Anita. Behav., 1973, 21, 579-584

SOCIAL VARIETY IN THE YELLOW-BELLIED MARMOT

(MARMOTA FLA VIVENTRIS)

BY DAVID P. BARASH* Biology Department, State University College, Oneonta, New York 13820

Abstract. The social behaviour of high and medium elevation colonies of yellow-bellied marmots in Rocky Mountain National Park, Colorado, were studied. The medium elevation animals were con- sistently larger, exhibiting greater physical spacing, lower frequencies of greeting and upright play- fighting, and a higher agonistic component than did those of the more closely-integrated high-elevation colony. These results are interpreted as part of a complex series of adaptations within the genus Mar- mota to variations in ecology related to growing season.

Previous studies of marmot behaviour (Barash 1973) have suggested a correlation of growing season and sociality, considering the woodchuck (Marmota monax), yellow-bellied marmot (M. flaviventris) and Olympic marmot (M. olympus). Thus, the woodchuck inhabits a long-growing- season environment, matures and disperses at an early age, and is basically solitary and aggressive (Grizzell 1955; de Vos & GiUespie 1960). By contrast, the Olympic marmot inhabits sub-alpine and alpine meadows with an extremely short growing-season, matures and disperses quite late (during the third year) and is colonial and relatively tolerant (Barash 1973). The yellow-bellied marmot at inter- mediate elevations in the Rocky Mountains exhibits correspondingly intermediate ages of maturation and dispersal and an intermediate condition of moderate aggressiveness and social- ity (Armitage 1962). I further tested this system by studying yellow-bellied marmot colonies at both high and medium elevations in Rocky Mountain National Park, Colorado. I hoped to determine whether the interspecific correlation of species ecology and behaviour described above applies intraspecifically, between yellow- bellied marmot populations living at different elevations. The value of such studies of intra- specific variations in social organization has recently been emphasized by Crook (1970).

Methods The primary study areas were two marmot colonies, one located in the central meadow of Moraine Park, at an elevation of 2650 m (referred to as the medium-elevation colony) and the other �89 km from Trail Ridge Road, along Tombstone Ridge at an elevation of 3850 m (referred to as the high-elevation or alpine colony). Animals at both study areas were *Present address: Psychology Department, University of Washington, Seattle, Washington 98195, U.S.A.

trapped with live traps manufactured by the National Live Trap Corp., Tomahawk, Wis- consin, using peanut butter as bait. Following trapping, they were sexed, marked with dis- tinctive patterns of ear notches, weighed and released within 3 min.

Observations were made from convenient locations within each meadow, generally within 25 to 100 m of the subjects. I used both binocu- lars and 20 • spotting scope, transcribing data directly into field notebooks. The study lasted from 2 June to 15 August 1971, during which 327 hr of direct field observations were obtained. Since yellow-bellied marmots tend to be inactive in the early afternoon (Armitage 1962) a valid comparison of two colonies requires that ob- servations from noon to 15.00 hours (the relatively inactive period) be comparably dis- tributed for both colonies. Such observations occupied 18 hr (12 per cent) at the medium and 24 hr (14 per cent) at the high elevation colony. Pelage differences permitted individual recog- nition at a distance without use of artificial marking.

Results The medium-elevation colony was composed of two adult males, three adult females, three yearlings and two litters of four young each. The high-elevation colony consisted of one adult male, two adult females, two yearlings and one litter of three young. Despite different total numbers, the two colonies were similar in proportional make up (Table I). Young ap- peared at both colonies 9 to 12 July and no mortality or dispersal of any animals occurred during the study period. However, two yearlings (presumed dispersers from another colony) spent several days at the periphery of the middle elevation colony in early July. For all age and sex classes, the medium-elevation animals were

579

580 A N I M A L B E H A V I O U R , 21, 3

Table I. Percentage of Individuals in Each Class

Adult male Adult female Yearlings Young Total no. ind.

High elevation 12.5 25 25 37 8

Medium elevation 12-5 18.75 18.75 50 16

consistently heavier than their alpine counter- parts (Fig. 1), probably reflecting different growing seasons in the two environments.

