Temnothorax Rugatulus Research Articles

Symmetry breaking and pivotal individuals during the reunification of ant colonies.

Maintenance of a social group requires the ability to reach consensus when faced with divisive choices. Thus, when migrating colonies of the ant Temnothorax rugatulus split among multiple sites, they can later reunify on the basis of queen location or differences in site quality. In this study, we found that colonies can reunify even without obvious cues to break the symmetry between sites. To learn how they do so, we observed both symmetric reunifications (between identical nests) and asymmetric reunifications (between nests of unequal quality) by colonies of individually marked ants. Both reunification types were accomplished by a tiny minority that carried nestmates from the 'losing' to the 'winning' site. Reunification effort was highly skewed in asymmetric splits, where the majority of the work was done by the first ant to transport, which nearly always came from the winning site. This contrasted with symmetric splits, where the initiator did not play an outsize role and was just as likely to come from the losing site. Symmetric reunifications were also characterized by high transporter attrition, which may help to prevent deadlocks. Tandem runs were abundant in both types and were typically led by transporters as they returned to the losing site to fetch another nestmate. Few tandem followers joined the transport effort, suggesting that tandem runs do not serve to recruit transporters but may have another, as yet unidentified role. Our results underscore the potentially large contribution of highly active individuals to group behaviour, even in decentralized societies such as ant colonies.

Peripheral sensory organs vary among ant workers but variation does not predict division of labor

The neural mechanisms underlying behavioral variation among individuals are not well understood. Differences among individuals in sensory sensitivity could limit the environmental stimuli to which an individual is capable of responding and have, indeed, been shown to relate to behavioral differences in different species. Here, we show that ant workers in Temnothorax rugatulus differ considerably in the number of antennal sensory structures, or sensilla (by 45% in density and over 100% in estimated total number). A larger quantity of sensilla may reflect a larger quantity of underlying sensory neurons. This would increase the probability that a given set of neurons in the antenna detects an environmental stimulus and becomes excited, thereby eliciting the expression of a behavior downstream at lower stimulus levels than an individual with comparatively fewer sensilla. Individual differences in antennal sensilla density, however, did not predict worker activity level or performance of any task, suggesting either that variation in sensilla density does not, in fact, reflect variation in sensory sensitivity or that individual sensory response thresholds to task-associated stimuli do not determine task allocation as is commonly assumed, at least in this social insect. More broadly, our finding that even closely related individuals can differ strongly in peripheral sensory organ elaboration suggests that such variation in sensory organs could underlie other cases of intraspecific behavioral variation.

Parallel vs. comparative evaluation of alternative options by colonies and individuals of the ant Temnothorax rugatulus

Both a single ant and the colony to which it belongs can make decisions, but the underlying mechanisms may differ. Colonies are known to be less susceptible than lone ants to “choice overload”, whereby decision quality deteriorates with increasing number of options. We probed the basis of this difference, using the model system of nest-site selection by the ant Temnothorax rugatulus. We tested the applicability of two competing models originally developed to explain information-processing mechanisms in vertebrates. The Tug of War model states that concurrent alternatives are directly compared, so that choosing between two alternatives takes longer than accepting a single one. In contrast, the Sequential Choice Model assumes that options are examined in parallel, and action takes place once any option reaches a decision criterion, so that adding more options shortens time to act. We found that single ants matched the Tug of War model while colonies fitted the Sequential Choice model. Our study shows that algorithmic models for decision-making can serve to investigate vastly different domains, from vertebrate individuals to both individuals and colonies of social insects.

Inform: Efficient Information-Theoretic Analysis of Collective Behaviors.

The study of collective behavior has traditionally relied on a variety of different methodological tools ranging from more theoretical methods such as population or game-theoretic models to empirical ones like Monte Carlo or multi-agent simulations. An approach that is increasingly being explored is the use of information theory as a methodological framework to study the flow of information and the statistical properties of collectives of interacting agents. While a few general purpose toolkits exist, most of the existing software for information theoretic analysis of collective systems is limited in scope. We introduce Inform, an open-source framework for efficient information theoretic analysis that exploits the computational power of a C library while simplifying its use through a variety of wrappers for common higher-level scripting languages. We focus on two such wrappers here: PyInform (Python) and rinform (R). Inform and its wrappers are cross-platform and general-purpose. They include classical information-theoretic measures, measures of information dynamics and information-based methods to study the statistical behavior of collective systems, and expose a lower-level API that allow users to construct measures of their own. We describe the architecture of the Inform framework, study its computational efficiency and use it to analyze three different case studies of collective behavior: biochemical information storage in regenerating planaria, nest-site selection in the ant Temnothorax rugatulus, and collective decision making in multi-agent simulations.

