to dolphins’ echolocation system.

How do they manage this?

In one experiment, a complex curve was “drawn” in the water along a pool, and then seals were released, and they moved along this curve, although to our eyes, and indeed anyone’s, there was nothing left of it. This is despite the fact that the ocean water is constantly moving. The swimming fish leaves what is called a hydrodynamic trail – water vortices that last for some time (minutes), and the seals’ whiskers detect them.

It should be mentioned that seals have so-so vision, plus the water is significantly opaque (when we talk about a hundred meters) in their habitats, plus this experiment was also conducted with the vision of the seals being blocked (I don’t know how to translate blindfolded better). Vision is generally mediocre both underwater and above water for almost everyone. Birds, then humans, various felines, and a few others are exceptions.

But vortex flows are also created by the whiskers themselves — how do seals distinguish those created by the fish? It turns out that seal whiskers have a wavy structure, thickening and thinning in certain areas, which helps suppress the excitement caused by the vortex flows of the seal itself and the whiskers, and against this background, enhance the vortex flows from fish that swam in that place earlier. Ultimately, this feature increases the sensitivity of the whiskers to the hydrodynamic trails left by prey, allowing seals to hunt effectively even in conditions of poor visibility. And this is unique to seals. For instance, walruses and sea lions have simpler whiskers, and they are not as good at tracking hydrodynamic trails.

By the way, there is something similar in all fish — an organ called the lateral line. It looks like a thin line on both sides of the body, stretching from the gill slits to the base of the tail. When you are cutting up a fish, take notice, it’s quite noticeable in many of them. Lateral line organs help fish navigate, sense the direction and speed of currents, as well as detect prey or enemies, and of course, to swim synchronously in a school without bumping into each other. The sensitivity there is far from the aforementioned in seals, of course, but it is claimed that some fish can thus detect the disturbance from an insect on the surface.

Also interesting, catfish have a unique ability to perceive taste with their entire body – they essentially have taste buds located all over their surfaces from head to tail.

By the way, if a fish in an aquarium is looking at you, it appears as “fish sideways” and not “fish nose at me”. Most likely, a typical fish sees almost nothing directly in front of itself, and the area of maximum clarity is to the left or right side. So if you imagine that it is examining you, then stand by its side.

Well, I hope this was interesting. I don’t understand how some can read, I quote, “600 books a year”. When do they think? When do they stop and google? Sigh.