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The social system of the coyote (Canis latrans) in a forested habitat
F. MESSIER AND C. BARRETTE De'partement de biologie, Universite' Laval, Que'bec (Que'.), Canada G I K 7P4
Received July 3 1 , 198 1
MESSIER, F., and C. BARRETTE. 1982. The social system of the coyote (Canis latrans) in a forested habitat. Can. J. Zool. 60: 1743-1753.
We studied the social organization of forest-living coyotes (Canis latrans) for 20 months. The four breeding groups in our study area were territorial. The size and shape of their territories remained unchanged despite the sudden and profound change in prey distribution in December as white-tailed deer (Odocoileus virginianus) congregated yearly in a winter yard. Solitary adults lived on overlapping areas that ignored the breeding groups' territories. Some juveniles lived on their parents' territory but were not always associated with them. During the winter (November-April) 35% of the coyotes were in packs of three to five animals, 28% in pairs, and 37% solitary. Animals that were usually solitary almost never congregated to form temporary groups, and members of pairs were almost always together. We conclude that territoriality is essential to insure pup survival by increasing the foraging efficiency of parents that must feed sedentary pups. We suggest that individual and immediate advantage is sufficient to explain the late dispersal of pups resulting in the formation of packs. We therefore question the traditional view that larger group size in coyotes and other social carnivores living in extended families evolved to increase foraging efficiency.
MESSIER, F., et C. BARRETTE. 1982. The social system of the coyote (Canis latrans) in a forested habitat. Can. J. Zool. 60: 1743-1753.
Nous avons ktudik durant 20 mois I'organisation sociale du coyote (Canis latrans) vivant en milieu forestier. Les quatre groupes reproducteurs vivant dans notre aire d'dtude ktaient territoriaux. Les dimensions, localisations et formes de ces territoires sont demeurkes constantes meme si en ddcembre de chaque annde la distribution des proies a change considdrablement alors que les cerfs de Virginie (Odocoileus virginianus) se sont concentrds dans un ravage. Les adultes solitaires occupaient des aires non-exclusives qui ne respectaient par les frontikres des territoires des groupes reproducteurs. Certains juvdniles vivaient sur le territoire de leurs parents tout en &ant dissocids d'eux. Au cours de I'hiver 35% des coyotes vivaient en meutes de 3 a 5 animaux, 28% en paires, alors que 37% dtaient solitaires. Les membres des paires ne se sont presque jamais sdparks, et les coyotes habituellement solitaires ne se sont que trks rarement regroupds et alors seulement temporairement. Nous concluons que la territorialitd est essentiellement pour assurer la survie des chiots, en amiliorant I'efficacitd de la recherche de la nourriture par des parents qui doivent nourrir des chiots ddentaires. Nous croyons que les avantages individuels et immkdiats que retirent les jeunes suffisent a expliquer leur dispersion tardive, cette dernikre menant la formation de meutes. Nous contestons donc la vision traditionnelle qui veut que la taille des groupes chez le coyote et d'autres carnivores sociaux ait dvoluke dans le but d'assurer une plus grande efficacitd dans la recherche et la ddfense des proies.
Introduction Interest in animal social systems has expanded in the
past decade (Wilson 1975). A major objective of social system studies is to determine how external environ- mental forces interact with a species to mold its social structure (i. e. , breeding and feeding group composition, spatial organization, and dispersion patterns; Crook et al. 1976). Such studies have shown the adaptive value of a species' social structure, particularly in regard to resource exploitation and predator avoidance (Crook 1965, 1970; Eisenberg et al. 1972; Barash 1974; Clutton-Brock 1974; Jarman 1974; Geist 1974; Clutton- Brock and Harvey 1977; Mills 1978; and others). It is also becoming clear that a species is not typified by a unique social structure (e.g., Estes 1969; Kruuk 1972; Barash 1973; Monfort-Braham 1975; Armitage 1977; Macdonald 1977, 1979a). Plasticity in social structure is certainly evident in mammalian carnivores.