While the two colonies were similar in number and kinds of inhabitants, their social organ- izations were quite different. Thus, the alpine colony was a closely-integrated social unit while the medium-elevation animals were loosely organized, spatially as well as socially. The high- elevation colony consisted of only two resident burrows: one occupied by the parous female and her litter and the other containing the remaining four animals. The main entrances to these burrows were 14-m apart and most social interactions occurred within an approximate elipse with these two points as epi-centres and an area of 300 m2. By contrast, the medium-elevation colony occupied seven appar- ently distinct resident burrows covering an area

exceeding 5000 m 2. Each adult and one yearling occupied a separate burrow while two yearlings shared a burrow. A region of 15 m 2 immediately surrounding the animal 's resident burrow was always included within its home range, and that region enjoyed minimum overlap with the ranges of other animals. Thus, I observed fourteen greetings (Armitage 1962) between residents of the medium-elevation colony, of which only three (21 per cent) occurred on these core areas. These data are particularly striking, since much of an animal 's time was spent near its resident burrow.

The two adult males at the medium-elevation colony occupied non-overlapping home ranges of about 2000 m 2 each. These may have been defended territories, since I observed only three cases of a male intruding upon another 's range, and in all cases the resident male immediately

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med ium high medium high medium high elevation elevation elevation

A D U L T A D U L T Y E A R L I N G MALES FEMALES MALES

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medium high medium high medium high elevation elevation elevation

Y E A R L I N G YOUNG YOUNG FEMALES MALES FEMALE~

Fig. 1. Comparative weights of all medium and high elevation animals. Based on means of two weighings for each animal during each month (August data represent only days 1 to 15).

BAR.ASH: SOCIAL VARIETY IN THE YELLOW-BELLIED M A R M O T 581

chased the intruder, who retreated to its range. The adult males wandered greatly, however, with mutual avoidance replacing territorial aggression in the remaining, apparently neutral, meadow region. Home ranges of the adult females over- lapped extensively and were poorly-defined due to infrequent use of the periphery. Immediately following parturition, the home ranges of the two parous females decreased as they spent more time in the immediate vicinity of their resident burrows, while the range of the non- parous females increased accordingly. Within 1 week, the parous females had resumed their original home ranges while the non-parous female also returned to her previous range. These observations suggest that interactions between the individuals concerned, either direct aggress- ion or passive avoidance, contributed to the determination of home ranges in the adult females at the medium-elevation colony.

Similar factors were implicated in the deter- mination of yearling home ranges. Thus, the one yearling female occupying a marshy, dis- tinctly sub-optimal home range was also clearly the most subordinate. It was involved in forty- six brief chases with adults and the other year- tings in which it was always being chased, never the chaser. The two yearlings (one male, one female) sharing a resident burrow also shared a small range in better habitat. On one occasion, these two animals appeared to co-operate in chasing the marsh-dwelling yearling from the core area surrounding their resident burrow. Figure 2 presents a schematic diagram of the home range relationships at the medium- elevation colony. The situation at the alpine colony was strikingly different: I was unable to identify ranges for any one animal, distinct from that of any other. The entire feeding area was a free region in which no spatial restrictive- ness was apparent for any animal

The medium-elevation colony accordingly experienced greater individual spacing than did the high-elevation colony. I recorded the position of all visible individuals at each colony on an enlarged photograph of the colony. collecting data every 10 min during a 5-hr period at each colony in mid-July. By con- necting the points so as to produce a figure of minimum length, measuring that length and dividing by one less than the number of individ- uals present in each case, a measure of linear distance between individuals of the two colonies was obtained. Through this technique, the alpine colony averaged a mean inter-individual dis-

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Fig. 2. Schematic representation of the home ranges of all animals at the medium-elevation colony. The black dot within each range represents the most commonly-used entrance to each residence burrow. Note the lack of overlap in the region of these entrance~ and between the ranges of the two adult males.

tance of 6.2 m while the medium-elevation colony averaged 14.4 m. Treating each measure- ment as a separate datum, the differences were significant (P<0.01, t-tes 0.

