Tiny Worms Can Make Complicated Decisions

They consider several factors before deciding how to attack.

C. elegans worm (right) escaping the predatory P. pacificus worm
C. elegans worm (right) escaping the predatory P. pacificus worm.

Salk Institute

Depending on the animal, they have to make so many decisions. They choose where to go, what to eat, and whether to make a run for it or stand up to a predator. Even the tiniest, simplest worms make complex decisions, researchers have found.

They discovered that worms can consider many factors when choosing between two possible actions. The complicated process is surprising considering worms have only 302 neurons compared to about 86 billion in people.

“Humans are capable of considering many factors at once to make amazingly complex and rational decisions. But how much of this is uniquely human, and how much of this process can also be achieved with a much simpler nervous system?” first author Kathleen Quach, a postdoctoral fellow in Salk Institute’s Molecular Neurobiology Laboratory, tells Treehugger.

“By understanding the kinds of decisions that a worm can make with only about 300 neurons, we can start to separate out which decisions require 100,000 neurons (fruit flies), 70 million neurons (mouse), or the 86 billion neurons humans have. In order to understand how intelligence emerges from increasingly intricate brains, we have to push the limit of what the simplest nervous systems can do.”

Researchers studied the nematode Pristionchus pacificus, a type of roundworm. They were curious about the worm’s movements when attacking competing prey.

“The P. pacificus worms we study have about 300 neurons—that is a stupefyingly small number of neurons. It is reasonable to assume that this small of a nervous system would have limited decision-making,” Quach says. “Simple decision-making involves responding rigidly or habitually to a single or few elements in the environment. The rules for responding are also simple, such as moving towards stimuli that are associated with food and moving away from stimuli that are associated with harm.”

Those are the kind of simple decisions most often observed with the nematodes studied in their lab.

“By contrast, complex decision-making considers the outcomes of actions and how those outcomes contribute toward a goal,” Quach says. “This kind of decision-making makes behavior flexible, which means that an animal can make large or fine-tuned adjustments to its behavior to optimize its chances of achieving its goal.”  

Studying Decision-Making

Scientists, in the past, have focused on studying the cells and brain connections that might be involved in the decision-making process.

They would have an animal perform a different action for each choice they wanted. For example, a mouse might press one lever to get sugar water or another to get saltwater. The mouse makes a choice and researchers see what they want to eat.

But it’s more difficult to understand the process when the decisions and outcomes aren’t as black and white.

“How do you assess why an animal performs an action when that action can lead to two different outcomes?” Quach says.

That was the challenge researchers faced because P. pacificus can kill and eat other worms, like C. elegans, but it prefers to eat more nutritious bacteria. So it's competing with its prey for nutritious bacteria.

“When P. pacificus attacks C. elegans, it is not immediately clear whether P. pacificus bites in order to kill C. elegans as prey, or to get rid of competitors for bacterial food,” Quach explains. “Worms can't talk to us about why they do the things they do, so we had to come up with a different way to get into the mind of a worm.”

A Complex Predatory Response

For their study, researchers presented the worm with either adult or larval prey, as well as varying amounts of bacteria. They knew that P. pacificus could kill and eat C. elegans in larval form because they are smaller and don’t eat a lot of bacteria.

In those cases, P. pacificus’ biting behavior was mostly considered to be a predatory response.

But researchers were surprised that P. pacificus also bites C. elegans adults. The adults are much larger and require hours of biting in order to be killed. They wondered why a predator would spend so much time and effort to attack prey when they could eat bacteria instead.

“We hypothesized that P. pacificus may bite adult C. elegans to defend bacterial food (territorial biting), rather than to kill it for prey (predatory biting),” Quach says.

They could determine whether the worm bites for predatory or territorial reasons based on how the biting behavior changed when they offered adult or larval worms, as well as bacteria.

Researchers used what is known as neuroeconomics to predict the way biting should change in each condition, depending on whether the goal is to kill prey or defend bacteria food.

“Neuroeconomics tells us how a person (or animal) should act when its action can lead to multiple potential outcomes (such as in gambling), in order to gain the most optimal rewards,” Quach says. “One of our most notable predictions concerned the value of biting when bacteria are absent: Predatory biting should be most useful since prey is the only food option, while territorial biting should be useless because there is no bacteria to defend.”

They discovered that the worms’ biting matched their predictions. It most often bites the rival larval worm for predatory reasons, and typically bites the adult for territorial ones.

“We were surprised that the behavior of P. pacificus matched our predictions, because our predictions assumed that this predatory worm was rational and could consider the outcomes of its actions,” Quach says. “It could have easily been the case that P. pacificus always bites C. elegans for predatory purposes, even if doing so would be irrational.”

The findings were published in the journal Current Biology.

Weighing the Options

The worms appeared to weigh the pros and cons of potential choices before deciding how and when to bite. The researchers said that was pretty impressive for a creature with so few neurons. Scientists had always assumed they were simple and that they only bit because they were predators.

“Our results are particularly exciting, because it suggests that there may be a plethora of animal behaviors that are actually more complex than they seem—we just have to dig deeper and work harder to find them,” Quach says.

“This means taking the time to understand how behaviors are relevant to an animal's natural life, and then using that information to elicit and assess complex decisions that matter to that animal.”

It can be difficult for researchers to understand motivation, which is why studies have focused on behaviors with easy-to-understand motivations.

“However, the cells and circuit mechanisms we uncover about behavior can only be as complex as the behavior itself,” Quach says. “Our research promotes the perspective that well-designed behavioral assays can do a lot towards gaining insights into an animal's motivation and how it makes decisions, all before we start looking at neurons.” 

View Article Sources
  1. Quach, Kathleen T., and Sreekanth H. Chalasani. "Flexible Reprogramming of Pristionchus Pacificus Motivation for Attacking Caenorhabditis Elegans in Predator-Prey Competition." Current Biology, vol. 32, no. 8, 2022, pp. 1675-1688.e7., doi:10.1016/j.cub.2022.02.033

  2. first author Kathleen Quach, a postdoctoral fellow in Salk Institute’s Molecular Neurobiology Laboratory