Although the environment constitutes the ultimate evolutionary force (Crook et al. 1976), the social system
is maintained by the social behavior of groups' mem- bers (Alexander 1974; West-Eberhard 1975). Many authors have emphasized that the evolution of social behavior among large carnivores reflects their proximate need for cooperative foraging through an increase of hunting success (Kleiman and Eisenberg 1973; Fox 1975; Kruuk 1975; Wilson 1975; Zimen 1976; Mills 1978) and (or) prey defence (Lamprecht 1978; Bekoff and Wells 1980; Bowen 1981).
It is not clear whether group formation evolves as a direct response to available prey size, or as a response to other selective pressures. Prey size may only facilitate the cohesion of groups formed for other reasons. This is relevant to an understanding of the social system of a medium-sized carnivore such as the coyote. The coyote, with its flexible social structure, can efficiently meet its energy requirements by feeding on a large range of prey, from small rodents to large ungulates (Bekoff 1977a). However, even if group size can be correlated with prey size (Bowen 198 I), cooperative hunting or
0008-4301 /82/071743-11$01 .OO/O 01982 National Research Council of Canada/Conseil national de recherches du Canada
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1744 CAN. I . ZOOL. VOL. 60, 1982
TABLE 1. Some of the data collected on the radio-collared coyotes
Total Social No. of Estimated home range size (km2) Coyote Age at Weight length group localiza-
No. Sex capture (kg) (cm) sizea Tracking periodb tions Pups Juveniles Adults
M Adult M Adult M Adult M Adult M Pup F Pup M Pup F Pup F Adult F Pup M Pup M Pup F Pup M Pup F Adult F Juvenile F Adult M Adult
15.0 130 - 23 Oct. 1976 - 25 Oct. 1976 14.5 125 3 20Nov. 1976- 9Dec. 1976 15.0 133 1 14Dec. 1976- 8Feb. 1977 15.3 141 2 18 Jun. 1977 - 29 Mar. 1978 4.5 99 - 12 Jul. 1977 4.9 95 1 18 Jul. 1977- 13Dec. 1977 4.9 99 1 19 Jul. 1977 - 14 Mar. 1978 4.2 93 1 19 Jul. 1977 - 4 Mar. 1978
13.4 132 1 24 Jul. 1977 - 2 Feb. 1978 4.4 96.5 1 27 Jul. 1977- 5Dec.1977 3.8 88.5 1 4Aug.1977- 1Feb.1978 8.3 114 5 12 Aug. 1977 - 28 Mar. 1978 5.2 93 5 16 Aug. 1977 - 28 Mar. 1978 6.8 101 - 16 Aug. 1977 - 18 Dec. 1977
12.1 139 2 18Aug.1977-30Mar.1978 10.8 125 - 5 Nov. 1977 - - 5 20 Jun. 1977 - 28 Mar. 1978 - - 2 3 Dec. 1977 - 30 Mar. 1978
"Size of social group determined by analysis of distinctive tracks during winter. bThe first date is the date of capture. 'Area used by coyote during tracking period; does not correspond to home range estimate (see text). dComputed from pooled localizations of all pups of the same family. Note that one group of pups occupied consecutively two distinct home ranges.
prey defence can be considered a prime driving force of coyote socialization only if we can demonstrate that a larger group results in an improvement of foraging efficiency in terms of kilograms of meat foraged (killed or scavenged) per unit time per member of a group.
In light of a 20-months-long field study on coyote behavioral ecology (Messier 1979) and a consideration of the current literature, we discuss the adaptive nature of socialization in coyotes. It is proposed that the co- operative-foraging hypothesis is not sufficient to explain the social system of the coyote or even other canids.