Dominance relations were well-established at the medium-elevation colony, but virtually non-existent at the higher altitude. Even when medium-elevation animals experienced over- lapping home ranges, one animal was clearly dominant. Dominance was often expressed by passive avoidance on the part of the subordinate, resulting in temporal distinctions in home range occupancy. On four occasions I observed direct encounters between two adult females in over- lapping parts of their ranges. In all cases the dominant animal was clearly victorious, twice- immediately chasing the subordinate into an unshared part of the latter's range, once initiating an attempted mounting and once a prolonged grooming (about 10 s) of the subordinate's neck and back. Both these latter behaviours also re- sulted in retreat by the subordinate, although the dominant female did not chase her. A dom- inant animal (the adult males excepted) usually permitted intruders within its home range and agonistic behaviour was generally initiated only when the animals were within about I0 m

582 A N I M A L B E I - - I A V I O U R , 2 1 , 3

of each other. By contrast, the alpine animals were often seen feeding alongside each other.

Animals of the medium-elevation colony were invariably dominant within a few metres of their burrow. Thus, the most subordinate female once briefly chased an adult male who strayed near her burrow. By contrast, the social status of animals at the high-elevation colony was not affected by proximity to their resident burrow. On eleven occasions I observed animals entering burrows in which they were not resid- ent, six times with the resident just outside. By contrast, I never observed a medium-elevation animal enter a burrow in which another animal was resident (excepting the mother- young and yearling-yearling association).

The greater social intolerance characteristic of the medium-elevation colony was reflected in several other aspects of social behaviour at the two colonies. Yellow-bellied marmots char- acteristically engage in oral greeting behaviour, presumably involved in some degree in indi- vidual recogntion (Armitage 1962). As such, it provides a convenient index of sociality. The relative frequencies of such greetings in the two colonies is presented in Fig. 3, and indicates a significantly higher greeting levet in the high- altitude colony (P<0.01, t-test).

Table II details the involvement of each class

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Fig. 3. Comparative greeting frequencies at the medium- and high-elevation colonies. Based on the following number of observation hours:

June July August

1-15 16-30 1-15 16-31 1-15 in these greetings. The adult males at both colon- ies consistently contributed more than their Medium elevation 29 33 share to the total number of greetings observed, High elevation 32 38

Table H. Percentage Involvement in Greetings of Individuals in Each Class

26 25 40 30 40 34

June July August Total

1-15 16-30 1-15 16-31 1-15

Adult male High elevation 42 (.1.29.5) 56 (.1.33.5) 35 (,1,22.5) 25 (.1.12.5) 19 (,1,6.5) 39 (.1.26.5) Medium elevation 50 (.1.37-5 44 (.1.3t'5) 30 (.1.27-5) 12-5 (0) 17 (.1.4.5) 28 (,1,1,15"5)

Adult female High elevation 25 (0) 14 (--11) 23 (--2) 22 (--3) 19 (--6) 20 (--5) Medium elevation 0 (--18.75) 37 (.1.18.25) 20 (--1.25) 32 (,1,13.25) 33 (,1,14.25) 29 (.1.1.10"25)

Yearling High elevation 33 (+8) 30 (+5) 19 (--6) 19 (--6) 31 (+6) 27 (+2) Medium elevation 50 (,1,31.25) 19 (-t-0.25) 30 (.1.11.25) 12.5 (--6.25) 0 (--18.75) 17 (--33)

Young High elevation - - - - 23 (--14) 34 (--3) 31 (--56) 14 (--23) Medium elevation - - - - 20 (--30) 43 (--7) 50 (0) 26 (--24)

Parentheses indicate the difference between this scoge for each class and the representation of each class in the colony populations; thus, they indicate the difference between the observed greeting frequency for each class and the expected frequency, assuming that representation is proportional to numbers of individuals present (e.g. -t-8 for high-elevation yearlings 1 to 15 June was obtained by subtracting 25, the percentage of yearlings at the high elevation colony, from 33, the percentage of yearlng involvement in greetings during that interval).

