Interspecific aggression in captive male lemmings

Anim . Behav., 1979,27,1014-1021

INTERSPECIFIC AGGRESSION IN CAPTIVE MALE LEMMINGS BY

EDWIN M . BANKS, U . WILLIAM HUCK & NICHOLAS J . MANKOVICH

Department of Ecology, Ethology and Evolution, University of Illinois, Urbana, Illinois 61801

Abstract. Two experiments were conducted with lemmings to determine which of two species, Lemmus trimucronatus and Dicrostonyx groenlandicus, was capable of exercising aggressive social dominance over the other in staged, dyadic encounters, and which was able to outcompete the other for a single shelter . Agonistic behaviour occurred in all dyadic encounters, but dominance was not established by either species . A sequential analysis of encounters revealed that each species had a distinct fighting strategy. In shelter competition, prior residence was a crucial factor in the outcome . However, one species did not completely exclude the other . Further evidence to support species differences in fighting strategies was derived from the shelter experiments . These results are discussed in relation to ecological considerations . aggressive and that whereas differences in diet and habitat preference may account largely for relative population differences, interference in runways and burrows may be an important contributing factor . This possibility is supported by radio-tracking studies demonstrating that, in areas of patchy habitat, Lemmus and Dicrostonyx exist in close proximity and may, in fact, occupy overlapping home ranges (Banks & Brooks, unpublished observations) . However, there have been no systematic field or laboratory observations of the relative ability of the two in interspecific agonistic encounters . In a review article, Grant (1972) has surveyed the rodent literature on interspecific competition . Documentation for numerous cases in which an aggressively dominant species restricts the movements of a sympatric species exist, e .g . chipmunk species in western North America (Brown 1971 ; Heller 1971 ; Sheppard 1971 ; Chappell 1978) . The present laboratory experiments were designed to test the hypotheses that (1) one of the two species is capable of dominating the other in dyadic encounters, and (2) given a single hide or shelter, one will outcompete the other for possession of this shelter .

The collared lemming, Dicrostonyx groenlandicus, and the brown lemming, Lemmus trimucronatus, are species of Arctic microtine rodents that are distributed sympatrically over much of their range in North America (Hall & Kelson 1959) . Both species have been studied in the field to gain an understanding of their characteristically cyclic population dynamics (Krebs 1964 ; Pitelka 1973) . Relative population density changes reported by Krebs (1964) for Lemmus and Dicrostonyx populations in Canada reveal that for the summers of 1959 to 1962, Lemmus was present in greater numbers than Dicrostonyx . In another region of overlap, the coastal plain near Barrow, Alaska, the same trend in relative abundance of the two species has been noted (Pitelka 1973) . However, at another site in Alaska, Prudhoe Bay (Feist 1975), Dicrostonyx was more abundant than Lemmus . Batzli (unpublished manuscript) reported that in some areas of the Meade River, Alaska, study site Dicrostonyx was more abundant than Lemmus, but the reverse held in other areas of the patchy Meade River site . The habitat and nutritional requirements of the two species differ . Broadly speaking, Lemmus occurs in wet tundra and its diet consists mainly of sedges and grasses, whereas Dicrostonyx typically occurs on dry ridges, a restricted tundra habitat, and feeds primarily on dicot herbs and shrubs (Thompson 1955 ; Batzli 1975) . It may well be that these factors play a primary role in the relative population densities of these two species . However, it is worth noting that during the extended study of Krebs (1964), when Lemmus populations increased, it was at the apparent expense of habitat typically occupied by Dicrostonyx . Pitelka (1973) suggested that of the two species, Lemmus appears to be the more

Experiment 1 : Dyadic Encounters Methods Subjects. Twenty male Dicrostonyx and 20 male Lemmus were used in this experiment. The Dicrostonyx were derived from an original stock of 100 trapped at Fort Churchill, Manitoba, and Baker Lake, N . W . T., during the summers of 1967, 1968 and 1971 . The Lemmus were descended from 100 animals trapped at Barrow, Alaska, during the summers of 1972 and 1973 . Age and weight data are summarized in Table I . 1014



