2017 | St Andrews, UK
Dwarf gourami (Trichogaster lalius) water shooting
Today’s post is about a little tropical fish with an unusual talent. When it sees a tasty morsel overhead, it creates a thin jet that shoots out of the water, ambushing unsuspecting insects. And it does so with great accuracy—perhaps you could even describe it as a little archer. But no, it’s not the fish you’re thinking of.
Throwing up
The title of Jane Goodall’s first published report on wild chimpanzee tool use (in March 1964) mentioned only one specific activity: aimed throwing. Termite fishing and leaf sponges were in the mix, but Goodall was careful to emphasise how she’d seen the Gombe chimpanzees deliberately throw rocks at baboons and a National Geographic photographer, seemingly out of frustration or aggression. And yet, despite the likely importance of throwing in human evolution—prominent psychologist James Gibson wrote that, ‘The missile that can be thrown is perhaps the earliest of tools’—work on hurled objects has played second fiddle to handheld tools in the intervening decades.
Projectile studies on our primate relatives are rare, certainly, but spare a thought for researchers dealing with animals with no hands. If all a creature can do is use its mouth or fins or wings to flick, swat or spit, is whatever it pushes around really a tool? When a killer whale pod creates a wave to wash a cowering seal from its ice floe, for example, is that actually tool use? You can decide for yourself: here’s what that looks like in action, via BBC Earth:
The way that the killer whales manipulate the water around them is deliberate, controlled and coordinated. It has a clear goal. To the unlucky seal, it’s being hit by an essentially solid object wielded by a group of bloodthirsty hunters. If one of the orcas had picked up floating driftwood and swiped the seal off the ice it’d be unquestionably tool use: is the wave-strike any different?
All of which sets the stage for today’s featured animal, the dwarf gourami (Trichogaster lalius). Like the orcas, gouramis have developed an ability to use water as if it were a weapon. And just like stone throwing by the Gombe chimpanzees, this is a behaviour that was noted and discussed in the 1960s and then essentially set aside. Instead, if nowadays you hear of a fish making ballistic use of water jets, odds are it’s the work of the aptly-named archerfish (Toxotoes spp.).
Here’s an archerfish in action, showing the complete control it has over the impact of its strike:
However, there is a new front emerging in this fishy projectile publicity war. For the past few years, Nick Jones and a team from the University of St Andrews have been conducting the first modern studies of gourami water shooting. Their discoveries show that these often colourful little swimmers may not be as prodigious as archerfish in their water-pistol-work, but they can still pack a well-aimed punch.
Performance drop
Between November 2017 and February 2020, Jones and colleagues in Scotland, Germany and Finland set out a series of tasks for their captive group of dwarf gouramis. Their results, published in the Journal of Experimental Biology in December 2021, concentrate on three main questions:
Do their gouramis even show any kind of water-spitting behaviour around live insects (animal behaviour research has to start from first principles like this surprisingly often!);
Is there a female/male difference in how often or how well the gouramis shoot water jets; and
What happens if the experimenters move the target to different heights—do the fish persist in their efforts?
The analytical method for dealing with that first question is admirably clear and to the point:
As we merely wanted to confirm whether fish shot down and ate prey, this experiment was observational only.
They gave five untrained, female fish opportunities to shoot down live insects—either fruit flies or baby black crickets—sitting on a dowel 2cm above the water surface. One of the gouramis never took the opportunity, but the other four were keen to not throw away their shot, taking at most three trials to get the job done. On to the next question.
The idea that male and female gouramis might have differing water-jet abilities comes from their nesting behaviour. Like some frogs and eels, male gouramis build bubble nests at the water surface that help protect fertilised eggs as they develop. Females don’t take part in this nesting activity. The research team therefore hypothesised that the water jet behaviour might either relate to or be a side-effect of this bubble-making task, in which case males would be more likely to both shoot and learn to shoot jets.
