Metabolic rate, or energy use, changes with the size of the organism. This general pattern has been observed across different species, as well as among individuals of the same species. But while the broad pattern holds, individuals of the same species and the same size can also vary in the amount of energy they use.

Some studies have shown that individuals have lower metabolic rates as population numbers go up, but no one really knows why. Metabolic rates increase following food intake, so one plausible explanation is that competition for food in crowded conditions reduces food intake and, in turn, metabolic rates.

Melanie Lovass and her supervisors Dustin Marshall and Giulia Ghedini have run a series of experiments to investigate this possibility.

The team used the model species Bugula neritina to test their ideas. They ran a series of experiments where they were able to measure metabolic rates in individual Bugula colonies and they manipulated food, oxygen concentration, water flow and chemical cues to try and tease apart what was causing a reduction in metabolic rates in dense populations.

In the model system, each of these measures are influenced by how sparse or dense the population is. As expected, food availability affected metabolic rates but the team was surprised to find that chemical cues from individuals of the same species are also able to drive changes in metabolic rates.

Metabolic rates were lower in colonies that were starved, but metabolic rates were not affected by changes in water flow and oxygen concentrations.

More interestingly, Melanie and her supervisors found that metabolic rates were suppressed in Bugula colonies that were kept in ‘pre-conditioned’ water. This water had been exposed to other Bugula colonies overnight and so incorporated any chemical cues released from these other colonies. Melanie thought that chemical cues from fellow colonies might signal a reduction in feeding rates. To check if this was the case, she counted the number of feeding structures active in colonies exposed to ‘pre-conditioned’ versus normal seawater.

They found no differences in feeding rates indicating that the chemical cues from Bugula colonies were suppressing physiological processes rather than reducing feeding rates. So, while the chemical cues from other Bugula colonies reduce energy use, this reduction is in processes other than feeding activity.

While searching for food is energetically costly; keeping up feeding activity may be worth the costs and become even more important when access to food is very competitive.

This research is published in the Journal of Experimental Biology.