BANKS ET AL . : INTERSPECIFIC AGGRESSION

1 01 5

Table I . Age and Weight Data

Species

Age (days)

Weight (g)

N

Mean (±sE) Range

Mean (±sE) Range

20 20

85 . 5 (+0 .6) 82-89 85 . 7 (+0 . 6) 83-89

52 . 8 (±1 . 5) 37 .9-65 . 7 54 . 6 (±1 . 7) 42 .0-682

22 22

103 . 9 (±7 . 8) 78-130 102 . 1 (+9 . 1) 76-138

50 . 6 (+4 . 7) 39 . 0-72 . 6 52 . 1 (+5 . 1) 382-76 . 4

Experiment 1 Dicrostonyx

Lemmus Experiment 2 Dicrostonyx

Lemmus

Animals were kept in a cold room maintained at 15 + 3 C with a photoperiod of 18L :6D . All subjects were derived from litters of multiparous females . At birth, the litters were reduced to three individuals . Weaning occurred at 18 days and consisted of removing the mother from the cage in which the young had been reared . The littermates remained together until 28 days, at which time they were transferred to individual plastic cages (29 x 18 x 13 cm), each provided with a 5-mm wire-mesh top. The plastic cages contained sawdust and cotton batting nest material . All lemmings remained in individual cages until testing . Food (Purina Laboratory Rabbit Chow) and water were provided ad libitum. Cages were changed weekly throughout the study . Apparatus. The dyadic encounter arena used in this study was similar to that used by Banks (1968) and Allin & Banks (1968) . It was approximately semi-circular in shape with sides of galvanized tin and a plywood floor . The glass front wall was 50 cm in length and, like the sides of the arena, 25 cm high . Floor space totalled 1645 cm 2 . The floor was covered with litter material . The recording system consisted of a 20-pen event recorder activated by a push-button keyboard . Chart speed was 15 . 0 cm/min . Behavioural acts were read to the nearest 0 . 25 s . A 25-W white bulb, suspended 1 m above the floor of the arena, provided illumination . Observations were made from behind a one-way screen . Procedure. On the day prior to testing, each subject was given an individual 20-min trial to become familiar with the arena . The following procedure was used for each dyadic encounter . (1) The two test animals were removed from their home cages and placed into the encounter arena . A double-layer mesh partition prevented the animals from achieving physical contact . (2) After a 2-min habituation period the partition was removed and the 10-min test started. (3) At

the end of each encounter the arena was cleaned and the bedding replaced . The behavioural elements (Table II) recorded in the encounters were described by Allin & Banks (1968) and Banks & Popham (1975) . The frequencies and durations of the behaviours are briefly summarized in Table III . Each subject was tested only once . Only animals that weighed within 5 g of one another were paired . Results Means and standard errors for frequencies of behaviours observed during dyadic encounters are summarized in Table III . Wilcoxon twosample tests (Ghent 1974) indicated that Lemmus obtained significantly higher scores for approach, retreat, chase, perineal investigation, and vocalization . Dicrostonyx, on the other hand, obtained higher scores for flee and attack . In addition, the results of a Chi-square test indicated that a significantly higher number of Dicrostonyx tooth-chattered (8) during the dyadic encounters (X2 = P < 0 . 01) (none of the Lemmus were observed to tooth-chatter) . Discussion Several facts emerge from experiment 1, in which dyadic encounters were staged between Dicrostonyx and Lemmus. In all 20 contests, agonistic behaviour was displayed by both contestants . It is clear that there are significant quantitative differences between the two species with respect to many of the components in their agonistic display repertories (Table III) . A marked qualitative difference between the two species is documented in the pattern of toothchattering, a common rodent audible signal displayed during stressful situations (Brooks & Banks 1973) . Dicrostonyx displays this signal ; Lemmus does not . In fact, over the past seven years during which we have maintained laboratory colonies of both species, we have never



