Insectes soc. 46 (1999) 186–191 0020-1812/99/020186-06 $ 1.50+0.20/0 © Birkhäuser Verlag, Basel, 1999
Insectes Sociaux
Research article
Behavioural differences between male and female replacement reproductives in Kalotermes flavicollis (Isoptera, Kalotermitidae) L. Maistrello and G. Sbrenna Dipartimento di Biologia, Sezione Biologia Evolutiva, Università di Ferrara, Via L. Borsari 46, I-44100 Ferrara, Italy, e-mail:
[email protected] Received 15 September 1997; revised 24 March and 13 August 1998; accepted 10 September 1998.
Summary. An ethogram type study was performed on representative colonies of Kalotermes flavicollis to investigate the behavioural repertoires of replacement king and queen. Our observations suggested a sex-based behavioural specialisation in the two reproductives, affecting colony organisation and equilibrium. Interesting differences were detected in overall activity of the two reproductives, with the male showing the greatest frequency of movements in the nest, vibratory acts, and the highest rate and range of social contacts. In addition to his importance for periodical insemination of the queen, the king might also play a basic role in the social structure and dynamic development of the colony, as a “mediator” in social interactions between the queen and the other colony members. Key words: Social behaviour, polyethism, reproductives, Kalotermes flavicollis, Isoptera.
Introduction The termites are one of the few cases of conjugal life in the insect world. After the nuptial flight, males and females form tandem pairs, shed their wings together and cooperate in excavating a nest chamber. The male remains with the female, inseminating her periodically for the whole duration of his life. At the beginning of colony formation king and queen cooperate in rearing the first brood. During the nest founding period and in incipient stages of colony growth, males and females exhibit similar behavioural patterns, giving no indication of a sex-based division of labour and supporting the role of mate assistance in the evolution of termite monogamy (Nalepa and Jones, 1991; Rosengaus and Traniello, 1991). However, there are no indications of what happens in already well established colonies. Lüscher (1974) and Springhetti (1985), working on royal pheromone pro-
duction in K. flavicollis, pointed out that the primary king and queen differ in their capability to control caste differentiation and their influence is complementary and concurrent. Lack of ethological investigations on the functions played by the male and the female in isopteran society, induced us to examine the behavioural roles of the reproductive pair. This study, as a part of a greater investigation performed on colonies of K. flavicollis, whose members suitably represented all castes and developmental stages, is focused on the behavioural repertories of replacement reproductives. Materials and methods Three colonies of Kalotermes flavicollis, coming from three distinct logs (spaced no more than 500 m each other) collected in Orbetello (Tuscany, Italy), were placed in a transparent artificial nest made of two glass sheets (80 ¥ 60 ¥ 4 mm), between which was interposed a Plexiglas sheet (5 mm thick) with a rectangular hole (43 ¥ 32 ¥ 5 mm), containing a piece of fir-wood (Abies alba) (43 ¥ 32 ¥ 2 mm) as substratum and food source, and a 0.2 g pile of moist vermiculite. Each colony consisted of one male and one female replacement reproductive, one soldier and some members of non terminal stages: 4 larvae (3 – 4 stage), 12 pseudergates and 8 nymphs. All termites (except the small larvae) were individually labelled by lightly marking the thorax and abdomen with Testors enamel paint. Previous trials demonstrated this marking technique did not considerably affect termite behaviour. Nests were placed on a revolving platform and maintained in an acoustically and thermally insulated room at 27 ± 2 °C with 80 ± 10 % relative humidity. The observations started 24 hours after the labelling operation by videorecording each colony at pre-programmed intervals evenly arranged during 24 hours, for a 16 days period. Each observation lasted 2 hours, during which six 5 second scans (one every 20 minutes) were performed using a JVC series BY-10E videocamera and a cold light illumination source. The light was switched on at least 10 min before the recording, allowing termites to acclimatise to observation conditions. Using the software package The Observer (Ver. 3.0, Noldus, 1993) and a PC as event-recorder, data were collected by scan-sampling all individuals of each colony observing the videorecorded images. Classification of behaviours is reported in Table 1. Active and passive
