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Nest Relocation named house-hunting ants (genus Temnothorax) rely entirely on preformed cavities, and many Sarah Bengston1 and Terry McGlynn2,3 species of army ants have forgone a nest alto- 1City University of New York: Baruch College, gether and instead build a shelter (bivouac) out New York, NY, USA of their own bodies. Yet, it is not only species with 2Department of Biology, California State very low investment in nest structures that relo- University Dominguez Hills, Carson, CA, USA cate readily. For example, the western honey bee, 3Entomology Department, Natural History Apis mellifera, builds large combs that store sig- Museum of Los Angeles County, nificant food resources and yet these colonies Los Angeles, CA, USA move so frequently that they have become a model species for understanding nest relocation.

Synonyms Types of Nest Relocation Nest moving; Nest transfer; Nomadism The propensity and ability of a colony to relo- As summarized by McGlynn [3], nest relocation cate some or all components of their nest varies can generally be classified along a continuum of widely across social insects. Some species are four types: nomadism, unstable nesting, adventi- relatively sessile, such as large, mature Atta leaf- tious nest relocation, and intrinsic nest relocation. cutting ant colonies, which may find it very diffi- Nomadic species display a colony history depen- cult to move a very large colony (with or without dent upon movement. They often show behavioral its substantial fungus gardens) to a new site. Other traits specifically tailored to constant movement, species move readily, either as a result of their such as the bivouacs mentioned above. Colonies intrinsic life history or external stresses and envi- may stay in place temporarily, particularly to ronmental stimuli. Indeed, it is likely that nest exploit an ephemeral resource, but move on after relocation by social insects is a much more fre- that resource has been depleted. Unstable nesting quent occurrence than is generally perceived species may also be adapted to exploit ephemeral [3]. Some of this underestimation results from resources and as such use low-cost nest sites, such the assumption that colonies should be reticent as under dried foliage in leaf litter. These distur- to leave structures that require so much invest- bance specialists can often be found in areas ment in labor and materials. Certainly, for some, with high levels of human disruption. Extrinsic especially mobile species, there is limited invest- disturbance, damage, or environmental change ment in nest structure. As examples, the aptly (such as the arrival of a predator) may also induce

© Springer Nature Switzerland AG 2020 C. Starr (ed.), Encyclopedia of Social Insects, https://doi.org/10.1007/978-3-319-90306-4_84-1 2 Nest Relocation relocation in species that would otherwise remain Resource Deficiency sessile. This strategy is known as adventitious Generally, nomadic species are the most likely to nest relocation. It is likely that most social insect routinely relocate in response to insufficient species are capable of adventitious nest relocation resource availability. However, non-nomadic spe- if the extrinsic threat is significant enough, though cies may use proximity to resources as a trait observing such a stochastic process poses many affecting nest site selection, often preferring a difficulties. Intrinsic nest relocation, as opposed to site closer to better resources when other nest the other categories that rely on external stimulus, site traits are comparable. For example, in the occurs as a result of a species’ colony history. ▶ slave-making ants Formica subintegra and Season events such as migration or seasonal F. pergandei, colonies are more likely to relocate ▶ polydomy trigger relocation of some or all of their nest when raiding distances to host colonies the colony to a new or secondary nest site. increased. These nest relocations considerably shorten raiding distances by putting them into closer proximity to host colonies. Additionally, the new raiding distances are shorter than the Causes of Relocation distance traveled to relocate, meaning colonies are simply not evacuating their current nest for a There is not one overarching reason why a colony nearby site but instead tracking resource availabil- may abandon a nest and relocate to a new site. ity and moving accordingly. Similar tactics may Certainly, one obvious explanation may be that be used for seasonally polydomous species; satel- the colony is facing extrinsic threat or stress in its lite nests may be established closer to resources current site. This may include problems with the while the primary nest remains in place. nest itself or the immediate vicinity. Predators and Parasites Compromised Nest External attacks from predators or parasites may A nest structure may become compromised, forc- elicit an adventitious nest relocation response if ing a colony to relocate. This may be the result of the damage or threat is severe. If predatory attacks direct damage to the nest or a decrease in its are rare and episodic, the resulting relocation may quality through time. Physical damage to the be a one-time move to a new nest that must be nest may occur from any external trauma, though established. However, species under continual most common when structures supporting the nest threat of predation have adapted to this threat by fail. For example, the ▶ giant honey bee, Apis maintaining multiple nests that they may move dorsata, builds large, exposed nests on the between at regular intervals or flee to when branches of trees or on rocky cliff overhangs. detected by a predator. For example, when army These are prone to damage when tree branches ants detect and attack the desert ant Pheidole fall or the rock formation shifts. Nests in leaf litter desertorum, the colony immediately evacuates to or flood prone subterranean nests may also expe- a nearby unoccupied nest they had previously rience frequent disturbance, resulting in impres- established. Similar to attacks from a predator, sive adaptations such as the well-documented slave-making social parasites may provide selec- rafting behavior of red imported ▶ fire ants, tive