Matthew D. Hall, Ben L. Phillips, Craig R. White, and Dustin J. Marshall
This is a plain language summary for a Functional Ecology research article which can be found here.
When attacked by a virus or bacteria, our body is predicted to use more energy to try and fight infection and repair any damage. To keep up, our bodies are predicted to work harder. Across a wide range of animal species, from insects to mammals, this change in biological work has been captured by measuring an animal’s metabolic rate before and after infection. For many types of animals, however, successfully fighting off infection didn’t always mean using more energy overall. One reason why these results may differ is that animals respond to various pathogens, including bacteria and viruses, in unique ways. In fact, some of the concepts related to infection and energy use are reflected in the well-known saying “feed a cold, starve a fever,” which can be traced back to an English dictionary from 1574.
But not all animals fight infection in the same way, and not all pathogens, even if they’re the same type, do the same amount of damage. In this study, we used a small crustacean Daphnia magna, known as the water flea, to test how variation in metabolic rate and overall energy use can arise from within a single encounter between an animal and a pathogen. Here the pathogen was a species of bacteria that quickly enters the water fleas’ body, fills all available space with spores, and causes an infected animal to stop all reproduction and die much faster than normal. Using genetically distinct types of both the water flea and the pathogen, we measured the metabolic rates of normal animals, animals that encountered the bacteria but successfully fought off infection, and animals that instead became sick. Measurements were made three times, spaced out over most of the animal’s life.
Our findings show that the metabolic rate of an animal changes quickly upon being attacked by a pathogen and is maintained long after the initial encounter. We found that water fleas that successfully prevented infection experienced a sustained reduction in their metabolic rates, and therefore their potential to do biological work of any kind. The reverse was true for animals that instead became sick. Their metabolic rates remained elevated over most of their life. Underlying these changes were differences in the energetic burden that different pathogen backgrounds imposed on the water fleas, as well as changes in the way host genetics and the outcome of infection shaped the underlying relationships between host body mass and metabolic rates. Our results show that variation in the magnitude and direction of change in metabolic rates will be a natural part of defending against pathogens.
Functional Ecology: Plain Language Summaries 