Shoots flies on leaves

2

January 9, 2014 by comparativelypsyched

PSM_V12_D319_Archer_fish

At first glance, archer fish aren’t particularly interesting. Their dreary colouring and dull patterning is not striking, and at 10cm they are no goliaths of the seas. They are not rare, found in brackish water off Australasian landmasses, and are also popular aquarium fish. However, if you ever get the chance to watch an archer fish, you will find this seemingly ordinary animal possesses a suite of fascinating behavioural talents.

Archer fish are named after one particular skill. All seven species can force water against a groove in their mouth, causing a stream of water to shoot into the air above them- as long as they are close to the water’s surface and correctly orientated. These carefully aimed blasts can dislodge insects from branches and blades of grass from up to two metres away (a distance twenty times the length of the largest archer fish!). The video below captures this extraordinary behaviour in all its glory.

In this video the fish are shooting at almost stationary targets. The narrator explains the inherent complexity of this skill- the archer fish must account for refracted light and gravity when aiming- but, the archer fish’s talent is not limited to stationary prey.

In 2006, Stefan Schuster and his colleagues reported a series of elegant studies that examined the archer fish’s remarkable ability to shoot moving targets. They presented captive fish with targets travelling at a range of speeds and heights. If a fish hit a target, a researcher would drop a piece of food into the fish tank, simulating the reward received in the wild when prey is struck. It was not surprising that the archer fish were initially dreadful, however, the task was shortly mastered, confirming anecdotal observations of this behaviour in more natural environments. But how did the fish adapt their behaviour to accomplish this feat? What strategies were implemented?  

The authors examined a number of possible explanations. Fish didn’t shoot wider streams of water, and they didn’t shoot more often. Two novel shooting strategies were identified: The “leading strategy”, and the “turn and fire” approach. The leading strategy required the fish to adapt it’s shooting-orientation to account for the movement of the target. Instead of aiming to where the target is, the fish would aim to where it will be at the time of impact; basically, a clever correction to take movement into account.

The second strategy, the “turn and fire”, has a bit more flair. The fish does not shoot while stationary but tracks the moving target, changing its orientation so that it is always facing the target. The projectile is released while the fish is still moving and the momentum of the turning fish is transmitted to the shot, correcting for the movement of the target.

The archer fish’s ability to learn these new strategies demonstrates great flexibility, but one additional finding by Schuster and his colleagues greatly contributes to the allure of this fish. The fish were housed in small social groups during this study. In one tank there was one particularly belligerent male that chased other fish away during training sessions, monopolising all opportunities to shoot the targets. Like the fish in other tanks, the dominant male’s performance improved over time, and once his training was complete he was removed from the tank to allow his group mates an opportunity to partake. Without the benefit of practice it was expected that these naive fish would follow a similar learning pattern to the others- i.e. start out poor but quickly improve. Surprisingly, this wasn’t the case. The fish that had no opportunity to practice shooting, but plenty of chances to observe the behaviour, were able to hit the target at a rate comparable to a well rehearsed fish (see the figure below from Schuster et al. 2006, p 382).

archer2

The solid green bar in the chart above shows the average number of hits on the target by a naive fish without any previous experience of hitting a moving target; the white bar shows the average hit rate after hundreds of practice trials. The solid red bar represents the performance of the observer fish, the individuals that had never practised shooting but had extensive opportunities to observe the dominant male. While we do not know how these fish can learn this behaviour through observation alone (see this paper for a good technical review of various social learning processes), we do know that no other fish species has demonstrated an ability to learn a behaviour of this complexity from solely watching a group mate.

Uninteresting? Dull? The archer fish hunts like no other fish, can quickly adapt its behaviour in a range of situations, and can learn complex hunting strategies from observation alone. The archer fish may not have the aesthetic flair of the Figure Eight Puffer or the Siamese Fighting Fish, but it has a behavioural beauty, and attraction, all of its own.

References:

Schuster, S., Wöhl, S., Griebsch, M., & Klostermeier, I. (2006). Animal cognition: how archer fish learn to down rapidly moving targets. Current biology, 16(4), 378–83. doi:10.1016/j.cub.2005.12.037

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2 thoughts on “Shoots flies on leaves

  1. Sam Hardman says:

    These fish are fascinating, I would love to see them in the wild. Do you if any other species show similar behaviours? Close relatives maybe?

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