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A method for the construction of dominance hierarchies in the field cricket Teleogryllus oceanicus (Le Guillou)
Anita. Behav., 1982, 30, 216-220
A METHOD FOR THE CONSTRUCTION OF DOMINANCE HIERARCHIES IN THE FIELD CRICKET TELEOGRYLLUS OCEANICUS (Lv, GUILLOU) BY W I N S T O N J. BAILEY & J. A. S T O D D A R T
Department of Zoology, University of Western Australia, Nedlands, Western Australia 6009 Abstract. The post-fight display of the field cricket, Teleogryllus oeeanicus (Le Guillou) is evaluated as a means of providing an index of level of aggression. This display comprises a bouncing motion of remarkably constant form. Hierarchies constructed using an index based on this behaviour correlated well with those produced by more classical methods and also provided an effective measure of small changes i n aggressive levels undetected by these classical methods. The dominance hierarchy is a ranking of animals in a social group and is constructed on the basis of their relative success in aggressive encounters with each other. ASsessment, by members of a group, of levels of aggression may serve to .reduce the amount of combat occurring in the group, as members are able to avoid highly aggressive animals with whom they stand little chance of success in combat. Pre-contact signals m a y provide cues which reinforce these dominance hierarchies.
Methods Experimental The adult male crickets used in this study were drawn randomly from a large culture and placed into the observation terrarium, a 30 x 30 • 60 cm lidless glass-walled box. The terrarium temperature was kept at 25~2 C in a constant-temperature room. Illumination was by fluorescent light on a 12:12 light-dark regime. All crickets used were outwardly perfect specimens on introduction (except in those cases where the antennae were experimentally removed) and were individually marked with a small, coded dot of paint on the pronotum. In those experiments where females were introduced, care was taken to ensure that they only contacted the male selected for copulation. This was accomplished by partitioning off the section of the terrarium containing both the female and the appropriate male. This procedure allowed a distinction to be made between crickets which had contacted a female and those which had not.
Alexander (1961) carried out a detailed examination of interactions between conspecific male crickets belonging to the genus Acheta and found that certain behaviour patterns seen during the interactions were conspicuous because of their regular use and constant form. In this situation it might be possible to use the frequency of one of these components to assess the level of aggression of crickets and so to construct a dominance hierarchy. Gauthreaux (1978), in reviewing the ecological significance of behavioural dominance, points out that an argument may be made for a more observationally based measure of dominance, which should include some descriptive breadth, as a foundation for an empirical measure of dominance. The behaviour described in this paper, being variable, can be assessed in relation to the hierarchy by observational means, so going beyond the simple all-or-none win/loss measure. Our primary aim is therefore to test the use of the bounce behaviour as an index of aggression within the field cricket Teleogryllus oeeanicus (Le Guillou), rather than to elucidate the significance of the bounce behaviour itself. However, we do briefly discuss the possible function of this behaviour.
Observations As many of the aggressive encounters occurred at high speed, a videotape recorder was used for observations. As well as slowing actions for detailed observation this allowed single-frame playback (40ms per frame) which enabled filmed sequences to be timed. Because of the need to film discrete periods of behaviour, crickets were only observed during the lights-on periods. Interactions recorded with the aid of a red light during lights-off were so similar in content to the lights-on interactions that the latter were taken to be representative of both. Observation periods lasted for half an hour or an hour, being extended to longer periods if less than 10 encounters had been ob216
BAILEY & STODDART: DOMINANCE HIERARCHIES IN CRICKETS served in the preceding half hour. In order to allow time for the males to settle down within the terrarium, the first observations were made several days after the introduction of the initial five males. Hierarchies based on our index of aggression were compared with conventional hierarchies constructed using the percentile rank method of Komai et al. (1959) and the tabular method of Davis (1966). The degree of association between rankings produced by conventional methods and those produced by our method was calculated using Spearman's rank correlation coefficient (rs) with correction for tied ranks (Siegel 1956) where these occurred. Construction of the Index
The prime requirement of a behavioural component to be used in the construction of an index is that it be objectively quantifiable. Alexander's (1961) detailed descriptions of aggressive encounters between male crickets show that such encounters may be broadly divided into three sections; approach and prefight stridulation; fighting, involving kicking, biting, and grappling with bouts of singing; and a post-fight display by the successful male. In T. oceanicus, components of this final category were observed which appeared to satisfy the need for objective quantification. These components all appeared in what we term the bounce behaviour. Video film of the bounce behaviour revealed a remarkable stereotypy in form, not only be-
^
" J
b
c
t~
d
Fig. 1. Schematic representation of the bounce in Teleogryllus oeeanieus. The sequence has been transcribed from video-film, showing the lowering of the body prior to bounce (a), 'standing erect' (b), 'recoil' (e), and forward 'thrust' (d).
