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Thursday, December 19, 2013

Ants weigh their options

When choosing between options, we have to weigh the importance of different factors. For example, if I’m deciding where to spend my next vacation, I have to consider cost, location, travel logistics, and many other aspects. Some of those aspects are more important  to me than others. Thus, I might be willing to compromise on my desire to go somewhere exotic if spending time with friends is a higher priority. However, if my friends become unavailable, I have to reweigh my options.

Ants also make choices, though not about whether to go on vacation. Their biggest decisions involve selecting a new nest. Temnothorax rugatulus ants live in rock crevices and prefer them to have dark interiors and small entrances.

Takao Sasaki and Stephen Pratt from Arizona State University gave ants a choice between a dark nest with a larger entrance and a nest with more internal light but a smaller entrance. In other words, the ants were asked to weigh the two attributes (light level and entry size) to decide which one mattered more to them.

Once their preferences were established, the researchers gave ants a new set of choices. Half the colonies (group A) could pick a home with the same light level and as the nest they had been ‘encouraged’ to vacate, or choose one with more light. The other colonies (group B) chose between a nest that was the same as the one they had just left or one with a larger entry hole. All the colonies picked a new nest that was identical to their old, standard, nest, rejecting the clearly inferior choices.

Finally, the ants were given their original choice back: smaller entry with more light or larger entry and less light. They were not allowed to stick with a standard nest, they had to pick one attribute over the other.

Figure 1.

Pre-treatment: An initial binary choice between sites E (small entry) and L (low light level) showed how colonies weighted entrance size and interior light level.
Light/entrance treatment: ants chose between a standard nest (S) and another that was inferior to the standard nest in one attribute, but identical to it in the other. For half the colonies (group A), the inferior attribute was light (
IL); for the other half (group B) it was entrance size (IE). 

Post-treatment: colonies repeated the original choice to determine whether experience had altered their preferences.

Colonies were more likely to prefer the darker nest with the larger entry hole after they had made a point of rejecting lighter nests (group A). Meanwhile, the group B colonies that had rejected the larger entry became more likely to pick the lighter nest with a small hole. Once a colony had decided, ‘we don’t like nests that have large entry holes’, they continued to find entry hole size more important that other attributes. And the same was true for colonies that had explicitly rejected homes that were too bright. They decided that light level was the most important thing to look for in a new nest.

This suggests that circumstances can affect which attributes matter most to ants. Rather than being mere automatons, ants are capable of being influenced by their experiences. Which means they aren’t as different from us as we thought.


Sasaki T, & Pratt SC (2013). Ants learn to rely on more informative attributes during decision-making. Biology letters, 9 (6) PMID: 24196516