Ants are remarkably busy creatures, ranging from navigators that scamper across vast desert pans to aphid-farming garden ants and wood-infesting species, yet little is known about their use of colour vision as they rush around. Although several species have eyes that are equipped with light-sensitive cells that are tuned to ultraviolet, green and even blue shades, Ayse Yilmaz, from the University of Würzburg, Germany, says, ‘No attempt, so far, has been made to analyse how ants form memories of colour’. Knowing that several Camponatus species (popularly known as carpenter ants) have a well-developed memory for odours, Yilmaz and colleagues Adrian Dyer, Wolfgang Rössler and Johannes Spaethe decided to investigate whether Camponotus blandus have a favourite colour and how long they can recall memories of colours that they associate with a tasty treat.
Having selected C. blandus – because of their relatively large eyes and day-active lifestyle – from the University of Würzburg's current collection of 15 ant species, Yilmaz identified keen foragers that were prepared to scale a vertical stick in the centre of their foraging arena in search of food to ensure that they were motivated to participate in her experiments. Then she offered these individuals a choice between two colours (combinations of UV, blue and green) and found that the ants had a strong preference for UV shades.
However, this fondness presented problems when the team was trying to figure out whether the insects could genuinely distinguish between UV and the other two colours, or were simply selecting their favourite colour. Yilmaz offered the ants a choice between two of the colours and rewarded them with a sugary treat until they reliably identified one of the two shades. They quickly learned to distinguish UV from the two other colours, even when the brightness was turned down. However, the insects were never able to distinguish blue from green. And when Adrian Dyer, from RMIT University, Australia, built computer simulations of how the ants might respond to different colours if they were equipped with one, two or three photoreceptors tuned to various wavelengths, it was clear that the simulation of two photoreceptors (tuned to 360 nm for UV and 390–560 nm for blue/green wavelengths) reproduced the ants’ abilities to distinguish UV from blue and green hues.
But how long would the ants hold on to their colourful memories? This time, Yilmaz took advantage of the ants’ UV preferences and trained them to recognise blue or green in favour of UV before checking their memories between an hour and as much as 7 days later. Impressively, the ants were still able to select the colour that they had been trained to recognize 7 days later and were equally capable after 1 h, 12 h, 1 day and 3 days, suggesting that the ants are capable of storing the recollection in their medium- and long-term memories.
‘Our results show that ants can use chromatic information in a way that should promote efficient foraging in complex natural environments’, says Yilmaz, who is now keen to learn more about the molecular and neuronal basis of long-term visual memory.