Pattern formation and optimization in army ant raids

Army ant colonies display complex foraging raid patterns involving thousands of individuals communicating through chemical trails. In this paper we explore, by means of a simple search algorithm, the properties of these trails in order to test the hypothesis that their structure reflects an optimized mechanism for exploring and exploiting food resources. The raid patterns of three army ant species, {em Eciton hamatum}, {em Eciton burchelli} and {em Eciton rapax}, are analysed. The respective diets of these species involve large but rare, small but common, and a combination of large but rare and small but common, food sources. Using a model proposed by Deneubourg and collaborators, we simulate the formation of raid patterns in response to different food distributions. Our results indicate that the empirically observed raid patterns maximise return on investment, that is, the amount of food brought back to the nest per unit of energy expended, for each of the diets. Moreover, the values of the parameters that characterise the three optimal pattern-generating mechanisms are strikingly similar. Therefore the same behavioural rules at the individual level can produce optimal colony-level patterns. The evolutionary implications of these findings are discussed.Postprint (published version

A Model for Collective Dynamics in Ant Raids

Ant raiding, the process of identifying and returning food to the nest or bivouac, is a fascinating example of collective motion in nature. During such raids ants lay pheromones to form trails for others to find a food source. In this work a coupled PDE/ODE model is introduced to study ant dynamics and pheromone concentration. The key idea is the introduction of two forms of ant dynamics: foraging and returning, each governed by different environmental and social cues. The model accounts for all aspects of the raiding cycle including local collisional interactions, the laying of pheromone along a trail, and the transition from one class of ants to another. Through analysis of an order parameter measuring the orientational order in the system, the model shows self-organization into a collective state consisting of lanes of ants moving in opposite directions as well as the transition back to the individual state once the food source is depleted matching prior experimental results. This indicates that in the absence of direct communication ants naturally form an efficient method for transporting food to the nest/bivouac. The model exhibits a continuous kinetic phase transition in the order parameter as a function of certain system parameters. The associated critical exponents are found, shedding light on the behavior of the system near the transition.Comment: Preprint Version, 30 pgs., 18 figures, complete version with supplementary movies to appear in Journal of Mathematical Biology (Springer

Emerging robot swarm traffic

We discuss traffic patterns generated by swarms of robots while commuting to and from a base station. The overall question is whether to explicitly organise the traffic or whether a certain regularity develops `naturally'. Human driven motorized traffic is rigidly structured in two lanes. However, army ants develop a three-lane pattern in their traffic, while human pedestrians generate a main trail and secondary trials in either direction. Our robot swarm approach is bottom-up: designing individual agents we first investigate the mathematics of cases occurring when applying the artificial potential field method to three 'perfect' robots. We show that traffic lane pattern will not be disturbed by the internal system of forces. Next, we define models of sensor designs to account for the practical fact that robots (and ants) have limited visibility and compare the sensor models in groups of three robots. In the final step we define layouts of a highway: an unbounded open space, a trail with surpassable edges and a hard defined (walled) highway. Having defined the preliminaries we run swarm simulations and look for emerging traffic patterns. Apparently, depending on the initial situation a variety of lane patterns occurs, however, high traffic densities do delay the emergence of traffic lanes considerably. Overall we conclude that regularities do emerge naturally and can be turned into an advantage to obtain efficient robot traffic

Individual rules for trail pattern formation in Argentine ants (Linepithema humile)

We studied the formation of trail patterns by Argentine ants exploring an empty arena. Using a novel imaging and analysis technique we estimated pheromone concentrations at all spatial positions in the experimental arena and at different times. Then we derived the response function of individual ants to pheromone concentrations by looking at correlations between concentrations and changes in speed or direction of the ants. Ants were found to turn in response to local pheromone concentrations, while their speed was largely unaffected by these concentrations. Ants did not integrate pheromone concentrations over time, with the concentration of pheromone in a 1 cm radius in front of the ant determining the turning angle. The response to pheromone was found to follow a Weber's Law, such that the difference between quantities of pheromone on the two sides of the ant divided by their sum determines the magnitude of the turning angle. This proportional response is in apparent contradiction with the well-established non-linear choice function used in the literature to model the results of binary bridge experiments in ant colonies (Deneubourg et al. 1990). However, agent based simulations implementing the Weber's Law response function led to the formation of trails and reproduced results reported in the literature. We show analytically that a sigmoidal response, analogous to that in the classical Deneubourg model for collective decision making, can be derived from the individual Weber-type response to pheromone concentrations that we have established in our experiments when directional noise around the preferred direction of movement of the ants is assumed.Comment: final version, 9 figures, submitted to Plos Computational Biology (accepted

