vibratory alarm signals in two sympatric higher termite species: pseudacanthotermes spiniger and p....

Journal of Insect Behavior, Vol. 12. No. 3, 1999

Vibratory Alarm Signals in Two Sympatric HigherTermite Species: Pseudacanthotermes spiniger andP. militaris (Termitidae, Macrotermitinae)

Sophie Connetable,1,3 Alain Robert,1 Francois Bouffault,2 andChristian Bordereau1

Accepted June 3. 1998; revised July 2, 1998

When they are disturbed, soldiers of the two termite species Pseudacanthotermesspiniger and P. militaris hit the substratum with their head, thereby producingsounds. High-speed video recordings allowed us to analyze the movement. Thesound emissions were recorded and their temporal structure was analyzed.Artificial stimulation proved that head-banging acts as an alarm signaltransmitted through the vibrations produced in the substratum. Perception ofthese vibrations induced a polyethic response. Workers reacted to head-bangingby escaping. Minor soldiers reacted by escaping, becoming immobile, orhead-banging, thereby indicating the existence of positive feedback in signalproduction. Differences in the time patterns of the drumming appeared betweenboth species but could not be shown to play a role in species recognition.

KEY WORDS: communication; alarm behavior; vibrations; Isoptera; Pseudacanthotermes.

INTRODUCTION

The transmission of alarm is vital for termite species that harvest in large groupson the ground and are exposed to predators such as ants. Pseudacanthotermesspiniger and P. militaris are two fungus-growing termite species living in equa-torial Africa. Both species build a hypogeous nest and thousands of workers

1CNRS UMR 5548. "Developpement Communication Chimique," Universite de Bourgogne, 6 BdGabriel, 21000 Dijon, France. Fax: (33) 03 80 39 62 89. e-mail; [email protected].

2 CNRS URA 671, Station Zoologique, 06230 Villefranche/mer, France.3To whom correspondence should be addressed.

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0892-7553/99/0500-0329$16.00/0 C 1999 Plenum Publishing Corporation

and soldiers leave the nest to harvest leaves and wood on the ground. In orderto protect the group, the minor soldiers of both species stand immobile aroundthe harvesting site, which is progressively covered by soil shields built by theworkers. Disturbing this group induces the production of sounds due to soldiersdrumming with their head on the substratum.

The drumming behavior of termites has been reported and considered asan alarm signal in many species since its first description by Konig in 1779 butits role and characteristics still remain unclear. Howse (1962, 1964, 1965) wasthe first to describe this behavior in the lower termite Zootermopsis angusticollis(Termopsidae). He showed that the drumming or head-banging behavior is struc-tured into bursts made up of short series of shocks on the substratum, called beatsor VOM (vertical oscillatory movement) (Howse, 1962). Studying the speciesZootermopsis angusticollis and Z. nevadensis, Stuart (1963, 1988) and Kirchneret al. (1994) suggested that the drumming behavior of these termite species, liv-ing in small groups inside wood, was primitive in comparison with the alarmcommunication system of higher termite species that live in bigger colonies andharvest on the ground. As yet, this hypothesis has been confirmed only by theobservations of Stuart (1988) on Coptotermes lacteus (Rhinotermititidae), andthe drumming of Termitidae has never been studied in detail.

The aim of the present study was to investigate the role of the drummingbehavior of Termitidae in communication. Therefore, we determined the charac-teristics of the head-banging movements of P. spiniger and P. militaris. As thesecharacteristics appeared to be species-specific, they were artificially reproducedand tested on homo- and heterospecific individuals in order to see if they areinvolved in species recognition.

MATERIALS AND METHODS

P. spiniger colonies were established from pairs of alates collected in Gabonduring the dispersal flight in 1988 and P. militaris colonies were founded inCongo in 1992. All experimental colonies were mature comprising several tenthousands of individuals and producing alates. They have been raised since theirfoundation at the laboratory in Dijon in altuglas containers (50 x 70 x 70 cm),under 28 ± 3 D C, a 12L: 12D daily cycle, and 80% relative humidity, and fedwith decayed wood.

All experiments were carried out on harvesting groups during the darkperiod since this is the period of greatest activity. In laboratory colonies, theharvesting group of P. spiniger is made up of 64.5 + 13.3% major and minorworkers, 2.8 ± 1.3% major soldiers, and 32.7 ± 15.4% minor soldiers (n = 12).The number of termites varies constantly. A study lasting 18 days showed thatthe number of termites on a small piece of wood (dry weight, ca. 60 g) rangedfrom 158 to 1250 (Connetable and Labaune, unpublished data). The harvesting

330 Connetable, Robert, Bouffault, and Bordereau

groups of P. militaris were estimated to be similar to the harvesting groups ofP. spiniger. Very few major soldiers attend the harvest in laboratory as well asin field colonies. Therefore, only minor soldiers and major and minor workerswere studied.

