A new review of ant research, published in the Annals of the Entomological Society of America, illuminates how ant behavior is driven by a cascade of individual decisions among colony members. Insights on ant movement and encounter rates also suggest parallels to collective behavior in another population under the microscope in 2020: humans. (Photo via Flickr/Insects Unlocked, public domain)

By Leslie Mertz, Ph.D.

The pattern of city traffic comprises many individual drivers, all making decisions about how to get where they want to go, and many of those route choices are influenced by the drivers’ perceptions, such as the sights or sounds of traffic jams ahead. The traffic patterns of ants are also made up of individuals that are making decisions, but, rather than using sight and sound, they base their movements on touch and smell, as well as environmental cues. In both cases, individual decisions modify the larger traffic pattern, while, at the same time, the larger traffic pattern also modifies individuals’ decisions.

Leslie Mertz, Ph.D.

The idea that the movements of ants and people share some similarities has been especially evident during 2020’s coronavirus pandemic, according to Deborah Gordon, Ph.D., biology professor at Stanford University. She just published a new article in Annals of the Entomological Society of America that not only reviews research into ant movement and behavior but also notes analogous human patterns as the coronavirus has swept through the population.

“There’s recently been a new wave of interest in how encounter patterns and spatial patterns are linked and how they organize what social insects do. And more generally, there is also a basic lesson (in such patterns) that the pandemic really brought out, because all of us started changing how we move to influence how we interact and reduce our individual chances of infection,” she says. Concurrently, those individual changes ultimately play into the collective good, because, as the rate of infection among individuals slows, the overall spread of the virus slows. Gordon says, “The pandemic shows that it is difficult for people, especially in this country, to realize that their behavior has an impact on the whole, because each person’s decisions affect the spread of the disease.”

In this diagram, colored lines each track individual ant movements over time (tunnels to the nest are depicted as white circles). Results of this work are published in “Effect of Interactions between Harvester Ants on Forager Decisions,” by Jacob D. Davidson, Roxana P. Arauco-Aliaga, Sam Crow, Deborah M. Gordon, and Mark S. Goldman, Frontiers in Ecology and Evolution, 2016. (Image courtesy of Jacob Davidson, Ph.D.)

In the review article in Annals of the Entomological Society of America, Gordon notes numerous examples of how ant movements affect how often they encounter one another, and this in turn has an impact on what they do next, such as whether they forage, how to search a new space, or when it’s time to move to a new nest. “All of those behavioral decisions are the outcomes of a pattern in how they move around and meet locally,” she says. “For example, in harvester ants, each forager ant leaves the nest to search for a seed and then comes back and waits inside the nest, and it uses the rate at which it meets other ants returning with seeds to decide whether to leave again on its next trip. This creates a kind of positive feedback: The more food there is out there, the more quickly ants find it and come back, and the more quickly more ants go out again,” she says. “No ant needs to make any global assessment of how much food is out there. It’s just responding to the rate at which it meets other ants.”

The idea that rate of encounter regulates collective behavior is evident not only in ants and other social insects but also in other animal groups that space themselves as a function of population density. “Catherine Markham’s work shows how baboon groups use the rate at which they meet to decide which group gets to use which waterhole,” Gordon says. Likewise, the same kind of encounter–behavior connection occurs with other biological systems, including the nervous system, in which a neuron “accumulates electrical charge from other neurons, and when it reaches some threshold, it fires,” or in embryonic development, during which local cell-to-cell interactions influence whether an undifferentiated cell ultimately becomes bone, liver, or some other tissue.

While reviewing the research into ant movement and behavior, Deborah Gordon, Ph.D., biology professor at Stanford University, noticed similarities in the ways human movement patterns have changed during the pandemic. Here, Gordon observes ant traffic on tree branches in Mexico. (Photo by Felipe Campos)

Shown here conducting field work in Arizona, Deborah Gordon, Ph.D., studies ant movements and how the rate of ant-to-ant encounters affects behavior, such as whether an ant continues to forage or begins another task. (Photo by Katie Dektar)

“Networks of interactions and rates of encounter regulate how the coronavirus or any disease spreads through a population, and also how cells differentiate, how ant colonies forage; in fact, collective behavior regulates how any biological system works. No one individual knows what every other individual is doing, and there is no central control,” Gordon says. “This is a general idea in nature: Collective outcomes depend on the rate of encounter.”

Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.