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The establishment of a dominance rank order in male Peromyscus maniculatus and its stability with time
Anim. Behav ., 1970,18, 55-59
THE ESTABLISHMENT OF A DOMINANCE RANK ORDER IN MALE PEROMYSCUS MANICULATUS AND ITS STABILITY WITH TIME BY R . M . F . S . SADLEIR
Department of Biological Sciences, Simon Fraser University, Burnaby 2, B .C., Canada One of the 81-cm sides was of clear Perspex sheeting so that the animals could be observed through a small eyeslit in a screen 61 cm away. The arena was empty apart from sawdust on the floor and was illuminated by a 60 W red bulb 91 cm above the floor . The animals were maintained under reversed 12 hr daylight schedule and the original and duplicate bouts were conducted between 07 .00 and 09 .00 hours or between 13 .00 and 15 .00 hours over a 10-day period . Two animals marked by clipping the back fur) were placed in the arena on either side of a dividing partition which was lifted after 5 min and the contestants observed for a further 15 min . All actions were described into a taperecorder and later transcribed . The behaviour of Peromyscus has been described in detail by Eisenberg (1962) and the units referred to below are based on his paper . To rate male hostility the behavioural units of each separate animal were classified as being either positive, negative, neutral, or as mutual actions . Generally speaking positive actions are those having aggressive significance while negative ones are usually of a submissive nature . Individual positive actions are : (1) chasing opponent ; (2) grooming opponent ; (3) investigating opponent's anal region ; (4) leaping onto opponent ; (5) attempting to mount opponent ; (6) following opponent ; (7) threat uprights . Negative actions are : (1) being chased by opponent ; (2) being groomed by opponent ; (3) submitting to investigation of anal region by opponent ; (4) running away from opponent ; (5) showing submission to opponent ; (6) showing defeat to opponent ; (7) submissive uprights. Neutral actions are : (1) any exploratory activity (movement, jumping, climbing) ; (2) exploratory uprights ; (3) washing, (4) being followed by opponent . Mutual actions are (1) mutual nasonasal or naso-anal contacts ; (2) fighting . The submissive upright was the only unit observed during these encounters which was not clearly described in Eisenberg's paper, and is similar to that described by Grant & Mackintosh (1963) for Mus musculus . Here the animal htQ14S
Seasonal changes in the agonistic behaviour of male deermice, Peromyscus maniculatus, have been measured in connection with a field study of the species population dynamics (Sadleir 1965) by observing the arena behaviour towards introduced strange laboratory males . Although hostility could be measured by a series of encounters between field animals, some of the variables involved cannot be demonstrated as nothing is known of the previous behavioural history of a field animal . Consequently this paper describes an attempt to determine a dominance rank order amongst male Peromyscus kept in the laboratory which was subsequently used to test the hostility of field animals . Apparently there have been no previous rank order studies on deermice although the behavioural repertoire of the species has been well described (Balph & Stokes 1960 ; Eisenberg 1962). For instance Eisenberg's chapter on behaviour in a recent monograph (1968) on the species makes no reference to quantified agonistic behaviour studies . A number of similar studies have, however, been carried out on Mus musculus (Catlett 1961 ; Lagerspetz 1964 ; Ropartz 1968), but the most similar approach to the present report is found in papers by Seward (1945a, b) on laboratory Rattus norvegicus . Methods Eight adult male Peromyscus maniculatus austerus, all weighing between 17 and 19 g, were used . Animals A, B, C and H were trapped as adults of unknown age and had been in captivity for 2 months prior to testing, in contact with females but not males . D, E, F and G were laboratory born of wild caught parents and were between 4 and 6 months old . D, E and G had previously been briefly in contact with each other and other males in a population cage, but like all animals used, they had been maintained as stud males for the majority of the period prior to testing. The eight males were matched against each other in a random order and then the whole series duplicated, a total of fifty-six bouts . The bouts were conducted in an arena measuring 81 cm x 53 cm x 81 cm . 55
56
ANIMAL
BEHAVIOUR, 18, 1