곤충의 섭식 및 반포식 행동에서 나타나는 유연전략과 고정전략

학위논문(박사)--서울대학교 대학원 :자연과학대학 생명과학부,2020. 2. 피오트르 야브원스키.This dissertation presents my theoretical and empirical studies of flexibility focused on two topics of insect ethology. The first topic is the collective foraging strategy of the ants. Army ants were chosen as an example of extremely low behavioral flexibility in foraging; and on the other hand, carpenter ants were used to study mechanisms of extraordinarily high flexibility. My second interest is the behavioral control of aposematism, the phenomenon in which a defended prey animal gives a warning signal to predators who may recognize its advertised unprofitability. In my first study, I investigated theoretical benefits potentially associated with the evolution of specialized foraging behavior performed by army ants. It is known that a typical colony of Neotropical army ants (subfamily Ecitoninae) regularly raids a large area around their bivouac by forming a narrow directional column that can reach up to one hundred meters in length. Then the raid is finished and then relaunched 12–17 times, each time toward different orientations before the colony relocates to a new area. A hypothetical alternative to this foraging mode is raiding radially and symmetrically by expanding the search front in every direction like a circular bubble. Using an existing agent-based modeling software that simulates army ants behavior, I compared the two possible modes of foraging in different food distributions. Regardless of the food patch abundance, the radial raiding was superior to the directional raiding when food patches had low quality, and the directional raiding was favored when the patches were rich. In terms of energy efficiency, the radial raiding was the better strategy in a wide range of conditions. In contrast, the directional raiding tended to yield more food per coverage area. Based on this model, I suggest that the directional raiding by army ants is an adaptation to the habitats with the abundance of high-quality food patches. This is the first theoretical argument for the adaptive value of the army ant behavioral syndrome which agrees with cumulated body of existing empirical measurements and descriptive models. This conclusion fits well with the known ecological conditions of army ants and their habitat. In the second study, I conducted field experiments using wild colonies of carpenter ants (Camponotus japonicus). Unlike the army ants which obligatorily maintain their tight marching column, C. japonicus shows considerable variation in the coherence when foraging in a group. My investigation on this variability was centered on three sub-questions. First, I observed if higher group dispersion resulted in the lower chances of reaching the food source. As a dispersed group would cover a wider search area collectively, I believed that there would be some balancing disadvantage that can explain the