In cooperative systems exhibiting division of labor, such as microbial communities, multicellular organisms, and social insect colonies, individual units share costs and benefits through both task specialization and exchanged materials. Socially exchanged fluids, like seminal fluid and milk, allow individuals to molecularly influence conspecifics. Many social insects have a social circulatory system, where food and endogenously produced molecules are transferred mouth-to-mouth (stomodeal trophallaxis), connecting all the individuals in the society. To understand how these endogenous molecules relate to colony life, we used quantitative proteomics to investigate the trophallactic fluid within colonies of the carpenter ant Camponotus floridanus. We show that different stages of the colony life cycle circulate different types of proteins: young colonies prioritize direct carbohydrate processing; mature colonies prioritize accumulation and transmission of stored resources. Further, colonies circulate proteins implicated in oxidative stress, ageing, and social insect caste determination, potentially acting as superorganismal hormones. Brood-caring individuals that are also closer to the queen in the social network (nurses) showed higher abundance of oxidative stress-related proteins. Thus, trophallaxis behavior could provide a mechanism for distributed metabolism in social insect societies. The ability to thoroughly analyze the materials exchanged between cooperative units makes social insect colonies useful models to understand the evolution and consequences of metabolic division of labor at other scales.

Since the dawn of life, transfers of metabolized material between individuals have led to great innovations of evolution. When metabolized material is transferred from one individual’s body to another (as with sperm, eggs, milk, symbionts), secondary manipulative molecules that induce a physiological response in the receiver are often transferred along with the primary cargo. The bioactive and transfer-supporting components in these socially transferred materials have evolved convergently to the point where they can be used in applications across taxa and type of transfer. Because these materials’ composition is typically highly dynamic and context-dependent, their focused study will allow deeper understanding of their transformative evolutionary and physiological role. We synthesize a conceptual framework for their study, and discuss future directions.

No abstract

Socially exchanged fluids, like seminal fluid and milk, present a direct and effective means through which an individual can influence conspecifics. As organisms heavily adapted to social life, social insects have evolved a suite of behavioral, morphological and molecular mechanisms to ensure cooperation and inclusive fitness benefits for all group members, just as multicellular organisms have. As a part of their social physiology, some species have developed a social circulatory system, where exogenous food and endogenously produced material, such as hormones, proteins and small molecules, are transferred mouth-to-mouth from the foregut of one individual to another. This fluid transfer – stomodeal trophallaxis – ensures that both food and endogenously produced components are distributed throughout the social network.To understand how the endogenous materials in trophallactic fluid relate to colony life, we investigated trophallactic fluid of the carpenter ant Camponotus floridanus, monitoring this fluid within colonies under different biotic and abiotic conditions and in different individuals within colonies. Using quantitative proteomic analyses of over 100 colony and single-individual trophallactic fluid samples, we established a set of core trophallactic fluid proteins. By combining frequentist, empirical Bayesian and machine-learning classification tools, we identified sets of proteins that are significantly induced in trophallactic fluid under different conditions: proteins that differ between young and mature colonies in the field, proteins that differ between colonies in the field and in the lab, and proteins that differ between the trophallactic fluid of nurses and foragers.Results reveal that different stages of the colony life cycle utilize classic metabolic processes in different ways, with young colonies prioritizing direct carbohydrate processing, while mature colonies prioritizing transmission of stored resources over the trophallactic network. Further, proteins from pathways that govern the fecundity-longevity trade-off and that have been previously implicated in social insect caste determination are being transferred between individuals within colonies, potentially acting as superorganismal hormones. Nurses in particular show higher abundances in proteins and pathways involved in defense against aging. Thus, we show that the protein composition of ant trophallactic fluid varies with social and developmental conditions both at the colony and at the individual level, suggesting that the trophallactic fluid proteome plays a key role in the social physiology of colonies.

Behavioral innovations can be ecologically transformative for lineages that perform them and for their associated communities. Many ecologically dominant, superorganismal, and speciose ant lineages use a mouth-to-mouth fluid exchange behavior - trophallaxis - to share both exogenously sourced and endogenously produced materials across their colonies, while lineages that are less abundant, less cooperative and less speciose tend not to perform this behavior. How and why this behavior evolved and fixed in only some ant lineages remains unclear and whether this trait enables ants' ecological dominance is not yet understood. Here we show that trophallaxis evolved in two major events ~110 Ma in lineages that today encompass 36% of ants, and in numerous smaller and more recent events. We found that trophallaxis evolved early only in ant lineages that had reduced intra-colonial conflict by losing workers ability to reproduce. Our causal models indicate that this signature behavior of superorganismal ants required social cooperation and ecological opportunism, and likely contributed to the large colony sizes and speciation patterns of the ants that use it and dominate our landscapes today. We hypothesize that the early evolution of trophallaxis was brought about by a major shift in terrestrial ecosystems through the origin and diversification of flowering plants and the consequent opportunistic inclusion of nectar and sap-sucker honeydew in the ant diet.