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Table 1. Classification of behaviours Behaviour
Code
Definition
Resting Walking Chewing
REST WALK CHEW
Being motionless, standing still Walking or running about Handling objects by means of mouthparts
Dig hole Handle Pulsing
HOLE HAND PULS
Vibrating
VIBR
Nibbling wood inside galleries Handling objects by means of legs Performing a single vibratory movement, violently Performing bursts of vibratory movements, remaining in place (Sbrenna et al., 1992)
Mandibulating Active grooming
MAND GROO
Slowly opening and closing mandibles Using mouthparts to palpate and clean body surface and appendices of another individual (Howse 1968; Nel et al., 1969; Jones 1980; Iwata et al., 1987)
Passive grooming
GRBY
Receiving grooming by another individual (belonged to the class of “passive behaviours”)
grooming were distinguished as separate behavioural classes (Noldus, 1993), allowing for the possibility that the same individual could be involved in this behaviour both as an actor and as a target subject. All grooming acts were exclusively allogrooming, since in our experiments self-grooming was never observed. During active grooming the animal was the actual initiator of the interaction (the one licking the partner), while during passive grooming it was the one to be groomed (the passive partner receiving attentions). Probable oral and anal trophallactic events are included in this behavioural category. The complete ethogram of all individuals in the colony, based on 19440 scans obtained in 240 observation hours, was made up of 19 elements, grouped in 5 behavioural categories (individual activity, feeding, nest care, communication and social interactions). King and queen repertories, based on 720 scans each obtained in 240 observation hours, are made up of 14 elements, grouped in 4 behavioural categories (individual activity, feeding, communication and social interactions). Data, in the form of frequencies of exhibition of behaviours for the observation period, were analysed by means of non parametric tests which are quoted in the text, together with the significance level. Kendall’s coefficient of concordance was used to assess individual variation among members of the same sex, allowing combination of data for further elaborations.
Modifiers
Objects: CORP = corpse, EXUV = exuvia, FECI = solid faeces, VERM = vermiculite, WOOD = wood Objects (see above)
Mode of vibration: SLOW = regular, slow CONV = irregular, convulsive Individual receiving grooming (specifying its name or number: KING, QUEEN, SOLD = soldier, 1, 2.,……8 = nymph, 9, 10,……20 = pseudergate, 21,……24 = larva); Focal body part (MOUT = mouthparts, HEAD = head, THOR = thorax, ABDO = abdomen, PROC = proctodeum), specifying if it happened in the superior part (TOP), in the flanks (SIDE), or in the inferior part (UNDER) Individual performing the grooming act Focal body part, (details as above)
Results Activities The behavioural repertories of the king and queen were similar in content but different in the frequency of some exhibited behaviours (Table 2). Comparisons of activities and social interactions using Wilcoxon signed-ranks test, showed highly significant differences for resting (Z = 4.77, P < 0.0001) and walking (Z = 3.32, P < 0.001) behaviours (Fig. 1 A). The male reproductive was the most active member of the colony (72 % resting and 17 % walking) while the queen was motionless in 82 % of the observations and moving in 11.5 %. There were differences also in feeding and vibrating behaviours, although not significant (respectively, Z = 1.22, P > 0.05 and Z = 1.63, P > 0.05): the king was observed eating and performing jerking movements with its whole body almost twice as frequently as its mate (Fig. 1 B). The queen ate wood and exuviae with the
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Table 2. Relative frequencies of behavioural acts performed by female and male replacement reproductives in colonies of K. flavicollis Behav. category
Behavioural act
QUEEN
KING
Individual activity
Rest Walk
83.179 11.188
72.184 17.207
RE-WA
94.367
89.390
Chew wood Chew exuvia Mandibulate
0.270 0.270 0.309
1.196 0.154 –
FEED
0.849
1.350
Nest care
NEST
–
–
Communication
Puls Vibrate slow Vibrate convulsively
0.579 2.238 0.154
1.235 3.318 –
COMM
2.971
4.552
Groom larvae Groom pseudergates Groom nymphs Groom soldier Groom queen Groom king
0.270 0.694 0.540 – – 0.309
0.733 0.810 1.042 0.424 1.775 –
Feeding behaviour
Social interactions
INTER
1.813
4.784
same frequency, while the king chewed mainly wood (Table 2).
