Since the invention of the internal combustion engine, traffic jams have become an inevitable part of life. ‘One of the problems faced by any transportation system is dealing with changes in traffic congestion’, says Tanya Latty, from the University of Sydney, Australia. ‘Sometimes there are simply more cars on the road than the system can handle’, she explains. And ants frequently face the same dilemma as they scurry along well-established odour superhighways that are continually reinforced by use as they forage. However, Latty explains, ‘Food resources can change in quality’, causing congestion as the determined insects have to respond to a change in circumstances. Knowing that the creatures of habit are almost as wedded to their tried and tested routes as we are to tarmac, Latty and Madeleine Beekman wondered whether Argentine ants can track changes in food quality and, if so, whether they could adjust their transport networks to make the most of changes in their circumstances.
Providing mini ant colonies with a choice of three feeders arranged at the apexes of either a small or a large triangle – one feeder was stocked with a sticky (1 mol l−1) sucrose treat, while the other two provided a more dilute (0.5 mol l−1) food supply – Latty filmed the ants’ antics as they scampered between their nest and each of the reservoirs. Then, Latty systematically switched the location of the tempting concentrated sucrose solution to the position of each of the two other weaker feeders to find out how the ants responded. ‘The hardest thing in the study was coming up with a rigorous, unbiased way to define “trails”’, admits Latty, who recalls with a chuckle, ‘We initially tried to do this by eye, but we quickly realised that the human brain is very good at seeing patterns, even when they don't exist’. However, after teaming up with Michael Holmes and James Makinson to investigate the ants’ preferences, patterns eventually began to emerge.
‘In the first few minutes of the experiment, the ants built many trails’, says Latty, adding that the feeder that was filled with the strongest sugar solution rapidly acquired the largest number of trails: the colonies allocated more than 60% of their foragers to plunder the sticky treat. However, when she moved the location of the concentrated sucrose feeder to one of the other feeder sites, Latty was impressed to see that the ants were able to adapt to the changes. Also, instead of constructing new trails from scratch, the ants reinforced existing trails that had previously been underused, while cutting the number of trails leading to the newly diluted feeder. ‘There were enough trails built early in the process for most of the network adaptation to be driven by pruning alone’, she says, explaining that pheromone trails that had been heavily used in their heyday were lost when they fell into disuse and the guiding scents faded.
‘The ants have a remarkable capacity to deal with the various constraints and trade-offs inherent in building a transportation system’, says Latty, who admits that she was surprised by the ants’ versatility. And, having discovered that ant transport networks are significantly more flexible than previously thought, she is eager to learn about the roles of individual ants in shaping their transportation networks and how their memories may help the industrious insects forge new transportation networks as their fortunes change.