coexistence of coherent and dispersed foraging behaviors. Second, I explored if there is any correlation between the group coherence and the behavior of the scout, the key individual that first discovers the food source and subsequently summons multiple nestmates to the site. As the scouts of related species were known to have central control over their groups of recruits in various ways, I hypothesized that C. japonicus scouts would be also involved in the determination of group behavior. Third, I tested what would happen if I remove the scout from a group. I believed that the lack of scout pheromone would signal dispersion, as the scout seemed to be the source of coherence signals. After my analysis, I reached the following conclusions. First, higher group dispersion leads to lower success rates in correctly finding the food source. Second, the mobility of the scout in the pre-recruitment stage, her characteristic stroking behavior during recruitment, and the dispersion of the recruited group were correlated to each other. Third, the simple lack of scout signal was not adequate to explain the observed variable reactions of the abandoned followers, and their response was linked to the pre-recruitment behaviors of the scout. I believe that C. japonicus possesses one of the most complicated recruitment strategy among the entire Formicidae, and the above findings rendered this species one of the best-understood ants in terms of mechanisms through which the flexible group foraging is controlled. From the above two studies, I revealed each one of the ultimate and proximate mechanisms that maintain different levels of flexibility in ants foraging strategy. My final topic, the behaviorally controlled aposematism, is a variant of aposematism in which the defended prey animal can choose to give different levels of anti-predatory warning signals depending on the circumstances. This switching may occur in reaction to predators approach (pre-attack signals) or attack (post-attack signals). The switchable aposematism has been relatively poorly studied, but it could possess a variety of benefits. First, the switching could startle the predators (deimatism). Second, it could facilitate the aversive learning. Third, it could minimize the exposure or energetic expense, as the signal can be switched off. These potential benefits might offset the cost of developing, maintaining and utilizing the switchable traits. Here I focused on the third benefit of switchability, the cost-saving aspect, and developed an individual-based computer simulation of predators and prey. In 88 128 model runs, I observed the evolution of permanent, pre-attack, or post-attack aposematic signals of varying strength. I found that, in general, the pre-attack switchable aposematism may require moderate predator learning speed, high basal detectability, and moderate to high signal cost. On the other hand, the post-attack signals