Lunch Break

Eating is essential for fueling our bodies with energy, but it is not always clear how animals decide when and how much food to consume. A new theoretical model of feeding, derived from observing the tiny roundworm Caenorhabditis elegans, suggests that feeding is not only for taking in nutrients but also for learning about the environment. 

A C. elegans worm living in a Worm-Spa, which allows researchers to observe its feeding behavior.

Kyung Suk Lee and Erel Levine

Researchers at the University of Chicago and Harvard University created “WormSpas”, or small, fluid-filled chambers that allowed them to observe and feed C. elegans worms a selection of fluids containing varying concentrations of the worms’ bacterial food. These worms feed themselves with their pharynx, a pump-like mechanism that sucks in small samples of bacterial fluid. Because the worms are transparent, it was easy to see when they were pumping versus pausing. 

At all levels of WormSpa food availability—low, medium, or high—the worms showed bursts of pumping interspersed with pauses. But the frequency and duration of pumping bursts increased with the concentration of bacterial food. This was surprising to Erel Levine, one of the study’s researchers. “One would think that with more food in the environment, the animal gets more with every burst, so it should be able to ‘rest’ more, which is not what really happens.” 

To explore this puzzling pattern, the research team created a decision theory-based mathematical model that treated food intake as a source of both energy and information. The model assumed that worms cycled through three states: First, the worm samples how much food is around. Second, based on the information obtained through sampling, it commits to continuously pumping or taking a break. Third, it resets its estimate of available food and continues the cycle.

By testing their model against actual behavior, the researchers found that the worms trade off speed against accuracy while exploring and exploiting their food environment. The worms did not always perfectly track fluctuations in food availability. “Sometimes the worm is completely wrong,” says Levine, explaining that worms go between perfect tracking and making mistakes in a pattern they call stochastic, or random, feeding. Their model suggests that in some cases, when more food is available, more information is required too.

Levine acknowledges that the WormSpas prevent the worms from moving around as they naturally would, so the simple three-state model that they tested may not capture the complexity of the worms’ feeding choices. Still, their insights – that the worms don’t perfectly track food availability, instead using random feeding -- provide a simple but powerful building block for better understanding how animals respond to their environment. (Proceedings of the National Academy of Sciences)