1016

ANIMAL BEHAVIOUR, 27,

observed Lemmus to produce tooth-chattering . Moreover, we have never witnessed toothchattering by Lemmus during three years of field work on a natural population at Barrow, Alaska. The strategy used by each species in agonistic encounters appears to be quite different . Lemmus may be characterized as more assertive and probing ; more apt to initiate momentary

4

contact. However, once contact is established Dicrostonyx is more prone to launch an attack . A consequence of these two distinct strategies is that a dominance-subordinance relationship in any pair cannot be discerned . Both species lack a definitive defensive posture frequently described for murid rodents such as rats and mice . The display of defensive postures in these latter

Table II. Description of Behavioural Acts Recorded during Dyadic Encounters Behaviour Approach (APR) Retreat (RET) Autogroom (AGM) Perineal investigation (PIN) Naso-nasal contact (NSN) Chase (CHS) Flee (FLE) Attack (ATK) Bite (BIT) Dig (DIG)

Vocalization (VOC) Tooth-chattering

Description Bodily movement of one animal toward the other Bodily movement of one animal away from the other Self-licking, scratching with hind leg, or rubbing forelegs over the head and face Vibrissal, nasal, or oral contact with the perineal region of another animal Animals simultaneously maintain close 'nose-to nose' or vibrissal contact Rapid and prolonged movement toward a retreating animal Rapid and prolonged movement away from an approaching animal Characteristically a jump toward the other animal, usually from an upright posture Self explanatory Vigorous strokes of the forelegs propel bedding material past the side of the animal or under it . Frequently hindlegs are used to expel material accumulated under the animal Includes all audible sounds except toothchattering Rapid clicking produced when lower incisors are vibrated against upper incisors

Table III. Mean Total Frequency Scores and Standard Errors for Behaviour Performed during 10-min Tests (by 20 Pairs)

Lemmus Behaviour* Approach Retreat Autogroom Naso-nasal contact Naso-nasal (duration)++ Chase Flee Attack Bite Perineal investigation Digging Digging (duration)++ Vocalization

Mean 28 . 3 26.7 1 .8 2 .6 0.1 2 .6 1 .3 0. 4 4.7 20 . 0 18 .7

(±sE)

Dicrostonyx Mean

(2 . 9) 14 . 1 (3 . 3) 16 . 7 (0 . 6) 2.6 2 .4 (0 .6) 0 . 6 (0 .8) (1 .0) 0. 1 (0 .1) 2 .6 (0 .9) 7.1 (0 .7) 1 .0 (0 . 1) 0.1 (1 .0) 2.3 (4 .7) 9.0 17 . 2 (4 . 1)

(±sE)

Pt

(2 . 5) (2 .8) (0 .5)

P < 0 .001 P < 0 .05

(0 . 1) (1 .0) (1 . 0) (0 . 4) (0 . 1) (0 . 6) (2 . 9) (7 . 8)

P < 0 . 005 P < 0 .005 P < 0 . 005

*Unless otherwise indicated, scores represent frequencies . tBased on Wilcoxon two-sample tests . $In seconds.

