Blind cave-dwelling fish are able to discriminate between different quantities, scientists say.
The fish, found beneath the deserts in Somalia, learned to identify the greater of two groups of sticks placed at opposite ends of a tank.
Researchers say it is the first time non-visual numerical abilities have been shown in fish.
They do not know whether the sightless fish have inherited the skills or evolved them to find food.
The findings are published in The Journal of Experimental Biology.
Previous studies have shown that fish, mammals and birds can determine quantities and solve different numerical tasks.
The Italian-based research team chose the fish, Phreatichthys andruzzii, because they have evolved for two million years in completely dark caves and have lost their sense of sight.
Cavefish are thought to use their lateral line - a system of sense organs with specialised cells - to detect changes in the water around them, sense nearby objects and to tell the difference between three-dimensional shapes.
'While mammals and birds have often been tested using visual and non-visual stimuli, all fish studies have been restricted to the visual modality,' team member Christian Agrillo, from the University of Padova said.
'Hence we asked if fish that clearly cannot use visual modality are able to process numerical information as well.'
The fish were housed in a circular tank and two sets of sticks were introduced at opposite ends. The fish were encouraged to swim towards the larger quantity of sticks by placing food near them.
Cavefish might need to use quantity information to find food sources'
End Quote Christian Agrillo University of Padova
Researchers found the fish learned to correctly select between two and four sticks and two and six sticks, even without the food rewards.
To ensure the fish were actually using numerical ability the team also carried out experiments which removed physical clues to the quantity of sticks.
They found they were still able to train the fish to tell the difference between two and four sticks when they kept the surface area, volume or density of both sets of sticks the same.
'In this sense, cavefish - as many other fish - display numerical abilities as they might have inherited from a common ancestor, even though the possibility exists that they might not need to use these abilities as other species,' Mr Agrillo said.
'Of course the possibility exists that this is a case of convergent evolution. Cavefish might need to use quantity information to find food sources and might have developed such cognitive skills independently of the numerical abilities of other vertebrates.
'Our study of course cannot dissociate between these two hypotheses.'
The fish could not discriminate between two and three sticks, which could suggest that vision improves numerical accuracy.
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