Central to communal living is providing for a common defense. Defensive mechanisms range from construction of protective barriers to coordinated responses to a particular threat, a predator, competitor, or pathogen. These ethological suites require complex coordination, either in behavioral repertoires or physiological responses, and may be associated with concomitant specializations in morphology, themselves building on or coopting developmental pathways to achieve such functions. In essence, a successful defense, whether of a single mother defending her brood or a colony of millions, is the result of a plethora of evolutionary changes honed to increase the collective survivorship of the individuals participating in communal life. Alarm signaling is a defensive strategy that has evolved independently in many social animals, as it increases the fitness of colonies in danger (1–5). A warning signal emitted by a colony member can be used by nearby nestmates to rapidly respond to by displaying defensive or evading behaviors in order to prevent or limit casualties (6). All social insects have evolved complex defensive traits, including morphological specializations, chemical defenses, structural nest modifications, and alarm behaviors, as inherent components of colony defense (7–10). The emergence of specialized defensive castes and complex behaviors (11–13) presumably contributed to the ecological success of termites and led to their extraordinary abundance throughout the tropics (4, 5, 11, 14–16). Unlike social Hymenoptera, termites are hemimetabolous insects and the foraging parties mostly consist of juvenile individuals, wingless and unsclerotized (17).
The first line of termite defense is passive, and consists in the physical isolation of the colony from the hostile environment, built and maintained by vulnerable workers. The active defense strategies are best exemplified by large soldiers (such as in Mastotermes, Macrotermes, Syntermes, Cornitermes, Labiotermes, or Cubitermes), which can inflict deep wounds with their mandibles, sometime coupled with the release of toxic or anti-healing compounds making termite bites unforgettable (13, 18, many personal observations). These toxins may be released in copious amounts, as is the case in Coptotermes, in which the frontal gland secretion represents over a third of soldier body weight (19). Other strategies are as peculiar as closing entrance holes with soldier heads (phragmosis) or strikes by modified snapping mandibles causing devastating wounds to invertebrates (12, 20, 21).
Alarm signals occur in all major eusocial insect groups (termites, ants, bees, and wasps), and are transmitted by two distinct sensory channels: vibroacoustic and chemical. Vibroacoustic communication is considered the most ancient and taxonomically widespread forms of communication (22), and more than 90% of insects use substrate vibrations alone or in concert with other forms of mechanical signaling (23). In, eusocial insects, the vibroacoustic signals are produced either via body vibrations or stridulation, and act as either short-range (tactile) signals (24–26), or via air- or substrate-borne vibrations perceived by distant nestmates through the Johnston’s (air-borne) or subgenual (substrate-borne) organs (27, 28). The vibroacoustic signaling may carry various messages, such as alarm, recruitment, or begging for food (26, 29–31). A specific means of vibroacoustic communication were observed in termites but not other social insects, such as alarming nestmates in response to pathogen encounter (32), evaluation of volume of the remaining wood (33) or perception of approaching competitor (34).
Apart of stridulatory organs in many ant groups, the vibroacoustic signals are generated by inconspicuous body parts showing little to no specialization to this particular task (26). The alarm pheromones provoke strong dose- and context-specific responses, resulting in retreat or attack, the latter usually accompanied with fast changes in caste or age-category proportions of the insects involved (13, 35–37). While the glandular origins of the alarm pheromones are diverse and taxon-specific (37–40), it is important to note that all alarm pheromones are produced by abdominal glands in cockroaches, while exclusively by cephalic glands in termite soldiers (12, 13, 30, 41).
Direct observations repeatedly revealed the importance of alarm communication in all social insects (26, 37, 42). In termites, any disturbance triggers seemingly erratic movements leading to effective defensive responses (13, 35, 36, 43, 44), and enhanced protection of the colony due to the increase of the soldiers-to-worker ratio at the place of disturbance, making the whole group unpalatable even to specialized vertebrate predators (45). Our understanding of the proximate mechanisms of alarm communication is still limited, and because termites, ants, bees, and wasps are known to respond to the threat stimuli in a context-dependent fashion, the acquisition of empirical data and their interpretation remains challenging (46).
In spite of the crucial importance of alarm communication for termite colony survival, only fragmented reports have hitherto been published about this topic, most of which focused on either vibroacoustic or pheromonal communication of isolated species (30, 36, 43, 44, 47–55). The evolutionary trajectories of alarm signals, and their significance within complex ecological constraints across extant termite lineages, have not previously been investigated, and there is no report on alarm communication in soil-feeding termites, which represents over half of termite diversity (54). In this work, we carried out a detailed study on nine termite species that, combined with existing knowledge on alarm communication in cockroaches and termites, includes members of all major lineages and ecological strategies (Dataset 1). We then studied the evolution of alarm characters in a phylogenetic context, and in relation to a series of social and ecological features, which are key factors influencing communication abilities in social animals (42).
We use the following definitions for the behaviors (all shown in Movie S1) observed in this study:
Locomotion speed is the average speed-of-motion of two individuals, either workers or soldiers, independently, per experiment, expressed in mm/s.
Burst is a sequence of oscillatory movements with stable spans between the beats at low or high frequency. It can be performed as a tremulation, drumming or head-banging sequence.
Tremulation (body shaking) is the longitudinal oscillatory movements sensu Howse (55), during which the head or the abdomen rarely hits the substrate. Tremulation signals are used at either low (≤ 15 Hz) or high frequency (> 15 Hz), which we refer to as “low tremulations” and “high tremulations”, respectively.
Drumming is the vertical oscillatory movements sensu Howse (55) during which the abdomen hits the substrate. Drumming is present in both workers and soldiers and is always displayed at high frequency (> 15 Hz).
Head-banging is performed exclusively by soldiers hitting the substrate with their heads at high frequency (> 15 Hz).