As you may know, there is no such thing as a solitary ant. These eusocial insects need the colony to function. Studies show that individuals without colonies live only one tenth of the duration than that of normal eusocial lives. So I stand corrected, there are solitary ants. Solitary dead ants. With such an interdependency on the entire colony, the different tasks required to maintain colony functioning have driven the development of different castes. The most complex ant societies are the fungus cultivating leaf-cutter ants. In these tropical species, there can be up to 9 different castes within a single ant nest. New research exposes that not only do these different castes have different roles, but actually have different susceptibility to pathogens and disease.
In 2014 Eddie K. H. Ho and Megan E. Frederickson studied 9 ant species with just two types of ants; workers and reproductive individuals. The ant species selected in this study are made up of workers which are sterile females, reproductive females called gynes and a small subset of fertile males. Some designate these three types of individuals in different castes, but for the sake of simplicity, I’ll refer to these as just two castes-workers and reproducers. The majority of ant species living from the forest floor in temperate ecosystems are more simply structured like this. Using more complex, multi-tiered colonies would be incredibly difficult to untangle, so these less structured colonies work perfectly in this study. Before they carried out their research, these researchers realized the trade-offs associated with pathogen resistance. A mating individual with a high resistance to pathogens must allocate more of its own resources to that particular function, as opposed to using those resources towards producing more offspring. This information was the basis of their main research question.
Testing the pathogen susceptibility of the entire colony doesn’t truly show what’s going on here. This research is the type of work I really admire, because of its comprehensiveness. These researchers sat down and thought about how ant colonies function. The best research always starts with critical thinking and an in-depth understanding of the research topic. Worker ants are sterile individuals that leave the nest constantly. They don’t have to save resources for reproduction since they don’t reproduce, and they come in to contact with a mélange of pathogens as they are out and about foraging. Taking a close look at worker’s roles, these researchers hypothesized that these ants would have more pathogen resistance. The more resistant these workers are, the less pathogen risk the mating castes will have. On the other hand, the mating members of the colonies that stay within the confines of the ant nest should allocate most of their effort to reproduction. Using an empirical study, these two researchers support their hypotheses with data, finding a significant difference in pathogen susceptibility between the two castes.
By exposing the different castes of each of the 9 ant species to a generalist entomopathogenic fungus called Beauveria bassiana, they were able see resistance differences. Beauveria bassiana has a worldwide distribution and infects over 200 different insect species. Easy to cultivate, the pathogen is used a lot in ant immunity studies. What they found was that 7 of the 9 ant species had increased mortality when facing the fungal pathogen. Not only that, but in the 7 species negatively affected, the separate castes too had different immunological responses. The reproductive males and females had significantly higher mortality compared to worker ants. Dying much sooner than worker ants, this research shows that reproductive individuals allocate more energy and resources towards reproduction than pathogen resistance.
Ultimately, this research reveals that reproductive individuals depend on their worker ants not only to forage for food and do the heavy lifting for the colony, but also to indirectly enhance their own pathogen resistance. The sterile worker ants need not save resources and energy for reproductive purposes so instead, fully invest in enhancing their pathogen resistance. Over millions of years, these patterns in eusocial insects have evolved to maximize colony fitness. The health of these colonies depends on the greater immunity of the foraging worker castes. With their greater resistance to fungi like entomopathogenic Beauveria bassiana, these workers allow the reproductive castes to specialize on reproduction. The complexity of the forest floor never ceases to amaze me. Members of the same species actually have a different susceptibility to pathogens based on the function they were born to carry out.