NS

Ns

P < 0 . 05 Ns NS

P < 0 . 05

BANKS ET AL . : INTERSPECIFIC AGGRESSION species is commonly an important criterion in the evaluation of dominance in an interacting pair. In the present case, it may be that the encounter period of 10 min was simply too brief to permit the animals to establish a well-defined dominance-subordinance relationship . However, intraspecific dyadic encounters staged for a 10-min period proved to be of adequate duration for the establishment of dominance among conspecific pairs of each species (Allin & Banks 1968 ; Banks & Popham 1975) . And, on the very rare occasions when a spontaneous intraspecific encounter was observed in the field between Lemmus males, the action was of short duration with one or both individuals leaving the site of conflict within 2 min (Banks, unpublished observations) . Sequential Analysis Procedure. The dyadic encounters involved a substantial amount of activity on the part of both contestants . Although the behaviour frequencies reported in Table III provide a rough idea of what transpired, they provide no information as to the sequential pattern of the behaviours . To obtain such data, 16 of the encounters were transcribed into behavioural sequence form . This entailed assigning a single letter code for each behaviour in the animal's repertory . For each transcription and analysis, the sequences were constrained to alternate strictly between animal 1 and 2 . Thus the ordering of behaviours in an encounter was always 121212 . . . with each animal's behaviour being represented by one of the letter codes available in the joint repertory . In reality, however, a dyadic encounter does not involve a game of trade with discrete units but is a continuum of action in two interacting animals . In order to approximate this situation while maintaining simple data structure, no attempt was made to reflect time in the sequences . Behaviours were only noted when a transition occurred . Thus, when one animal switched behaviours from B to C and the other remained constant at A, the resulting sequence appeared as . . .CABA . . even though no change of state occurred in one animal . This does not severely limit the applicability of the representation as long as this convention is considered during interpretation of the results . Within the sequential analysis catalogue there is a behaviour designated pause (PAU) that is not detailed in the descriptions in Table II . This behaviour has been added in order to represent

1017

accurately multiple repetitions of a behavioural act. This is necessary when the record of the encounter shows a sequence such as . . .ARAR . . In this example animal 2 is retreating when approached by animal 1 . According to our convention some change of state must have occurred in order to record four behaviours rather than two . Upon closer examination of both the record and the experimenter's use of the recorder it was discovered that certain seemingly continuous behaviours contain an implicit pause before the next bout of that behaviour can occur . Thus, when given a record showing an animal approaching twice, the original action included a pause before the second approach . This use of an implicit behaviour allows the experimenter to restrict the recording of behaviour and glean only the frequencies, latencies, and durations that are desired . All of the sequential analyses were performed by computer . The programs used are written in PL/I or FORTRAN IV for the IBM 360/75 or the Cyber 175, respectively . The computer programs are available upon request from NJM . Results and Discussion The results of the sequential analyses are shown as conditional probability transition matrices in Tables IV and V . To facilitate comparison of the two matrices, the cell values have been converted into conditional probabilities . If I represents the row act and J the column act, then each cell value in this matrix indicates the probability of animal 2 performing act J given that animal I is engaged in act I . This was calculated by dividing each cell in the transition matrix by its row sum. The row and column sums are also shown to provide an indication of the absolute frequencies of the behaviours . These tables can be viewed as a concise description of the 16 encounters . A certain amount of caution is necessary in the interpretation of these matrices . As mentioned above, the processing of sequential data is an attempt to perceive underlying patterns in a group of contiguous behaviours . Therefore much information is necessarily discarded in this simplification . Table IV shows only the Lemmus to Dicrostonyx transitions and Table V counts only those from Dicrostonyx to Lemmus . Earlier, we suggested that the behavioural strategies used by the two species differed . Lemmus was characterized as more probing, whereas Dicrostonyx was more prone to launch an attack when approached by Lemmus . Some of



ANIMAL BEHAVIOUR, 27, 4

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BANKS ET AL. : INTERSPECIFIC AGGRESSION

the trends disclosed by the sequential analysis (Tables IV and V) support this contention . For example, approach (APR) by Lemmus led to attack (ATK) by Dicrostonyx with a higher probability than the converse situation in which Dicrostonyx approached . Approach by Dicrostonyx was more likely to lead to retreat (RET) by Lemmus . Similarly, when Dicrostonyx retreats, Lemmus was likely to approach or chase (CHA), whereas retreat by Lemmus was more likely to be followed by a pause (PAU) by Dicrostonyx .