217
tween bounces on different occasions for one cricket, but also between individuals. Figure 1 depicts a single bounce. The initial phase of the bounce behaviour involved the cricket lowering its body before raising it as high as possible while keeping the feet firmly planted. This was termed the 'standing erect' posture and in some instances was the only component of the display. However, if the bounce continued the next two phases would always occur in sequence. The second, or 'recoil', phase consisted of the body being drawn downwards and backwards as far as possible while keeping the feet stationary; this phase ended with the head being posterior to the foretibia. Immediately after assuming this posture, the third, or 'thrust', phase was initiated, in which the whole body was propelled vigorously upwards and as far forward as possible, with stationary feet; this was followed by resumption of the resting posture. The tegmina were often raised during the thrust phase, but without an accompanying stridulation. The whole cycle could be repeated a number of times in rapid succession. The timing of the three phases of the bounce was determined using frame-by-framc playback. Whereas the length of time the cricket remained in the standing erect posture varied, the duration of the succeeding phases was remarkably constant. Analysis of 50 bounces by five different individuals showed that the time taken between the conclusion of the standing erect posture and the completion of the recoil phase was 0.20 s (so 0.02 ) while the thrust phase occupied only 0.12 s (so 0.01 s). We have quantified the bounce behaviour by dividing it into four categories in order to establish a base for an index, here termed the 'bounce index'. These categories reflected discrete responses which were non-randomly distributed with respect to a cricket's position in the hierarchy (Fig. 2). The occurrence of a multiple bounce sequence in any group of males is disproportionately low compared to the earlier phase of the bounce behaviour, giving a crudely logarithmic function. By applying uneven scores to these bounce categories we hoped to produce a linear index. The categories and their scores are as follows: Category 1. Erect posture only: 1 point. Category 2. One or two bounces: 2 points. Category 3. Three or four bounces: 5 points. Category 4. More than four bounces: 15 points.
ANIMAL
218
BEHAVIOUR,
In the course of the observation periods each cricket was observed in a number of fights. At the end of each fight the number of bounces was recorded and the appropriate point score allocated to the bouncing cricket. An index could then be calculated at the conclusion of the period by dividing the sum of a cricket's bounce scores by the number of its successful encounters (crickets were never observed to bounce after losing a fight). This index could then be used to construct a ranking of the animals in a group or to estimate a single animal's aggressiveness at any one time. Results Utility of the Hierarchy
One feature of the estimation of rankings by the percentile rank method or tabular rank method is that these methods use only win/loss criteria and cannot therefore indicate the level of aggression in each encounter. By applying the bounce score to each win and looking at this over all the winning encounters for an individual one may produce at any one time a quantified measure of hierarchy status for that cricket. Table I illustrates results for five crickets which had established hierarchies in the terrarium over a 7-day period. They were observed for one hour on day 7 and their ranks were subsequently determined by both percentile and Rank
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tabular rank methods. Comparisons of the hierarchy based on the bounce index with those calculated from the classical methods of win/loss revealed significant correlations. In order to test the validity of the index under varying conditions, as assessed by its correlation with win/loss methods, we carried out two manipulations designed to alter existing hierarchies. These were (a) removing the antennae from a male during a sequence of observations, and (b) allowing a male to mate between observation periods. The tests were not intended to shed light on their effects on behaviour per se, but rather to have them act as simple modifiers for the purpose of hierarchy comparison. The manipulations were primarily chosen for their demonstrated ability to affect aggression (Alexander 1961). After a half-hour period during which the dominance hierarchy was stable, an antenna was removed from the dominant male: this resulted in a decrease in its bounce score (Table IIA). There was an equivalent change in its position in the hierarchy constructed during this period. The within-observation-period change in position is responsible for the non-significant correlation coefficient of Table IIA, as the hierarchies constructed by the three methods respond differently to such a change. Table IIB shows the position of the hierarchy after a further half-hour period. The cricket with the antenna removed is now low in the hierarchy and the animals have formed a rank order which is consistent across the methods of assessment. Copulation influences male aggression: males are more aggressive after than before (Alexander 1961). When this change in state is estimated by the percentile or tabular rank methods, a change
:::::::::::::::
Table I. Dominance Hierarchies Derived from Two Standard Methods Using Win/Loss, Compared to That Using the Bounce Index
10--
0
...... 1
(6.6)
..... 2
H
5
r~ 4
(55.9) (40.5) (15.7) Bounce category
5
(5.3)
Fig. 2. Distribution of the grades of post-fight bounce behaviour between the five crickets in a dominance hierarchy. The data represent 125 encounters recorded over 12 half-hour periods. Figures in parentheses under the bounce categories represent the percentage of encounters in which that category occurred. The zero category is equivalent to no bounce following a victory.
