We know that the rate at which organisms use energy (metabolic rate) varies substantially between individuals of the same species, even after accounting for size and temperature. What we are less sure about, is, why we see this variation.
When Amanda Pettersen and her colleagues thought about this question they considered it plausible that the competitive environments that individuals find themselves in might be important in determining whether a faster or slower metabolic rate is selected for. They wanted to find out whether variation in competition affects selection on metabolic rates and whether that could account for the variation in metabolic rate that persists more generally.
To test this idea Amanda used the model species Bugula neritina because it allowed the team to collect larvae in the lab, measure size and metabolic rate of larvae before assigning the larvae to one of three ‘competition’ treatments. Once the larval measurements were made, larvae were randomly assigned to either a no-competition environment, a competitive environment where they were put with other Bugula neritina (intraspecific competition) or, a competitive environment with an established mixed-species community (interspecific competition).
Individuals were then returned to the field and monitored weekly for survival, growth, age when first become reproductive and fecundity (total reproductive output).
The team already knew that higher metabolic rates are linked to faster growth, earlier onset of reproduction, and a shorter lifespan, while low metabolic rates are associated with a slow pace-of-life (slow growth, late onset of reproduction and long lifespan).
This experiment showed that individuals with higher metabolic rates were more likely to survive, more likely to reproduce and had greater numbers of ovicells at the start of reproduction, in the more intense, interspecific competition treatment. The team speculates that the individuals with higher metabolic rates grow more quickly enabling them to reach resources such as food and oxygen that are less available to smaller, slower growing organisms. There is a downside: these higher metabolic rate individuals also had a shorter lifespan.
The expectation amongst evolutionary biologists is that where there is strong selection pressure for a particular trait then the variation within that trait is reduced. So even accounting for the shorter lifespan, the overall increase in reproduction should mean that a higher metabolic rate is strongly selected for in competitive environments.
But the team also found that individuals with lower metabolic rates had a higher probability of living for longer in the absence of competition and would have continued to reproduce long after the individuals with a higher metabolic rate had died.
Bugula live on hard substrates and these areas commonly vary in their composition. Complex three-dimensional, highly diverse communities are interspersed with sparse populations of a few species and patches of bare areas. Individuals with lower metabolic rates that happen to arrive at a bare patch will have a long reproductive period and high overall fecundity. In contrast, individuals with a high metabolic rate that settle in amongst other species will be reproductive quickly and so successfully produce large numbers of offspring.
Because Bugula larvae are likely to find themselves in a variety of different competitive environments – even when they settle relatively close to each other – there is a strong evolutionary argument to explain the persistence of variation in metabolic rate, for Bugula at least.