BAR.ASH: SOCIAL VARIETY IN THE YELLOW-BELLIED MARMOT 583

while the young contributed less (in part be- cause they were not included in the June data). The two colonies did not suffer greatly in the relative involvement of different classes in greeting behaviour, except that yearlings at the medium elevation colony were involved in pro- portionately fewer greetings than were yearlings at high elevation. This suggests that to some extent the reduced sociality at medium elevation reflects reduced integration of the yearlings in particular.

In addition to differences in greeting fre- quencies, the nature of the greetings themselves differed in the two colonies: at medium- elevation it involved a brief sniffing at the cheek of the animal being greeted, as described by Armitage (1962) for animals at an equivalent elevation m Wyoming. At high elevation, greeting often involved direct contact between open mouths with occasional tooth-locking, and was therefore more lengthy: twenty arbitrarily- chosen greetings at the medium-elevation colony averaged 1-9 s in duration (sD = 0-6), as com- pared to 3.3 s (so = 0.8) at the high-elevation colony. The difference was significant (P<0.01 ; t-test). This more prolonged contact may reflect greater social tolerance at the alpine colony.

Play involving adults or yearlings was com- mon among the high-altitude animals, most commonly manifested in upright play-fights. The participants sat on their haunches or occa- sionally stretched only on their hindlegs and engaged ventro-ventrally with another animal, either boxing with the forefeet or straining against each other, in an apparent effort to push the partner over on its back. Play at the medium- elevation colony was limited to young or mother and young, and never occurred among yearlings or adults as in the alpine colony, and never involved the upright posture. Play itself, and especially the upright posture with its concom- mitant prolonged individual contact, may also require a degree of social tolerance lacking in medium-elevation marmot society.

Grooming appeared to" reflect dominance relations at the medium-elevation colony. I observed five instances of grooming, in all cases with the dominant animal grooming the subordinate. Significantly, I did not observe any grooming at the high-elevation colony. Dominance relations were also indicated by mounting behaviour, with dominant animals mounting the subordinates. I observed eight such mountings at the medium-elevation colony and three mountings at the alpine colony. Of

the latter, two involved yearlings mounting the adult male and one, a yearling mounting an adult female; in neither situation did these behaviours at the high elevation colony reflect a dominance-subordinance relationship.

Chases, in which one animal aggressively chased another a distance exceeding several metres, occurred at both colonies. As shown in Fig. 4, the frequency was consistently higher at the medium-elevation colony and the differ ences are significant (P<0.05, t-test). This

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Fig. 4. Comparative frequencies of chases at the medium and high elevation colonies. Data based on the observa- tion regime reported for Fig. 3.

difference is particularly striking, given the generally lower level of social interaction at the medium-elevation colony. Furthermore, chases at the medium-elevation colony resulted either in the pursued animal entering its burrow or simply escaping, while 55 per cent of the chases observed at the alpine colony resulted in an upright play-fight or other seemingly playful behaviour. On three occasions I observed young at the medium-elevation colony being chased by older animals (once a yearling and twice by an adult female). No chasing of young by older animals occurred at the high elevation colony.

Table III shows the percentage involvement of each class in all observed chases. Despite the great difference in total chase frequency at the two colonies, the distribution of chases among the classes within each colony is quite similar. Thus, the adult males are the predominant chasers, contributing well over their share in both cases, while the yearlings at both colonies endure the largest proportion of chases.