Who needs 'lazy' workers? Inactive workers act as a 'reserve' labor force replacing active workers, but inactive workers are not replaced when they are removed.

Social insect colonies are highly successful, self-organized complex systems. Surprisingly however, most social insect colonies contain large numbers of highly inactive workers. Although this may seem inefficient, it may be that inactive workers actually contribute to colony function. Indeed, the most commonly proposed explanation for inactive workers is that they form a ‘reserve’ labor force that becomes active when needed, thus helping mitigate the effects of colony workload fluctuations or worker loss. Thus, it may be that inactive workers facilitate colony flexibility and resilience. However, this idea has not been empirically confirmed. Here we test whether colonies of Temnothorax rugatulus ants replace highly active (spending large proportions of time on specific tasks) or highly inactive (spending large proportions of time completely immobile) workers when they are experimentally removed. We show that colonies maintained pre-removal activity levels even after active workers were removed, and that previously inactive workers became active subsequent to the removal of active workers. Conversely, when inactive workers were removed, inactivity levels decreased and remained lower post-removal. Thus, colonies seem to have mechanisms for maintaining a certain number of active workers, but not a set number of inactive workers. The rapid replacement (within 1 week) of active workers suggests that the tasks they perform, mainly foraging and brood care, are necessary for colony function on short timescales. Conversely, the lack of replacement of inactive workers even 2 weeks after their removal suggests that any potential functions they have, including being a ‘reserve’, are less important, or auxiliary, and do not need immediate recovery. Thus, inactive workers act as a reserve labor force and may still play a role as food stores for the colony, but a role in facilitating colony-wide communication is unlikely. Our results are consistent with the often cited, but never yet empirically supported hypothesis that inactive workers act as a pool of ‘reserve’ labor that may allow colonies to quickly take advantage of novel resources and to mitigate worker loss.

Temnothorax rugatulus ant colonies consistently vary in nest structure across time and context.

A host of animals build architectural constructions. Such constructions frequently vary with environmental and individual/colony conditions, and their architecture directly influences behavior and fitness. The nests of ant colonies drive and enable many of their collective behaviors, and as such are part of their ‘extended phenotype’. Since ant colonies have been recently shown to differ in behavior and life history strategy, we ask whether colonies differ in another trait: the architecture of the constructions they create. We allowed Temnothorax rugatulus rock ants, who create nests by building walls within narrow rock gaps, to repeatedly build nest walls in a fixed crevice but under two environmental conditions. We find that colonies consistently differ in their architecture across environments and over nest building events. Colony identity explained 12–40% of the variation in nest architecture, while colony properties and environmental conditions explained 5–20%, as indicated by the condition and marginal R2 values. When their nest boxes were covered, which produced higher humidity and lower airflow, colonies built thicker, longer, and heavier walls. Colonies also built more robust walls when they had more brood, suggesting a protective function of wall thickness. This is, to our knowledge, the first study to explicitly investigate the repeatability of nestbuilding behavior in a controlled environment. Our results suggest that colonies may face tradeoffs, perhaps between factors such as active vs. passive nest defense, and that selection may act on individual construction rules as a mechanisms to mediate colony-level behavior.

The sensory arrays of the ant, Temnothorax rugatulus

Individual differences in response thresholds to task-related stimuli may be one mechanism driving task allocation among social insect workers. These differences may arise at various stages in the nervous system. We investigate variability in the peripheral nervous system as a simple mechanism that can introduce inter-individual differences in sensory information. In this study we describe size-dependent variation of the compound eyes and the antennae in the ant Temnothorax rugatulus. Head width in T. rugatulus varies between 0.4 and 0.7 mm (2.6–3.8 mm body length). But despite this limited range of worker sizes we find sensory array variability. We find that the number of ommatidia and of some, but not all, antennal sensilla types vary with head width.The antennal array of T. rugatulus displays the full complement of sensillum types observed in other species of ants, although at much lower quantities than other, larger, studied species. In addition, we describe what we believe to be a new type of sensillum in hymenoptera that occurs on the antennae and on all body segments. T. rugatulus has apposition compound eyes with 45–76 facets per eye, depending on head width, with average lens diameters of 16.5 μm, rhabdom diameters of 5.7 μm and inter-ommatidial angles of 16.8°. The optical system of T. rugatulus ommatidia is severely under focussed, but the absolute sensitivity of the eyes is unusually high.We discuss the functional significance of these findings and the extent to which the variability of sensory arrays may correlate with task allocation.

Task switching is associated with temporal delays in Temnothorax rugatulus ants.