pressure for colonies to move more Solenopsis invicta. Additionally, colonies may frequently. relocate if they perceive a new site as of higher Both internal ▶ nest parasites and external par- quality than the current site [1]. The characteris- asitic threat may result in nest relocation. Nest tics determining the quality of a nest site vary movement may be the result of a colony moving based on species-specific criteria; Temnothorax away from burdensome parasites or may instead ants prefer a small nest opening, while be the result of host manipulation by the parasites. Aphaenogaster araneoides prefers larger nest. Certain parasites and pathogens have been shown to manipulate host behavior and thus are able to Nest Relocation 3 oblige host movement away from the nest as a year after year. A similar pattern was found in the method of parasite dispersal. While this has not highly polydomous and invasive ▶ Argentine ant been demonstrated to result in whole colony relo- (Linepithema humile). Colonies show a repeated cation, substantial proportions of infected pattern of retraction to one or few nests in the ▶ Polistes dominula colonies have been found winter and expansion in the spring. Inter-nest aggregating away from their original nests. and foraging trails also shift seasonally to main- tain connectivity between nests. They show little Colony Growth fidelity to satellite nests, as nearly half were occu- The growth of a colony, particularly in relation to pied for less than a single month and 90% were worker and brood number, has not been clearly occupied for less than 3 months. Laboratory demonstrated to result in nest relocation in social experiments have demonstrated that much of insects. What is seen more commonly is that their nest relocation was dependent upon humid- mature or rapidly growing colonies accommodate ity. Colonies are more likely to relocate to a site expanding numbers with seasonal polydomy. with higher humidity than to one with better food After alates have dispersed, the colonies contract resources. for the winter (see below). However, for species that reside in preformed cavities or other unstable, space-constrained nest sites, it stands to reason Collective Decision-Making that if worker number exceeds what can comfort- ably fit in the nest, the colony must move. For a colony to relocate to a new nest site, it must Crowding within the nest may also inhibit signals find, assess, and select a new site. This requires that promote cohesion; in honey bee colonies, substantial coordination and information flow when the density of workers increases within the among many individuals and a mechanism for nest, queen pheromones are diluted, resulting in reaching a decision. This process falls under the the rearing of virgin queens and subsequent more general concept of collective decision- swarming. making, and researchers have borrowed many concepts and theories from sociology and Seasonality psychology [5]. True seasonal nest relocation is mainly known Before a decision can be made, information from tropical bees and wasps. Often, relocation must be gathered, particularly about the quality events occur within microclimates, such as mov- of any potential nest site. This requires the assess- ing the colony from a high-quality but exposed ment of numerous noisy variables, such as size of nest site to a lower-quality but more sheltered site potential sites and of entrance passage ways, as during periods associated with heavy seasonal well as proximity to neighbors and resources. rainfall. Colonies of the western honey bee often Accurately sampling these variables takes time, relocate significant distances between the dry and resulting in an inherent trade-off between the wet seasons, effectively a seasonal migration. accuracy and the speed of the assessment. Faster Rather than a complete seasonal relocation, sampling will be less correct than taking a longer ants are biased towards seasonal polydomy. time to repeatedly sample the variables. Gener- Many species show a significant expansion in ally, the cost associated with the decision deter- colony size via nearby satellite colonies during mines how much error is acceptable. If nest spring and summer, followed by a contraction of relocation is particularly risky, colonies are more colony size during a dry or winter season. For likely to take the time to make a more accurate example, the dolichoderine ant Dolichoderus decision. mariae expands from one or two overwintering In social insects, it is not always necessary for nests up to 60 nests during the spring and summer. individuals to visit multiple sites for a decision to Colonies also maintain high fidelity to these sat- be made. For example, in laboratory experiments ellite nests and often use the same sites repeatedly with Temnothorax ants, this decision-making 4 Nest Relocation occurs through an entirely decentralized process. A challenge experienced by both social insect The ants have a strong preference for large cavi- colonies and noninsect social groups making col- ties with small entrances and minimal light. When lective decisions is the management of irrational presented with two potential nests of varying behavior of some group members. Best demon- quality, scouts may only visit one of the two strated in honey bees and Temnothorax while sites to assess quality. They then choose to initiate nesthunting experience with irrelevant alterna- recruitment to that site based on quality; the tives can influence preference of individuals higher the quality, the more likely they are to within the group but not the group as a whole. recruit to it. Subsequently recruited individuals For example, when presented with two nests that then also assess the site and in turn do or do not trade-off between low light and nest entrance size, recruit based on quality. As a result, there is strong experience with irrelevant nests of low quality in positive feedback into the colony when a high- one of these two variables (e.g., bright inside the quality site is discovered. Once recruitment to a nest or large nest entrance) biases lone site reaches a specific threshold, or quorum, the Temnothorax workers towards the relevant nest focus of recruitment switches from scouts to more of high