Percentile rank
Tabular rank
Bouncerank
2 5 4 3-1
2 5 4 3
2 (2.33) 5 (1.50) 4 (0.67) 3 (0.29)
1
1 (0.13)
rs Percentile and Bounce = 0.975 (P < 0.05) r~ Tabular and Bounce = 1.00 (P < 0.05)
Observation period 1000-1100 hours, day 7. Hierarchies were computed from 14 aggressive encounters; cricket numbers are listed in descending order of rank, with bounce index in parentheses.
BAILEY & STODDART: DOMINANCE HIERARCHIES IN CRICKETS
219
i n r a n k m a y be observed if the increase is large e n o u g h a n d lasts long enough. However, if the change is small, or more i m p o r t a n t l y if the cricket is already the d o m i n a n t male, n o change will be observed. The b o u n c e index also changes after crickets copulate. After the observation period in Table IIB, cricket 4 courted a n d copulated with a n i n t r o d u c e d female. I t rose to a new, d o m i n a n t position i n all hierarchies (Table IIIA). However, only the change i n b o u n c e index clearly showed the direction of this change in hierarchy (0.5 to 4.3). O n a n o t h e r occasion, when a new cricket was i n t r o d u c e d to an established hierarchy after h a v i n g copulated in a separate terrarium, it immediately established a position as the domin a n t male a n d the extent o f this d o m i n a n c e was again reflected by the b o u n c e index (0.8 had been the score o f the previously d o m i n a n t male c o m p a r e d to 4.0 for the new male, Table IIIB). The index reveals that this c o p u l a t i o n - i n d u c e d rise in a p p a r e n t aggressive levels m a y be transitory. Hence only 24 h after being d o m i n a n t cricket 4 (Table I I I A , B) h a d reverted to a lower b o u n c e score a n d thus a lower position (4.3 in Table I I I A to 0 in Table IIIB).
Thus the b o u n c e index appears to provide a n accurate measure of the level of aggression in each male within a group a n d appears to be sensitive to small changes over short time periods. F o r example cricket 6 (Table IIIC), i n t r o d u c e d to the group after copulation, became a s u b - d o m i n a n t male within one h o u r after its i n t r o d u c t i o n ( b o u n c e index fell from 4.0 to 1.0, T a b l e I I I A - C ) . U n d e r all test conditions, except for a withinperiod r a n k shift, hierarchies constructed using
Table H. Effect on the Hierarchies of Antennal Damage to the Alpha Cricket (No. 2, Table I) over Two Observation Periods A. Observation period 1330-1400 hours, day 8
Cricket No. 2 has no antennae. Hierarchies were computed from 22 aggressive encounters; cricket numbers are listed in descending order of rank.
Table III. Effect of the Introduction of a Recently Copulated Male (No. 4) on the Hierarchies and Bonnce Indexes (A), and the Replacement of Cricket No. 2 by a Second Recently Copulated Male (No. 6) on tile Next Day (B), with a Third Introduction of a Recently Copulated Male (No. 5) the Following Day (C) A. Observation period 1515-1545 hours, day 10
Percentile rank
Tabular rank
Bounce rank
4
4
4 (4.30)
1
1
1 (1,80)
2-3 5
2-3 5
3 (0.60) 2 (0.30) 5 (0) rs Percentile and Bounce = 0.975 (P < 0.05) r~ Tabular and Bounce = 0.975 (P < 0.05)
B. Observation period 1400-1430 hours, day 11
Percentile rank
Tabular rank
Bounce rank
4 1-3-5 2
4 4 (2.0) 1 1 (0.92) 3 3-2 (0.66) 2 5 (0.30) 5 rs Percentile and Bounce = 0.459, Ns rs Tabular and Bounce : 0.975 (P < 0.05)
Percentile rank
Tabular rank
Bounce rank
6 1
6 1
6 (4.00) 1 (0.80)
3
3
5 (0.66)
5 5 3 (0.60) 4 4 4(0) r~ Percentile and Bounce = 0.90 (P < 0.05) r~ Tabular and Bounce = 0.90 (P < 0.05)
Cricket No. 2 has only t of both antennae left at the end of this period. Hierarchies were computed from 14 aggressive encounters; cricket numbers are listed in descending order of rank.
Hierarchies were computed from 12 aggressive interactions; cricket numbers are listed in descending order of rank.
B. Observation period 1430-1500 hours, day 8
C. Observation period 1500-1530, day 12
Percentile rank
Tabular rank
Bounce rank
Percentile rank
1
1
1 (1.45)
5 6 1-3 4
3-4 3 3 (0.66) 2 4 4 (0.50) 5 2-5 2-5 (0) re Percentile and Bounce = 0.947 (P < 0.05) rs Tabular and Bounce = 1.00 (P < 0.05) Cricket No. 2 is as in conclusion of Table IIA. Hierarchies were compiled from 16 aggressive encounters; cricket numbers are listed in descending order of rank.