Methods Field work was conducted in a 155-km2 woodland area in
southern Quebec, Canada (70'25 ' W, 46'00' N). Access to the area was controlled. The topography consists of rolling hills typical of the Appalachian region. Annual precipitation averages 100 cm, of which 35% is snowfall. Snow accumula- tion frequently reaches 70 cm and covers the ground from mid-November to April. The study area is in the Great Lakes - St. Lawrence forest region, Eastern Townships section (Rowe 1972). Two important features of the study area are the absence of wolves (Canis lupus) and the presence of a 36-km2 white-tailed deer winter yard. Each winter, about 600 deer from surrounding areas find shelter in this yard from early December until April (Pichette 1979). Given the private land status of the study area, man-induced mortality upon the coyote population was very limited. From 1975 to 1978, only three coyotes were reported killed within the study area.
Field work was conducted from September 1976 to May
1978. Fifteen coyotes (Table 1) were equipped with radio collars. Radio tracking was conducted at ground level using four stationary tracking stations plus a portable hand antenna. The inherent error in the system, as determined by field testing, was less than 5'. This was considered sufficiently precise for the purpose of this study. We attempted to locate each coyote once a day during the field work. Eighteen coyotes, including the 15 radio-collared ones, were marked by foot-pad ablation which regularly permitted recognition of individual coyote tracks during winter. This proved invaluable in correlating the spatial organization with the group affiliation of such highly mobile animals, especially since it was impossible to make direct observations.
Group composition was obtained by track surveys. A predetermined circuit of logging roads, trails, and waterways was patrolled each day, weather permitting, during the two winters of the study. The purpose of these track surveys (n = 13 1) was to correlate space use with prey distribution. Each set of tracks was located on an aerial photographic mosaic. The chance that a given set of tracks, recorded as one group, was the result of more than one group passing at different times between two surveys was considered negligible given the frequency of the surveys and the relatively low coyote population.
Results Spatial organization
We located coyotes 869 times (56% from radio- telemetry and 44% from observation of marked tracks; Table 1). We analysed the spatial distribution of each
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MESSIER AND BARRETTE 1745
animal with regard to its group association and its %
breeding status. Each pair or pack occupied a distinctive territory (Fig.
1 a). Territories remained unaltered from June 1977 until March 1978 despite the drastic change in prey distribu- tion in December resulting from the formation of the deer yard. Analysis of 437.6 km of known tracks during the winter 1977-1978 revealed that the range of each group was limited by the range of the others and
5+
that scent marking constituted an important proximate ----- Llmll ol Ihe deer yard
mechanism of spacing (Barrette and Messier 1980). - P ~ b l l c road - LOQQlnQ road
Thus, reproductive groups clearly exhibited territorial a behavior.
The pattern of space use in solitary adults differed markedly from that of reproductive groups. As shown in Fig. 1 b for a female, the living area of solitary animals overlapped territories of groups, as well as the living area of other solitary animals. Furthermore, the two solitary adults tracked were nomadic, making frequent shifts in the areas they used (e.g., female No. 6-77, Fig. 1 b). Weak home-range fidelity is probably the rule for solitary adults and, therefore, the term "living area" is perhaps more appropriate than home range for these animals. It would certainly be misleading to draw a polygon enclosing all the points in Fig. 1 b and call that a home range.