quality on that axis. Accordingly, workers passive members of the colony that are otherwise who have experienced a nest with a very large not involved in the decision-making process. entrance will prefer nests with a smaller entrance While this process is decentralized, it does not despite that nest being more brightly lit. This mean that individual variation between workers preference, however, does not carryover to the is unimportant. In Argentine ant colonies, having colony as a whole. This group rationality, in more workers who are highly exploratory spite of individual irrationality, may best be improves both the speed and accuracy of collec- explained by scouts only assessing a single site tive nest choice. when in a collective setting. Scouts are not then Like Temnothorax ants, honey bees also use a biased by unnecessary comparisons [4]. quorum-based collective decision-making pro- Previous experience with nest sites may also cess. When a colony buds, the dispersing colony lead to in learning, with consequences for nest site establishes a bivouac structure, or a swarm, often selection. Temnothorax ants show short-term on a tree branch. Scouts then fly out to inspect learning during emigrations, with increases in potential nest sites. These scouts report potential their migration speed if migrating multiple times sites to the colony using the ▶ waggle dance, with over several days. They also become more accu- high-quality sites yielding more vigorous dancing rate in their decision-making process, and experi- (100+ repetitions) while moderate- and low- enced colonies are less likely than naive colonies quality sites yield less enthusiasm (10–20 repeti- to split between multiple nest sites. Colonies also tions). Multiple sites of similar quality or two sites learn to avoid low-quality nests and will avoid of high quality both result in colonies taking lon- poor nests that they have already experienced, ger times to reach a quorum, though a consensus even if the other option is of identical quality [5]. between two competing sites does not need to be reached before the colony makes a decision [6]. Interestingly, behavioral variations between Implications of Nest Relocation colonies may also affect how a decision is reached but not necessarily how long it takes to reach a As a result of the likely underestimation of the decision. When 17 swarms were each presented incidence of colony relocation, not much is with three new hives, there were consistent behav- known about its specific implications for social ioral differences between colonies in the number insects. However, some predictions can be drawn of dances performed and scouting activity, yet this from the better known effects of animal move- did not affect how long it took colonies to select a ment generally. For example, relocation may new nest site [7]. have effects on competition between colonies or between species. Surprisingly, competition has Nest Relocation 5 not been shown to induce colony relocation or the population or the connectedness of different otherwise affect colony movement [3], though populations. Instead, researchers must more care- spatial distribution of colonies and competition fully consider nest relocation as a critical compo- for finite resources may affect other aspects of a nent to social insect biology, as well as work to species’ natural history. Patterns of over- better understand the factors inducing and shaping dispersion of nest sites seen in some social insect nest relocation. systems may not be directly explained by a colony moving to directly avoid competition. Rather, this pattern may arise through avoiding competition Cross-References when selecting a new nest site, after already being induced to move by another intrinsic or extrinsic ▶ Supercolonies factor [2, 3], such as predation risk or seasonal ▶ Temnothorax migration. Regardless of why colonies relocate, a neces- sary result of relocation is gene flow and increased genetic diversity within populations. This is per- References haps best measured as a consequence of seasonal migration in Apis dorsata, where relatedness 1. Dornhaus, A., Franks, N. R., Hawkins, R. M., & Shere, H. N. S. (2004). Ants move to improve: Colonies increases between colonies in close proximity to of Leptothorax albipennis emigrate whenever they find one another, while differentiation among a superior nest site. Animal Behaviour, 67, 959–963. populations is mediated by seasonal nest reloca- 2. Gordon, D. M. (1992). Nest relocation in harvester ants. tion. Similar patterns have been seen in highly Annals of the Entomological Society of America, 85, ▶ 44–47. invasive species, particularly when colonies 3. McGlynn, T. P. (2012). The ecology of nest movement relocate quickly and frequently, resulting in sig- in social insects. Annual Review of Entomology, 57, nificant gene flow between geographically distant 291–308. colonies. This has been thought to result in the 4. Sasaki, T., & Pratt, S. C. (2011). Emergence of group ▶ rationality from irrational individuals. Behavioral evolution of unicoloniality ( supercolonies)by Ecology, 22, 276–281. homogenizing populations, particularly after a 5. Sasaki, T., & Pratt, S. C. (2018). The psychology of population bottleneck, thus furthering the inva- superorganisms: Collective decision making by insect sive potential of species such as the Argentine ant. societies. Annual Review of Entomology, 63, 259–275. 6. Seeley, T. D., Visscher, P. K., & Passino, K. M. (2006). Because nest relocation may be more common Group decision making in honey bee swarms: When than expected and holds significant implications 10,000 bees go house hunting, how do they coopera- for much of a species’ biology and evolution, tively choose their new nesting site? American researchers must approach the question with cau- Scientist, 94, 220–229. 7. Wray, M. K., & Seeley, T. D. (2011). Consistent tion. Assuming spatial permanence, for example, personality differences in house-hunting behavior but in spatially explicit models, may miss significant not decision speed in swarms of honey bees (Apis and important within-species variation. It may mellifera). Behavioral Ecology and Sociobiology, 65, also not accurately reflect the genetic structure of 2061–2070.