Social interactions For each subject not only the behaviour in which it was engaged was recorded, but also whether it was alone or had some interactions with other individuals. Interactions were divided into two categories: slight, which included antennal touchings and body contacts, and intense, which included performing or receiving grooming acts. As shown in Figure 2, the queen was observed alone with a greater frequency than its mate, and this difference was highly significant (Z = 3.25, P < 0.001, Wilcoxon signedranks test). The king was involved in intense interactions and antennal touchings with its nestmates with a significatively higher frequency than the queen (respectively, Z = 2.09, P < 0.05 and Z = 2.42, P < 0.05). The frequency of body contacts was slightly higher for the king, although not significantly (Z = 1.42, P > 0.05). The king and queen received grooming with almost the same frequency (Z = 0.11, P > 0.05) (Fig. 3); however, a highly significant difference was detected for active grooming (Z = 3.26, P < 0.001). The king groomed its nestmates with a frequency that was almost three times higher than that of the queen. If we closely examine active grooming behaviour, considering both the receiving subjects (Fig. 4A) and the target
Figure 1. Direct comparison between the behavioural repertories of female and male replacement reproductives (** = P < 0.001, Wilcoxon test). Percent values of the average frequencies exhibited during the entire experiment. REST = standing still; WALK = walking or running about; EAT = nibbling; VIBR = performing bursts of jerking movements
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INTENSE INTERACTIONS
INTENSE
Figure 2. Direct comparison between the repertories of female and male replacement reproductives in their modes of interactions with nestmates (* = P < 0.05; ** = P < 0.001, Wilcoxon test). Percent values of the average frequencies exhibited during the entire experiment. NONE = absence of interactions; INTENSE = performing or receiving grooming acts; ANTE = antennal touchings; BODY = body contacts
Figure 3. Direct comparison between the repertories of female and male replacement reproductives in their intense interactions with nestmates (** = P < 0.001, Wilcoxon test). Percent values of the average frequencies exhibited during the entire experiment. ACTIVE GROOMING = performing grooming acts; PASSIVE GROOMING = receiving grooming acts
Figure 4. Analysis of active grooming behaviour performed by the female and male replacement reproductives, considering: A) the individuals towards whom acts where directed, and, B) body regions involved in the interactions. LARV = larvae; PSEU = pseudergates; NYM = nymphs; SOLD = soldiers; REPR = queen or king; PROC = proctodeum; ABDO = abdomen; THOR = thorax; HEAD = head; MOUT = mouthparts
body regions (Fig. 4B), the king groomed all nestmates, including the soldier, in all regions, mainly posterior ones, and directed almost one third of his attentions to the queen. The queen mostly groomed members of non-terminal stages in their proctodeal and abdominal regions, and directed one sixth of her attentions to the king. With regard to passive grooming, both reproductives received attentions from all nestmates, except the soldier (Fig. 5A): the queen was groomed mostly in the posterior parts, while the king in the tho-racic region (Fig. 5B) and this difference was significant (c2 = 11.64, P < 0.05).
Discussion We engaged in a series of experiments on laboratory colonies of K. flavicollis, which allowed us to shed light on behavioural patterns of different castes and developmental stages of this lower termite (Maistrello and Sbrenna, 1996; 1998). With this study, that is the result of a deeper ethogram type survey based on a different sampling technique, we present the first indications of the existence of some important differences in the repertories of male and female replacement reproductives.