The duration of parent-offspring bonds has a pro- found influence on the spatial organization of juveniles (6-20 months old). Of the eight juveniles successfully tracked, two (Nos. 9-77, 10-77) were still associated with their parents in March 1978 when they were approximately 11 months old. The area used by those juveniles corresponds closely to their parents' temtory (Fig. 2a); they only occasionally went on short excur- sions outside their parents' territory. In the few cases where we were able to observe their distinctive tracks, the juveniles travelled alone (two cases) or with other juveniles (presumably of the same litter) (two cases). Two juveniles remained within the parental territory without apparent social bonds with them. For example, male No. 4-77 was never observed with its parents after December 1977, but it intensively used the parental territory until the end of the study in March 1978 (Fig. 2b). After performing two excursions at least 8 km away from its home range, No. 4-77 returned to its parents' territory. Few short excursions were also observed for that individual. Juvenile No. 8-77 also left its parents' territory in February after 3 months of no apparent social bond with them (we never saw its tracks with its parents', and never located it with its parents by radio-telemetry during that time). Four juveniles (Nos. 3-77, 5-77, 7-77, 11-77) dispersed in December, presumably shortly after disruption of their social bonds with the family group. Juvenile No. 5-77 dispersed to the deer yard 3 km west of its native range and exhibited
O . .... ~errntoroal boundary of the lour breedlng Q ~ O U P ~ (eslabl from llgure la1
= public road
Ihe eslabl
FIG. 1 . (a) Localizations of two adult males (A, No. 1-77; and *, No. 15-77) and two adult females (0, No. 12-77; and I, No. 14-77), each a member of four different reproductive groups (see Table 1). (6) Localizations of an adult solitary female (No. 6-77); A , July and August; I, September and October; 0, November and December; *, January and February.
nomadic wanderings similar to the solitary adults. On 4 March 1978 it dispersed further west and we lost radio contact with it.
Ten pups (0-6 months) were fitted with radio collars in midsummer. They were members of three families as determined by radio locations. We consistently located the pups within a restricted area less than 0.4 km2 from July to October and, for one family, until November (see Fig. 2a). However, the parents used a much greater area and were frequently located away from the pups at these times. This separation may ensure more freedom for the adults until the pups acquire sufficient mobility or the necessary behavior required for hunting. We were able to identify only one restricted area used by the pups in two families, whereas the pups of the third family used first one area from 18 July to 5 August, and then shifted to a different one 6 km away. This is similar to rendez-vous sites in wolves (Joslin 1967).
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CAN. J. ZOOL. VOL. 60. 1982
A - - - Terrllortal boundary 01 the pup 5 .. . . . Terrnlornsl boundary ol the three
- other breedlng groups - Publlc road -
parents
- - - - ~err!toroal boundary 01 the pups Parents .. .. . ~err~torbal boundary ol Ine three ...- - olner breedlng groups =., ..... -.....- - Public road
b 3 .m -
FIG. 2. (a) Localizations of a pup (No. 9-77) which has maintained parental affiliation; 0, 12 August 1977 to 30 November 1977; A , 1 December 1977 to 28 March 1978. (b) Localizations of a pup (No. 4-77) for the period (from 1 November 1977 to 16 March 1978) when it was not affiliated with its parents, but did not disperse either.
Home range estimates "The area in which an animal normally lives, exclu-
sive of . . . unusual erratic wanderings, is commonly referred to as its home range" (Brown and Orians 1970, p. 240; Burt 1943). In computing home-range size we have discarded such erratic wanderings by using only 95% of all the localizations available for an animal. We also treated separately cases where animals occupied what we previously called a "living area" for a while, and then suddenly and repeatedly shifted to a new one.
We used the minimum convex polygon method (Mohr 1947) to delineate home ranges. That technique does not provide information on how the coyotes used their home ranges, but we are aware that this is an important component of space use behavior (see Ford and Krumme 1979). Home range estimate is a function of the number of localizations (Odum and Kuenzler 1955). A coyote exhibited home range fidelity, if the estimate increased rapidly and soon reached a plateau (e.g., No. 12-77, Fig. 3a). We assumed that the estimate represented adequately the size of a home range
1 SEP I H W I M C I J A N I F E B I M A R I
JUL I 70-
60 - 50 -
40 - 30-
20 - 1C).
AUC 1 SEP I 30-
25 -
20 - 15 -
10.