Another distinct difference in agonistic strategies between the two species can be seen in sequences involving naso-nasal contacts (NSN) . Naso-nasal contact is a bout of behaviour in which both animals participated simultaneously. This can be seen in both matrices by noting the high probability of naso-nasal contact given that the other animal was also engaged in this act. Therefore, any incidence of a non-nasonasal contact behaviour following a bout may be considered a contact-breaking behaviour . Cast in this light, the species differences were readily apparent . Examination of corresponding rows in the matrices shows Lemmus with a propensity to break contact by retreating whereas Dicrostonyx initiated an attack . Differences in strategies were even more apparent in sequences involving chase and attack . Chase, when initiated by Lemmus, was followed by Dicrostonyx fleeing (FLE), retreating, or vocalizing (VOC) . Table V shows that the reciprocal case with Dicrostonyx chasing was never observed. Attacks by Dicrostonyx were usually followed by a Lemmus attack whereas Lemmus attacks were followed by Dicrostonyx retreating, vocalizing, or attacking with approximately equal probability . Sequences involving biting (BIT) suggested Lemmus was more likely to return a bite when bitten by Dicrostonyx. On the other hand, when bitten by Lemmus, Dicrostonyx was more apt to retreat . The foregoing observations indicate that the strategies outlined in the discussion of experiment 1 were accurate, simple descriptions of the respective species behaviour repertories. Lemmus retained its tendency to initiate momentary contact while Dicrostonyx seemed more prone to launch an attack . However, once a fight had been initiated, Lemmus was more likely to return attacks and bites than was Dicrostonyx .

1 01 9

Experiment 2 : Competition for Shelter Methods Subjects. Twenty-two male Dicrostonyx and 22 male Lemmus were used in this experiment . All animals were of the same stock described in experiment 1 . The breeding schedule was slightly different ; litters were not culled . At weaning, littermates were separated into groups of two or three . These sibling groups were broken up at 28 days, and individuals were kept in isolation until the start of the experiment . Apparatus. The arena used was identical to that in experiment 1 . The floor was covered with 2 cm of bedding material . A nest box was provided that was constructed from a metal can 10 . 0 cm in diameter and 13 . 5 cm high . The can was weighted and placed, open end down, near the arena wall. An entrance hole, 6 .0 cm high and 5 . 5 cm wide, faced the arena centre . The nest box was provided with a small amount of cotton batting . Water was available from a suspended bottle and an ample amount of food (Rabbit Chow) was always available on the arena floor . Procedure. Prior to testing, all animals between the ages of 90 and 150 days were weighed . Animals were matched within 10 g of each other . The observation cage and nest box were washed with soap and water and then rinsed with methyl alcohol prior to each test . The arena was then set up as described above. The animal designated as the resident in each pair was placed in the arena 24 h before the intruder animal was introduced . At the same time that the resident was put into the arena, the intruder was handled and returned to its cage. The observation session began when the intruder was placed in the arena . The number of approaches and nest entries of both the intruder and resident were noted for 2 min . The location of each animal was then recorded every 10 min for I h and once every hour until 8 h had elapsed . Two replications were run concurrently with the observations from the second arena lagging by 2 min . This procedure was followed for 22 pairs of animals . Half of these replications had Lemmus as the resident ; in the other half Dicrostonyx was resident . Occupancy scores were calculated from the observations for both of the sampling periods (hour 1 and hours 1 to 8) . These scores were computed by counting the number of times the resident was observed in the nest box and dividing by the total number of observations . Only those observations with an animal in the nest



1 020

ANIMAL BEHAVIOUR, 27, 4

box were counted . Observations with both animals outside of the box were judged as contests with no outcome and were excluded . For example, if 6 of 8 observations showed one of the animals in the box and, of these, only 4 were the resident animal, the score would be 100 x 4/6 or 67 . These scores reflect the percentage of resident animal occupancy observed . Results The results of a two-way analysis of variance of occupancy scores appears in Table VI for the 10-min samples and in Table VII for the hourly samples . As described above, occupancy scores for a group of subjects were calculated at each sample time . Thus, the group is the unit of experimental interest, and group scores over the time period represent repeated measures . This feature of the design and the presence of many subjects per group forces us to interpret the ANOVA conservatively . In addition, because groups are not replicated in treatments, we are unable to test the species x time interaction . Consequently we are led to consider only the most highly significant main effects . Discussion The conclusion drawn from the 10-min sample ANOVA in Table VI is that the occupancy score of an animal depended neither on the duration of stay nor on the species of the resident. If this score were not dependent upon resident or intruder status, the expected mean score would be 50 . 0. This was not the case, and we conclude that the mean occupancy score of 90 . 5 indicates that the important characteristic determining occupancy of the shelter during the first hour was prior residence . Significant differences in shelter possession between resident and intruder are documented in Table VIII . The ANOVA for hours 1 to 8 (Table VII) presents a more complex picture . In this analysis both the species of the resident and