584 A N I M A L B E H A V I O U R , 21 , 3

Table HI. Percentage Iatvolvement in Chases as Either Chaser or Chased for Individuals of Each Class in Each

Colony. Parentheses Calculated as for Table II

Chaser Chased

Adult High elevation 48 (+35.5) 4 (--8.5) male Medium elevation 46 (+33.5) 19 (+6-5)

Adult High elevation 29 (--4) 40(+ 15) female Medium elevation 31 (+12,25) 18 (--7.5)

Yearling High elevation 20 (--5) 52 (+27) Medium elevation 20 (--1.25) 58 (+39.25)

Young High elevation 3 (--34) 4 (--33) Medium elevation 3 (---47) 5 (---45)

By dividing the data from Table III by the number of individuals in each age class at each colony, a measure of the percentage involve- ment in all chases per individual in each class is obtained. In this way, for example, each adult female is seen to have been chased in 20 per cent of all chases observed at the high elevation colony (40 per cent/2) and 6 per cent of all chases observed at medium elevation (18 per cent/3). Similarly, each yearling was chased in 26 per cent of chases observed at high elevation, and 19 per cent at medium elevation. Chases were thus more equitably distributed between yearlings and adult females at the high elevation colony (26 per cent and 20 per cent respectively) than at the medium elevation colony (19 per cent and 6 per cent). This disparity, combined with the higher absolute number of chases at the medium elevation colony shown in Fig. 4, suggests a distinctly more aggressive treatment of yearlings at this colony.

D i s c u s s i o n

The two colonies of yellow-bellied marmots observed in this study supported my initial hypothesis: medium-elevation animals generally resembled the colony studied by Armitage (1962) and approached the woodchuck type in social organization relative to the high-elevation, alpine colony that approached the Olympic marmot type with comparatively high sociality and tolerance (Barash 1973). These intra- specific differences in social organization of M.

f laviventris might conceivably be explained as a direct response to physiographic differences between the high and medium-elevation habitats. For example, vegetation in the alpine meadow on Tombstone Ridge is visibly sparser th~n at Moraine Park. However, there is no evidence that feeding is restricted to certain limited areas, thereby necessitating greater sociality among the alpine inhabitants. Thi~ behavioural variety

could conceivably be a simple result of existing intercolony diversity (between colonies even at equivalent altitude). Thus, Armitage (1962) has found great year-to-year variation in ag- gressiveness within the same colony. Further- more, Downhower & Armitage (1971) have reported variation in harem size, although social interactions were not considered. Only further studies will reveal whether the correlation described here between altitude and social organization is a consistent phenomenon.

However, I believe the apparent altitudinal variation in behaviour of the yellow-bellied marmot is real and reflects a complex series of adaptations to different growing seasons. This parallels the interspecific variation from wood, chuck at low elevation, to yellow-bellied marmot at intermediate elevations, to the Olympic marmot at high elevations. Higher elevations experience shorter growing seasons, which result in retarded growth rate, delayed dispersal and sexual maturation, and a consequent evolution- ary advantage to social tolerance which enables prolonged mother-young contact and resulting greater size of dispersers. In addition, increasing- ly severe environments may necessitate increas- ingly precise control of local colony population size, and such control may be more efficiently maintained in socially tolerant and closely- organized colonies (Barash 1973). This theory will be treated in greater detail in subsequent publications.

A c k n o w l e d g m e n t s This research was supported by grants from The Research Foundation of the State University of New York, The Theodore Roosevelt Memorial Fund of The American Museum of Natural History and The Society of the Sigma Xi.

R E F E R E N C E S Armitage, K. B. (1962). Social behaviour of a colony

of the yellow-bellied marmot (Marmota flaviven- tris). Anim. Behav., 10, 3--4, 319-331.

Barash, D. (1973), The social biology of the Olympic marmot. Anita. Behav. Monog., 6, 171.

Crook, J. H. (1970). Social organization and the environ- ment. Anita. Behav., 18, 197-209.

Downhower, J. F. & Armitage, K. B. (1971). The yellow-bellied marmot and the evolution of polygamy. Am. Nat., 105 (944), 355-370.

Grizzell, R. A. (1955). A study of the southern wood- chuck, Marmota monax. Am. Midl. Nat., 53, 257-293.

Vos, A. de & Gillespie, I. (1960). A study of wood- chucks on an Ontario farm. Can. Fld Nat., 74, 140-145.

(Received 14 October 1971 ; revised 17 February 1972; second revision 18 May 1972; MS. number: A1245)