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Arnica Sheaffer

Courtney Earl

Claire Henderson

Isabella Peet

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Eusociality is a highly organized social system that typically consists of a few defining characteristics. Some of these characteristics include cooperative care of young, overlapping generations, and a division of labor, called a caste, that puts individuals into either reproductive or non-reproductive categories.

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Ants are considered eusocial animals because they live in highly organized colonies. Within these colonies, there is a division of labor, that leads to the highest fitness of all the ants in the colony. In these colonies, individuals specialize in different roles, such as workers, soldiers, and reproductive queens, all working together for the colony's survival. Workers forage for food, protect the nest, and care for the queen's offspring, while the queen's primary role is reproduction. This cooperative behavior among the colonies is a key characteristic of eusociality, which helps the ants survive, together.

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Roles & Responsibilities within an Ant Colony 

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Roles & Responsibilities within an Ant Colony 

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What is a caste system? A caste refers to a distinct group within a colony whose members are specialized for certain roles and responsibilities that they perform consistently over extended periods of time. As shown in the video, this commonly includes the queen, workers, soldiers, and nurses. These groups are usually characterized by specific genetic, anatomical, or physiological features that set them apart from other castes within the same colony. This differentiation helps maintain the structure and efficiency of the social system. (Wilson, 1979).

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Example of Variation in Ant Morphology Across Species

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Example of Variation in Ant Morphology Across Species

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Morphology is the study of the shape and structure of organisms. Basically, it’s about understanding how their bodies are formed and put together.

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).

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What is the difference between monomorphism and polymorphism in ants? 

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What is the difference between monomorphism and polymorphism in ants? 

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Worker polymorphism, as shown above, occurs in 29% of all ant species.

(La Richelière et al., 2022)

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Our main research question: Is there a link between eusociality intensity and morphological variation of ants?

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Our main research question: Is there a link between eusociality intensity and morphological variation of ants?

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The interaction of different factors such as genetics, the environment, and the caste system impact both eusociality and the morphologies of ants.

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🧬 Genetic Influence on Morphology and Eusociality

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🧬 Genetic Influence on Morphology and Eusociality

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The study by Romiguier et al. (2022) reveals that genetics played a foundational role in shaping both the morphology and eusocial behavior of ants. By sequencing ant genomes, the researchers identified a striking burst of positive selection in genes along the branch leading to the formicoid clade—a group known for the most complex forms of eusociality. Histone acetylation was the key gene marker that researchers identified as being essential for the forminoid clade to function and exist as we know ants today. Histone acetylation regulates gene expression and is crucial for caste differentiation, worker behavior, and queen longevity. The selection of histone acetyltransferase genes, in particular, is linked to the morphological development of worker polymorphism and caste-specific roles. Furthermore, histone acetylation is linked to the unique autophagy we see within ants. These processes are critical for regulating caste differentiation, worker sterility, colony rhythms, and queen longevity—hallmarks of eusocial organization. While these genetic changes provided the molecular groundwork for complex eusocial traits, variation in social complexity among formicoid species suggests that additional factors influence their expression, indicating that gene selection favored—but did not solely determine—eusociality and morphological specialization.

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🧬 Phylogenetic Tree Highlighting Histone Acetylation Under Positive Selection and the Resulting Colony Size and Polymorphism of Ants

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🧬 Phylogenetic Tree Highlighting Histone Acetylation Under Positive Selection and the Resulting Colony Size and Polymorphism of Ants

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🧬 Genome Erosion: Social Parasites in Ant Colonies

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🧬 Genome Erosion: Social Parasites in Ant Colonies

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Essentially all ant species exhibit eusociality. However, specific genome erosion over evolutionary history has led to a specific kind of ant who is not eusocial called social parasites. These social parasites are morphologically (smaller traits such as the mandible and brain size) and behaviorally different (non cooperative). Dr. Schrader and colleagues looked at three social parasite species P. argentina, A. charruanus, and A. insinuator have substantially diverged from their closely related Acromyrmex hosts. These ants invade non-related ant colonies (in this case Acromyrmex colonies) and exploit the eusocial nature of the colony to benefit themselves and their offspring.

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).

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🌎 Influence of Foraging Behaviors and Diet on Morphology.

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🌎 Influence of Foraging Behaviors and Diet on Morphology.

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The foraging behavior of ants, selected by the environment, plays a large role in morphologies. Researchers found that different colonies of the same ant species prey on different foods due to the varying environments. Consequently, species that prey on bulkier, irregularly shaped food developed specialized caste workers with larger heads and mandibles. This morphological specialization allows colonies to maximize their foraging efforts when it comes to the different food present. This adaptation highlights the link between ecological foraging demands that create the differences in distinct worker morphologies, and shows that it is because of more than just colony size or phylogeny (Powell & Franks, 2006).

