Courtney Earl

Claire Henderson

Isabella Peet

In this article, researchers investigated the variation of patterns in morphology in Pheidole. To do this, they had four sections of analysis. The first section looked at the structure of the rage of major and minor workers. Next they looked at major and minor worker levels of integration of their morphologies. The third looked at the organization of both major and minor worker morphological variation. Finally, they did a confirmatory analysis of intercaste differences. Specimens were obtain via high-quality digital images available from Wilson's (2003) treatise of New World Pheidole. A total of approximately 231 species were measured. Researchers measured the various body dimensions of the obtained samples. In order to rule out the morphological traits as being due to shared history, a series of simulations were conducted. These simulations generated datasets similar to that used in their study, however any correlated variation between individuals was solely due to having a shared history. It found that a large portion of the evolutionary morphological differences among Pheidole are attributed to a variation in their size. The simulations performed indicate that the level of concentration found for the explained variance is unlikely to be due to shared history alone. It was found from their first test that traits that are functionally related tend to evolve in tandem and seem to be somewhat independent from other traits. Overall, it was found that many of the morphological differences among the species is attributed to the variation in size and how it attributes to the functions of the species.

Powell explores how nesting ecology influences the evolution of soldier ants in turtle ant species. This study examines whether environmental factors, like nesting habits, drive the development of specialized soldier castes. By analyzing different species and their nesting preferences, the author finds that ants in scarce or competitive nesting sites evolve unique soldier morphologies. For example, some develop head shapes that act as barricades to block nest entrances. These findings suggest that competition for nesting spaces drives the evolution of distinct physical traits in turtle ants. Overall, this study highlights the significance of how ecological pressures can shape the evolution of ants, leading to specialized adaptations.

In this article, the authors investigate the ecological and evolutionary factors that contribute to the morphological diversity of workers in the Eciton genus of army ants. Researchers observed different colonies of Eciton army ants in their natural habitats to gather data on worker ant characteristics and behaviors. Next, they compared the different species morphologies since they have different evoronmental pressures and foraging strategies. To collect data, they measured physical traits of the worker ants, such as size and shape, and analyzed these characteristics in relation to the environment and colony tasks (like foraging or defense). Using this data they concluded that the different morphologies are a response to different environmental and ecological factors like food availability, habitat type, and colony organization.

As shown in the image above, ants display a wide range of mandible (jaw) sizes. The shape and size of an ant’s mandibles often reflect its role in the colony, with each caste benefiting from mandibles specialized for its specific duties. For example, soldier ants typically have large and powerful mandibles suited for defense and combat. Worker ants, responsible for tasks like foraging, nest construction, and transport, have versatile mandibles that allow them to manipulate a wide range of objects. Nurse ants, who care for the brood, possess smaller and more precise mandibles, ideal for gently handling eggs and larvae. (Schmidt, 2004).

La Richelière, F., Muñoz, G., Guénard, B., Dunn, R. R., Economo, E. P., Powell, S., ... & Lessard, J. P. (2022). Warm and arid regions of the world are hotspots of superorganism complexity. Proceedings of the Royal Society B: Biological Sciences, 289(1968), 20211899. https://doi.org/10.1098/rspb.2021.1899 

Pie, M. R., & Traniello, J. F. A. (2006). Morphological evolution in a hyperdiverse clade: The ant genus Pheidole. Journal of Zoology, 271(1), 99–109. https://doi.org/10.1111/j.1469-7998.2006.00239.x 

Powell, S. (2016). A comparative perspective on the ecology of morphological diversification in complex societies: Nesting ecology and soldier evolution in the turtle ants. Behavioral Ecology and Sociobiology, 70(7), 1075–1085. https://doi.org/10.1007/s00265-016-2080-8 

Powell, S., & Franks, N. R. (2006). Ecology and the evolution of worker morphological diversity: A comparative analysis with Eciton army ants. Functional Ecology, 20(6), 1105–1114. https://doi.org/10.1111/j.1365-2435.2006.01184.x

Schmidt, C. A. (2004). Morphological and functional diversity of ant mandibles. Tree of Life Web Project. http://tolweb.org/treehouses/?treehouse_id=2482

Schrader, L., Pan, H., Bollazzi, M., Schiøtt, M., Larabee, F. J., Bi, X., ... & Rabeling, C. (2021). Relaxed selection underlies genome erosion in socially parasitic ant species. Nature Communications, 12(1), 2918. https://doi.org/10.1038/s41467-021-23178-w  

Wilson, E. O. (1979). The evolution of caste systems in social insects. Proceedings of the American Philosophical Society, 123(4), 204–210.

The researchers created a phylogenic tree for this study by complementing 4,415 ortholog genes from 65 known ant species. After cleaning and preparing the data using extensive analysis, they were able to conduct several types of analyses, including divergence dating analyses, positive selection analyses, game predictions, and gene family analyses. The main two types of analyses they were able to perform were supermatrix and supertree approaches. In a supermatrix, the genes are concentrated to estimate a single species tree. A supertree approach involves estimating a species tree from a gene tree. One important discovery supported the theory that the Leptanillinae and Martialinae ant groups form a monophyletic group (now referred to as leptanillomorph clade). They also report findings about the genes which are positively selected for and involved in eusociality. To investigate this, the researchers conducted positive selection analyses on 4,151 ortholog genes. The branch of the phylogenic tree that led to the formicoid clade had 30 times the rate of positive selection for these genes than the average of the other tree branches. Formicoid clade ants typically exhibit the most extreme forms of social organization. There were 110 positive selections on the formicoid ant branch compared to the average number of 3.1 on the other branches. Remarkably, the most significantly over-represented function of the 110 genes was histone acetylation. Histone acetylation is involved with many other essential aspects vital to caste system development, including foraging behavior, colony rhythms, trade-offs in workers, and caste determination of honeybees through royal jelly. The results further show a positive selection of histone acetyltransferase as being relevant to eusociality and has been linked to the regulation of worker polymorphism. The second most relevant function of histone acetyltransferase was autophagy, which is linked to the longevity of the queen's life. The last function histone acetyltransferase seemed to be important for was processes that lead to the sterilization of worker ants. In conclusion, the positive selection of these genes appears to be important for the complex division of labor and social systems of the ants within the formicoid clade. On the other hand, there is variation in the extent to which ants within the formicoid clade are eusocial, so gene selection favored eusociality but did not cause complex social systems.

(La Richelière et al., 2022)

(La Richelière et al., 2022)

These ants have key phenotypic differences especially in their olfactory system not giving them the same chemosensory functionality as other ants, which inhibits their ability to communicate effectively. Specifically, these ants have erosion on their odorant receptor (OR) gene. However, these ants rely more on the care behavior and social infrastructure provided by their host colony rather than the nuanced communication systems. Therefore, social parasitic ants can thrive while not engaging in brood-care, colony founding, foraging, or nest-defense behaviors (Schrader et al., 2021).