Hitting the Mark

An archerfish shooting a fly. Image from Wikimedia

One of the things we humans used to think made us unique was making and using tools. However, we now know that plenty of animals use tools and sometimes make them. Most of the evidence for tool-use in non-humans comes from apes and corvids. Fish, despite having nothing with which to grab a tool except their mouths, do use tools and, on occasion, make (or modify) them. There are some pretty cool examples of this, from cichlids that use leaves as platters to transport their eggs [1], to wrasse that crack shellfish by throwing them against rocks ([2] which, to be pedantic, doesn’t qualify as tool-use under most definitions; if they threw the rock at the mollusc, rather than the other way around, it would).

The star tool-user amongst fish, though, must surely be the archerfish. Archerfish suck water into their mouths, place their ‘lips’ right at the surface, and shoot a jet of water at unsuspecting insects sitting on branches over the water. The jets of water knock the insects into the water and the archerfish eat them. If you’ve never seen this, there are videos of it all over the internet (like this one). So archerfish use water as a tool; in fact, they use it as a weapon, in the same way that riot police use water-cannons (except that the cops fire on conspecifics who they then do not consume, usually).

Whenever comparative psychologists observe this sort of behavior they immediately ask the question: how flexible is it? In other words, is this a simple reflexive behavior (say, like your knee-jerk reflex), or does the fish ‘understand’ something about the physics of what it does, which might allow it to modify the behavior in response to changes in the situation (like your ability to throw a ball fast or slow or curved)?

By filming archerfish at very high frame rates, researchers have found that their shots are tuned in a lot of different ways. They can hit objects with breathtaking precision at ranges from a couple of centimeters to almost two meters away. They adjust the amount of water they shoot to the distance and size of their target (more water to knock down larger prey), correct the angle of their shot for the visual distortion caused by the transition from water to air, and can learn to hit rapidly moving targets simply by watching another fish do so [3]. Let’s pause for a second to marvel at that last one. When they first see a moving target, archerfish are very bad at hitting it. It takes a lot of practice until they get good. However, other fish that merely watch this practice happening (and probably heckle), without ever getting to shoot at the moving target themselves, are almost as good as the practiced fish.

Most impressively, in my opinion, archerfish modify the speed of the water leaving their mouths so that the back of the jet is moving more quickly than the front. This means that as the water jet flies through the air, the back catches up to the front so that all the water hits the prey at the same time, as a blob, delivering a much stronger punch [4]. They even adjust this according to the object’s distance, so that the maximal focusing of the blob happens just as it reaches the target. This has been taken by some people as evidence that they are ‘shaping’ their liquid weapon: not just using a tool but making one as well.

This is one sort of flexibility in the behavior, and it’s pretty impressive. Very recently, however, it has been found that archerfish will also use jets of water under the water. Researchers gave the fish a piece of food buried under some sand in a bowl and the fish used jets of water to blow away the sand and expose the food. Interestingly, they used the same sequence of mouth movements as they do when shooting down prey outside the water [5]. This is especially interesting from a cognitive perspective because it suggests that the fish can adaptively use their tools for different, possibly new, things. Kind of like MacGyver (the original, not the remake). This kind of flexibility requires that you know something about the properties of your tool and how it interacts with other objects in the world (sometimes referred to as the ‘affordance’ of the tool). It may be a bit early to claim that archerfish have this level of understanding, since blowing sand off food is likely something they also do often in the wild, so it isn’t a completely novel use of their tool (we’d be less impressed with MacGyver if we knew that he practices making tanks out of shoelaces and olive oil every evening).

Finally, there is one more thing that makes archerfish exciting to researchers. One of the difficulties in doing research on fish is getting them to make distinct choices. Usually, animals make choices in experiments by moving. Fish, however, move a lot (compared to, say, rats) and it is hard to make them choose one spot and stay there long enough for you to reward them for it. One of the reasons for this is that movement is cheap for fish: they don’t have to support their own weight and experience almost no friction, so there is very little cost to them in going to the wrong place first. This tends to mess up learning experiments. Archerfish, however, make distinct choices (what to shoot at) which are quite costly in terms of energy. Researchers are increasingly using this to show that they can learn all sorts of amazing things, such as telling apart human faces [6]. So they can spit in your eye, from two meters away, while you’re moving.

1. Keenleyside MHA, Prince CE (1976). Spawning-site selection in relation to parental care of eggs in Aequidens paraguayensis (Pisces: Cichlidae). Canadian Journal of Zoology, 54:2135-2139.
2. Bernardi G (2012). The use of tools by wrasses (Labridae). Coral Reefs, 31:39-39.
3. Schuster S, Wöhl S, Griebsch M, Klostermeier I (2006). Animal cognition: how archer fish learn to down rapidly moving targets. Current Biology, 16:378-383.
4. Gerullis P, Schuster S (2014). Archerfish actively control the hydrodynamics of their jets. Current Biology, 24:2156-2160.
5. Dewenter J, Gerullis P, Hecker A, Schuster S (2017). Archerfish  use their shooting technique to produce adaptive underwater jets. Journal of Experimental Biology. doi: 10.1242/jeb.146936.
6. Newport C, Wallis G, Reshitnyk Y, Siebeck UE (2016). Discrimination of human faces by archerfish (Toxotes chatareus). Scientific Reports, 6. doi: 10.1038/srep27523.