Since disturbance appeared to modify the termites' behavior, harvestinggroups could not be moved from the highly decayed wood to a test arena. Mea-surements of substrateborne vibrations in reproducible conditions were thereforenot possible and the temporal characteristics of the drumming were obtainedfrom acoustical recordings of the airborne sounds produced.

Incipient colonies of both species, founded with alates from mature labora-tory colonies, were opened in order to observe the behavior of the reproductives,presoldiers, and larvae after a perturbation (in this case, opening the nest).

Video and Audio Recordings

Several stimuli—puff of air and odor (manipulator breathing), small jetof air from a pipette (with a disposable Pasteur pipette; 5-ml volume), strongshock (with a hammer) on the substratum, sudden light, and intrusion of acockroach—were tested and the reactions they elicited were filmed with a video-camera (Sony CDD-Iris) linked to a video cassette recorder.

The oscillation movement of a minor soldier was then analyzed using ahigh-speed experimental camera shooting up to 40 images at a speed of 300frames/s, with a special resolution of 190 x 122 pixels (Bouffault et al., 1997).

Sound recordings of the harvesting groups were obtained with a microphone(Sennheiser MD211N), held 1 cm above the termites, and a tape recorder (NagraIV-D; tape speed, 9 cm/s).

Minor soldiers of both species were isolated from the group and placedon filter paper (Whatman; 150-mm P) in a petri dish 1 cm under the micro-phone, and the sounds were recorded using the same apparatus. One hundredthree isolated minor soldiers of P. spiniger and 117 isolated minor soldiers of P.militaris were stimulated by a puff of air and recorded individually until theystopped drumming (692 bursts for P. spiniger and 646 bursts for P. militaris).The number of head-bangs composing each burst was counted by slowing downthe audio tape by a factor of four (2.25 cm/s). Results obtained on both specieswere compared using a nonparametric comparison test (Mann-Whitney U test).

The temporal structure of the sounds was analyzed using a computer pro-grame (Hypersignal V 2.01; Hyperception) to determine the beat frequency (BF)and the burst frequency (BR; burst rate). For this analysis, 135 bursts by P.spiniger (i.e., 663 time intervals between beats) and 143 bursts by P. militaris(616 time intervals between beats) were recorded using the conditions describedabove.

Alarm Behavior in Termites 331

Group Effect

One soldier was isolated in a petri dish and the number of bursts producedafter one stimulation with a disposable Pasteur pipette was counted. The sameexperiment was performed with groups of 5, 10, and 20 soldiers and the resultswere compared with a Mann-Whitney U test (each test was repeated 10 times).

Artificial Stimulation

A loudspeaker replaying a termite recording was used to determine if therewas an acoustic perception of the sounds produced by the vibration movements.The loudspeaker was first placed 1 cm above the group and the reactions of thetermites were observed. It was then applied directly on the piece of wood.

The time patterns of the signals were reproduced using a function generator(Hewlett Packard HP 33120A; set to produce regular bursts of white noises)coupled with a loudspeaker on which a small piece of metal was clipped tohit the piece of wood harvested by the termites. This experimental system wasused to modify the characteristics (BF, BR, and number of beats in a burst) andinvestigate their role in the perception of the signals. The tests were carried outon harvesting groups of both species, keeping two of the three parameters totheir mean value (i.e., BF = 20 beats/s, BR = 0.7 burst/s, 4 beats/burst) andchanging the third one. Tested values ranged from 1 to 300 beats/s for BF, 0.1to 10 bursts/s for BR, and from 1 to 20 beats/burst. Ten repetitions on differentgroups of termites were made for each value.

Species Specificity of Vibratory Signals

Figure 1 represents the experimental arena used to study the transmission ofvibrations from one species to another. This system allows individuals to com-municate only by substrateborne signals and we studied the reaction of one groupafter the disturbance of the other by a puff of air (with a disposable Pasteurpipette; 5-ml volume). The groups were composed of 20 minor soldiers. Trans-mission was considered positive when more than 50% of the unstimulated minorsoldiers drummed simultaneously with the stimulated ones. A control test wascarried out by placing individuals on one side only and applying the stimuluson the other side to check the efficiency of the barrier to stop the air movement.

