“House Hunters” for ants

Have you ever wondered why ants build a nest right next to your mailbox? And when you trample on it, why they would dare to build another one just a foot away? A couple of researchers from Arizona State University did, and they discovered that ants choose their living spaces very similarly to the way humans do—by choosing whichever one has the qualities they liked in the previous one.

Researchers Takao Sasaki and Stephen Pratt, whose study is published in the November 6 issue of Biology Letters, wanted to know how ants made decisions. So they studied colonies of the ant Temnothorax rugatulus, a species that usually lives in rock crevices and can collectively choose a new home if needed.

T. rugatulus forager drinking dyed sucrose solution. Photo by Zack Shaffer.

T. rugatulus forager drinking dyed sucrose solution. Photo by Zack Shaffer.

The experiment took ants living in a standard nest and put colonies in a situation where they had to choose between two new living sites: one with a small entrance but a bright interior, and one with a larger entrance but a dark interior.

The colonies then had to change to another nest with the same choices: either a bright interior and a small entrance, or a dark interior and a larger entrance. Researchers observed the second choice more closely than the first, because the decision pointed to the ants’ desired living standards.

The situation was not too different from a couple of homeowners who move from one state to another and are faced with the choice of a house with larger rooms but fewer windows or a house with lots of light but cramped spaces. As the homeowners would do, ants scout out both spaces before choosing one over the other, but when they do decide, they don’t tend to bicker (like people) over the choice.

In fact, ants make their decision about a new home very carefully. In another study in Bristol, England, Dr. Elva Robinson at the University of Bristol studied how rock ants (Temnothorax albipennis) chose between a poor nest and a good nest. When a colony needs a new living site, several scouting ants look at several locations and assess them. They do not, however, compare them by revisiting them multiple times. When they choose one, some of the scouts recruit others from the colony to assess the nest as well. The new scouts visit the nests, and when the majority of the scouts land at one nest, that nest is designated as the new home, and the entire colony begins to migrate over (Robinson).

During the first migration, the ants in Sasaki and Pratt’s study randomly selected either a home with a lot of light or with a large entrance. The second migration was more telling. Ants that had chosen the homes with a bright interior tended to choose homes with a larger entrance and darker interior the second time, while ants that had initially chosen homes with a larger entrance the first time tended to make that choice again the second time.

“Specifically, increases in the weighting of light level could be detected as an increase in preference for L, whereas increases in the weighting of entrance size could be detected as an increase in preference for E.,” the authors say in the article.

Unlike humans, who compare and contrast several options and often go back and forth between the best options, ants compare options to an internal scale based on the present living situation. The scientists limited the attributes to only two to aid in decision-making.

It is possible, authors hypothesize, that ants may adapt to whatever their present environment is, changing their expectations when forced to choose another location. If ants cannot find a location with their favored attribute, for instance, they may settle for something with an inferior attribute, and that inferior attribute may eventually have greater value if the ants find only sites with the inferior attribute.

Although Sasaki and Pratt did not conduct the experiment for pest management purposes, their results can be used to find new ways to control ant populations by forcing their choice in living spaces. Temnothorax ants live in crevices, so their choice of spaces that had a low light level and wide entrance make sense. By understanding the preference of an insect species, we can possibly make the environment less tolerable by removing the desired attributes of a living space, forcing the population to migrate elsewhere.

References:

Sasaki, T. & Pratt, S.C. (2013, November 6). Ants learn to rely on more informative attributes during decision-making. Biology Letters. Retrieved from http://rsbl.royalsocietypublishing.org/content/9/6/20130667.full.pdf html. DOI: 10.1098/rsbl.2013.0667.

Robinson, E.J.H., Smith, F.D., Sullivan, K.M.E. and Franks, N.R. (2009, April 22) Do ants make direct comparisons? Proc. R. Soc. B. DOI: 10.1098/rspb.2009.0350.

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