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Figure 5. Analysis of passive grooming behaviour in female and male replacement reproductives, considering: A) the individuals from which they received acts, and, B) body regions involved in the interactions (for abbreviations see Fig. 4) (* = P < 0.05, Chi Square test)
Overall, such differences in behavioural frequencies agree with Springhetti (1980), who observed that in small colonies of K. flavicollis with primary reproductives, the king displayed a greater tendency than the queen to move around within the nest, and to play an active role in the movements of the couple. Our observations show indeed that the king has a greater inclination to occupy an active role in the colony. The continuous shiftings in the nest and the abundance of vibratory movements, together with the high frequency of social contacts and peculiarity of grooming patterns, seem to confer to the king a very important role in the termite society and suggest a sex-based behavioural specialisation between the two reproductives in colony organisation and equilibrium. In Z. nevadensis the nest-founding male and female reproductives were observed to exhibit a dynamic division of labour in response to the availability of nitrogen and throughout the pre- and initial post-zygotic stages of reproduction (Shellman-Reeve, 1990). During the period prior to egg-laying females significantly reduced their total activity, when the feeding source was poor in nitrogen, whereas when fed nitrogen-rich diets, the royal couple appeared to specialise on different tasks with males spending more time collecting water and females collecting pulp. These behavioural asymmetries were not observed in wood-feeding pairs, where both reproductives spent similar amounts of time in overall activity, contributing almost equally to the nest care and the nutritional requirements necessary for offspring production. In the same way, no indication of division of labour between partners was found during colony foundation in wood-fed Zootermopsis angusticollis (Rosengaus and Traniello, 1991),
the behavioural repertories of the primaries being identical. Absence of behavioural specialisation in the incipient stages of colony growth might be explained by considering that the king and the queen are responsible for all labour until their offspring reach the third instar (Howse, 1968; King and Spink, 1974; Garnier-Sillam, 1983). Hence, by maintaining a high level of behavioural flexibility they increase their ability to cope with the fluctuating and potentially dangerous environmental contingencies, like nest climate and predators (Calabi and Traniello, 1989). However, what happens when the colony increases in size and its members differentiate into the main castes, behaviourally and morphologically adapted to different tasks? Relieved from providing parental care and colony labour, the royal pair can behaviourally specialise in the reproductive and caste controlling tasks. It may be hypothesised that in K. flavicollis, the male-female asymmetry in overall activity represents an adaptive response that maximises the queen’s acquisition of nutrients stores prior to and during egg production. That might hold true not only for founding pairs, but also for developing colonies in presence of replacement reproductives and, in our experiments, the queen’s greatest consumption of exuviae might be a proof confirming this suggestion. Usually the reproductives are the ones to receive the most care in the colony (Howse, 1968; Nel et al., 1969; Maistrello and Sbrenna, 1996). Our observations showed that both male and female received the same amount of attention from nestmates, indicating that they really seem to be the “centre of a behaviour gradient” (Emerson, 1950), and play a key role in receiving and transmitting signals on which the regulation of
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social equilibrium is probably based. It may be suggested that the role of the male reproductive might acquire importance as a co-ordinator of colony activities, because of the female’s inactivity, which remains motionless to reduce the consumption of precious energetic reserves. Vibratory movements, abundantly performed by the king, could play a relevant role in interindividual communication of messages of still unknown meaning but probably associated with short-term colony needs (Maistrello and Sbrenna, 1996). The great abundance of interactions, in form of antennal touchings and active grooming directed towards all nestmates, and, mostly, to his partner qualifies the king, in our opinion, as a “mediator” in social interactions between the queen and the other colony members. Even considering that our conclusions are based on observations performed on few colonies, the considerable observation time and the number of scans performed on all colony members, represent something not definitely negligible. Although more interesting and general conclusions might be drawn from further detailed investigations, including the comparisons with primary reproductives repertories, our findings on replacement reproductives seem to suggest a possible evolutionary explanation for the retention of a king in isopteran society. Beyond the importance for periodical insemination of the queen, it might be the need for a behavioural mechanism that could allow the female to maximise her reproductive investment, as well as maintaining a central role in regulating caste equilibrium: the presence of an efficient partner who actively exchanges messages between her and their offspring.
Acknowledgements Very special thanks to Prof. Michael Lenz and Dr. Christine Nalepa for their precious and helpful comments on this manuscript. We are also grateful to Dr. Dimitri Giunchi and Dr. Milvia Chicca for their suggestions and to Isabella Angelini, Donatella Palazzi and Vanni Verona for help in experimental activities and data collection. This research is part of a programme supported by funds (40 %) from M.U.R.S.T.
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