Tabular rank
Bounce rank
5 5 (3.57) 6 6 (1.00) 3 3 (0.71) 1 1 (0.60) 4 4(0) r~ Percentile and Bounce = 0.975 (P < 0.05) r~ Tabular and Bounce = 1.00 (P < 0.05) Hierarchies were computed from 18 aggressive encounters; crickets are listed in descending order of rank.
220
ANIMAL
BEHAVIOUR,
our index were significantly correlated (at the 5 ~ level) with hierarchies constructed using classical methods. Discussion
Post-fight displays have apparently not been used as measures of aggression in other groups. In most vertebrates the hierarchy acts to prevent overt aggression and little occurs in settled groups. Crickets often call after winning a fight (Alexander 1961), and as they occur in sub-social aggregations (Campbell & Shipp 1979) such a signal could be effective over some distance. However, the relatively short signalling range of the bounce behaviour suggests that it is used in close-contact encounters and it may be that the forceful nature of the bounce allows the signal to be transmitted through substrate vibration. Morris (1971) reports a tremulation of male conocephaline bush crickets during the approach phase of their aggressive interaction, and suggests that this is used to transmit information about an animal's level of aggression; in addition male tettigoniids bounce when in close contact with females, providing information through the substrate which may be used for mate localization. It seems likely, therefore, that the bounce transmits information about the aggressive state of the bouncer. A possible explanation of the significance of the bounce behaviour may be formulated in terms of its ability to allow males to be freed from the constant pressures of fighting long enough to mate successfully. Mate attraction by males involves the use of a courtship song and our observations support Cade's (1979) suggestion that aggressive males may also be attracted by a singing male. I f the bounce display by a victorious male informs the loser that this male's level of aggression is high and the result of the fight is unlikely to be reversed by further combat, it may enable the victor to court long enough to attract a female and mate before having to fight again. Low bounce levels may suggest that re-engagement might produce a reversal. Whatever the overall significance of the system, the stereotypy of the bounce component of the post-fight display makes it an easily quantified way of measuring aggression in an objective fashion. Our index of aggression calculated
30,
1
using this display was designed to provide a linear estimation of the level of aggression for individuals. Hierarchies constructed from this index were correlated significantly with those produced from the classical win/loss methods of Komai et al. (1959) and Davis (1966) under all except one of the test conditions. All three methods produced similar estimates of crickets' ranks, with the exception of a shift of rank within an observation period. However, unlike the win/loss methods the bounce index provides a measure of the differences between individuals and between overall aggressiveness for animals in different hierarchies. Once the hierarchy has been established, small changes, even those not producing a shift in rank, may be measured by the index. REFERENCES
Alexander, R. D. 1961. Aggressiveness, territoriality and sexual behaviour in field crickets (Orthoptera: Gryllidae). Behaviour, 17, 130-223. Cade, W. 1979. The evolution of alternative male reproductive strategies in field crickets. In: Sexual Selection and Reproductive Competition in Insects
(Ed. by M. S. Blum & N. A. Blum), pp. 343377. New York: Academic Press. Campbell, D. J. & Shipp, E. 1979. Regulation of spatial pattern in populations of the field cricket Teleogryllas eommodus (Walker). Z. Tierpsychol., 51, 260-268. Chauvin, R. 1968. Animal Societies. London: Gollancz. Davis, D. E. 1966. Integral Animal Behaviour. New York: Macmillan. Gauthreaux, S. A. 1978. The ecological significance of behavioural don-finance. In: Perspectives in Ethology, Vol. 3 (Ed. by P. P. G. Bateson & P. H. Klopfer), pp. 17-54. New York: Plenum Press. Kaufman, H. 1965. Definitions and methodology in the study of aggression. Psychol. Bull., 64, 351-364. Komai, T., Craig, J. O. & Wearden, S. 1959. Heritability and repeatability of social aggressiveness in the domestic chicken. Poultry Sei., 38, 356-359. Morris, G. K. 1971. Aggression in male conocephaline grasshoppers (Tettigoniidae). Anita. Behav., 19, 132-137. Siegel, S. 1956. Non-parametric Statistics for the Behavioral Sciences. New York: McGraw-Hill. Wiley, R. H. 1973. Territoriality and non-random mating in the sage grouse, Centroeereus urophasianus. Anita. Behav. Monogr., 6, 87-169. Wood-Gush, D. G. M. 1955. The behaviour of the domestic chicken: a review of the literature. Br. J. Anita. Behav., 3, 81-110. (Received 15 July 1980; revised 27 March 1981 ; MS. number: 2032)
A METHOD FOR THE CONSTRUCTION OF DOMINANCE HIERARCHIES IN THE FIELD CRICKET TELEOGRYLLUS OCEANICUS (Lv, GUILLOU) BY W I N S T O N J. BAILEY & J. A. S T O D D A R T
Department of Zoology, University of Western Australia, Nedlands, Western Australia 6009 Abstract. The post-fight display of the field cricket, Teleogryllus oeeanicus (Le Guillou) is evaluated as a means of providing an index of level of aggression. This display comprises a bouncing motion of remarkably constant form. Hierarchies constructed using an index based on this behaviour correlated well with those produced by more classical methods and also provided an effective measure of small changes i n aggressive levels undetected by these classical methods. The dominance hierarchy is a ranking of animals in a social group and is constructed on the basis of their relative success in aggressive encounters with each other. ASsessment, by members of a group, of levels of aggression may serve to .reduce the amount of combat occurring in the group, as members are able to avoid highly aggressive animals with whom they stand little chance of success in combat. Pre-contact signals m a y provide cues which reinforce these dominance hierarchies.