its body in a vertical position while balancing on its hind paws with the eyes usually slitted and the ears laid back . The forepaws are always held close to the body with the digits curled under the pads . During the transcription of the recorded data a 10-s time unit was used, an action being scored for the interval if it occurred once inside it . If more than one action of the same sort occurred inside the interval it was only scored once, but more than one sort of action could be scored inside a single interval . This procedure is similar to that of Seward (1945a) except that he used a 15-s time unit . The result of each bout was determined as follows . A win/lose decision was recorded either when the number of positive actions of the `winner' exceeded the number of his negative actions in a proportion falling outside the limit of two standard errors (5 per cent) from a 1 :1 ratio on binomial probability paper, or when the number of negative actions of the `loser' exceeded the number of his positive actions by the same criterion or when both occurred . In fortytwo of the fifty-six bouts conducted, one animal clearly won, while the other clearly lost by this simple criterion . Determining the aggressive rank of an individual from his series of encounters can be carried out by either a `relative' or by an 'absolute' method . His `relative' rank can be shown by the distribution of the final decisions of each bout with each of his opponents . In its simplest form a male can be considered more aggressive than individuals whom he clearly defeats and less aggressive than those whom he is defeated by. This method does not allow inclusion of the indecisive bouts in a male's evaluation . To do this it is necessary to rate the result of each bout numerically and then determine each male's average rating as done by Seward (1945b). The results of indecisive bouts have thus been further subdivided and ratings to the bout decisions given as follows : Rating 5 = Win Rating 4 = Positive actions exceed negative actions by three or more Rating 3 = Positive actions within three of negative actions Rating 2 = Negative actions exceed positive actions by three or more . Rating 1 = Lose The `absolute' rank of an individual male can be determined by the counting of his behavioural actions . The proportion of negative actions for
all bouts of the series can be compared for each male and the animals placed in an order . This method, however, has two major disadvantages in that it will only agree with the `relative' method when the individuals used in the experiment : (a) cover the whole range in behavioural phenotypes present in the species, and (b) are representatively distributed throughout this range . These requirements can only be met by complete cross-testing of a very much larger number of individuals than used here so that the `absolute' position is considered less representative in this study than the `relative' position . However, the proportions of behavioural actions are useful in other ways, as demonstrated below . Results (a) The Order of Individuals in the Rank Order Table I shows the results of bouts in which a clear win/lose decision was recorded and is henceforth called the standard order . With the exception of the G, E, C triangle a clear singleline ranking can be seen. Each individual was defeated by all the individuals above and defeated in turn most of those below. The main discrepancy found was the double defeat of E by C, and the single defeat of G by E so that G and E are considered as equal apha males . The mean scores of each individual are given in Table II, the results of each bout being rated as described above (i .e . after Seward 1945b) . At first sight the order appears to be somewhat different from the standard order but all the discrepancies are due to the relative positions of animal B . This animal participated in a high proportion of indecisive bouts (seven out of its fourteen) so that its rating has been biased . With this exception the order of individuals in ranking by this method is consistent with the standard order. Positive and negative actions are shown as a proportion of total actions (all bouts summed) in Fig . 1, which demonstrates that G, E and D showed the most positive actions with H, C, F and B being approximately the same and mouse A having the highest proportion of negative actions. The only explanation for D's high proportions lies in its previous history ; it was laboratory born of wild caught mother and was placed as a juvenile in a population maze with adult males . As has been observed in other experiments, this resulted in considerable fighting, inhibition of growth, and delay of sexual maturity while in the maze, although D was of comparable weight to the other males during the bout period .
