What do ants, plants, and your immune system have in common?

Biologist studying Paraponera clavata ants in Costa Rica. [1]

All organisms live in a very competitive world, where a strong defense system could mean the difference between life and death. Thus many organisms have specific defenses, often consisting of many specialized components. For example, you may be familiar with the human immune system, which consists of five different types of white blood cells that all have different functions. However, this kind of specialized defense is not just limited to humans, of course. Other animals and even plants have it, too. In fact, groups of animals can sometimes also be thought of as having something like an immune system, as they must defend their group as a whole.

Cephalotes varians left: soldier, right: worker [2]

A superorganism is comprised of many organisms and is most often used to describe a group of eusocial animals. Eusociality characterizes all ant species as well as some bees, wasps, termites and aphids. The three main characteristics of these animals are 1) communal care of young by adults, 2) overlap of generations, and 3) the division of labor between reproductive and nonreproductive groups. This last trait is the most interesting to defense because it allows for physically specialized individuals whose main purpose is defense. This division can be very clearly seen in many turtle ant species where there are very different looking queens, workers and soldiers. Our lab, in collaboration with Scott Powell’s lab at George Washington University, is studying Cephalotes varians, a species of turtle ant found in the Florida keys that has specialized soldiers with large, round, plated heads that they use to block the entrances to the colony’s nests – yes, nests! These ants require multiple nests for one colony, which makes their division and allocation strategy of soldiers and resources all the more important.

 Cephalotes varians left: soldier guarding the entrance to a nest,
right: worker [3]

This ant system has many parallels to the aforementioned human immune system. For example, the ant soldiers have very different physical characteristics and behavior patterns from the workers, and queens. This is similar to the white blood cells in humans, which are made with different structures from other cells to aid their specialized function of protecting the organism. This means that both of these systems struggle with the cost-benefit balance of defense, for if they produce more defenders, they have less energy and resources to commit to other important functions. Both of these systems’ approaches to dealing with this problem are similar and depend on external factors like available resources and immediate threats. Thus the study of one system may help us understand the other (Houston et al. 2007, Viney et al. 2005, Powell and Dornhaus 2013).

Pineapple plant with heavily defended fruit [4]

Another similar defense system is that of plants. I know we don’t usually think of plants as having an immune system, but they too have many predators, from hungry herbivores to bacterial parasites. These organisms also face the cost-benefit conundrum. Additionally, they are able to allocate existing resources to specific tissue types depending on their relative value (Zangerl and Bazzaz 1992). For example, fruits can be considered highly valuable to the plant, as they must invest a lot of nutrients into making them and they directly produce the next generation of plants. However, their high nutritional value is extremely attractive to animals, thus many plants spend a lot of energy protecting their fruit. These defenses can be physical (like spines) or chemical (like acids that make the fruit bitter) that the plant specifically allocates to the fruit (and often the outside layers of the fruit) in an attempt to protect it. In ant colonies we can see a similar preference for defending high quality nests (those with large cavities and thus the capacity to fit a lot of eggs). Although these systems have many similarities, different aspects of each have yet to be explored. Therefore, studying and relating these different systems can give us new insights and perspectives on how each works and possibly how other defense systems function and evolve (Kaspari and Byrne 1995).

Further Reading:

Hölldobler, Bert and Wilson, Edward. The Ants. Cambridge, MA: The Belknap Press of Harvard University Press, 1990. https://books.google.com/books?id=R-7TaridBX0C&lpg=PA1&ots=sK1RgdxL7h&dq=the%20ants%2C%20wilson&lr&pg=PA1#v=onepage&q=the%20ants,%20wilson&f=false

Houston, A.I, MacNamara, J.M., Barta, Z., Klasing, K.C. (2007) “The effect of energy reserves and food Availability on optimal immune defence.” Proceedings of the Royal Society B 274(1627):2835-2842 http://rspb.royalsocietypublishing.org/content/274/1627/2835.short

Viney, M.E., Riley, E.M., Buchanan, K.L. (2005) “Optimal immune responses: immunocompetence revisited.” Trends in Ecology and Evolution 20(12):665-669. http://www.sciencedirect.com/science/article/pii/S0169534705003204

Powell, S., Dornhaus, A. (2013) “Soldier-based defences dynamically track resource availability and quality in ants.” Animal Behaviour 85:157-164. http://www.sciencedirect.com/science/article/pii/S0003347212004800

Zangerl, A.R., Bazzaz, F.A. (1992) “Theory and pattern in plant defense allocation.” Plant Resistance to Herbivores and Pathogens: Ecology, Evolution and Genetics. Ed. Robert Fritz and Ellen Simms. Chicago and London: University of Chicago Press, 1992. 363-391. https://books.google.com/books?id=ffDnW0ANW9wC&lpg=PA363&ots=VJE71bz6Ew&dq=zangerl%20and%20bazzaz%201992&lr&pg=PA363#v=onepage&q&f=false

Kaspari, M., Byrne, M.M. (1995) “Caste allocation in litter Pheidole: lessons from plant defense theory.” Behav Ecol Sociobiol 37:255-263. http://link.springer.com/article/10.1007/BF00177405#page-1

Media Credits:

[1] Alexander Wild: http://www.alexanderwild.com/Ants/Myrmecology-the-Study-of-Ants/Personalities/i-mDNN3K8/A

[2] Alexander Wild: http://www.alexanderwild.com/Ants/Taxonomic-List-of-Ant-Genera/Cephalotes/i-fS7WtNr/A

[3] Alexander Wild: http://www.alexanderwild.com/Ants/Taxonomic-List-of-Ant-Genera/Cephalotes/i-mpwfQM2/A

[4] Chang’r: https://www.flickr.com/photos/chang-er/3754097832