The major evolutionary transitions often result in reorganization of biological systems, and a component of such reorganization is that individuals within the system specialize on performing certain tasks, resulting in a division of labor. Although the traditional benefit of division of labor is thought to be a gain in work efficiency, one alternative benefit of specialization is avoiding temporal delays associated with switching tasks. While models have demonstrated that costs of task switching can drive the evolution of division of labor, little empirical support exists for this hypothesis. We tested whether there were task-switching costs in Temnothorax rugatulus. We recorded the behavior of every individual in 44 colonies and used this dataset to identify each instance where an individual performed a task, spent time in the interval (i.e., inactive, wandering inside, and self-grooming), and then performed a task again. We compared the interval time where an individual switched task type between that first and second bout of work to instances where an individual performed the same type of work in both bouts. In certain cases, we find that the interval time was significantly shorter if individuals repeated the same task. We find this time cost for switching to a new behavior in all active worker groups, that is, independently of worker specialization. These results suggest that task-switching costs may select for behavioral specialization.

Queen location and nest site preference influence colony reunification by the ant Temnothorax rugatulus

Social insect colonies adeptly make consensus decisions that emerge from distributed interactions among colony members. How consensus is accomplished when a split decision requires resolution is poorly understood. We studied colony reunification during emigrations of the crevice-dwelling ant Temnothorax rugatulus. Colonies can choose the most preferred of several alternative nest cavities, but the colony sometimes initially splits between sites and achieves consensus later via secondary emigrations. We explored the decision rules that govern reunification using artificially split colonies. When monogynous colonies were evenly divided between identical sites, the location of the queen played a decisive role, with 14 of the 16 colonies reuniting at the site that held the queen. This suggests a group-level strategy for minimizing risk to the queen by avoiding unnecessary moves. When the queen was placed in the less preferred of two sites, all 14 colonies that reunited did so at the preferred nest, despite having to move the queen. These results show that colonies balance multiple factors when reaching consensus, and that preferences for physical features of the environment can outweigh the queen’s influence.

Workers ‘specialized’ on inactivity: Behavioral consistency of inactive workers and their role in task allocation

Social insect colonies are often considered to be highly efficient collective systems, with division of labor at therootoftheirecologicalsuccess.However,inmanyspecies, a large proportion of a colony's workers appear to spend their time completely inactive. The role of this inactivity for colony function remains unclear. Here, we investigate how inactivity is distributed among workers and over time in the ant Temnothorax rugatulus. We show that the level of inactivity is consistent for individual workers, but differs significantly among workers, that is, some workers effectively specialize on 'inactivity'. We also show that workers have circadian rhythms, although intra-nest tasks tend to be performed uni- formly across the whole day. Differences in circadian rhythms, or workers taking turns resting (i.e., working in shifts), cannot explain the observation that some workers are consistently inactive. Using extensive individual-level data to describe the overall structure of division of labor, we show that 'inactive workers' form a group distinct from other task groups. Hierarchical clustering suggests that inactivity is the primaryvariableindifferentiatingbothworkersandtasks.Our results underline the importance of inactivity as a behavioral state and the need for further studies on its evolution.

Latitudinal variation in behaviors linked to risk tolerance is driven by nest-site competition and spatial distribution in the ant Temnothorax rugatulus

Geographic range has long been noted to be associated with many organismic and ecological traits such as body size and species richness. However, much less is known about whether and how ecological variation across latitudinal gradients reflects behavioral variation. Ant colonies may also show behavioral variation, and Temnothorax rugatulus show a colony-level behavioral syndrome that seems to reflect risk tolerance across their North American range. While it is presumed that this pattern is the result of adaptation to local environmental conditions, which ecological factors are driving this variation are unknown. Here, we test if colony risk tolerance is affected by competition, predation, resource availability, or environmental stress at each site. Our results show that increased competition, specifically for nest sites, as well as increased spatial clustering of colonies predicts higher risk tolerance. Additionally, the spatial clustering of colonies influences the structure of the risk-taking syndrome, i.e., which colony-level behaviors are correlated and how strongly. This emphasizes the need for understanding large-scale geographic variation in behavior, as it may explain how ecological factors drive the evolution and maintenance of intraspecific behavioral variation across populations.

