In the wake of the global pandemic, we're all a bit more aware of how diseases can spread and the effects they can have. Other recent scares, including bird flu, have also shown us that other animals can come down with infections as well. Even animals as small as ants aren't immune to infections. As ants are social insects, having an infection can cause problems within their societies, such as shortening an ant colony's life cycle or causing more females to be born than should be. However, not all bacteria are harmful. Some bacteria – such as Wolbachia – could even provide benefits to their insect hosts. With this in mind, Rohini Singh and Timothy Linksvayer of the University of Pennsylvania, USA, teamed up with Sachin Suresh, Jennifer Fewell and Jon Harrison of Arizona State University, USA, to see whether invasive pharaoh ants (Monomorium pharaonis) might receive a beneficial boost from the bacteria.
First, the team wanted to test whether the bacteria might cause the queens to lay more eggs. So, after setting up over 30 new colonies containing 20 recently mated queens and 50 workers, the researchers counted how many eggs each queen laid over the span of 50 days. The queens from infected colonies laid more eggs during the 50 day trial than the queens whose colonies weren't infected with Wolbachia. The infection was making the queens better at producing eggs, but could this also be true across the queen's lifetime? Pharaoh ant queens typically live between 9 and 12 months, so Singh and colleagues counted the number of eggs that 20 infected queens laid when they were 1, 3, 4, 6 and 9 months old. The team found that the infected queens laid more eggs than the uninfected queens when they were younger, but they saw no difference as they became older. This suggests that being infected with Wolbachia makes queen ants better at producing progeny, but at what cost?
Generally, animals that have lots of babies don't live as long as those that have fewer young. Do the infected queens live as long as their uninfected counterparts? When the researchers measured how long each queen survived, they found that the infected queens lived just as long as the uninfected ones (about 200–230 days). So, the bacteria allow the ant queens to lay more eggs without the cost of a shorter lifespan. But, laying eggs requires a lot of energy, so does the bacteria give a helping hand here as well? Sometimes. When the ants were 1 month old, the infected queens had the same metabolic rate as the uninfected ants even though they laid more eggs. However, 3-month-old infected queens burned more energy than their uninfected peers. By the time the queens were 9-months-old, both the infected and uninfected ants had high metabolisms. This implies that only younger infected queens get a benefit from the Wolbachia, but then the benefit disappears as the animal ages before coming back again when the queen is elderly. However, ant queens are only a very small part of the colony. Wolbachia infections are passed down from mother to offspring, meaning that the queen's offspring are also infected. So, what happens to the Wolbachia-infected workers?
As pharaoh ant workers don't lay eggs, Singh and colleagues measured how long the worker ants lived. Amazingly, infected workers had a longer lifespan (∼70 days) than those without the infection (∼60 days). While Singh and colleagues are quick to point out that many of these findings could be due to the laboratory conditions the ants are kept in and findings may be different in the wild, there's no doubt that, sometimes, bacteria are beneficial.