Methods Experimental The adult male crickets used in this study were drawn randomly from a large culture and placed into the observation terrarium, a 30 x 30 • 60 cm lidless glass-walled box. The terrarium temperature was kept at 25~2 C in a constant-temperature room. Illumination was by fluorescent light on a 12:12 light-dark regime. All crickets used were outwardly perfect specimens on introduction (except in those cases where the antennae were experimentally removed) and were individually marked with a small, coded dot of paint on the pronotum. In those experiments where females were introduced, care was taken to ensure that they only contacted the male selected for copulation. This was accomplished by partitioning off the section of the terrarium containing both the female and the appropriate male. This procedure allowed a distinction to be made between crickets which had contacted a female and those which had not.
Alexander (1961) carried out a detailed examination of interactions between conspecific male crickets belonging to the genus Acheta and found that certain behaviour patterns seen during the interactions were conspicuous because of their regular use and constant form. In this situation it might be possible to use the frequency of one of these components to assess the level of aggression of crickets and so to construct a dominance hierarchy. Gauthreaux (1978), in reviewing the ecological significance of behavioural dominance, points out that an argument may be made for a more observationally based measure of dominance, which should include some descriptive breadth, as a foundation for an empirical measure of dominance. The behaviour described in this paper, being variable, can be assessed in relation to the hierarchy by observational means, so going beyond the simple all-or-none win/loss measure. Our primary aim is therefore to test the use of the bounce behaviour as an index of aggression within the field cricket Teleogryllus oeeanicus (Le Guillou), rather than to elucidate the significance of the bounce behaviour itself. However, we do briefly discuss the possible function of this behaviour.
Observations As many of the aggressive encounters occurred at high speed, a videotape recorder was used for observations. As well as slowing actions for detailed observation this allowed single-frame playback (40ms per frame) which enabled filmed sequences to be timed. Because of the need to film discrete periods of behaviour, crickets were only observed during the lights-on periods. Interactions recorded with the aid of a red light during lights-off were so similar in content to the lights-on interactions that the latter were taken to be representative of both. Observation periods lasted for half an hour or an hour, being extended to longer periods if less than 10 encounters had been ob216
BAILEY & STODDART: DOMINANCE HIERARCHIES IN CRICKETS served in the preceding half hour. In order to allow time for the males to settle down within the terrarium, the first observations were made several days after the introduction of the initial five males. Hierarchies based on our index of aggression were compared with conventional hierarchies constructed using the percentile rank method of Komai et al. (1959) and the tabular method of Davis (1966). The degree of association between rankings produced by conventional methods and those produced by our method was calculated using Spearman's rank correlation coefficient (rs) with correction for tied ranks (Siegel 1956) where these occurred. Construction of the Index
The prime requirement of a behavioural component to be used in the construction of an index is that it be objectively quantifiable. Alexander's (1961) detailed descriptions of aggressive encounters between male crickets show that such encounters may be broadly divided into three sections; approach and prefight stridulation; fighting, involving kicking, biting, and grappling with bouts of singing; and a post-fight display by the successful male. In T. oceanicus, components of this final category were observed which appeared to satisfy the need for objective quantification. These components all appeared in what we term the bounce behaviour. Video film of the bounce behaviour revealed a remarkable stereotypy in form, not only be-
^
" J
b
c
t~
d
Fig. 1. Schematic representation of the bounce in Teleogryllus oeeanieus. The sequence has been transcribed from video-film, showing the lowering of the body prior to bounce (a), 'standing erect' (b), 'recoil' (e), and forward 'thrust' (d).