RESULTS

Characteristics of the Head-Banging Movements

Video recordings showed that the sounds are produced when the ventral partof the head, the postmentum, hits the substrate. Minor soldiers can also react tostimulation by "freezing" (i.e., becoming immobile) or escaping (i.e., running


Alarm Behavior in Termites

Fig. 1. Arena used to study the transmission of the vibra-tions through the substratum from one group of termites toanother. (A) Flexible barrier (plastic film); (B) paper sheet.

back to the nest). No recruitment of soldiers ever occurred in the tests. Workersalways reacted to perturbation by running back to the nest. Observations carriedout by opening young nests showed that major soldiers and presoldiers can alsoexhibit head-banging movements inside the nest. In larvae, workers, and alates,head-banging was never observed.

The drumming behavior could be elicited with any of the tested stimuli,but the association of an odor and a puff of air (manipulator breathing) was thestrongest stimulus (Table I). All the tested stimuli induced escape among workers.

The head-banging behavior is characterized by the occurrence of series,or bursts, of single vibration movements called beats (Fig. 2). Minor soldiers

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Table I. Effect of Several Stimuli on the Release of the Drumming Behavior of Minor Soldiersof P. spiniger and P. militarisa

P. spinigerP. militaris

Odor andpuff of air

++++++++++

Small jet of airfrom a pipette

++++++

Suddenlight

++

Cockroachintrusion

++

Strongshock

++

a+++++, over 50% of the minor soldiers drummed in each test; +++, between 10 and 30%; +, lessthan 10%. Each stimulation was tested 10 times.

Connetable, Robert, Bouffault, and Bordereau

Fig. 2. Time analysis of head-bangings of P. spiniger, induced by a puff of air(amplitudes in arbitrary unit). (A) Time analysis of a sequence of six bursts by agroup of minor soldiers. (B) Time analysis of a succession of five bursts by anisolated minor soldier; BR, burst rate. (C) Time-expanded view of the first burstshown above, made up of 11 beats. BF, beats frequency.

334


Fig. 3. Spectrographic analysis of a P. .spiniger isolated minor soldier's burst: motifs, amplitude indB (decibels). The absence of frequency modulation and harmonic characterizes a sound producedby a shock.

located on the same piece of wood exhibited synchronized bursts. In contrast,two groups of minor soldiers located on two pieces of wood or on the samebut separated by more than 40 cm did not drum simultaneously. This last valuecould only be estimated because of the heterogeneity of the harvesting sites.

Spectral analysis of the sounds produced by the head-banging (Fig. 3)showed no frequency modulation and no harmonic, as would be expected from asound produced by a shock. Therefore, as sound frequencies and amplitudes relymostly on the nature of the substratum and on its ability to conduct vibratorywaves, they have not been investigated here.

The beat of a single minor soldier was analyzed using the experimentalcamera. Figure 4 shows the images of two successive beats of a P. spiniger minorsoldier. The movement is divided into three phases. In the first one, lasting 13ms, the minor soldier raises its head and thorax by extension of the forelegs. Thisis followed by a 3-ms immobility period. In the third phase, which lasts 6 ms,the head and thorax go down and the sound is produced when the postmentumhits the substratum. The complete movement lasts 22 ms. The beat of the P.militaris minor soldier is similar to this one and lasts 21 ms. Those results wereobtained with one series of images for each species.

Bursts are regular repetitions of these movements. The number of beats ina burst ranges from 1 to 10 for P. spiniger (n = 103 minor soldiers) and from1 to 8 for P. militaris (n =117 minor soldiers). Other observations showed a P.spiniger minor soldier drumming up to 13 successive beats and a P. militaris12 successive beats. Figure 5 shows the frequencies of the different kinds of

335


Fig. 4. Video sequence of two heats by a P. spiniger minor soldier(recording speed: 300 frames/s): frames 2 to 10. first heal; frames16 to 25. second heat.

bursts. The most frequent drumming movements in P. spiniger are composedof 3 and 4 beats, while the most frequent ones in P. militaris are composed of1 or 2 beats. A Mann-Whitney U test carried out on those results shows thatP. spiniger minor soldiers produce significantly more beats in a burst (n = 646bursts, U' = 348,683, P = 0.001) than P. militaris minor soldiers.

The frequency of the beats (BF) was determined on 135 bursts (663 timeintervals between beats) of P. spiniger and 143 bursts (616 time intervals between

336

337

Fig. 5. Comparison of the number of beats in a burst in P. spiniger and P. militaris.

beats) of P. militaris. Results showed that this frequency ranges from BF = 6 to50 beats/s (X = 18.99 ± 4.11 beats/s) for P. spiniger and from 6 to 26 beats/s(X = 14.12 ± 2.74 beats/s) for minor soldiers of P. militaris. A Mann-WhitneyU test carried out on these results showed that this frequency is significantlyhigher for minor soldiers of P. spiniger (n = 616, U' = 354,500, P = 0.0001).