may arise under slow predator learning, low basal detectability, and high signal cost. When predator population turnover is fast, it may lead to the evolution of post-attack aposematic signals that are not conforming to the above tendency. I also suggest that the high switching cost may exert different pressure on the pre-attack and post-attack switchable strategies. To my knowledge, these are the first theoretical attempts to systematically explore the evolution of the switchable aposematism relative to permanent aposematism in defended prey. These findings in common provided novel view into the proximate and ultimate mechanisms of behavioral flexibility found in insects. In addition, they exposed limitations of our understanding that called for future studies. First, I expect agonistic interactions with competitors or prey animals would considerably affect the collective foraging, aposematism, and food aversion, but I could not successfully provide universal background for such effects within the currently available datasets and literature in spite of many efforts. Second, the qualitative difference of food items such as carbohydrate- or protein-richness is known to be pivotal in insect trophic ecology, but the scarcity of information and logistical limitations left me being unable to incorporate relevant enquiries into this dissertation. I predict that further discussions and experiments regarding these questions will reveal valuable information about the balance between the needs of resource exploitation and defense, which might greatly influence the behavioral flexibility of insects observed in the ecosystem.이 논문은 곤충 행동학상의 두 가지 주제를 중심으로 하여 유연성에 대한 이론적 및 실험적 연구들을 제시한다. 첫 번째 주제는 개미의 집단적 섭식 전략이다. 이 논문에서는 먹이찾기 행동에서 매우 낮은 행동적 유연성을 보이는 예시로 군대개미를 선택하였고, 매우 높은 유연성의 기전을 연구할 대상으로는 왕개미를 택하였다. 두 번째 주제는 방어능력을 갖춘 피식동물이 포식자에게 정보를 주어 그 부적합성을 알게 하는 현상, 즉 경고신호 (aposematism) 의 행동적 통제이다. 첫 번째 연구에서는 군대개미의 특화된 섭식 행동이 진화할 수 있게 한 이론적인 이점들을 조사하였다. 신열대구 군대개미 (Ecitoninae아과) 의 집락은 일반적으로 100여 미터에 이르는 가느다란 행렬을 특정 방향으로 지향하여 영소 (bivouac) 주변의 광활한 지역을 강습하는 것으로 알려져 있다. 이러한 강습 행동이 매번 다른 방향으로 12-17번 반복된 후 집락은 새로운 지역으로 이주한다. 이와 같은 먹이찾기 방법에 대해 하나의 가설적인 대안은 방사적, 대칭적인 형태로 강습하여 수색대의 최전선이 점차 확장되는 원형을 형성하도록 전진하는 것이다. 이와 같은 두 가지 먹이찾기 방법을 다양한 먹이자원 분포 하에서 비교해보기 위해, 군대개미의 행동을 시뮬레이션하는 기존의 개체 기반 모델링 소프트웨어를 활용하였다. 먹이패치의 질이 낮은 경우, 먹이패치의 풍부도와 무관하게 방사형 강습 행동은 지향성 강습보다 우월한 결과를 보였으며, 반면 먹이패치의 질이 높은 경우 지향성 강습이 유리하였다. 에너지 효율의 측면에서는 광범위한 조건에 걸쳐 방사형 강습이 더 나은 전략인 것으로 밝혀졌다. 그에 반해, 지향성 강습은 탐색 면적 당 더 많은 먹이를 구할 수 있는 경향이 있었다. 이 연구에서는 이 모델을 기반으로 하여, 군대개미의 지향성 강습 행동이 양질의 먹이패치가 풍부한 서식지 환경에 적응한 결과임을 주장하였다. 이는 지금까지 누적된 실험적 측정값 및 기술적 모델들과 일치하면서 군대개미 행동군 (behavioral syndrome) 의 적응적 가치를 보여주는 최초의 이론적 논증이다. 이러한 결론은 군대개미와 그 서식지에 대해 기존에 알려진 생태학적 지식과 잘 일치하는 것이다. 두 번째 연구에서는 왕개미 (Camponotus japonicus)의 야생 집락을 대상으로 야외 실험을 진행하였다. 응집된 행진 대열만을 절대적으로 고수하는 군대개미와 달리, C. japonicus는 집단적으로 먹이를 찾을 때 응집도의 변산이 상당히 크다. 이 연구에서는 다음의 세 가지 하위 질문을 중심으로 이 현상에 대한 조사를 진행하였다. 