,x-X-X-X Pup-Juvenile r-X-X-X
,?.x,x ,
Number of successive localizations
FIG. 3 . (a) Home range estimate for an adult female (No. 12-77) member of a breeding pair, (b) a lone adult female (No. 6-77), and ( c ) a pup (No. 9-77) whose transition to the status of juvenile was monitored.
when 20 additional daily localizations resulted in an increase of less than 10% of the area used. Generally, this condition was satisfied within 50 daily localizations (see also Bowen 1982; Smith et al. 1981). For our solitary adult coyotes the home range estimate increased constantly with the number of localizations (e.g., No. 6-77, Fig. 3 b) and reached a value far greater than that observed for coyotes with home range fidelity (e.g., 69.0 krn2 for No. 6-77 compared with 14.3 km2 for No. 12-77). This is an artifact of the method used since we considered the cumulative localizations, including some areas that the animal no longer used (see Fig. l b for an example). That type of curve reveals that the animal frequently changed its living area, preventing an adequate estimation of its home range.
In pups, the area utilized increased slowly through the summer. But within a few weeks, generally in the fall, the index rapidly increased until it reached an asymptote
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MESSIER AND BARRETTE 1747
(e.g., No. 9-77, Fig. 3c), We believe that this marked change corresponds to the time when the pups begin to regularly accompany their parents. Although some individual differences occurred, namely in the timing of the greater mobility, the observation-area curve for four other young showed the same pattern as No. 9-77. The abrupt change in mobility of the young constitutes a useful and significant demarcation between pups and juveniles.
Home range estimates of two adult females (Nos. 12-77, 14-77) were 14.3 and 22.8 km2, respectively. The difference may be related to group size (two and five, respectively). Recently Bowen (1982) found a positive correlation between home range size and group size in coyotes. For three juveniles (Nos. 4-77, 9-77, 10-77), we obtained 36.7, 29.0, and 25.2 km2 as home range estimates. Finally, pups' home ranges were evalu- ated at 1.1, 2.0, 3 .O, and 4.0 km2 (see Table 1 for details). Table 1 also contains the size of area used for five additional coyotes for which the observation-area curve does not reach an asymptote.
Group composition Group composition was established from 1468 tracks
observed along a 118 krn permanent circuit (Messier 1979). The circuit was patrolled 13 1 times (sometimes partially) during two winters (mid-November-April) (1976- 1977 and 1977- 1978). We found that overall, 43% of the tracks were solitary, 34% were in pairs, and 23% in groups of three to five. There were no significant differences between years (p > 0.05). However, varia- tion in group composition did occur throughout the winter (Fig. 4). The frequency of groups of three or more tracks fell sharply from November to January (40 to 20%). At the same time the frequency of single tracks increased from 35 to 50%. We believe that this pattern represents a gradual separation of juveniles from family groups in early winter: of the five instances of dispersing juveniles monitored, four occurred in December and one in February. A second change was noticed just prior to the breeding season in February and early March (Kennelly 1978): the frequency of paired tracks in- creased suddenly from 23 to 40% between January and February, and remained at that level until the end of the winter. The drop of the single tracks suggests that new pairs were being formed.
Group stability was established from 181 sets of marked tracks; this was to verify if social grouping can be accurately deduced from track observations. Five known coyotes were consistently observed alone (98% of 58 observations). Thus, temporary aggregation of normally solitary coyotes, reported in Wyoming (Camen- zind 1978) was not common in our study area. Known pairs (n = 3) showed a high degree of stability as well, at least during winter: mates were observed together in
NOV. DEC. JAN FEE. MAR.
FIG. 4. Variations in group composition throughout the winter; . , solitary coyotes; X , pairs, , packs (three to five coy0 tes) .
96% of 9 1 observations. However, Andelt et al. (1979) reported frequent pair splitting in Nebraska, especially in spring and summer. We therefore postulate that solitary and paired coyotes produced solitary and paired tracks, respectively.