Flexibility of collective decision making during house hunting in Temnothorax ants

Many social animals cooperatively process information during decision making, allowing them to concentrate on the best of several options. However, positive feedback created by information sharing can also lock the group into a suboptimal outcome if option quality changes over time. This creates a trade-off between consensus and flexibility, whose resolution depends on the information-sharing mechanisms groups employ. We investigated the influence of communication behavior on decision flexibility in nest site choice by colonies of the ant Temnothorax rugatulus. These ants divide their emigration into two distinct phases separated by a quorum rule. In the first phase, scouts recruit nestmates to promising sites using the slow method of tandem running. Once a site's population surpasses a quorum, they switch to the faster method of social transport. We gave colonies a choice between two sites of different quality, and then switched site quality at different points during the emigration. Before the quorum was met, colonies were able to switch their choice to the newly superior site, but once they began to transport, their flexibility dropped significantly. Close observation of single ants revealed that transporters were more likely than tandem leaders to continue recruiting to a site even after its quality was diminished. That is, tandem leaders continued to monitor the quality of the site, while transporters instead fully committed to the site without further assessment. We discuss how this change in commitment with quorum attainment may enhance the rapid achievement of consensus needed for nest site selection, but at a cost in flexibility once the quorum is met.

Be meek or be bold? A colony-level behavioural syndrome in ants.

Consistent individual variation in animal behaviour is nearly ubiquitous and has important ecological and evolutionary implications. Additionally, suites of behavioural traits are often correlated, forming behavioural syndromes in both humans and other species. Such syndromes are often described by testing for variation in traits across commonly described dimensions (e.g. aggression and neophobia), independent of whether this variation is ecologically relevant to the focal species. Here, we use a variety of ecologically relevant behavioural traits to test for a colony-level behavioural syndrome in rock ants (Temnothorax rugatulus). Specifically, we combine field and laboratory assays to measure foraging effort, how colonies respond to different types of resources, activity level, response to threat and aggression level. We find evidence for a colony level syndrome that suggests colonies consistently differ in coping style--some are more risk-prone, whereas others are more risk-averse. Additionally, by collecting data across the North American range of this species, we show that environmental variation may affect how different populations maintain consistent variation in colony behaviour.

‘Lazy’ in nature: ant colony time budgets show high ‘inactivity’ in the field as well as in the lab

Social insect colonies are models for complex systems with sophisticated, efficient, and robust allocation of workers to necessary tasks. Despite this, it is commonly reported that many workers appear inactive. Could this be an artifact resulting from the simplified laboratory conditions in most studies? Here, we test whether the time allocated to different behavioral states differs between field and laboratory colonies of Temnothorax rugatulus ants. Our results show no difference in colony time budgets between laboratory and field observations for any of the observed behaviors, including ‘inactivity’. This suggests that, on the timescale of a few months, laboratory conditions do not impact task allocation at the colony level. We thus provide support for a previously untested assumption of laboratory studies on division of labor in ants. High levels of inactivity, common in social insects, thus appear to not be a laboratory artifact, but rather a naturally occurring trait.

Morphological differences between extranidal and intranidal workers in the ant Temnothorax rugatulus, but no effect of body size on foraging distance

Most ant genera are thought to have monomorphic workers, indicating perhaps a high degree of flexibility in task allocation, and the well-studied genus Temnothorax is an example of this. However, considerable size variation may exist between individuals. In addition, though workers can show flexible behavior, it has been shown that individuals may consistently differ in their task profiles. Here we test whether body size variation among workers affects foraging behavior. Two main hypotheses were tested: first, whether larger ants forage at greater distance from the nest, and second, whether larger individuals show a higher propensity to work outside of the nest. Our results showed that ant body size does not significantly affect foraging distance. However, larger ants were more likely to be found outside the nest. Though Temnothorax ants are a common model system, this is the first study demonstrating task allocation based on body size, which is fixed in adults. Our study suggests that particularly small species may have to be examined carefully for body size variation before concluding that body size is uniform and therefore irrelevant for task allocation.

The frequency of multi‐queen colonies increases with altitude in a Nearctic ant

1. Most ants in boreal and alpine habitats are facultatively polygynous, i.e. their colonies may contain one or several queens. It was investigated how the proportion of polygynous colonies varies along an elevation gradient from 60 to 2700 m in the Nearctic ant Temnothorax rugatulus (Emery).2. Across all populations, the proportion of polygynous colonies was positively correlated with altitude. The correlation was considerably stronger when only populations in the narrow area of the Chiricahua Mts, Arizona, were compared.3. The dominance of polygynous colonies at high altitudes may be associated with selection against solitary colony founding by young queens. In areas with short summers and long winters, hibernation mortality of solitarily founding queens may select for alternative reproductive tactics, such as polygyny and colony founding by budding.4. Colony founding tactics need to be taken into account to more fully understand altitudinal and latitudinal patterns of ant faunas.

A context-dependent alarm signal in the ant Temnothorax rugatulus.

