A team of researchers from North America and Asia has demonstrated that the nematode Caenorhabditis elegans, a brainless organism with a simple nervous system, can display basic fear-like responses. The observation emerged from experiments reported in Genetics.
In the study, scientists examined how the worm reacts to a brief electric shock. When the current was applied, the nematode exhibited rapid escape behavior that resembled fear. Experts suggest that the duration and intensity of this negative state are likely controlled by a specific neural circuit rather than by direct motor stimulation from the electricity itself.
Researchers defined an emotional response in animals as behavior that persists after the stimulus ends, varies with the strength of the stimulus, dominates other responses, and remains appropriate to the nature of the trigger. The nematode’s reaction to the shock fulfilled at least three of these four criteria, indicating an emotional component to its behavior.
The length of time the worm remained in a heightened state depended on how long it was exposed to the electric current. In these trials, the creatures largely forwent feeding, suggesting that the negative situation dampened positive stimuli and priorities.
One of the authors noted that signaling through neuropeptides appears essential for fear-like brain states. This implies that evolutionarily conserved molecular mechanisms may regulate fear in C. elegans, and this mechanism shows parallels with mammalian systems, including humans [Genetics study, 2024].
Earlier work in the field has shown that the absence of a vertebrate brain does not stop learning. Even organisms with simple neural architectures, such as jellyfish, can modify behavior based on past experiences, underscoring the idea that learning and emotion-like states can emerge from relatively basic neural circuits [Genetics study, 2024].