the elapsed time from introduction of the intruder were important . The elapsed time effect was barely significant at the a = 0 .05 level, and, as noted earlier, may represent a repeated measure artifact . The species effect was highly significant at the a = 0 . 001 level, and we therefore consider it to be genuine . The basic species differences in the occupancy scores were indicated by the mean score for Lemmus of 84 . 1 and that for Dicrostonyx of 67 .9 . In addition, Dicrostonyx exhibited a slightly more pronounced decrease in occupancy over the eight hours than that shown by Lemmus . General Discussion The purpose of these experiments was to test the hypothesis that one of the species shows higher levels of agonistic behaviour, and that as a consequence, one could both dominate the other in interspecific encounters and exclude it from a shelter. Our data failed to show such a clear-cut relationship . The results of experiment 1 indicate that Lemmus males are highly investigatory and assertive . Although less prone to initiate an attack, Lemmus is more apt to bite when bitten than is Dicrostonyx . Dicrostonyx, on the other Table VU . Occupancy Scores ; Hourly Samples : Hours 1-8 ; Two-way ANOVA Source of variation

SS

df

F

Species Time

11 670 . 0 3050.6

1 1

19 . 8261 5 . 183

Regression Error Total

14354-8 7652 . 1 22006 . 9

2 13 15

12 . 1941

*P < 0 .05 . 1P < 0 . 001 . Table VIII. Tabulated Counts and X2 Values for 22 Shelter Competition Encounters* Animal in box Resident species Resident

Table VI. Occupancy Scores ; 10-min Samples : Six Observations in 1 h ; Two-way ANOVA Source of variation Species Tim-, Regression Error Total *NS

Lemmus Dicrostonyx

Intruder

X 2f

10-min samples (hour 1) 42 3 32 .089$ 53 7 33 .750$ Hourly samples (hours I to 8) 58 11 30 . 667$ 56 28 8 . 679$

SS

df

F

614 . 7 404 . 9

1 1

2.247 1 .480

Ns* NS

Lemmus Dicrostonyx

1057 .8 2462 .3 3520 . 1

2 9 11

1 . 933

Ns

*The cell values indicate the total number of times that a particular species, acting as resident or intruder, was observed in the nest box during the sample period . 1Chi-square with Yates correction for continuity . $P < 0 .01 .

not statistically significant .

BANKS ET AL . : INTERSPECIFIC AGGRESSION

hand, avoids initiating contacts, but once contacted, is more prone to launch an attack . Just how these differences in agonistic strategies may affect these species' competitive ability in the field remains an open question . If Lemmus is capable of out-competing Dicrostonyx for limited resources, this capacity is but weakly demonstrated in the shelter experiment . Neither species maintained exclusive use of the shelter when this resource was provided as a focal point of competition . The differences in agonistic behaviour that emerged from this study were unpredicted and became clear only because of the care exercised in observing and recording the encounters . One wonders whether reports of interspecific aggression and outcomes of dyadic encounters in which other rodent species have been used are as meaningful as they might be . Tabulations of won/loss frequencies inevitably hide the behaviours used and the sequences in which component acts are displayed. The population density of the two species in areas of sympatry may ultimately be ascribed to habitat and dietary factors . Our data suggest that the role of interspecific behaviour in the population dynamics seems worth pursuing . Banks & Fox (1968) reported a study of the interspecific aggression of the lemming, D . groenlandicus, and the vole, Microtus pennsylvanicus, living sympatrically in the area of Churchill, Manitoba . It was found that the outcomes of interspecific dyadic encounters of newly trapped males depended, in part, on the local habitat from which the animals had been trapped . Thus, although lemmings tended generally to win over voles, when vole contestants trapped from optimal vole habitat, and the lemmings from suboptimal lemming habitat, fought, voles won over lemming opponents . It appeared that quite subtle interactions of behaviour and habitat were at work . Enclosure studies of the type described in Grant (1972) are likely to shed light on this problem . Acknowledgments Support from NSF BNS 76-82029 is gratefully acknowledged. Computing services for this research were provided by the Computing Services Office at Urbana-Champaign . We would also like to