첫째, 집단의 분산도가 높으면 먹이원에 이르는 성공률이 낮아지게 되는가를 관찰하였다. 분산된 집단은 전체적으로 보았을 때 더 넓은 면적을 탐색하게 된다는 점을 감안할 때, 그에 대응되는 단점이 존재하여야 응집형 및 분산형 집단이 공존하는 현상을 설명할 수 있으리라 판단하였다. 둘째, 처음으로 먹이원을 발견하고 이후에 다수의 동료들을 불러모으는 중심 개체인 정찰병의 행동이 집단적 응집도와 어떠한 연관성을 지니는지 탐색하였다. 근연종의 정찰병들이 소집된 집단에 대해 다양한 방식으로 통제력을 행사하는 것이 알려져 있었으므로, C. japonicus 정찰병 역시 집단 행동의 결정에 관여할 것이라는 가설을 수립하였다. 셋째, 집단으로부터 정찰병을 제거하는 경우 어떤 반응이 나타나는지 시험하였다. 정찰병이 응집 신호의 근원인 것으로 생각되었으므로, 정찰병 페로몬의 부재는 곧 분산 신호로 해석될 것이라 판단하였다. 분석 결과, 이 연구는 다음과 같은 결론들에 도달하였다. 첫째, 집단의 분산도가 높으면 먹이원을 올바르게 찾아내는 성공률이 낮아진다. 둘째, 소집 전단계에서 정찰병의 이동성, 소집 중 정찰병의 획긋기 행동, 소집된 집단의 분산도는 모두 서로 연관되어 있다. 셋째, 정찰병 신호의 단순 부재만으로는 남겨진 추종자들의 다양한 반응을 모두 설명할 수 없으며, 이들의 반응은 소집 전단계에서 정찰병이 보였던 행동과 연결되어 있다. C. japonicus는 개미과를 통틀어 가장 복잡한 소집전략을 지닌 종 중 하나로 생각되는데, 위와 같은 발견들을 통해 집단적 섭식행동의 유연성 및 그 제어방법이라는 영역에서 이 종에 대한 이해도는 개미 중 최고 수준에 이르게 되었다. 위의 두 연구에서는 개미의 섭식 전략에서 다양한 수준의 유연성이 유지되게 하는 궁극적 및 직접적 기전 한 가지씩을 규명하였다. 이 논문의 마지막 주제는 행동적으로 통제 가능한 경고신호, 즉 방어능력을 갖춘 피식동물이 상황에 따라 다른 수준의 반포식 경고신호를 택할 수 있는 현상에 대한 것이다. 이와 같은 신호의 전환 은 포식자의 접근에 대응하여 발생할 수도 있고 (공격전 신호) 또는 공격에 대응하는 것일 수도 있다 (공격후 신호). 전환가능한 경고신호는 비교적 연구가 잘 되어 있지 않으나, 다양한 이점을 지닐 것으로 보인다. 첫째, 전환 행동 자체가 포식자를 놀라게 할 수 있다. 둘째, 회피 학습을 촉진할 수 있다. 셋째, 신호를 꺼 놓음으로써 불필요한 노출이나 에너지 소모를 최소화할 수 있다. 이와 같은 잠재적인 이점들은 전환형 형질을 발달시키고 유지하는 데 필요한 비용을 상쇄할 가능성이 있다. 이 연구에서는 전환가능성의 세 번쨰 이점, 즉 비용 절감 측면에 초점을 맞추어 포식자와 피식자에 대한 개체 기반 컴퓨터 시뮬레이션을 개발하였다. 88,128회의 모델 구동을 통하여 다양한 강도의 영구적, 공격전 및 공격후 경고신호가 진화하는 과정을 관찰하였다. 일반적으로, 공격전 경고신호는 중등도의 포식자 학습속도, 높은 기저 발견률, 중등도에서 고도의 신호비용을 요구하리라는 점을 발견하였다. 그에 반해 공격후 진호는 낮은 포식자 학습속도, 낮은 기저 발견률, 높은 신호비용 하에서 나타날 가능성이 높았다. 포식자 개체군의 회전이 빠른 경우, 위의 경향성을 벗어난 공격후 신호의 진화가 가능하였다. 또한 이 연구는 높은 전환비용이 공격전 및 공격후 신호 전략에 대해 서로 다른 압력을 가하리라는 예측을 제시하였다. 이와 같은 발견들은 방어능력이 있는 피식동물에서 영구적 경고신호에 대해 전환가능한 경고신호가 진화하는 과정을 체계적으로 탐색하고자 한 최초의 이론적 시도이다. 이와 같은 발견들은 공통적으로 곤충에서 나타나는 행동적 유연성의 직접적 및 궁극적 기전에 대해 새로운 시각을 제공하였다. 또한 이 연구들은 후속 연구를 필요로 하는 이해의 한계를 노출하기도 하였다. 첫째, 경쟁자나 피식자와의 적대적 상호작용이 집단섭식, 경고신호 및 회피학습에 상당히 큰 영향을 끼칠 것으로 예상되나, 많은 노력에도 불구하고 현재 가용한 데이터와 문헌으로는 이같은 효과에 대해 범용적인 배경지식을 생산할 수 없었다. 둘째, 탄수화물 또는 단백질 함량과 같은 먹이자원의 질적 차이가 곤충의 영양생태에 중대한 영향을 준다는 것이 알려져 있으나, 정보의 희박함과 자원의 한계로 인하여 이와 관련된 조사를 본 논문에 포함시키지 못하였다. 이러한 질문들에 대한 추가적인 논의와 실험이 이루어진다면, 생태계에서 관찰되는 곤충의 행동적 유연성에 큰 영향을 주고 있을 자원 획득과 방어의 균형에 관한 가치 있는 정보들이 밝혀지리라 예상한다.CHAPTER 1: INTRODUCTION 1 1. 1 Opening remarks 1 1. 2 Behavioral flexibility in ant foraging 3 1. 3 Behavioral flexibility in aposematism 5 CHAPTER 2. WHY DO ARMY ANTS, AND ONLY ARMY ANTS, FORAGE IN COLUMNS? 7 2. 1 Introduction 8 2. 2 Methods 9 2. 3 Results 11 2. 4 Discussion 12 CHAPTER 3. COLUMNAR FORAGING WITH VARYING COHESIVENESS: BEHAVIOR OF Camponotus japonicus 20 3. 1 Introduction 20 3. 2 Methods 24 3. 3 Results 33 3. 4 Discussion 35 CHAPTER 4. SWITCHABLE APOSEMATISM: A BEHAVIORALLY CONTROLLED WARNING SIGNAL 65 4. 1 Introduction 66 4. 2 Methods 71 4. 3 Results 90 4. 4 Discussion 93 4. 5 Appendix 122 CHAPTER 5. GENERAL CONCLUSION 140 REFERENCES 145 국 문 초 록 159 ACKNOWLEDGMENT 164Docto

