What are the benefits of division of labor? Or: specialization does not predict individual efficiency in an ant
Anna Dornhaus
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The ecological success of social insects is often attributed to an increase in efficiency achieved through division of labor between workers in a colony. However, the important assumption that specialists are indeed more efficient at their work than generalist individuals – the “Jack-of-all-trades is master of none” hypothesis – has rarely been tested. Using individually marked workers of the ant Temnothorax albipennis, I could show that individual efficiency is not predicted by how specialized workers were on a task for several tasks. Worker efficiency is also not consistently predicted by that worker’s overall activity or delay to begin the task. My results demonstrate that in an ant species without morphologically differentiated worker castes, workers may nevertheless differ in their ability to perform different tasks. Surprisingly, this variation is not utilized by the colony – worker allocation to tasks is unrelated to their ability to perform them. What, then, are the adaptive benefits of behavioral specialization, and why do workers choose tasks without regard for whether they can perform them well? We are still far from an understanding of the adaptive benefits of division of labor in social insects.
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Spatial Distribution of Bumble Bees (Bombus impatiens) Inside the Nest: Evidence for Spatial Fidelity Zones among Workers
Jenny Jandt, Anna Dornhaus
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We show that in colonies of bumble bees, Bombus impatiens (1) workers tend to remain at a specific distance from the colony center independent of their age, and thus the spatial pattern of workers on the nest is not random; (2) smaller individuals maintain smaller spatial zones and tend to be closer to the center; and (3) individuals who are more likely to perform brood care tend to remain in the center of the nest, whereas foragers are more often found on the periphery of the nest when not foraging. Spatial preferences observed in workers do not seem to depend on the exact location of brood or honeypots inside the nest. Instead, spatial patterns may be the result of a self-organized sorting mechanism that plays a role in the division of labor among workers.
We also have been quantifying ovary development of workers after the queen was removed. Task performance, i.e. foraging or brood care, during the ergonomic phase of the colony does not seem to predict ovary development (dominance rank) later on.
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Task allocation mechanisms and colony-level performance
Anna Dornhaus, Franziska Kluegl
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In spite of much research on potential mechanisms for allocating tasks to workers, very little is known on how different such mechanisms impact
colony-level efficiency. We are using a computational model to predict which mechanisms, and what degree of specialization, would optimize colony function under
different conditions. This model will generate predictions as to which species should be expected to display a high degree of individual specialization,
and also whether colony size or environmental variability is more likely to influence this. Results from this project will also be useful for engineers, for
choosing the appropriate task allocation mechanism in artificial complex systems, such as distributed computing networks.
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Effects of scaling on performance and metabolic rate of superorganisms
Tuan Cao, Michele Lanan, Mitchell Pavao-Zuckerman, Anna Dornhaus
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We are interested in effects of scaling on superorganisms and the emergence of complexity at higher colony sizes, with a particular focus on determining the
effect of numbers of workers and mass involved. We are measuring metabolic rates of ant colonies of varying size. General scaling theory predicts lower mass-specific metabolic rate at larger body sizes, but predictions for superorganisms are unclear. If ant colonies can maintain high mass-specific metabolic rate at high colony sizes, that may explain their higher productivity and success compared to animals of a body mass that is similar to the mass of an ant colony. We will also investigate how effects of colony size may be mediated by individual body size differences.
We were already able to show that worker density inside a nest cavity affects colony metabolic rate and thus energy use.
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Worker performance and learning
Anna Dornhaus
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Why do workers differ in how well they perform tasks? We have marked naive individual workers of Temnothorax albipennis and quantified their performance at brood transports in colony emigrations and transports of nest material (sand grains). We will test whether performance is affected by experience (learning), and whether learning increases or decreases intrinsic performance differences among workers. We will also test whether workers increase their preference for tasks that they perform well.
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What is the adaptive benefit of worker polymorphism in bumble bees?
Anna Dornhaus; Maggie Couvillon
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Even within the same colony, bumble bees produce workers of strongly varying sizes. What is the adaptive function of this size variation? We are testing the hypotheses that (1) different-sized workers are adaptive as specialists for different tasks; (2) that they are the result of a trade-off between cheap, low-quality and expensive, high-quality workers; that (3) small workers have other benefits besides being cheaper to produce, even if they do not perform any task well; and (4) that they are the non-adaptive result of a biased larval care system.
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Individual search... rules!
Amelie Schmolke, Anna Dornhaus
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While recruitment to known food sources by ants has been studied extensively, it is not as clear how individuals discover new food sources. Moreover, the
search strategies used by ants lacking mass recruitment are not well known. We approach these questions by characterizing the foraging behavior
of Temnothorax rugatulus ants. We found that the search path of T. rugatulus foragers can be described as a random walk, i.e. the ants show no significant bias in direction choice at any point of their (unsuccessful) search. This is also reflected in the area the foragers cover during their search: foragers visit wide areas, and their space use is not influenced by the areas visited by their nest mates. Hence, T. rugatulus foragers act independently on their food search and do not divide up the nest surroundings into individual search areas.
While foragers act individually outside the nest, the nutritional status of the colony determines the number of ants leaving the nest to forage. The longer
time a colony remains without food intake, the more individuals will go out to search for food. Is this strategy of food search optimal for small
colonies? And does it depend on the distribution of food sources the ants tend to exploit? We introduce an individual-based model of ant foraging that allows the
comparison of different strategies with respect to colony-level foraging success.
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Collective decision-making in ants and populations of neurons
James Marshall, Rafal Bogacz, Anna Dornhaus, Robert Planque, Tim Kovacs, Nigel Franks
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The problem of how to compromise between speed and accuracy in decision-making faces organisms at many levels of biological complexity. Striking parallels are evident between decision making in primate brains an collective decision-making in social insect colonies: in both systems separate populations accumulate evidence for alternative choices, when one population reaches a threshold a decision is made, and this threshold may be varied to compromise between the speed and accuracy of decision-making. We analyze the properties of both of these systems, and show in which ways the ant collective decision making system may be statistically optimal.
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Parasite-host relatedness and other host characteristics in ant social parasitism
Ming Huang, Anna Dornhaus
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We used data from published literature to characterize commonalities and differences in host choice between the four major types of ant social parasitism: xenobiosis, temporary parasitism, dulosis, and inquilinism. Our analysis shows that xenobiotic parasites tend to have single or multiple host species that are very distantly related and have medium to large host colonies. Temporary parasites tend to have multiple host species that are very closely related and have medium host colonies. Dulotic parasites tend to have multiple host species that are slightly less related and have host colonies of any size. Lastly, inquiline parasites tend to have single host species that are very closely related and have medium host colonies. In addition, a parasite tends to be very closely related to its host when the parasite has a single host species or large host colony sizes.
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Communication, learning, and polydomy in weaver ants, Oecophylla smaragdina
Tuan Cao, Anna Dornhaus
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email.arizona.edu) directly.