The fish that divers come across in the wild have the ability to tell us apart – as long as we assist them with a few visual clues, such as the colour of our dive-gear.
This is the finding of a new study by the Max Planck Institute of Animal Behaviour (MPI-AB) in Germany, which was inspired by the experiences of its scientific divers at a Mediterranean research station in Corsica.
At some point in every field season the divers had found that wild fish would follow them and steal food intended as experimental rewards. Individual fish appeared to recognise whichever diver had previously carried the food and would follow only that diver while ignoring others.
Past research into fishes' ability to distinguish individual humans had however been very limited.
Captive-bred archerfish had been shown to recognise computer-generated images of human faces in laboratory experiments “but nobody has ever asked whether wild fish have the capacity, or indeed motivation, to recognise us when we enter their underwater world,” says co-first author of the new study MPI-AB doctoral student Maëlan Tomasek, from the University of Clermont Auvergne in France.
Willing volunteers
The scientists conducted a series of open-water experiments at a depth of 8m, with the fish participating in the study as “willing volunteers who could come and go as they pleased”, notes co-first author and MPI-AB bachelor student Katinka Soller.
In the first experimental phase Soller “trained” the fish, aiming to attract their attention while wearing a bright red vest, and feeding them while swimming over a 50m distance.
Over time she removed the conspicuous cues until she was wearing only plain dive gear. She would keep the food hidden and feed only those fish that had followed her the full 50m.
Of dozens of fish species inhabiting the marine station, two species of sea bream in particular engaged willingly in the training sessions, surprising the scientists by their curiosity and willingness to learn.
“Once I entered the water, it was a matter of seconds before I would see them swimming towards me, seemingly coming out of nowhere,” says Soller.
The same individuals would attend the sessions day after day, becoming so familiar that she could give them names, such as “Bernie with two shiny silver scales on the back and Alfie who had a nip out of the tail-fin”.
Two divers, different kit
After 12 days some 20 identifiable fish could be relied on to follow Soller on training swims.
The next experimental phase involved testing whether the fish could tell Soller apart from another diver, so she and Tomasek would wear differently coloured wetsuits and fins and swim in different directions from the same starting point.
On the first day the fish followed both divers equally, seeming to struggle to decide which one to pursue. Tomasek did not feed the fish following him, however, and on day two the number of fish following Soller increased significantly.
To check that the fish were learning to recognise the correct diver, the researchers focused on six fish and found that four showed strong positive learning curves over the course of the experiment.
“This is a cool result because it shows that fish were not simply following Katinka out of habit or because other fish were there,” says Tomasek. “They were conscious of both divers, testing each one and learning that Katinka produced the reward at the end of the swim.”
Colour vision
The scientists then repeated the trials while wearing identical diving gear, and found that the fish were no longer equipped to tell them apart. “Almost all fish have colour vision, so it is not surprising that the sea bream learned to associate the correct diver based on patches of colour on the body,” says Tomasek.
Human divers do much the same, he points out: “Faces are distorted by diving masks, so we usually rely on differences between wetsuits, fins or other parts of the gear to recognise each other.” Given more time, the scientists believe that fish might have learnt to distinguish divers using subtler human features such as hair or hands.
“We already observed them approaching our faces and scrutinising our bodies,” says Soller. “It was like they were studying us, not the other way around.”
“It doesn’t come a shock to me that these animals, which navigate a complex world and interact with myriad different species every minute, can recognise humans based on visual cues,” says senior author Alex Jordan, who leads a group at MPI-AB.
“I suppose the most surprising thing is that we would be surprised they can. It suggests we might under-estimate the capacities of our underwater cousins.”
“It might be strange to think about humans sharing a bond with an animal like a fish that sits so far from us on the evolutionary tree, that we don’t intuitively understand,” concludes Tomasek.
“But human-animal relationships can overcome millions of years of evolutionary distance if we bother to pay attention. Now we know that they see us, it’s time for us to see them.” The study has just been published in the journal Biology Letters.
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