Mechanisms for the Evolution of Superorganismality in Ants

Ant colonies appear to behave as superorganisms; they exhibit very high levels of within-colony cooperation, and very low levels of within-colony conflict. The evolution of such superorganismality has occurred multiple times across the animal phylogeny, and indeed, origins of multicellularity represent the same evolutionary process. Understanding the origin and elaboration of superorganismality is a major focus of research in evolutionary biology. Although much is known about the ultimate factors that permit the evolution and persistence of superorganisms, we know relatively little about how they evolve. One limiting factor to the study of superorganismality is the difficulty of conducting manipulative experiments in social insect colonies. Recent work on establishing the clonal raider ant, Ooceraea biroi, as a tractable laboratory model, has helped alleviate this difficulty. In this dissertation, I study the proximate evolution of superorganismality in ants. Using focussed mechanistic experiments in O. biroi, in combination with comparative work from other ant species, I study three major aspects of ant social behaviour that provide insight into the origin, maintenance, and elaboration of superorganismality. First, I ask how ants evolved to live in colonies, and how they evolved a reproductive division of labour. A comparative transcriptomic screen across the ant phylogeny, combined with experimental manipulations in O. biroi, finds that reproductive ants have higher insulin levels than their non-reproductive nestmates, and that this likely regulates the reproductive division of labour. Using these data, as well as studies of the idiosyncrasies of O. biroi’s life history, I propose a mechanism for the evolution of the first colonies. It is possible that similar mechanisms underlie the evolution of reproductive division of labour in other superorganisms, and of germ-soma separation in nascent multicellular individuals. Second, I ask how ant workers assess colony hunger to regulate their foraging behaviour. I find that workers use larval signals, but not their own nutritional states, to decide how much to forage. In contrast, they use their nutritional states, but not larval signals, to decide how much to eat, suggesting that in at least some ant species, foraging and feeding have been decoupled. This evolution of colony-level foraging regulation has occurred convergently in hymenopteran superorganisms, and is analogous to the evolution of centralised regulation of foraging behaviour in multicellular animals. Finally, I ask how an iconic collective foraging behaviour – the mass raids of army ants – evolved. I find that O. biroi, a relative of army ants, forages collectively in group raids, that these are ancestral to the mass raids of army ants, and that the transition from group to mass raiding correlates with expansion in colony size. I propose that the scaling effects of increasing colony size explain this transition. It is possible that similar principles underlie the evolution of disparate collective behaviours in other animal groups and among cells within developing animals. Together, these studies illuminate the life history of O. biroi, and suggest mechanisms for the evolution of core aspects of cooperative behaviour in ant colonies. I draw comparisons to the evolution of superorganismality in other lineages, as well as to the evolution of multicellularity. I suggest that there may be additional similarities in the proximate evolutionary trajectories of superorganismality and multicellularity

Pheromone-based Swarming for Position-less MAVs

Unlike existing aerial swarm systems, we aim at developing algorithms which do not require global or relative positioning information concerning agents and their neighbors. This alleviates the need for sensors which require calibration, are expensive and heavy or unusable because of environmental constraints. Rather than positioning, MAVs rely only on simple sensors (magnetic compass, speed sensor and altitude sensor) and local communication with neighbors. Our endeavor is motivated by an application whereby Micro Air Vehicles (MAVs) must organize autonomously to establish a robust communication network between users located on ground. Such a system is aimed towards the rapid and easy deployment of communication networks in disaster areas

Diversity, Biogeography and Community Ecology of Ants

Ants are a ubiquitous, highly diverse, and ecologically dominant faunal group. They represent a large proportion of global terrestrial faunal biomass and play key ecological roles as soil engineers, predators, and re-cyclers of nutrients. They have particularly important interactions with plants as defenders against herbivores, as seed dispersers, and as seed predators. One downside to the ecological importance of ants is that they feature on the list of the world’s worst invasive species. Ants have also been important for science as model organisms for studies of diversity, biogeography, and community ecology. Despite such importance, ants remain remarkably understudied. A large proportion of species are undescribed, the biogeographic histories of most taxa remain poorly known, and we have a limited understanding of spatial patterns of diversity and composition, along with the processes driving them. The papers in this Special Issue collectively address many of the most pressing questions relating to ant diversity. What is the level of ant diversity? What is the origin of this diversity, and how is it distributed at different spatial scales? What are the roles of niche partitioning and competition as regulators of local diversity? How do ants affect the ecosystems within which they occur? The answers to these questions provide valuable insights not just for ants, but for biodiversity more generally

Simulating the evolution of recruitment behavior in foraging Ants

Spatial heterogeneity in the distribution of food is an important determinant of species\u27 optimal foraging strategies, and of the dynamics of populations and communities. In order to explore the interaction of food heterogeneity and colony size in their effects on the behavior of foraging ant colonies, we built agent-based models of the foraging and recruitment behavior of harvester ants of the genus Pogonomyrmex. We optimized the behavior of these models using genetic algorithms over a variety of food distributions and colony sizes, and validated their behavior by comparison with data collected on harvester ants foraging for seeds in the field. We compared two models: one in which ants lay a pheromone trail each time they return to the nest with food; and another in which ants lay pheromone trails selectively, depending on the density of other food available in the area where food was found. We found that the density-dependent trail-laying model fit the field data better. We found that in this density-dependent recruitment model, colonies of all sizes evolved intense recruitment behavior, even when optimized for environments in which the majority of foods are distributed homogeneously. We discuss the implications of these models to the understanding of optimal foraging strategy and community dynamics among ants, and potential for application to ACO and other distributed problem-solving systems