Bursts were repeated several times by the termites after a perturbation. Thenumber of bursts induced by a single stimulation was linked with the numberof individuals and their level of excitement before the stimulation. An isolatedminor soldier produced only one burst after a single stimulation in more than90% of cases, while a group of 20 minor soldiers could emit more than 20 bursts(Fig. 5). Furthermore, a group of harvesting minor soldiers stopped head-bangingand left the harvesting site after the fifth or sixth stimulation. As it was impos-sible to control the number of individuals in the harvesting group or to count orisolate them without modifying their level of excitement, the number of burstsemitted by minor soldiers in the group has not been further investigated. Therepetition frequency of those bursts, BR, was estimated using the few record-ings of isolated minor soldiers producing more than one burst. BR ranged from0.5 to 1 burst/s for P. spiniger (four recordings) and from 0.4 to 1 burst/s forP. militaris (three recordings).

Artificial Stimulation

Table II shows a comparison of the head-banging characteristics of P.spiniger and P. militaris. Differences appear in the number of beats and theirfrequency in a burst. Therefore, artificial reproduction of head-banging move-



merits was used to investigate their role and the existence of a discriminationbetween homo- and heterospecific signals.

First, a loudspeaker reemitting the sounds produced by the drumming of thesoldiers and placed 1 cm above the substratum elicited no reaction in a harvestinggroup. The same loudspeaker placed on the substratum induced head-banging,escape or freezing among minor soldiers and escape among workers.

This shows that head-banging acts as a signal through the vibrations pro-duced in the ground.

Effect on Workers

Artificial vibratory stimuli always elicited escape response in workers overthe whole range of tested beat frequencies, burst frequencies, and number ofbeats per burst. Therefore, workers are sensitive to groundborne vibrations butthe structure of these vibrations does not seem to affect their reaction.

Effect on Minor Soldiers

Varying the number of beats in a burst did not change the minor soldiers'reaction: the response was similar to the one triggered by a puff of air. On theother hand, the response of the minor soldiers varied with BF and BR (Table III).In both cases the stimulation always induced three different behavior patterns:head-banging, freezing, or escaping. For values close to the ones observed onthe termites' recordings, the proportions of these three behavior patterns weresimilar to the ones obtained after a stimulation by a puff of air. Applying loweror higher values induced fewer head-bangings among minor soldiers and morefreezing and escape. Therefore, modifications of the time pattern of the signalmodifies the proportions of the various behavior patterns observed among minorsoldiers rather than inducing a precise behavior.

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Table II. Comparison Between Characteristics of Head-Banging in Minor Soldiers of P. spinigerand P. militarisa

Duration of a beatBeats per burst* (n = 646,

U' = 348643 P = 0.0001)

BR

BF* (X ± SD) (n = 616,U' = 354500, P = 0.0001)

P. spiniger

22 ms

From 1 to 10, mostlybetween 2 and 6

Between 0.5 and 1burst/s

18.99 ± 4.11 beats/s

P. militaris

21 ms

From 1 to 8, mostlybetween 1 and 3

Between 0.4 and 1burst/s

14.12 ± 2.74 beats/s

aBR, burst rate; BF, beats frequency in a burst.*Statistically significant differences between the species (Mann—Whitney U test).


Our results obtained with artificial stimulation show that minor soldiers areable to perceive the time pattern of the vibration bursts and respond most stronglyto the natural signal. On the other hand, our results indicate that these vibrationsare not suited for species recognition.

Species Specificity of Vibratory Signals

Disturbance of one group always induced simultaneous head-banging inthe group placed behind the barrier, whether they were homo- or heterospecific(Table IV). No reaction was obtained in the control tests when there were nosoldiers in the stimulated area (Fig. 1). Therefore, head-banging acts as a com-munication signal and can release further head-banging in nestmates. As in theartificial stimulation experiments, the reaction of the individuals does not dependon the species-specific characteristics of the signals.

Table III. Reaction of the Minor Soldiers of Both Species to Artificial Playbacks with VaryingValues of BF (Beat Frequency) and BR (Burst Rate)

BF(beats/s)


BR(bursts/s)


1

++ ++++

0.1

++

5

++ +++ +

0.2

++++

15

++++ +++++ +

0.4

++++ +++++ +

20 25

+++++ ++++++++ +++

0.7

++++ +++++ +

1

++++ +++++ +

50

+ ++ +

2.5

++ +++ +

100

+ ++ +

5

++++

200

++

7.5

++

300

++

10

++

a++++ + over 50% of the minor soldiers react by head-banging in each test; +++, between 20 and30%; ++, between 10 and 20%; +, less than 10%. Each value was tested 10 times.

