Getting Better May Get You Eaten
Immune response increases predation risk (2012) Otti et al., Evolution 66
Parasites and diseases cause a lot of problems for their hosts, stealing resources like blood, food and energy. But fighting off parasites is also a costly process, so hosts have to walk the thin line between using just enough resources to fight off the parasite and using too many, leaving them with nothing. The amount a host invests in their immune response will depend on the specific environment that they live in. For example, in an environment where resources are plentiful, a host may decide that it is worth shaking off a parasite or disease. In areas where resources aren’t, they may choose to save energy.
Introducing predation to a situation further complicates things. Having a lot of predators around naturally means energy conservation becomes even more important. This study examines the risk of predation for an organism that is fighting off an infection.
How it works
The authors infected field crickets (Gryllus campestris) with lipopolysaccharides, a substance that causes an immune response in crickets, much like a fever in a human. They wanted to see if the loss of energy caused by triggering the immune response not only lowered the crickets reaction times, but also if sick crickets were actually eaten more than healthy crickets.
In a lab environment, they measured the reaction times of sick and healthy crickets. If sick crickets are slower to react to a perceived threat, then they are more vulnerable to predation. To test the actual risk of predation, the authors set up enclosures containing the crickets with and without predators, in this case a species of shrew (Crocidura russula). Survival differences between the two different enclosures would show if sick crickets were more likely to be eaten than healthy crickets.
Did you know: Diabetic Insects
In addition to fever-like symptoms, insects can also suffer from diabetes! Intestinal parasites in dragonflies have been shown to cause high blood-sugar, obesity, and insulin resistance. This shows yet another example of how parasites, whilst troublesome in and of themselves, can cause a cascade of other negative side-effects.
What they found out
The authors found that sick crickets had a slower reaction time than healthy crickets. In addition, all crickets in the predator-free enclosure survived, both sick and healthy. The predator enclosure was a different story, as sick crickets were eaten almost five times as often as healthy crickets over the first day. However, survival rates were similar after the first day had passed.
The immune responses triggered by the cricket causes them to seek warmer areas, which in this case mean spending time outside the burrow, remaining exposed to predators longer.
The authors found that crickets who attempt to shake off infection quickly are more likely to be eaten, which tells us that parasites and diseases have a secondary and more subtle effect on their hosts. ‘
The manner of an immune response has a direct impact on predation risk. Sick crickets, much like sick humans, had a fever. These crickets tried to cool down by sitting outside of their burrows, putting them at risk of being seen by a shrew. This put them in a precarious situation, as the shrew could see them, but it was made even worse by the fact that they couldn’t escape as well as a healthy cricket.
This helps us to predict population fluctuations in species. For instance, we know that if a disease or parasite, even a minor one, is sweeping through a population, that we can expect higher mortalities from local predators. If this population is of particular risk, we can then take management steps to help its conservation, such as removing affected individuals or treating those affected.