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This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

Goldfish may spend most of their time swimming up and down a glass tank, but researchers have found they have a sophisticated navigation system that allows them to estimate distance.

While researchers have previously shown a wide array of fish can navigate efficiently, questions remained about the mechanisms involved. Understanding those, scientists say, could help shed light on whether similar brain cells are involved in the internal GPS of the human brain.

“We want to know where are those cells appearing on the evolutionary tree,” said Dr. Adelaide Sibeaux of the University of Oxford, who is first author of the latest research.

Writing in the journal Proceedings of the Royal Society B, Sibeaux and colleagues report how they created a tank with two-centimetre-wide black and white vertical stripes on the walls, connected by similar stripes across the floor.

The team trained nine goldfish to swim along the tank and, when waved at after they had travelled 70 centimetres, to return to their starting position.

The team then tested the fish on whether they could estimate the same distance without the aid of gestures.

For six goldfish, these results were compared with the distance they travelled when the pattern was switched to one-centimetre-wide vertical stripes, a checked pattern with two-centimetres squares, and to two-centimetre-wide horizontal stripes aligned with the fish’s direction of travel.

Each fish made the journey 45 times for each background pattern and was recorded on video.

Changing their surroundings reveals how the creatures gauge distance using the visual density of their environment. #Goldfish #Nature

The team found the goldfish travelled 74 centimetres on average, give or take 17 centimetres, when presented with the vertical, two-centimetre-wide stripes. Similar results were found when the background was switched to the checked pattern. However, the fish turned back markedly sooner when the stripes were vertical but narrow — overestimating the distance they travelled by about 36 per cent.

When horizontal stripes were used, the distance the fish swam varied enormously. “The fish were totally inconsistent,” said Sibeaux.

The team says the results suggest goldfish use a type of “optic flow mechanism” based on visual density of the environment — in other words, they kept track of how frequently the vertical pattern switched between black and white to estimate how far they had travelled. As the world appeared to pass by faster when the width of the stripes was reduced, the fish overestimated how far they had swum.

Sibeaux said a different optic flow mechanism is used by mammals including humans, based on the angular motion of the visual features. However, the results suggest the use of visually based distance information arose early in our evolutionary past, given it is widespread across different groups of animals.

The team says other mechanisms might also be at play, noting the goldfish were more accurate in gauging distance when their start position was closer to the end of the tank, while for some the number of fin beats they made was associated with how far they swam.

Prof. Colin Lever of the University of Durham, who was not involved in the study, said the research suggested goldfish at least partly use the rate of optic flow to estimate distance, although other clues might also be used.

“This study is important and novel because, although we already know that fish respond to geometric information regarding direction and distance, we don’t know how they estimate distances,” he said.

“It’s exciting to explore fish spatial mapping because fish navigation evolved before mammals, and fish navigation has to negotiate the vertical dimension more fully [than most mammals].”

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