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Simple lifeforms may benefit from complex decision making, study finds

A new study looks at two species of worms — one predator, and one prey — to explain how even simple lifeforms may use a form of cost-benefit analysis to determine which set of actions would provide the maximum reward in a given scenario.

(CN) — Scientists have determined that even small lifeforms such as worms are able to employ and benefit from relatively complex decision making to take down a food source or to defend their territory from competitors.  They hope their work can help to reveal more about how decision-making works in complex animals such as mammals.

Such decision-making abilities have long been observed in vertebrates, but until recently had not been confirmed in less complex organisms like worms. Previous work has mainly focused on the connections formed between relevant neurons in the brain, but more recently scientists have begun looking into their subjects’ real-world behavior.

The line between predation and territoriality can become especially blurred when dealing with a prey animal which also competes for the same food source as its predator. Does an attack occur because the predator wants to eat the prey? Or merely because it wants to run off the competition?

Researchers from the Salk Institute for Biological Studies studied two competing species of worms — P. pacificus, the predator, and C. elegans, the prey — to see if they could disentangle the predatory and competitive motivations for an attack. They published their findings Monday in a study in the journal Current Biology.

“Our study shows you can use a simple system such as the worm to study something complex, like goal-directed decision-making. We also demonstrated that behavior can tell us a lot about how the brain works,” said senior author Sreekanth Chalasani, associate professor in Salk’s Molecular Neurobiology Laboratory, in a related statement. “Even simple systems like worms have different strategies, and they can choose between those strategies, deciding which one works well for them in a given situation. That provides a framework for understanding how these decisions are made in more complex systems, such as humans.”

The authors found that what initially appeared to be failed attempts at predation on the part of P. pacificus toward C. elegans, were actually acts of territorial aggression meant to drive away the competing worm from a common bacterial food supply. While P. pacificus can effortlessly kill and eat larval C. elegans in a single bite, the worm is more difficult to kill in its mature form, and thus P. pacificus often chooses a non-fatal bite to shoo away adult competitors.

This led researchers to believe that P. pacificus is performing a delicate cost-benefit analysis on the spot to determine which course of action to take when confronted by an intruder to its domain.

P. pacificus can also be induced to switch from territorial to predatory biting by treating it with an octopamine receptor antagonist, according to the authors, demonstrating that its motivation for searching out prey can be altered by outside influences. The study shows that predatory and territorial motivations for biting can quickly change in response to the abundance of bacterial food sources in a given location; P. pacificus bites most often when food is scarce, but less so as its local food supply increases.

“We illustrate how P. pacificus weighs costs and benefits of biting outcomes, flexibly reprograms biting motivation to prioritize prey or bacterial food and orchestrates foraging strategies that are energetically commensurate with its food choice,” explain the authors in the study.

The team did encounter a few limitations during the course of their research, however. For instance, P. pacificus would sometimes bite even when that was predicted to have zero value, either because it wanted to test or confirm an existing belief, or perhaps because it was doing so randomly.

The authors also have yet to determine exactly how P. pacificus distinguishes between larval and adult C. elegans during their encounters. They suspect it may have something to do with peptide-mediated recognition, which allows the worm to recognize its own progeny among closely related species.

“Scientists have always assumed that worms were simple—when P. pacificus bites we thought that was always for a singular predatory purpose,” said first author Dr. Kathleen Quach, a postdoctoral fellow in Chalasani’s lab. “Actually, P. pacificus is versatile and can use the same action, biting C. elegans, to achieve different long-term goals. I was surprised to find that P. pacificus could leverage what seemed like failed predation into successful and goal-directed territoriality.”

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