C. elegans worm (right) escaping the predatory P. pacificus worm (left). Credit: Salk Institute
Humans boast 86 billion neurons in the brain—the most of any mammal in the world. At the other end of the spectrum, the predatory worm Pristionchus pacificus possesses just 302 neurons. Don’t count out the “underworm,” however, as scientists at Salk Institute have just shown how effectively P. pacificus can use all its neurons to make a complex decision.
The complex decision at the heart of this study led by associate professor Sreekanth Chalasani—do P. pacificus bite their prey for more than one reason?
While P. pacificus prefers to eat bacteria, researchers already knew they would use their teeth to attack and eat competitor prey Caenorhabditis elegans. Therefore, the team’s challenge was to determine the worm’s intentions when it bites.
In the study, published in Current Biology, Chalasani and his team created a laboratory system that allowed them to examine intentions by placing the predatory nematode P. pacificus, its competitor prey C. elegans and a shared bacterial food source within the same environment.
According to the results, P. pacificus consciously chooses between two different foraging strategies when this situation presents itself. The worm may choose the predatory strategy, in which its goal for biting is to kill C. elegans. Or, it may choose the territorial strategy, in which its goal for biting is to force C. elegans away from the food source.
The research team found that P. pacificus chooses the predatory strategy against larval C. elegans, which is easy to kill. In contrast, P. pacificus opts for the territorial strategy against adult C. elegans, which is difficult to kill and can somewhat easily outcompete the predatory worm for food.
“Scientists have always assumed that worms were simple—when P. pacificus bites we thought that was always for a singular predatory purpose,” says first author 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. It was surprising to find that P. pacificus could leverage what seemed like failed predation into successful and goal-directed territoriality.”
Overall, the team concluded that P. pacificus can weigh the costs and benefits of multiple potential outcomes of an action—behavior that’s very familiar in vertebrates but wholly unexpected in a worm.
“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,” said Chalasani, senior author on the paper.
The professor in Salk’s Molecular Neurobiology Laboratory says the study not only has important implications for the way researchers assess motivation and cognitive abilities in animals, but it also demonstrates that complex decision-making capabilities could be encoded in small biological and artificial networks.
“Our study shows you can use a simple system to study something complex, like goal-directed decision-making,” said Chalasani. “We also demonstrated that behavior can tell us a lot about how the brain works. The results provide a framework for understanding how these decisions are made in more complex systems, such as humans.”
In future research, the scientists want to investigate which of P. pacificus’ cost-benefit calculations are hardwired versus flexible, in hoped to further uncover the molecular underpinnings of decision-making—at all levels.