By Matthew Stock
Scientists have shown for the first time how a species of tropical fish can distinguish between human faces. The archerfish used in experiments could demonstrate the ability to a high degree of accuracy; despite lacking the crucial neocortex part of the brain which other animals use for sophisticated visual recognition.
The research, conducted by scientists from the University of Oxford and Australia's University of Queensland, wanted to test the long-held belief that differentiating between human faces could only be accomplished by more sophisticated animals, such as primates.
The archerfish, found largely in Australia and southeast Asia, was chosen for its ability to spit a jet of water; a technique it uses to shoot down insect prey even above the water level.
In laboratory-based tests, an archerfish was presented with two different images of human faces and trained to 'choose' one of them by shooting a jet of water at it.
"We present them with different stimuli, and it can be a whole range of different things. But what we do is we give them different options and then we train them by giving them a food reward to select a particular one. So this can give us a huge amount of information about what the fish is able to see and how they do it," explained lead author Dr. Cait Newport from the University of Oxford.
In subsequent tests, the archerfish were presented with the learned face and a series of new faces. Researchers found that the fish could discriminate one face from up to 44 new faces with up to an 81 percent success rate. They were able to do this even when features such as head shape and colour were removed from the images.
In her lab at the Department of Zoology, Newport demonstrated a similar level of visual perception in her Picasso triggerfish. The brightly coloured tropical fish were able to successfully pick out a black coloured disc mounted on a board surrounded by white discs.
Newport said the lack of a neocortex in fish and the fact they have no evolutionary need to recognise human faces makes the results of their research all the more surprising.
"It [the brain] is very large in primates, and it's highly folded, so there is a lot of different connections within the neurons happening in the brain; fish entirely lack that. When you look at a picture of a fish's brain it's only got what we consider the primitive sections of the human brain which are underneath that highly folded neocortex. And yet fish are still able to perform really complex behaviours; they can do facial recognition as we showed, they also build social systems, and there's some evidence of potential tool use," Newport told Reuters, adding that the fish could be applying their pattern recognition ability that evolved to detect aerial prey to the task of discriminating human faces.
The cognition demonstrated by fish sheds some light on their ability to recognise and return to the same territory year after year for breeding. This is something that could be under threat in the Great Barrier Reef due to the current mass bleaching of coral reefs.
Bleaching occurs when the water is too warm, forcing coral to expel living algae and causing it to calcify and turn white. Mildly bleached coral can recover if the temperature drops, otherwise it may die.
Australian scientists have said mass bleaching is likely to destroy half of the northern coral. This could prove devastating for the marine life there.
"These guys [the fish] are looking at colours and patterns and textures. And if all of that is bleached… we don't know if they're still going to be able to find their territories, their homes; we don't know how that will affect how they detect predators or potential prey," said Newport.
The study, published recently in the journal Scientific Reports, suggests that complicated brains are not necessarily needed to recognise human faces, even though subtle features need to be identified in order to differentiate.
Newport added that the research provides evidence that fish have much more impressive visual discrimination abilities than previously believed.
"It is amazing what they can do with a really simple brain, as humans like to call it. Although it seems a bit unfair to call it simple - I think their brains are perfectly adapted to what they do and that's what's important to remember about all this - brains can look different, but they've evolved for different tasks," she said.