Table IV. Response of Groups of Minor Soldiers to the Drumming of theGroup Placed Behind a Barrier

Stimulated species

P. spiniger

P. militaris

Tested species

P. spinigerP. militarisP. spinigerP. militaris

Response

++++++++++++++++++++

a+++++, over 50% of the tested soldiers react by simultaneous head-bangingin each test. Each test was reproduced 10 times.


Fig. 6. Number of successive bursts obtained after stimulation (of groups of different size of minorsoldiers of P. spiniger and P. militaris). Means (+SE) marked with the same letter are not significantlydifferent (Mann-Whitney U test). P. spiniger: U' = 63.5, P = 0.026. P. militaris: U' = 375, P =0.0001.

DISCUSSION

This study gives the first detailed description of the head-banging behaviorin two species of Termitidae. We showed that in harvesting groups of P. spinigerand P. militaris, a perturbation induces escape among workers and freezing,escape, or head-banging among soldiers. These head-bangings result in vibra-tions in the substratum that trigger escape among workers and thereby act as analarm communication signal that spreads throughout the whole group by posi-tive feedback. Soldiers tend to drum less after important perturbations and thiscould explain why, after spreading, the signal finally stops.

Due to the high variability of the nature of the substratum and its rapidmodification by Pseudacanthotermes (building of soil shields), the amplitudeand the propagation of the vibrations could not be accurately measured and theresults are based on recordings of airborne sounds.

The characteristics of the head-banging movements of P. spiniger and P.militaris are different, and experiments carried out using artificial reproductionof the signals showed that drumming appears to play no role in species recogni-tion. Nevertheless, this behavior is effective in communicating rapidly the alarmthroughout the whole harvesting group and inducing escape among workers evenif they did not perceive the original stimulus. The drumming could also be effec-tive in repelling predators like ants and should therefore be tested on the mainpredators of Pseudacanthotermes.

Studies of the head-banging behavior of the lower termite genus Zooter-

340

mopsis (Termopsidae) proved that termites dram only in direct response to dis-turbance (no feedback). Howse (1962, 1964, 1965), Stuart (1963, 1988), andKirchner et al. (1994) described all the characteristics of these movements andshowed that the termites perceive the vibrations produced by head-bangings. Stu-art (1988) also studied the drumming behavior of the termite species Coptoter-mes lacteus (Rhinotermitidae, Coptotermitinae). He briefly described a polyethicalarm response with soldiers' head-banging inducing other soldiers to drum (pos-itive feedback) and workers to retreat.

The signals of P. spiniger and P. militaris are very close in structure tothe ones produced by Zootermopsis and Coptotermes and probably to those ofMacrotermes (Rohrig et al., 1997). Unlike Zootermopsis but as in Coptotermesand Macrotermes, the only individuals to perform dramming in P. spiniger andP. militaris are soldiers. Thus, the response to a disturbance is polyethic: workersalways escape and minor soldiers respond by running, freezing, or head-bangingand the signal spreads through the minor soldiers by positive feedback.

In Zootermopsis, there is no positive feedback and signals should thereforebe transmitted over a shorter distance (Kirchner et al., 1994). This behavior iseffective for small colonies living and harvesting inside wood. A higher degreeof complexity is reached with the existence of a polyethic response and a positivefeedback for species such as Macrotermes subhyalinus, C. lacteus, P. spiniger,and P. militaris. For all these species, the size of the population and the expo-sure to predation during foraging are important compared to Zootermopsis. Thus,it is necessary for them to communicate the information to a large number ofindividuals.

Rohrig et al. (1997) showed a high level of variability in the behavior ofthe soldiers of the genus Macrotermes, and in Pseudacanthotermes, we haveobserved that the response of the soldiers varies with the circumstances. Thedrumming behavior of termites could therefore also be interpreted as a combinedalarm and alarm modulator system as suggested by Stuart (1963), though thisneeds further investigation.

ACKNOWLEDGMENTS

We are grateful to Yves Tupinier for his invaluable help on the acousticalanalysis, to Nicole Gache for allowing us to use facilities at the Ecole Superieurede Chimie, Physique et Electronique in Lyon and to Bernard Michaux from theLE2I laboratory in Dijon for his technical help for the conception of the artificialstimulation apparatus.

We wish especially to thank Professor Charles Noirot and Birgit Wobst fortheir comments that greatly improved the manuscript and David Jones for revis-ing the English.


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vibratory alarm signals in two sympatric higher termite species: pseudacanthotermes spiniger and p....

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