217
tween bounces on different occasions for one cricket, but also between individuals. Figure 1 depicts a single bounce. The initial phase of the bounce behaviour involved the cricket lowering its body before raising it as high as possible while keeping the feet firmly planted. This was termed the 'standing erect' posture and in some instances was the only component of the display. However, if the bounce continued the next two phases would always occur in sequence. The second, or 'recoil', phase consisted of the body being drawn downwards and backwards as far as possible while keeping the feet stationary; this phase ended with the head being posterior to the foretibia. Immediately after assuming this posture, the third, or 'thrust', phase was initiated, in which the whole body was propelled vigorously upwards and as far forward as possible, with stationary feet; this was followed by resumption of the resting posture. The tegmina were often raised during the thrust phase, but without an accompanying stridulation. The whole cycle could be repeated a number of times in rapid succession. The timing of the three phases of the bounce was determined using frame-by-framc playback. Whereas the length of time the cricket remained in the standing erect posture varied, the duration of the succeeding phases was remarkably constant. Analysis of 50 bounces by five different individuals showed that the time taken between the conclusion of the standing erect posture and the completion of the recoil phase was 0.20 s (so 0.02 ) while the thrust phase occupied only 0.12 s (so 0.01 s). We have quantified the bounce behaviour by dividing it into four categories in order to establish a base for an index, here termed the 'bounce index'. These categories reflected discrete responses which were non-randomly distributed with respect to a cricket's position in the hierarchy (Fig. 2). The occurrence of a multiple bounce sequence in any group of males is disproportionately low compared to the earlier phase of the bounce behaviour, giving a crudely logarithmic function. By applying uneven scores to these bounce categories we hoped to produce a linear index. The categories and their scores are as follows: Category 1. Erect posture only: 1 point. Category 2. One or two bounces: 2 points. Category 3. Three or four bounces: 5 points. Category 4. More than four bounces: 15 points.
ANIMAL
218
BEHAVIOUR,
In the course of the observation periods each cricket was observed in a number of fights. At the end of each fight the number of bounces was recorded and the appropriate point score allocated to the bouncing cricket. An index could then be calculated at the conclusion of the period by dividing the sum of a cricket's bounce scores by the number of its successful encounters (crickets were never observed to bounce after losing a fight). This index could then be used to construct a ranking of the animals in a group or to estimate a single animal's aggressiveness at any one time. Results Utility of the Hierarchy
One feature of the estimation of rankings by the percentile rank method or tabular rank method is that these methods use only win/loss criteria and cannot therefore indicate the level of aggression in each encounter. By applying the bounce score to each win and looking at this over all the winning encounters for an individual one may produce at any one time a quantified measure of hierarchy status for that cricket. Table I illustrates results for five crickets which had established hierarchies in the terrarium over a 7-day period. They were observed for one hour on day 7 and their ranks were subsequently determined by both percentile and Rank
~1 2~
i
I
3EEl
3O
41~A
i:~:!:!:!:!:!:~:
!i!i!i!ilii!~! ~ i!i#ii!!i! i:!:!:!:~:~:)~:
g-
5El
30,
1
tabular rank methods. Comparisons of the hierarchy based on the bounce index with those calculated from the classical methods of win/loss revealed significant correlations. In order to test the validity of the index under varying conditions, as assessed by its correlation with win/loss methods, we carried out two manipulations designed to alter existing hierarchies. These were (a) removing the antennae from a male during a sequence of observations, and (b) allowing a male to mate between observation periods. The tests were not intended to shed light on their effects on behaviour per se, but rather to have them act as simple modifiers for the purpose of hierarchy comparison. The manipulations were primarily chosen for their demonstrated ability to affect aggression (Alexander 1961). After a half-hour period during which the dominance hierarchy was stable, an antenna was removed from the dominant male: this resulted in a decrease in its bounce score (Table IIA). There was an equivalent change in its position in the hierarchy constructed during this period. The within-observation-period change in position is responsible for the non-significant correlation coefficient of Table IIA, as the hierarchies constructed by the three methods respond differently to such a change. Table IIB shows the position of the hierarchy after a further half-hour period. The cricket with the antenna removed is now low in the hierarchy and the animals have formed a rank order which is consistent across the methods of assessment. Copulation influences male aggression: males are more aggressive after than before (Alexander 1961). When this change in state is estimated by the percentile or tabular rank methods, a change
:::::::::::::::
Table I. Dominance Hierarchies Derived from Two Standard Methods Using Win/Loss, Compared to That Using the Bounce Index
10--
0
...... 1
(6.6)
..... 2
H
5
r~ 4
(55.9) (40.5) (15.7) Bounce category
5
(5.3)
Fig. 2. Distribution of the grades of post-fight bounce behaviour between the five crickets in a dominance hierarchy. The data represent 125 encounters recorded over 12 half-hour periods. Figures in parentheses under the bounce categories represent the percentage of encounters in which that category occurred. The zero category is equivalent to no bounce following a victory.