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🌎 Describing the Different Eating Habits of Five Ant Species...

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🌎 Describing the Different Eating Habits of Five Ant Species...

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(Powell & Franks, 2006)

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🌎 ... Resulting Morphisms of Head Size and the Ratio of Head Width to Back-leg Length and Body Size.

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🌎 ... Resulting Morphisms of Head Size and the Ratio of Head Width to Back-leg Length and Body Size.

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🌎 Influence of Nesting Type on Morphology.

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🌎 Influence of Nesting Type on Morphology.

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Specific nesting types play a crucial role in shaping the diverse morphologies found across ant species. These variations in nest types drive the evolution of specialized traits, such as soldier morphology, which support and reinforce the complex structure of their eusocial societies. Researchers have found that differences in nest entrance sizes are a key factor in the development of specialized soldier traits—especially in head shape and size—enabling ants to effectively block entrances and protect the colony. This relationship between nesting type and morphological specialization demonstrates how environmental pressures directly influence the social roles within a colony. In conclusion, these environmentally driven morphological differences encourage cooperative behavior, highlighting how ants thrive in a eusocial system where division of labor and specialization are essential for both survival and colony efficiency. (Powell, 2016).

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🌎 Example: Morphological Differences Linked to Nest Type

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🌎 Example: Morphological Differences Linked to Nest Type

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This image shows a soldier from the turtle ant species, using its rounded head to block the nest entrance, with the size of the entrance precisely matching the size of its head.

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🌎 Influence of Geographical Location on Morphology. 

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🌎 Influence of Geographical Location on Morphology. 

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In regions that are more arid and hot we see an increased likelihood that one will encounter an ant species that has worker polymorphism.

(La Richelière et al., 2022)

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🤝 Internal Influences (Caste Influences) →

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🤝 Internal Influences (Caste Influences) →

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🤝 Influence of the Caste System on Morphology. 

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🤝 Influence of the Caste System on Morphology. 

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Due to their abundance, many species of Pheidole are currently of interest in studying aspects of sociobiology such as division of labor, caste ecology, and caste determination (Pie & Traniello, 2006, pg. 99-100). Not only are Pheidole species of interest because of their eusocial characteristics, they are also of interest due to the drastic differences in not only the roles of workers but also in the worker morphologies. The minor workers in these species typically take on roles that are more "domestic", such as performing tasks within the nest and foraging. While major workers take on roles that consist of defense and/or food processing and storage (Pie & Traniello, 2006, pg. 100). Researchers Pie & Traniello (2006) found in their study of Pheidole species, that there were distinct morphological differences in size between the minor and major workers. This suggests that the differences in roles within a caste system could be another factor in the various morphological differences seen amongst many eusocial species.

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🤝 Examples of morphological differences that were used in (Pie & Traniello, 2006)

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🤝 Examples of morphological differences that were used in (Pie & Traniello, 2006)

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As colony size increases, workers and queens maximize their inclusive fitness by specializing in non-reproductive and reproductive roles. In other words as the colony size increases the complexity of the eusocial system also increases (Ferguson-Gow et al., 2014). One key finding from this study was that as colony size increased the variation in worker size variation also increased. This indicates that larger social groups have an influence on the variation that we see in worker size.

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Ferguson-Gow, H., Sumner, S., Bourke, A. F. G., & Jones, K. E. (2014). Colony size predicts division of labour in attine ants. Proceedings of the Royal Society B: Biological Sciences, 281(1793), 20141411. https://doi.org/10.1098/rspb.2014.1411

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.

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Annotation: Romiguier, J., Borowiec, M. L., Weyna, A., Helleu, Q., Loire, E., La Mendola, C., Rabeling, C.,  Fisher, B., Ward, P., & Keller, L. (2022). Ant phylogenomics reveals a natural selection hotspot preceding the origin of complex eusociality. Current Biology, 32(13), 2942-2947. https://doi.org/10.1016/j.cub.2022.05.001

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Annotation: Romiguier, J., Borowiec, M. L., Weyna, A., Helleu, Q., Loire, E., La Mendola, C., Rabeling, C.,  Fisher, B., Ward, P., & Keller, L. (2022). Ant phylogenomics reveals a natural selection hotspot preceding the origin of complex eusociality. Current Biology, 32(13), 2942-2947. https://doi.org/10.1016/j.cub.2022.05.001

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Contributor: Arnica Sheaffer

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.

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Annotation: 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.1365-2435.2006.01179.x

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Annotation: 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.1365-2435.2006.01179.x

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Contributor: Claire Henderson

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.

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Annotation: 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 

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Annotation: 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 

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Contributor: Isabella Peet

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.

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Annotation: 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 

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Annotation: 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 

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Contributor: Courtney Earl

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.