SADLEIR : DOMINANCE RANK ORDER IN PEROMYSCUS MANICULATUS
57
Table I . Total Number and Distribution of Conclusive Win/Lose Bouts in the Original and Duplicate Series Winner Loser
G
E
D
H
C
F
B
A
Total lost bouts
G
-
1
0
0
0
0
0
0
1
E
0*
-
0
0
2*
0
0
0
2
D
2*
2*
-
0
0*
0
0
0
4
H
2
2
1
0
0
0
0
5
C
1
0
2
1
-
0
0
0
4
F
2
2*
1*
2
2*
-
0
0
9
B
2
2
1*
0
1*
0*
-
0
6
A
2*
2*
2
1
1
2
1*
-
11
7
4
6
2
1
0
42
Total won bouts
11
11
*Decisions after 1 month . Table H. Mean Score Based on Rating the Decision of Each Bout Original bouts
Duplicate bouts Scale position
Score
Scale position
Score
A
1 .7
7=8
1 .3
8
B
2 .9
4=5
2.4
6
C
2 .6
6
3.3
4=5
D
3 .7
3
3.4
3
E
4.3
2
4.4
1
F
1 .7
7=8
2.1
7
G
4.8
1
4 .0
2
H
2 .9
4=5
3 .3
4=5
(b) The Temporal Stability of the Rank Order. I. Short-term stability. The results of the duplicate bouts closely paralleled that of the originals . There was no case in which the duplicate bout reversed the decision of the original and in sixteen duplicate bouts the result was identical . This indicates a very considerable degree of short term stability of the ranking . A similar result is seen in Table II where the mean scores are presented . The `absolute' rank of the individual based on the proportion of positive and negative actions is extremely constant between the original and
duplicate series (Fig. 1) . There is striking agreement in the proportion of positive actions for animals A, C, D, F and H . The proportion of negative actions stayed much the same for the top-ranking G, E and D, slightly decreased for the next four animals but markedly increased for the most submissive animal, A. II . Long-term stability . As the longest period between the original and duplicate bout had been 5 days, a month after the main series fifteen bouts between randomly selected pairs were carried out . Their results are shown in Table I . In the intervening month the males were kept under varying conditions . A marked effect was the much reduced social activity between the contestants after the intervening month . So much so that in six bouts a win/lose decision was based on less than ten positive/negative actions . Despite this in fourteen of fifteen bouts the status of both animals was the same as demonstrated in the standard order . In the exception, a bout of almost total inactivity, animal C showed one positive action and animal D showed three negative actions . With this inconclusive exception, the agreement in results shows that ranking is stable over a considerable period of time . Discussion The work of Seward (1945b) on albino rats is the only previous study of this kind and as his results were considerably different to those presented for Peromyscus it is worthwhile to discuss the reasons. Seward's rats showed nq
58
ANIMAL BEIhAVIOUR, 18, 1 I POSITIVE ACTION
W J
z
4
Ilii
NEGATIVE ACTION
J
0
NEUTRAL ACTION
a 100%
o
0
90% 80%_ 70%60l 50% 40% 30% 2 0% 10% G
E
D
H
C
F
B
A
STANDARD ORDER
Fig. 1 . The relative proportions of various types of actions by individuals (all bouts summed) in the original and duplicate series . individual consistency between bouts and he attributed this to the conditioning of avoidance responses by the defeated rat . Consequently, he was unable to demonstrate any dominance ranking . He was able to show from his results that the number of bouts where actual fights occurred decreased as his series progressed and that a defeated rat was less aggressive and less likely to win a bout after a defeat than before it . For Peromyscus neither of these effects occurred and this is apparently true also in Mus (Lagerspetz 1964) . The ratios of bouts with overt fights to total bouts for the main Peromyscus series on consecutive days was : 3 :8, 3 :8, 2 :4, 2 :4, 2 :4, 0 :3, 7 :9, 4 :7, 3 :6, 2 :4. Males defeated in morning bouts did not show any increased tendency to lose in afternoon bouts . The conclusion is then that Peromyscus does not show fear conditioning to the degree described by Seward, and that this lack of conditioning to defeat is one possible reason for the stable and clear cut rank order which the deermouse demonstrates . The only sign of fear conditioning seen in this study was in the case of A, who in a few duplicate bouts and in the repeat bouts after one month occasionally showed submissive behaviour immediately upon
being placed with another male . This effect was only ephemeral as A was able to completely defeat wild-caught males in the intervening period and afterwards . In his discussion, Seward (1945b) suggests a continuous genotype scale of innate aggression ranging from pugnacity (P) to fear (F) . He suggests that `one basic determiner of the stability of a hierarchy is the distribution of P-F amongst members of a group' . As his albino rats were from inbred laboratory stock, it can be inferred that their genotype represented a very limited range of potential aggressiveness . Possibly because of this compressed range, constant individual differences in aggressiveness could not be demonstrated . It seems likely that the stability of the Peromyscus ranking was due to the source of the eight males used . All were wild caught, or less than one generation removed from field animals, so that they were a sample of the range of aggressiveness of a wild population . Summary This study has demonstrated that a sample of eight wild male Peromyscus maniculatus, when repeatedly matched in an arena situation against each other, showed a virtually constant single-
SADLEIR : DOMINANCE RANK ORDER IN PEROMYSCUS MANICULATUS line ranking of aggressiveness which remained stable for 1 month. Acknowledgments This work was carred out in the Department of Zoology, University of British Columbia, while I was in receipt of a C .S .I .R.O . Overseas PostGraduate Studentship and later supported by N .R .C . of Canada funding to Dr D . H . Chitty . 1 wish to thank my good friend John Eisenberg for his continual help and advice . REFERENCES Balph, D . F. & Stokes, A . W. (1960) . Notes on the behaviour of deermice (Peromyscus maniculatus rufinus) . Proc. Utah Acad. Sci., 37, 55-62 . Catlett, R . H . (1961). An evaluation of methods for measuring fighting behaviour with special reference to Mus musculus . Anim . Behav., 9, 8-10. Eisenberg, J . F. (1962) . Studies on the behaviour of Peromyscus maniculatus gambelli and Peromyscus californicus parasiticus . Behaviour, 19, 177-207 .
