Hit, sunk
In the Indo-Pacific region it is dangerous to sit just above the water surface: flies and co are quickly the victims of a predatory spitting attack. Because there are hungry archer fish lurking in the water, which snatch their prey with a well-directed jet of water. Because this is actually as strenuous as it sounds, the animals adjust their energy expenditure to the size of the bite.
Do you have a tiger in your tank at home? We're not talking about the liquid food for your car, but a family of fish with unique abilities: the toxotidae or archer fish, which are known in their Asian homeland as tiger fish because of their black stripes on a shiny silver background. And unique in that they spit down part of their food.
As long as they can find food floating on the surface, they usually forego the feat. But if hunger drives them and they see something worthwhile within spitting distance, they take a mouthful of water, press their tongue against the palate, in which a groove is dug as a built-in spit-tube, then hit the previously opened gills against the body and thus squirt a well-aimed one Water jet on the unsuspecting victim. When it is hit, it falls into the water, where it is immediately devoured.
Sounds exhausting? It is. After all, the fish, which are about twenty centimeters in size, not only hunt flies in this way, but even small lizards - there must be something tasty behind the shot. Scientists used to think that the finned creatures always expend the same amount of energy, regardless of the size of the prey: shot, hit, sunk. Thomas Schlegel, Christine Schmid and Stefan Schuster came to different conclusions.
The scientists from the University of Erlangen-Nuremberg photographed their spitting aquarium inhabitants at 5000 frames per second and also recorded the mass and the initial speed of the ejected splashes. In doing so, they established a clearly linear relationship: the heavier the prey, the thicker the jet of water. Speed and pressure, on the other hand, remained the same regardless of prey size.
Is this feat a matter of practice? Not at all, the researchers noticed: even fish that didn't have to exert themselves very much for two years - every splash, no matter how small, was rewarded with plenty of prey - still expend more energy after two years with larger bites, although it wasn't necessary at all. The spitting program seems to be innate and unchangeable - unusual for a species that is considered to be extremely capable of learning.
Why the fish only change the amount of water but not the speed or pressure is probably due to a fundamental relationship that researchers uncovered in 2003. At that time they found that the maximum adhesion forces - despite the variety of types and adhesion mechanisms - all follow the same rule and are directly dependent on size. The force required for an archerfish to overcome its prey against these forces therefore increases linearly. And here the hunters play it safe: To be on the safe side, they invest ten times what is necessary.
You have chosen the most sensible way to adjust your energy expenditure. If they wanted to achieve the same effect with a correspondingly higher speed of the spit jet, this would be reflected in the square of the energy expenditure. Or to put it another way: With twice the strength, you need twice as much and not four times the effort. This arithmetic could certainly be learned, but if the internal programming takes over, nobody will complain - not even archer fish.
