Today’s featured guest is the fantastic archerfish, who merits this honor for several good reasons:

• It is totally cool.
• It performs highly complex cognitive tasks with tremendous efficiency.
• It raises interesting questions about how we define tool use.

Archerfish in Action

First, the totally cool.

The archerfish is a small fish that earns a living by shooting prey – insects, spiders and even small lizards – out of the sky, knocking them off twigs and leaves and into the water with an incredibly accurate jet of water launched from its mouth. Here’s a brief video that shows off the archerfish’s hunting skills:

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Pretty incredible, huh?

Complex Calculations

And this brings us to the topic of the archerfish’s specialized and complex cognitive abilities.

As the video notes, because the archerfish hunts from beneath the water’s surface, it must be able to take into account both the bending of light at the surface of the water and the curvature of the water stream it shoots toward a target perched as much as two meters away. I’m not aware of any studies on how adept humans are at shooting water pistols at above-ground targets while snorkeling, but in an accuracy contest, I’m betting on the archerfish.

Additionally, in a report published in Current Biology¹ in 2006, researchers from Erlangen-Nürnberg University in Germany showed that the archerfish not only aim accurately, but are able to save energy by estimating the size of their prey and modifying the amount of water they shoot. Using high speed photography, the researchers “discovered that archerfish transfer systematically larger maximum forces to larger targets … for any given size of prey, the fish apply about ten times the forces the adhesive organs of prey of that size could maximally sustain.”

Dinner is served! (photo credit: Peter Arnold)

In a later study published in Science², the same research group elaborated on the efficiency and speed with which archerfish are able to swim to the precise spot where their prey will land after being hit by a water jet. (Because archerfish hunt in groups and are surrounded by other surface-feeders, they have to be able to swim to fallen prey extremely quickly or they will lose it to another hungry mouth.)

The researchers found that archerfish are able react to the motion of falling prey and start swimming to the correct spot at the correct speed within as little as 40 milliseconds, 1/20 of a second. Moreover, the archerfish accomplish this complex task (which requires them to process a host of variables, including the initial height of the prey, the speed of the fall, and the direction in which it is falling) using relatively few neurons and without reference to a priori information regarding the trajectory of the water jet that hit the prey. As the researchers summarized it, “our data show that even complex decisions can be made rapidly and accurately by a relatively small number of neurons.”

So, as we consider the meaning of the archerfish’s impressive skills, we should bear in mind that sophisticated cognitive behavior can evolve to address the particular tasks and challenges facing a species, and that even an animal with a small, non-mammalian brain can accomplish “super-human” cognitive feats if those feats help the animal to successfully adapt to its ecological niche.

Tool Use?

One final question is whether the archerfish is engaging in tool use when it shoots down its dinner with jets of water. We touched on this in the earlier post regarding the fearsome clam-smashing tuskfish, noting Jane Goodall’s definition of tool use as “the use of an external object as a functional extension of mouth or hand in the attainment of an immediate goal.” While there will undoubtedly continue to be debate and disagreement over the definition of tool use, some points to ponder for now include:

• Does the water used by the archerfish constitute an “external object” within the meaning of the Goodall definition? On the one hand, the water was external to the archerfish until it decided to use it to shoot down prey; on the other, the water obviously is not external right at the moment it is launched.
• Does the “external object” need to be solid, as alluded to in the earlier tuskfish post? Why should the consistency of the object matter?
• Can one argue that the archerfish is transforming the nature of the water (from a surrounding environmental medium into a targeted projectile)? If yes, does this imply that the archerfish’s use is more sophisticated than, say, simply picking up a rock lying outside on the ground (or a monkey wrench hanging on an Ace Hardware rack) and using it as is?
• Some researchers have described behavior that meets some, but not all, of the requirements of a strict tool use definition as “proto” or “borderline” tool use. Is that what we are talking about here?
• Should the behavior speak for itself without attempting to attach a label to it? Why does it matter whether or not we categorize the behavior as tool use? Is there something anthropomorphic about the “tool use” label in the first place?

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¹Schlegel, T., Schmid, C., & Schuster, S. (2006). Archerfish shots are evolutionarily matched to prey adhesion Current Biology, 16 (19) DOI: 10.1016/j.cub.2006.08.082.

²Schlegel, T., & Schuster, S. (2008). Small Circuits for Large Tasks: High-Speed Decision-Making in Archerfish Science, 319 (5859), 104-106 DOI: 10.1126/science.1149265.