59
Eisenberg, J. F. (1968) . Behaviour patterns. In Biology of Peromyscus (Rodentia) (ed . by J . A . King) . Am. Soc. Mammal., Spec. Publ. No. 2 . Grant, E . C. & Mackintosh, J. H. (1963) . A comparison of the social postures of some common laboratory rodents . Behaviour, 21, 246-259. Lagerspetz, K . (1964) . Studies on the aggressive behaviour of mice. Ann. Acad. Sci., Fenn ., B, 131, 1-131 . Ropartz, P . (1968) . The relation between olfactory stimulation and aggressive behaviour in mice. Anim. Behav ., 16, 97-100 . Sadleir, R . M. F. S . (1965) . The relationship between agonistic behaviour and population changes in the deermouse Peromyscus maniculatus (Wagner) . J. anim . Ecol., 34, 331-352 . Seward, J . P . (1945a) . Aggressive behaviour in the rat. I . General characteristics, age and sex differences . J. comp . Psychol., 38, 175-197 . Seward, J . P. (1945b) . Aggressive behaviour in the rat . II. An attempt to establish a dominance hierarchy . J. comp . Psychol., 38, 213-224 . (Received 23 April 1969 ; revised 19 August 1969 ; MS. number: 892)
THE ESTABLISHMENT OF A DOMINANCE RANK ORDER IN MALE PEROMYSCUS MANICULATUS AND ITS STABILITY WITH TIME BY R . M . F . S . SADLEIR
Department of Biological Sciences, Simon Fraser University, Burnaby 2, B .C., Canada One of the 81-cm sides was of clear Perspex sheeting so that the animals could be observed through a small eyeslit in a screen 61 cm away. The arena was empty apart from sawdust on the floor and was illuminated by a 60 W red bulb 91 cm above the floor . The animals were maintained under reversed 12 hr daylight schedule and the original and duplicate bouts were conducted between 07 .00 and 09 .00 hours or between 13 .00 and 15 .00 hours over a 10-day period . Two animals marked by clipping the back fur) were placed in the arena on either side of a dividing partition which was lifted after 5 min and the contestants observed for a further 15 min . All actions were described into a taperecorder and later transcribed . The behaviour of Peromyscus has been described in detail by Eisenberg (1962) and the units referred to below are based on his paper . To rate male hostility the behavioural units of each separate animal were classified as being either positive, negative, neutral, or as mutual actions . Generally speaking positive actions are those having aggressive significance while negative ones are usually of a submissive nature . Individual positive actions are : (1) chasing opponent ; (2) grooming opponent ; (3) investigating opponent's anal region ; (4) leaping onto opponent ; (5) attempting to mount opponent ; (6) following opponent ; (7) threat uprights . Negative actions are : (1) being chased by opponent ; (2) being groomed by opponent ; (3) submitting to investigation of anal region by opponent ; (4) running away from opponent ; (5) showing submission to opponent ; (6) showing defeat to opponent ; (7) submissive uprights. Neutral actions are : (1) any exploratory activity (movement, jumping, climbing) ; (2) exploratory uprights ; (3) washing, (4) being followed by opponent . Mutual actions are (1) mutual nasonasal or naso-anal contacts ; (2) fighting . The submissive upright was the only unit observed during these encounters which was not clearly described in Eisenberg's paper, and is similar to that described by Grant & Mackintosh (1963) for Mus musculus . Here the animal htQ14S
Seasonal changes in the agonistic behaviour of male deermice, Peromyscus maniculatus, have been measured in connection with a field study of the species population dynamics (Sadleir 1965) by observing the arena behaviour towards introduced strange laboratory males . Although hostility could be measured by a series of encounters between field animals, some of the variables involved cannot be demonstrated as nothing is known of the previous behavioural history of a field animal . Consequently this paper describes an attempt to determine a dominance rank order amongst male Peromyscus kept in the laboratory which was subsequently used to test the hostility of field animals . Apparently there have been no previous rank order studies on deermice although the behavioural repertoire of the species has been well described (Balph & Stokes 1960 ; Eisenberg 1962). For instance Eisenberg's chapter on behaviour in a recent monograph (1968) on the species makes no reference to quantified agonistic behaviour studies . A number of similar studies have, however, been carried out on Mus musculus (Catlett 1961 ; Lagerspetz 1964 ; Ropartz 1968), but the most similar approach to the present report is found in papers by Seward (1945a, b) on laboratory Rattus norvegicus . Methods Eight adult male Peromyscus maniculatus austerus, all weighing between 17 and 19 g, were used . Animals A, B, C and H were trapped as adults of unknown age and had been in captivity for 2 months prior to testing, in contact with females but not males . D, E, F and G were laboratory born of wild caught parents and were between 4 and 6 months old . D, E and G had previously been briefly in contact with each other and other males in a population cage, but like all animals used, they had been maintained as stud males for the majority of the period prior to testing. The eight males were matched against each other in a random order and then the whole series duplicated, a total of fifty-six bouts . The bouts were conducted in an arena measuring 81 cm x 53 cm x 81 cm . 55
