Rock Ant Colonies Calculate the Quality of Nest Sites

close up photograph of black ants on yellow figure

Biological Strategy

Rock ant

Sam Stier

Image: Salmen Bejaoui / Unsplash / Free non-commercial use

Coordinate by Self-Organization

To create and maintain a healthy community of individuals and ecosystems requires that living systems coordinate their activities. Coordination doesn’t necessarily mean that there’s a leader orchestrating what happens. In nature, coordination is usually achieved through self-organization. In a flock of geese flying in a V-formation, for example, there’s no lead goose controlling where all of the others fly. The flock uses this formation because each goose gains energy from air vortices created by the goose in front of it. The lead goose doesn’t gain that benefit, so when it tires, it moves back and another goose takes the front position.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Rock ant colonies find better nests by calculating how much better a new site is on average compared to the old site.

Rock ants live throughout Europe and make nests for shelter in the various cracks that form in rocks. Individual scouts from a colony roam throughout an area near the nest in order to identify other potential nest sites that might be better than the existing one. If a better nest site is found, the entire colony will move to it. Rock ants determine if a potential nest site is superior based on how many scouts gather at the new potential site. If the number of scouts reaches a certain threshold, they will return to the colony and start moving to the new site.

What determines whether a new site is better than the old one? Ants will look for a number of features, but they usually prefer nest sites that are darker because it offers more protection from harmful weather. If a new site is darker than the old one, it is relatively easy for ants to decide to make the switch. But what happens when the choice is less obvious? For example, a new site that’s under a tree might be completely dark for 75% of the time on a windy day, and light only 25% of the time. If the old nest site is only moderately dark 100% of the time, how do the ants decide whether to switch sites or not?

The question is similar to deciding where to go to dinner based on how many people are in two different restaurants. It makes sense to go to the fuller restaurant. But what if you know that one restaurant serves consistently mediocre food, while the other varies in quality between better-than-mediocre and worse? How does one decide then?

Rock ants are able to navigate this difficult situation by determining the average quality of a new nest site. If a new nest site is of good quality 75% of the time and poor 25% of the time, they will transfer from an old nest site that is consistently (100%) of mediocre quality. However, if a new nest site is of poor quality 75% of the time and good only 25% of the time, they won’t move. Rock ants are able to calculate the average quality of a variable nest site because more scouts eventually spend more time in nest sites that are of higher quality. Thus, the probability of enough scouts gathering in higher quality sites is greater than in lower quality sites (where scouts leave after shorter periods of time).

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Rock ant scout leading another scout to a potential new nest site

Last Updated May 14, 2020

References

“The ants’ algorithm seems to be effective in estimating the average quality of a new nest site even when the quality of that nest site fluctuates through the assessment period. The key to this extraordinary ability almost certainly lies with the ants’ use of quorum sensing whereby they only commit to accepting and beginning an emigration to a new nest when a certain abundance of their nest mates have accumulated within such a nest site. One likely explanation is that individual ants simply spend time within a nest site as a function of its quality.” (Franks et al. 2015: 10)

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