Because collective cognition emerges from local signaling among group members, deciphering communication systems is crucial to understanding the underlying mechanisms. Alarm signals are widespread in the social insects and can elicit a variety of behavioral responses to danger, but the functional plasticity of these signals has not been well studied. Here we report an alarm pheromone in the ant Temnothorax rugatulus that elicits two different behaviors depending on context. When an ant was tethered inside an unfamiliar nest site and unable to move freely, she released a pheromone from her mandibular gland that signaled other ants to reject this nest as a potential new home, presumably to avoid potential danger. When the same pheromone was presented near the ants' home nest, they were instead attracted to it, presumably to respond to a threat to the colony. We used coupled gas chromatography/mass spectrometry to identify candidate compounds from the mandibular gland and tested each one in a nest choice bioassay. We found that 2,5-dimethylpyrazine was sufficient to induce rejection of a marked new nest and also to attract ants when released at the home nest. This is the first detailed investigation of chemical communication in the leptothoracine ants. We discuss the possibility that this pheromone's deterrent function can improve an emigrating colony's nest site selection performance.

Linear recruitment leads to allocation and flexibility in collective foraging by ants

Positive feedback in communication is central to the emergence of collective behaviour in animal groups. The nonlinearity hypothesis predicts that group behaviour will be defined in large part by the degree of cooperative interaction in this feedback. Strong interactions mean that the effectiveness of communication grows nonlinearly with signal quantity, leading to robust spatial patterning and consensus decision making. These predictions have been supported in many experimental systems, but the corresponding behaviour of linear systems, where signal effectiveness is proportional to signal quantity, has not been well explored. We examined the consequences of tandem running, a linear method of recruitment, for collective decision making by foraging colonies of the ant Temnothorax rugatulus. We found that colonies collectively chose the better of two unequal feeders, but they allocated foragers equally when feeders were identical. This result accords with theoretical predictions and contrasts with symmetry breaking seen in species with highly nonlinear trail pheromone recruitment. Colonies were also able to reallocate foragers when the quality of two unequal feeders was switched, again in accord with theoretical expectations. We built a model based upon the behaviour of individual ants and found that colony-level qualities of choice, allocation and flexibility can be predicted by two behaviours: quality-dependent linear recruitment and quality-dependent attrition. These experiments are the first thorough investigation of the consequences of linear recruitment for collective animal behaviour.

High social density increases foraging and scouting rates and induces polydomy in Temnothorax ants

Many organisms live in crowded groups where social density affects behavior and fitness. Social insects inhabit nests that contain many individuals where physical interactions facilitate information flow and organize collective behaviors such as foraging, colony defense, and nest emigration. Changes in nest space and intranidal crowding can alter social interactions and affect worker behavior. Here, I examined the effects of social density on foraging, scouting, and polydomy behavior in ant colonies—using the species Temnothorax rugatulus. First, I analyzed field colonies and determined that nest area scaled isometrically with colony mass—this indicates that nest area changes proportionally with colony size and suggests that ants actively control intranidal density. Second, laboratory experiments showed that colonies maintained under crowded conditions had greater foraging and scouting activities compared to the same colonies maintained at a lower density. Moreover, crowded colonies were significantly more likely to become polydomous. Polydomous colonies divided evenly based on mass between two nests but distributed fewer, heavier workers and brood to the new nests. Polydomous colonies also showed different foraging and scouting rates compared to the same colonies under monodomous conditions. Combined, the results indicate that social density is an important colony phenotype that affects individual and collective behavior in ants. I discuss the function of social density in affecting communication and the organization of labor in social insects and hypothesize that the collective management of social density is a group level adaptation in social insects.

Visual Navigation during Colony Emigration by the Ant Temnothorax rugatulus

Many ants rely on both visual cues and self-generated chemical signals for navigation, but their relative importance varies across species and context. We evaluated the roles of both modalities during colony emigration by Temnothorax rugatulus. Colonies were induced to move from an old nest in the center of an arena to a new nest at the arena edge. In the midst of the emigration the arena floor was rotated 60°around the old nest entrance, thus displacing any substrate-bound odor cues while leaving visual cues unchanged. This manipulation had no effect on orientation, suggesting little influence of substrate cues on navigation. When this rotation was accompanied by the blocking of most visual cues, the ants became highly disoriented, suggesting that they did not fall back on substrate cues even when deprived of visual information. Finally, when the substrate was left in place but the visual surround was rotated, the ants' subsequent headings were strongly rotated in the same direction, showing a clear role for visual navigation. Combined with earlier studies, these results suggest that chemical signals deposited by Temnothorax ants serve more for marking of familiar territory than for orientation. The ants instead navigate visually, showing the importance of this modality even for species with small eyes and coarse visual acuity.