Percentile rank
Tabular rank
Bouncerank
2 5 4 3-1
2 5 4 3
2 (2.33) 5 (1.50) 4 (0.67) 3 (0.29)
1
1 (0.13)
rs Percentile and Bounce = 0.975 (P < 0.05) r~ Tabular and Bounce = 1.00 (P < 0.05)
Observation period 1000-1100 hours, day 7. Hierarchies were computed from 14 aggressive encounters; cricket numbers are listed in descending order of rank, with bounce index in parentheses.
BAILEY & STODDART: DOMINANCE HIERARCHIES IN CRICKETS
219
i n r a n k m a y be observed if the increase is large e n o u g h a n d lasts long enough. However, if the change is small, or more i m p o r t a n t l y if the cricket is already the d o m i n a n t male, n o change will be observed. The b o u n c e index also changes after crickets copulate. After the observation period in Table IIB, cricket 4 courted a n d copulated with a n i n t r o d u c e d female. I t rose to a new, d o m i n a n t position i n all hierarchies (Table IIIA). However, only the change i n b o u n c e index clearly showed the direction of this change in hierarchy (0.5 to 4.3). O n a n o t h e r occasion, when a new cricket was i n t r o d u c e d to an established hierarchy after h a v i n g copulated in a separate terrarium, it immediately established a position as the domin a n t male a n d the extent o f this d o m i n a n c e was again reflected by the b o u n c e index (0.8 had been the score o f the previously d o m i n a n t male c o m p a r e d to 4.0 for the new male, Table IIIB). The index reveals that this c o p u l a t i o n - i n d u c e d rise in a p p a r e n t aggressive levels m a y be transitory. Hence only 24 h after being d o m i n a n t cricket 4 (Table I I I A , B) h a d reverted to a lower b o u n c e score a n d thus a lower position (4.3 in Table I I I A to 0 in Table IIIB).
Thus the b o u n c e index appears to provide a n accurate measure of the level of aggression in each male within a group a n d appears to be sensitive to small changes over short time periods. F o r example cricket 6 (Table IIIC), i n t r o d u c e d to the group after copulation, became a s u b - d o m i n a n t male within one h o u r after its i n t r o d u c t i o n ( b o u n c e index fell from 4.0 to 1.0, T a b l e I I I A - C ) . U n d e r all test conditions, except for a withinperiod r a n k shift, hierarchies constructed using
Table H. Effect on the Hierarchies of Antennal Damage to the Alpha Cricket (No. 2, Table I) over Two Observation Periods A. Observation period 1330-1400 hours, day 8
Cricket No. 2 has no antennae. Hierarchies were computed from 22 aggressive encounters; cricket numbers are listed in descending order of rank.
Table III. Effect of the Introduction of a Recently Copulated Male (No. 4) on the Hierarchies and Bonnce Indexes (A), and the Replacement of Cricket No. 2 by a Second Recently Copulated Male (No. 6) on tile Next Day (B), with a Third Introduction of a Recently Copulated Male (No. 5) the Following Day (C) A. Observation period 1515-1545 hours, day 10
Percentile rank
Tabular rank
Bounce rank
4
4
4 (4.30)
1
1
1 (1,80)
2-3 5
2-3 5
3 (0.60) 2 (0.30) 5 (0) rs Percentile and Bounce = 0.975 (P < 0.05) r~ Tabular and Bounce = 0.975 (P < 0.05)
B. Observation period 1400-1430 hours, day 11
Percentile rank
Tabular rank
Bounce rank
4 1-3-5 2
4 4 (2.0) 1 1 (0.92) 3 3-2 (0.66) 2 5 (0.30) 5 rs Percentile and Bounce = 0.459, Ns rs Tabular and Bounce : 0.975 (P < 0.05)
Percentile rank
Tabular rank
Bounce rank
6 1
6 1
6 (4.00) 1 (0.80)
3
3
5 (0.66)
5 5 3 (0.60) 4 4 4(0) r~ Percentile and Bounce = 0.90 (P < 0.05) r~ Tabular and Bounce = 0.90 (P < 0.05)
Cricket No. 2 has only t of both antennae left at the end of this period. Hierarchies were computed from 14 aggressive encounters; cricket numbers are listed in descending order of rank.
Hierarchies were computed from 12 aggressive interactions; cricket numbers are listed in descending order of rank.
B. Observation period 1430-1500 hours, day 8
C. Observation period 1500-1530, day 12
Percentile rank
Tabular rank
Bounce rank
Percentile rank
1
1
1 (1.45)
5 6 1-3 4
3-4 3 3 (0.66) 2 4 4 (0.50) 5 2-5 2-5 (0) re Percentile and Bounce = 0.947 (P < 0.05) rs Tabular and Bounce = 1.00 (P < 0.05) Cricket No. 2 is as in conclusion of Table IIA. Hierarchies were compiled from 16 aggressive encounters; cricket numbers are listed in descending order of rank.
