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Variation in Perception of Task Initiation and Completion Between Temnothorax Rugatul8us Nestmates Drives Division of Labor VARIATION IN PERCEPTION OF TASK INITIATION AND COMPLETION BETWEEN TEMNOTHORAX RUGATUL8US NESTMATES DRIVES DIVISION OF LABOR Item Type text; Electronic Thesis Authors Lynch, Colin Michael Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 29/09/2021 04:35:33 Item License http://rightsstatements.org/vocab/InC/1.0/ Link to Item http://hdl.handle.net/10150/631546 VARIATION IN PERCEPTION OF TASK INITIATION AND COMPLETION BETWEEN TEMNOTHORAX RUGATUL8US NEST- MATES DRIVES DIVISION OF LABOR By COLIN MICHAEL LYNCH ____________________ A Thesis Submitted to the Honors College In Partial Fulfillment of the minor with honors in Ecology and Evolutionary Biology THE UNIVERSITY OF ARIZONA DECEMBER 2018 Approved by: __________________ Dr. Anna Dornhaus Ecology and Evolutionary Biology Variation in perception of task initiation and completion between Temnothorax rugatulus ​ ​ ​ nest-mates drives division of labor. Abstract Division of labor is one of the defining characteristics of social insect societies. It is thought that division of labor arises through simple logical rules and requires only that workers respond to environmental stimuli they encounter. It has been proposed that the response threshold is one of the primary rules that drives division of labor. We test this hypothesis by measuring response thresholds in the ant species Temnothorax rugatulus to different task-associated stimuli and see ​ ​ whether or not they can predict which tasks the ant takes on in the colony. We found that the response threshold hypothesis successfully predicts the relationship between the sensitivity to task-associated stimuli and performance of the task itself for one task, but it fails at predicting the relationship of the other task. In fact, the results were the opposite of what the response threshold predicted, suggesting that there may exist an alternative rule that functions in the opposite direction of the response threshold and yet may also be capable of allocating tasks. Here we coin the term “satisfaction threshold” as a name for this alternative mechanism. Introduction Social insects are among the most dominant organisms on the planet. The biomass of ants alone rivals that of humans and they are one of the main turners of earth, moving some 50 tons of soil every year in a single square mile of land (Holldobler and Wilson, 1990). Their ecological success is often attributed to their division of labor (Robinson, 1992), which can be understood as the partitioning of work between specialists in a system (Duarte et al, 2011). As these colonies are not centrally controlled, this emergent behavior of colonies arises from the actions and decisions of individuals. These decisions are made through simple logical rules which convert signals from the environment into behavioral responses. While a number of different models have been proposed to explain the logic used by social insects (reviewed in Beshers and Fewell, 2001), one of the most widely used employs the use of innate response thresholds as a regulating mechanism of division of labor. Low threshold workers perform tasks at a lower level of stimulus than high threshold workers and are therefore specialists in a task, maintaining homeostasis in the task associated stimulus. High threshold workers for a particular task stimulus will perform a task only if the stimulus exceeds their higher thresholds. When the need for the task is kept low by the low threshold workers, then the high threshold workers will rarely be triggered. It has been demonstrated that variation in sensitivity to sucrose in young honeybees correlates with their future foraging behavior (Pankiw and Page, 2000). Workers with the lowest response threshold became water foragers, followed with increasing response thresholds by pollen foragers, nectar foragers, bees collecting both pollen and nectar, and finally those returning to the colony empty. It has also been shown that bumblebees vary in their response thresholds for thermoregulatory fanning behavior, such that some nestmates consistently fan at lower temperatures and others at higher temperatures, suggesting that behavioral differences might emerge from differences in sensitivity to temperature (Weidenmuller, 2004). However, outside of this work with bees, few empirical studies been done to show whether or not response thresholds in fact exist and that they regulate division of labor in social insects. To fill the knowledge gap, we measured the sensitivity Temnothorax rugatulus ants have to two ​ ​ different task-associated stimuli and tested whether or not this sensitivity was correlated to their behavior in the colony under normal circumstances. The response threshold hypothesis posits that the most sensitive individuals to a certain stimulus will be the individuals who specialize in the task associated with that stimulus. For example, the ants that respond low levels of brood pheromone should be more likely to be brood care workers. However, some preliminary experiments indicated that insensitive individuals may be the ones who specialize in a task. This was a wholly unexpected result, as there was no theoretical framework to understand why this would occur. Therefore the “satisfaction threshold” hypothesis was proposed to make sense of these results. To illustrate the differences between the two hypotheses, consider the following thought experiments. In the first, imagine that there are two roommates sharing a kitchen. The dishwasher is broken, so the roommates agreed to wash the dishes in the sink. One of these roommates is very clean, so he or she washes the dishes when they just start piling up. The other roommate, who has a higher tolerance for dishes, never does the task, as it never reaches the point where he or she is concerned with them. In this colony of two individuals, the person with the lower response threshold (the number of dishes required to initiate a response) ends up becoming the specialist without ever needing to appeal to higher-level cognition. For the second thought experiment, imagine that these two roommates are now working to fill the fridge with food. One of the roommates loves food, and is not satisfied with a fridge that is only partially filled. He or she will keep working until the fridge is completely filled. The other roommate, who is less fond of food, may be satisfied with a fridge that is only half full, and is therefore never inspired to work. Here again task specialization is achieved without higher-level cognition, as the person with the higher satisfaction threshold (the amount of food necessary to stop performing a task) ends up becoming the specialist. These thought experiments illustrate that there are at least two different types of task-associated stimuli which have yet to be explored. The fullness of the sink, for example, is an indicator of how much work needs to be done. When the sink is full, this indicates that dishwashing absolutely needs to be done. This subcategory of the task-associated stimulus is called the task initiation stimulus (TIS). On the other hand, the fullness of the fridge is an indicator of how much work has already been performed. When the fridge is full, this indicates that food gathering no longer needs to be performed. This subcategory of the task-associated stimulus is called the task completion stimulus (TCS). When a signal in the environment is perceived as a TIS, the response threshold hypothesis (or variations in the starting point of a task) is necessary to explain task allocation. When an individual is sensitive to this indicator that a job needs to be performed, they tend to overestimate how much of that signal is present in the environment, which means that they have a lower response threshold. This is akin to overestimating how many dishes are in the sink, which compels a person to start performing them sooner. On the other hand, when a signal in the environment is perceived as a TCS, the satisfaction threshold hypothesis is necessary to explain task allocation. When an individual is insensitive to this indicator of how much work has already been performed, they tend to underestimate how much of that signal is present in the environment, which means that they have a higher satisfaction threshold. This is akin to underestimating how much food is in the fridge, which compels a person to continue foraging for food. Sensitivity does not necessarily need to be addressed to create this variation in starting and stopping. It could very well be the case that each individual has a different cognitive rule that correlates with their thresholds. For example, a person may only start performing a task when they know they see 10 dishes in the sink, or they may stop filling the fridge when they see 10 food items. The purpose of this paper, then, is to demonstrate that sensitivity is associated with ​ task allocation, which seems to support the sensitivity-driven view of the thresholds rather than the cognitively-driven view. What is more, it seems that ants may use both response and satisfaction thresholds to organize labor, as for one task-associated stimulus, sensitive individuals performed the task in the colony under normal conditions, which supports the response threshold hypothesis. For the other
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