56
ANIMAL
BEHAVIOUR, 18, 1
its body in a vertical position while balancing on its hind paws with the eyes usually slitted and the ears laid back . The forepaws are always held close to the body with the digits curled under the pads . During the transcription of the recorded data a 10-s time unit was used, an action being scored for the interval if it occurred once inside it . If more than one action of the same sort occurred inside the interval it was only scored once, but more than one sort of action could be scored inside a single interval . This procedure is similar to that of Seward (1945a) except that he used a 15-s time unit . The result of each bout was determined as follows . A win/lose decision was recorded either when the number of positive actions of the `winner' exceeded the number of his negative actions in a proportion falling outside the limit of two standard errors (5 per cent) from a 1 :1 ratio on binomial probability paper, or when the number of negative actions of the `loser' exceeded the number of his positive actions by the same criterion or when both occurred . In fortytwo of the fifty-six bouts conducted, one animal clearly won, while the other clearly lost by this simple criterion . Determining the aggressive rank of an individual from his series of encounters can be carried out by either a `relative' or by an 'absolute' method . His `relative' rank can be shown by the distribution of the final decisions of each bout with each of his opponents . In its simplest form a male can be considered more aggressive than individuals whom he clearly defeats and less aggressive than those whom he is defeated by. This method does not allow inclusion of the indecisive bouts in a male's evaluation . To do this it is necessary to rate the result of each bout numerically and then determine each male's average rating as done by Seward (1945b). The results of indecisive bouts have thus been further subdivided and ratings to the bout decisions given as follows : Rating 5 = Win Rating 4 = Positive actions exceed negative actions by three or more Rating 3 = Positive actions within three of negative actions Rating 2 = Negative actions exceed positive actions by three or more . Rating 1 = Lose The `absolute' rank of an individual male can be determined by the counting of his behavioural actions . The proportion of negative actions for
all bouts of the series can be compared for each male and the animals placed in an order . This method, however, has two major disadvantages in that it will only agree with the `relative' method when the individuals used in the experiment : (a) cover the whole range in behavioural phenotypes present in the species, and (b) are representatively distributed throughout this range . These requirements can only be met by complete cross-testing of a very much larger number of individuals than used here so that the `absolute' position is considered less representative in this study than the `relative' position . However, the proportions of behavioural actions are useful in other ways, as demonstrated below . Results (a) The Order of Individuals in the Rank Order Table I shows the results of bouts in which a clear win/lose decision was recorded and is henceforth called the standard order . With the exception of the G, E, C triangle a clear singleline ranking can be seen. Each individual was defeated by all the individuals above and defeated in turn most of those below. The main discrepancy found was the double defeat of E by C, and the single defeat of G by E so that G and E are considered as equal apha males . The mean scores of each individual are given in Table II, the results of each bout being rated as described above (i .e . after Seward 1945b) . At first sight the order appears to be somewhat different from the standard order but all the discrepancies are due to the relative positions of animal B . This animal participated in a high proportion of indecisive bouts (seven out of its fourteen) so that its rating has been biased . With this exception the order of individuals in ranking by this method is consistent with the standard order. Positive and negative actions are shown as a proportion of total actions (all bouts summed) in Fig . 1, which demonstrates that G, E and D showed the most positive actions with H, C, F and B being approximately the same and mouse A having the highest proportion of negative actions. The only explanation for D's high proportions lies in its previous history ; it was laboratory born of wild caught mother and was placed as a juvenile in a population maze with adult males . As has been observed in other experiments, this resulted in considerable fighting, inhibition of growth, and delay of sexual maturity while in the maze, although D was of comparable weight to the other males during the bout period .