This is what a gourami bubble-nest looks like from above:
The St Andrews team first trained ten female and ten male fish to associate a black plastic bead in their tank with food—each time the fish bit or touched the 5mm bead they were given a dried bloodworm treat. Once the association was locked in, they moved the bead 2.5cm out of the water, so the only way the fish could keep getting fed via bead was to use a water spout. More males than females ended up taking a shot (9 vs 7 females) but that difference isn’t significant, which means the experiment didn’t find evidence of a difference in the tendency to shoot.
There was also no difference in how quickly either sex learned to associate the bead with food, or to successfully hit the bead once it was raised out of the water. Whatever the reason for the evolved gourami firepower, bubble-nesting doesn’t seem to hold the key.
Here’s a female gourami putting her skills on display, from the 2021 study. You’ll notice that the jet seems to be coming from a slightly different place than the fish’s head, which is a refractive effect (because air and water have different densities) that the fish also needs to compensate for:
Or if you prefer, this is a short (30s) video of the same event, showing how the female makes multiple attempts on the target, in rapid succession:
The team’s final question dove deeper into the ways that individual fish approached the shooting gallery. Could they adjust to different target heights, and if at first they didn't succeed, would they try, try again?
Here, the gouramis showed a level of sophistication that seems to go beyond mindless spitting. When the target was more than 3cm above the water, they took longer to line up the shot, and ended up making fewer attempts. They also missed more often as the experimenters moved the target from 1 to 5cm height. Perhaps the fish were aiming to conserve energy and maximise their returns, just like a basketball player taking fewer, riskier shots from outside the 3-point line (Steph Curry excluded). There was also an element of ‘the fish who hesitates is lost’, with the gouramis who were quicker to shoot also having the highest success rate.
Shooting range
The researchers are quick to point out that gourami water-tool use doesn’t quite measure up to that of their more famous archerfish cousins. Archerfish can accurately hit targets more than a metre away, while the gouramis get uncomfortable at anything over a few centimetres. As a commentary accompanying the main research paper put it:
dwarf gouramis can add their name to the list of water squirters capable of downing dinner with a ballistic jet, albeit a shorter dribblier imitation of the archerfish’s high-precision fluid missiles.
Still, evolution doesn't have a goal, and even an outside chance of capturing something from outside the water can increase a fish’s survival.
Wider interpretation of these results, including tracking its evolutionary history, is hampered by the fact that not enough is known about dwarf gourami behaviour in the wild. Even the fish used in these experiments were bought from a local ornamental fish wholesaler, with their lives lived in tanks rather than shallow tropical seas. Perception and biomechanics are important too: the role of prey size and movement in triggering gourami attacks is yet to be worked out, and it’s not clear whether archerfish have more specialised mouthparts that make their shots so much more effective.
Jones and colleagues have previously worked with archerfish, so they know the right kinds of questions to ask, and where to look for answers. Studies such as this one will only continue to raise the prominence of animals that use water as a tool. On this blog we’ve already seen how supersonic, super-heated underwater bubbles are an effective weapon for the snapping shrimp (Synalpheus regalis), and we’ll spend more time with the wavy killer whales in a future post.
Just as importantly, no matter what turns up in the future, this kind of behaviour continues to push against our understandings and definitions of tool use (is all displaced water a tool, or even wafted air? What even is an object?). Let’s not forget that this larger picture is a big part of the fun of exploring these topics!
Sources: Goodall, J. (1964) Tool-Using and Aimed Throwing in a Community of Free-Living Chimpanzees. Nature 201:1264-1266. || Gibson, J.J. (1979) The Ecological Approach to Visual Perception. New York, Psychology Press. || Jones, N. et al. (2021) Short-range hunters: exploring the function and constraints of water shooting in dwarf gouramis. Journal of Experimental Biology. 22:jeb243477. || Knight, K. (2021) Sharpshooting dwarf gouramis target dinner with dribbly jets. Journal of Experimental Biology. 22:jeb243865. || Jones, N. et al. Presence of an audience and consistent interindividual differences affect archerfish shooting behaviour (2018) Animal Behaviour 141:95-103.
Main image credit: Fishhi; https://www.flickr.com/photos/133633232@N05/29588017794 || Second image credit: Wikipedia; https://commons.wikimedia.org/wiki/File:Schaumnest.jpg || Third image credit: Jones et al. (2021)
