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G Gregarious Cockroaches potential of group members, an absence of task specialization, high levels of social dependence, Michel-Olivier Laurent-Salazar1,Sofia Bouchebti2 central place foraging, social information transfer, and Mathieu Lihoreau3 kin recognition, and a meta-population structure 1Faculty of Agriculture, Department of [3]. Subtropical Agro-Environmental Sciences, University of the Ryukyus, Okinawa, Japan 2Robert H. Smith Faculty of Agriculture, Food & Group Structure Environment, Department of Entomology, B. Triwaks Bee Research Center, The Hebrew Most cockroach species are gregarious in early University of Jerusalem, Rehovot, Israel stages of development. Some, such as the Amer- 3Research Center on Animal Cognition (CRCA), ican cockroach (Periplaneta americana) and Ger- Center for Integrative Biology (CBI), CNRS, man cockroach (Blattella germanica), remain University Paul Sabatier, Toulouse, France gregarious throughout their entire lives, while others, such as the firefly mimic cockroach (Schultesia lampyridiformis or S. nitor), only dis- Many of the approximately 4600 known cock- play gregarious behavior as adults. The roach species have been described as gregarious compostion of aggregations also varies according based on a tendency to remain in cohesive groups to species. In domiciliary species (e.g., P. ameri- at various stages of their life through phenomena cana and B. germanica), aggregations are com- of mutual attraction [1]. Gregarious cockroaches posed of all developmental stages of both males are emerging models in social insect research and females, while for some species, groups are because of the diversity of their social behavior typically composed of nymphs, females and one and their phylogenetic proximity with solitary territorial male (e.g., Gromphadorhina portentosa praying mantises and eusocial termites, constitut- and Nauphoeta cinerea), or no male at all (e.g., ing an important point of comparison with the Arenivaga grata and Ectobius albicinctus). While Hymenoptera (Fig. 1)[2]. Best studied taxa are cockroaches can sometimes form mixed-species the domiciliary cockroaches, about 25 species of aggregations, individuals preferentially aggregate Ectobiidae, Blattidae, and Blaberidae species that with conspecifics and even members from the have adapted to human habitats. These are char- same strain or matriline when a choice is present acterized by social traits that include the sharing of [4]. The size of an aggregation, from a dozen to a common shelter, overlapping generations of millions of individuals, depends on the amount of adults, non-closure of groups, equal reproductive food resources and the carrying capacity of © Springer Nature Switzerland AG 2019 C. Starr (ed.), Encyclopedia of Social Insects, https://doi.org/10.1007/978-3-319-90306-4_52-1 2 Gregarious Cockroaches Gregarious Cockroaches, Fig. 1 Simplified phylogenetic tree of cockroach families. (Modified from Ref. [2]). The four most studied species for social behavior are highlighted with drawings shelters available in the environment. Domiciliary aggregation are impaired. Social isolation experi- and cave-dwelling species form the largest cock- ments show that regular tactile stimulation roach aggregations and are only found in environ- through social contacts is key for normal devel- ments where food resources are regularly renewed opment. Although the precise nature of these con- (e.g., bats, birds, and human wastes). Most cock- tacts has not yet been identified, mechanical roaches are nocturnal, resting in groups during the stimulation provided by other insects (e.g., day in dark and humid shelters and foraging at locusts) or even applied artificially (e.g., with a night. These shelters are used repeatedly by the feather) is sufficient to accelerate the development same groups of individuals, showing a high level of isolated nymphs and reproduction in adults [6]. of site fidelity even if the shelter has been Presumably, these tactile cues act on the corpora disturbed. allata that control the production of the juvenile hormone responsible for nymphal development and sexual maturation. The more gregarious the species, the more important these developmental Isolation Syndromes effects of social isolation, therefore highlighting the strong dependence of these cockroaches on Although naturally living in groups, most gregar- their gregarious lifestyle. ious cockroaches can survive long periods of social isolation, which may explain their high success in colonizing unfavorable urban habitats [5]. Such experience can nevertheless cause phys- Collective Behaviors iological and behavioral disorders called “isolation syndromes.” In these individuals the imaginal Gregarious cockroaches commonly engage in col- molt and sexual maturation