SADLEIR : DOMINANCE RANK ORDER IN PEROMYSCUS MANICULATUS
57
Table I . Total Number and Distribution of Conclusive Win/Lose Bouts in the Original and Duplicate Series Winner Loser
G
E
D
H
C
F
B
A
Total lost bouts
G
-
1
0
0
0
0
0
0
1
E
0*
-
0
0
2*
0
0
0
2
D
2*
2*
-
0
0*
0
0
0
4
H
2
2
1
0
0
0
0
5
C
1
0
2
1
-
0
0
0
4
F
2
2*
1*
2
2*
-
0
0
9
B
2
2
1*
0
1*
0*
-
0
6
A
2*
2*
2
1
1
2
1*
-
11
7
4
6
2
1
0
42
Total won bouts
11
11
*Decisions after 1 month . Table H. Mean Score Based on Rating the Decision of Each Bout Original bouts
Duplicate bouts Scale position
Score
Scale position
Score
A
1 .7
7=8
1 .3
8
B
2 .9
4=5
2.4
6
C
2 .6
6
3.3
4=5
D
3 .7
3
3.4
3
E
4.3
2
4.4
1
F
1 .7
7=8
2.1
7
G
4.8
1
4 .0
2
H
2 .9
4=5
3 .3
4=5
(b) The Temporal Stability of the Rank Order. I. Short-term stability. The results of the duplicate bouts closely paralleled that of the originals . There was no case in which the duplicate bout reversed the decision of the original and in sixteen duplicate bouts the result was identical . This indicates a very considerable degree of short term stability of the ranking . A similar result is seen in Table II where the mean scores are presented . The `absolute' rank of the individual based on the proportion of positive and negative actions is extremely constant between the original and
duplicate series (Fig. 1) . There is striking agreement in the proportion of positive actions for animals A, C, D, F and H . The proportion of negative actions stayed much the same for the top-ranking G, E and D, slightly decreased for the next four animals but markedly increased for the most submissive animal, A. II . Long-term stability . As the longest period between the original and duplicate bout had been 5 days, a month after the main series fifteen bouts between randomly selected pairs were carried out . Their results are shown in Table I . In the intervening month the males were kept under varying conditions . A marked effect was the much reduced social activity between the contestants after the intervening month . So much so that in six bouts a win/lose decision was based on less than ten positive/negative actions . Despite this in fourteen of fifteen bouts the status of both animals was the same as demonstrated in the standard order . In the exception, a bout of almost total inactivity, animal C showed one positive action and animal D showed three negative actions . With this inconclusive exception, the agreement in results shows that ranking is stable over a considerable period of time . Discussion The work of Seward (1945b) on albino rats is the only previous study of this kind and as his results were considerably different to those presented for Peromyscus it is worthwhile to discuss the reasons. Seward's rats showed nq
58
ANIMAL BEIhAVIOUR, 18, 1 I POSITIVE ACTION
W J
z
4
Ilii
NEGATIVE ACTION
J
0
NEUTRAL ACTION
a 100%
o
0
90% 80%_ 70%60l 50% 40% 30% 2 0% 10% G
E
D
H
C
F
B
A
STANDARD ORDER
Fig. 1 . The relative proportions of various types of actions by individuals (all bouts summed) in the original and duplicate series . individual consistency between bouts and he attributed this to the conditioning of avoidance responses by the defeated rat . Consequently, he was unable to demonstrate any dominance ranking . He was able to show from his results that the number of bouts where actual fights occurred decreased as his series progressed and that a defeated rat was less aggressive and less likely to win a bout after a defeat than before it . For Peromyscus neither of these effects occurred and this is apparently true also in Mus (Lagerspetz 1964) . The ratios of bouts with overt fights to total bouts for the main Peromyscus series on consecutive days was : 3 :8, 3 :8, 2 :4, 