Tabular rank
Bounce rank
5 5 (3.57) 6 6 (1.00) 3 3 (0.71) 1 1 (0.60) 4 4(0) r~ Percentile and Bounce = 0.975 (P < 0.05) r~ Tabular and Bounce = 1.00 (P < 0.05) Hierarchies were computed from 18 aggressive encounters; crickets are listed in descending order of rank.
220
ANIMAL
BEHAVIOUR,
our index were significantly correlated (at the 5 ~ level) with hierarchies constructed using classical methods. Discussion
Post-fight displays have apparently not been used as measures of aggression in other groups. In most vertebrates the hierarchy acts to prevent overt aggression and little occurs in settled groups. Crickets often call after winning a fight (Alexander 1961), and as they occur in sub-social aggregations (Campbell & Shipp 1979) such a signal could be effective over some distance. However, the relatively short signalling range of the bounce behaviour suggests that it is used in close-contact encounters and it may be that the forceful nature of the bounce allows the signal to be transmitted through substrate vibration. Morris (1971) reports a tremulation of male conocephaline bush crickets during the approach phase of their aggressive interaction, and suggests that this is used to transmit information about an animal's level of aggression; in addition male tettigoniids bounce when in close contact with females, providing information through the substrate which may be used for mate localization. It seems likely, therefore, that the bounce transmits information about the aggressive state of the bouncer. A possible explanation of the significance of the bounce behaviour may be formulated in terms of its ability to allow males to be freed from the constant pressures of fighting long enough to mate successfully. Mate attraction by males involves the use of a courtship song and our observations support Cade's (1979) suggestion that aggressive males may also be attracted by a singing male. I f the bounce display by a victorious male informs the loser that this male's level of aggression is high and the result of the fight is unlikely to be reversed by further combat, it may enable the victor to court long enough to attract a female and mate before having to fight again. Low bounce levels may suggest that re-engagement might produce a reversal. Whatever the overall significance of the system, the stereotypy of the bounce component of the post-fight display makes it an easily quantified way of measuring aggression in an objective fashion. Our index of aggression calculated
30,
1
using this display was designed to provide a linear estimation of the level of aggression for individuals. Hierarchies constructed from this index were correlated significantly with those produced from the classical win/loss methods of Komai et al. (1959) and Davis (1966) under all except one of the test conditions. All three methods produced similar estimates of crickets' ranks, with the exception of a shift of rank within an observation period. However, unlike the win/loss methods the bounce index provides a measure of the differences between individuals and between overall aggressiveness for animals in different hierarchies. Once the hierarchy has been established, small changes, even those not producing a shift in rank, may be measured by the index. REFERENCES
Alexander, R. D. 1961. Aggressiveness, territoriality and sexual behaviour in field crickets (Orthoptera: Gryllidae). Behaviour, 17, 130-223. Cade, W. 1979. The evolution of alternative male reproductive strategies in field crickets. In: Sexual Selection and Reproductive Competition in Insects
(Ed. by M. S. Blum & N. A. Blum), pp. 343377. New York: Academic Press. Campbell, D. J. & Shipp, E. 1979. Regulation of spatial pattern in populations of the field cricket Teleogryllas eommodus (Walker). Z. Tierpsychol., 51, 260-268. Chauvin, R. 1968. Animal Societies. London: Gollancz. Davis, D. E. 1966. Integral Animal Behaviour. New York: Macmillan. Gauthreaux, S. A. 1978. The ecological significance of behavioural don-finance. In: Perspectives in Ethology, Vol. 3 (Ed. by P. P. G. Bateson & P. H. Klopfer), pp. 17-54. New York: Plenum Press. Kaufman, H. 1965. Definitions and methodology in the study of aggression. Psychol. Bull., 64, 351-364. Komai, T., Craig, J. O. & Wearden, S. 1959. Heritability and repeatability of social aggressiveness in the domestic chicken. Poultry Sei., 38, 356-359. Morris, G. K. 1971. Aggression in male conocephaline grasshoppers (Tettigoniidae). Anita. Behav., 19, 132-137. Siegel, S. 1956. Non-parametric Statistics for the Behavioral Sciences. New York: McGraw-Hill. Wiley, R. H. 1973. Territoriality and non-random mating in the sage grouse, Centroeereus urophasianus. Anita. Behav. Monogr., 6, 87-169. Wood-Gush, D. G. M. 1955. The behaviour of the domestic chicken: a review of the literature. Br. J. Anita. Behav., 3, 81-110. (Received 15 July 1980; revised 27 March 1981 ; MS. number: 2032)