are delayed, and many lective behaviors to locate and exploit resources in behaviors such as foraging, courtship, or their home range (Fig. 2a). In B. germanica and P. Gregarious Cockroaches 3 Gregarious Cockroaches, Fig. 2 Representation of the shelter for an individual, which is shown by a thinner black feedbacks involved in an (a) aggregation and (b) a collec- arrow in the dark shelter with more cockroaches. (b) The tive fleeing response. Black arrows represent the probabil- fleeing event begins when a group of individuals is stimu- ities of changing from one location/state to another. Green lated by an external, stressing, cue (orange arrow). The stop lines and red arrows represent negative and positive black arrow represents the probability of changing from the feedbacks (respectively) on the probabilities of changing immobile state to the moving state. Immobile individuals place/state. Thickness of stop lines and arrows represents have an inhibitory effect that reduces the probability of their relative amplitude of the probabilities. (a) The black individuals to start fleeing (green stop line). This negative arrows represent, first, the random probability of entering a feedback means that individuals in larger groups have a shelter, regardless of its quality. The green striped stop lower probability of starting to flee than individuals in lines represent the inhibitory effect of the quality of the smaller groups. Fleeing individuals on the other hand shelter on a newcomer (darker shelter equals more reten- have an amplifying effect on immobile individuals (red tion). The green solid stop lines represent the inhibitory arrow). This positive feedback means that as more individ- effect of conspecifics already sheltered. As the number of uals flee (move), the probability of an immobile individual sheltered individuals increases, the inhibitory effect also to start fleeing increases increases, further diminishing the probability of leaving the americana, collective decisions occur during the and stays. Through this “retention effect” that selection of a new shelter, for instance, if the resting individuals exert on newcomers, an aggre- previous shelter is overcrowded or if nearby gation can gradually develop, eventually leading food resources are depleted. An individual’s deci- to the selection of a unique shelter by the entire sion to settle in a new place depends on the shel- group. This behavioral model based on simple ter’s physical properties (darkness, size, height, positive feedback rules has been implemented in temperature, or hygrometry), as well as on the autonomous robots that successfully reproduce presence of conspecifics already resting in it as the aggregation behavior observed in cockroaches perceived by cuticular hydrocarbons passively and mimic their collective decision-making [7]. deposited on the substrate and volatiles emitted Similar aggregation dynamics are observed dur- by gut microbiota in the feces. When an exploring ing foraging. In B. germanica, as in shelter selec- cockroach perceives an occupied shelter, it tion, the selection of food sites depends on the switches from a search mode to joining and set- properties of food resources (nutritional value, tling. The larger the group in the shelter, the distance to the shelter, etc.) and the presence of higher the probability that the newcomer joins conspecifics already feeding on the source. 4 Gregarious Cockroaches Aggregation at food sources is based on social to a differentiation between groups. In turn, dif- facilitation for feeding, so that cockroaches in ferent group personalities regarding their time large groups feed longer than those in small spent outside or sheltered result in different shel- groups. To select a feeding site, a minimum of tering dynamics. group size (i.e., quorum) is required. Both resting and feeding aggregations are formed and maintained by positive feedbacks [8] (Fig. 2a). Kin Recognition Collective decisions can also occur in response to a stress event such as the presence of predators. Cockroach aggregations are typically composed In this case, the aggregation suddenly disperses of individuals from different parental lineages. B. based on positive and negative feedbacks, germanica uses cuticular hydrocarbons to dis- resulting in collective fleeing (Fig. 2b; [9]). On criminate familiar individuals according to kin the one hand, the alarm stimulus following a stress classes [12]. These chemical profiles consist of a event is spread within the group by fleeing indi- fixed number of compounds, but their relative viduals, which rapidly activates individuals that abundances covary with genetic relatedness and have not yet started to flee. On the other hand, the are not affected by social interactions, thus pro- number

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