2 :4, 2 :4, 0 :3, 7 :9, 4 :7, 3 :6, 2 :4. Males defeated in morning bouts did not show any increased tendency to lose in afternoon bouts . The conclusion is then that Peromyscus does not show fear conditioning to the degree described by Seward, and that this lack of conditioning to defeat is one possible reason for the stable and clear cut rank order which the deermouse demonstrates . The only sign of fear conditioning seen in this study was in the case of A, who in a few duplicate bouts and in the repeat bouts after one month occasionally showed submissive behaviour immediately upon
being placed with another male . This effect was only ephemeral as A was able to completely defeat wild-caught males in the intervening period and afterwards . In his discussion, Seward (1945b) suggests a continuous genotype scale of innate aggression ranging from pugnacity (P) to fear (F) . He suggests that `one basic determiner of the stability of a hierarchy is the distribution of P-F amongst members of a group' . As his albino rats were from inbred laboratory stock, it can be inferred that their genotype represented a very limited range of potential aggressiveness . Possibly because of this compressed range, constant individual differences in aggressiveness could not be demonstrated . It seems likely that the stability of the Peromyscus ranking was due to the source of the eight males used . All were wild caught, or less than one generation removed from field animals, so that they were a sample of the range of aggressiveness of a wild population . Summary This study has demonstrated that a sample of eight wild male Peromyscus maniculatus, when repeatedly matched in an arena situation against each other, showed a virtually constant single-
SADLEIR : DOMINANCE RANK ORDER IN PEROMYSCUS MANICULATUS line ranking of aggressiveness which remained stable for 1 month. Acknowledgments This work was carred out in the Department of Zoology, University of British Columbia, while I was in receipt of a C .S .I .R.O . Overseas PostGraduate Studentship and later supported by N .R .C . of Canada funding to Dr D . H . Chitty . 1 wish to thank my good friend John Eisenberg for his continual help and advice . REFERENCES Balph, D . F. & Stokes, A . W. (1960) . Notes on the behaviour of deermice (Peromyscus maniculatus rufinus) . Proc. Utah Acad. Sci., 37, 55-62 . Catlett, R . H . (1961). An evaluation of methods for measuring fighting behaviour with special reference to Mus musculus . Anim . Behav., 9, 8-10. Eisenberg, J . F. (1962) . Studies on the behaviour of Peromyscus maniculatus gambelli and Peromyscus californicus parasiticus . Behaviour, 19, 177-207 .
59
Eisenberg, J. F. (1968) . Behaviour patterns. In Biology of Peromyscus (Rodentia) (ed . by J . A . King) . Am. Soc. Mammal., Spec. Publ. No. 2 . Grant, E . C. & Mackintosh, J. H. (1963) . A comparison of the social postures of some common laboratory rodents . Behaviour, 21, 246-259. Lagerspetz, K . (1964) . Studies on the aggressive behaviour of mice. Ann. Acad. Sci., Fenn ., B, 131, 1-131 . Ropartz, P . (1968) . The relation between olfactory stimulation and aggressive behaviour in mice. Anim. Behav ., 16, 97-100 . Sadleir, R . M. F. S . (1965) . The relationship between agonistic behaviour and population changes in the deermouse Peromyscus maniculatus (Wagner) . J. anim . Ecol., 34, 331-352 . Seward, J . P . (1945a) . Aggressive behaviour in the rat. I . General characteristics, age and sex differences . J. comp . Psychol., 38, 175-197 . Seward, J . P. (1945b) . Aggressive behaviour in the rat . II. An attempt to establish a dominance hierarchy . J. comp . Psychol., 38, 213-224 . (Received 23 April 1969 ; revised 19 August 1969 ; MS. number: 892)