Synergy between social and private information increases foraging efficiency in ants

Abstract

Insect societies integrate many information sources to organize collective activities such as foraging. Many ants use trail pheromones to guide foragers to food sources, but foragers can also use memories to find familiar locations of stable food sources. Route memories are often more accurate than trail pheromones in guiding ants, and are often followed in preference to trail pheromones when the two conflict. Why then does the system expend effort in producing and acquiring seemingly redundant and low-quality information, such as trail pheromones, when route memory is available? Here we show that, in the ant Lasius niger, trail pheromones and route memory act synergistically during foraging; increasing walking speed and straightness by 25 and 30 per cent, respectively, and maintaining trail pheromone deposition, but only when used together. Our results demonstrate a previously undescribed major role of trail pheromones: to complement memory by allowing higher confidence in route memory. This highlights the importance of multiple interacting information sources in the efficient running of complex adaptive systems.

Figure 1.

Apparatus used to create a walkway marked by trail pheromone. An ant colony is split into two equal fragments. The donor fragment is allowed access to a 21 cm long walkway covered in printer paper with a 1 M sucrose feeder at one end. The middle 7 cm section has an additional paper overlay. Once either 1 or 20 ants had deposited pheromone at least once on the middle section, the 7 cm overlay is discarded and the 21 cm walkway cleared of ants. This was then used to replace the walkway in the test fragment colony. Simultaneously, the test ants are also visiting a sucrose feeder at the end of a 21 cm walkway. Ants are marked individually at the feeder and allowed to make one or three return visits. The trail pheromone-marked walkway from the donor colony is then used to replace the original walkway. A fresh 7 cm paper overlay is placed over the middle section to mask any possible marks, and discarded and replaced every time an ant walks over it. On the right are two representative paths of an experienced ant returning to the feeder. The blue path (left) is of an ant with one previous visit to the feeder. The red path (right) is an ant with three previous visits to the feeder. Both paths are outbound walks on a heavily pheromone-marked walkway.

Figure 2.

Comparing behaviours on middle and side trail sections. Side trail sections always have home-range markings plus trail pheromones where indicated. Middle sections are always unmarked. (a) Pheromone laying reduced when experienced ants step off a pheromone-marked section. (b,c) Walking speed increased and sinuosity decreased when experienced ants walk on substrate marked with pheromone. M, route memory; P, side sections marked with trail pheromone. Annotations refer to significance level (GLMM, ***p < 0.001, **p < 0.01, *p ≤ 0.05, n.s. = p > 0.05). Thick connecting lines present statistical comparisons. Whiskers on bars represent 2 s.d.