1021

thank A . W . Ghent and R . B. Selander for statistical advice . REFERENCES Allin, J . T. & Banks, E. M . 1968 . Behavioural biology of the collared lemming Dicrostonyx groenlandicus (Trail]) : I. agonistic behaviour. Anim. Behav., 16, 245-262. Banks, E. M . 1968 . Behavioural biology of the collared lemming Dicrostonyx groenlandicus (Traill) : II . sexual behaviour . Anim. Behav ., 16, 263-270 . Banks, E . M . & Fox, S. F. 1968 . Relative aggression of two sympatric rodents : a preliminary report . Commun. Behav. Biol., 1, Pt. A ., 51-58 . Banks, E. M . & Popham, T. 1975 . Intraspecific agonistic behavior of captive brown lemmings, Lemmus trimucronatus. J. Mammal., 56, 514-516 . Batzli, G . O . 1975 . The role of small mammals in arctic ecosystems . In : Small Mammals: Their Productivity and Population Dynamics (Ed . by F . B . Golley, K . Petrusewicz, & L. Ryszkowski), pp. 243-268 . Cambridge : Cambridge University Press . Brooks, R . J. & Banks, E . M . 1973 . Behavioural biology of the collared lemming [Dicrostonyx groenlandicus (Traill)] : an analysis of acoustic communication. Anim. Behav. Monogr., 6,1-83 . Brown, J. H . 1971 . Mechanisms of competitive exclusion between two species of chipmunks . Ecology, 52, 305-311 . Chappell, M . A . 1978. Behavioral factors in the altitudinal zonation of chipmunks (Eutamias) . Ecology, 59, 565-579 . Feist, D . 1975 . Population studies of lemmings in the coastal tundra of Prudhoe Bay, Alaska . Biol. papers of the University of Alaska, Special Report No . 2, 135-143 . Ghent, A . W. 1974 . Theory and application of some nonparametric statistics. II . normal approximations to the Wilcoxon two-sample and pairedsample tests, and two related tests . The Biologist, 56,1-31 . Grant, P . R . 1972 . Interspecific competition among rodents . Ann. Rev. Ecol. Syst., 3, 79-106. Hall, E. R . & Kelson, K. R. 1959 . The Mammals of North America. New York : The Ronald Press . Heller, H . C . 1971 . Altitudinal zonation of chipmunks (Eutamias) : interspecies aggression . Ecology, 52, 312-319 . Krebs, C . J . 1964 . The lemming cycle at Baker Lake, Northwest Territories during 1959-62. Arctic Institute North . Amer. Tech . Paper No. 15 . Pitelka, F . A. 1973 . Cyclic Pattern in Lemming Populations near Barrow, Alaska (Ed. by M . E . Britton), pp . 199-215 . Arctic Institute of North America Technical Paper No . 25 . Sheppard, D . H. 1971 . Competition between two chipmunk species (Eutamias). Ecology, 52, 320-329. Thompson, D . Q. 1955 . The role of food and cover in populations of the brown lemming at Point Barrow, Alaska. Trans . North. Am . Wildlf. Conf., 20,166-176 . (Received 7 June 1978 ; revised 10 November 1978 ; MS. number: A2179)