Packs appear to show a much greater flexibility in composition. For example, from observations of 32 tracks from one pack of 5 we computed a mean group size of 3.3 per observation (Fig. 5). (The pack contained three marked animals: one adult female, and two juveniles; and two unmarked ones: an adult male with a very distinctive track size whose sex was determined by observation of raised leg urinations typical of adult males (Wells and Bekoff 1981) and one coyote pre- sumed to be a juvenile based on a track size that was comparable to that of other juveniles.) It appears from this case that the degree of socialization of coyote families is underestimated when using track observa- tion; we knew this group to be 1.5 times greater than its size determined from track counts (5 +- 3.3). We computed very similar values from Camenzind's (1978) data, where the mean group size was derived from direct observations rather than tracks (1.5, 1.5, and 1.9 for known groups of 4-5, 5, and 6, respectively). Thus, applying a factor of 1.5 seems realistic in our case to obtain a correct picture of coyote packs from track surveys. On the other hand, temporary pack splittings result in a misleading increase in the frequency of single and paired tracks. We corrected for it as follows. Thirty- five percent of the coyotes were in packs (23% x 1 . 9 , but only 23% of the tracks were in packs. The 12% (35% - 23%) of tracks missing are those of pack members
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Adult female (1 4-77)
Adult male (unmarked)
Juven~le male (10-77)
Juven~le female (9-77)
Juven~le ( 7 ) (unmarked)
15 NOV 1 DEC 15 DEC l J A N 15JAN 1 FEE 15FEE 1 MAR 15MAR 1 APR
FIG. 5. Composition of one breeding group of five on each of the 32 occasions that its tracks were encountered. Each symbol represents a given coyote. Dotted lines link coyotes that were together on a given observation of tracks.
temporarily away from their pack, either alone or in two (we assume either possibility as equally likely). The groupings in our population during winter were there- fore evaluated as 35% in packs, 28% in pairs (34%- 6%), and 37% solitary (43% - 6%).
Group stability is generally affected by the size of prey exploited (Schoener 197 1 ). In winter (December- April), deer (killed or scavenged) composed nearly 80% of the diet of our coyotes (Messier 1979). With deer as a prey, a whole family can forage together. However, social factors may cause group disruptions. For ex- ample, the alpha pair in wolves shows a high level of aggressiveness toward other pack members during the breeding season (Zimen 1976). We suspect that the same was responsible for repeated separations of juveniles from the family in March (Fig. 5). Frequent family splittings in early winter may be related to the low availability of large prey. Furthermore, as pointed out earlier, juveniles may temporarily leave the family group to explore some areas away from their parents either within or even outside of the parents' territory. Obviously more data are needed to discern the specific role of social and environmental factors in family splitting. Camenzind (1 978) and Bowen (1 98 1) did observe a decrease of group stability during the summer when coyotes prey primarily on rodents and ungulate young. We were unable to quantify group stability in summer given the cryptic nature of coyotes in woodland areas. Although it is likely that the size of foraging groups decreases in the summer, this does not neces- sarily imply a change in the size of the social groups per se.
Discussion Spatial organization
The literature on coyote spatial organization is con- fusing. Ozoga and Harger (1966), Edwards (1975), and Danner (1976) observed a wide overlap between coyote
home ranges and did not consider coyotes territorial. However, none of those studies correlated space use with the social status of the animal, a prerequisite to understanding spatial organization. For example, we must make a clear distinction between solitary and group-living individuals. Berg and Chesness ( 1978) reported that only adult females exhibit territorial behavior and that they use considerably smaller home ranges than adult males (1 6 km2 compared with 68 km2). Once again, however, Berg and Chesness (1978) con- sidered neither the social context, nor the change of home range estimate with the number of localizations. In fact, it is unlikely that adult males and females show such a great difference in space-use behaviour as a strong pair bond persists throughout the year in coyotes (Kleiman 1977; Kleiman and Brady 1978; Andelt et al. 1979).
It is clear that in some areas, group-living coyotes use exclusive home ranges (Wyoming, Camenzind 1978; Alberta, Bowen 1982; Quebec, this study). In addition, our study of scent marking in coyotes (Barrette and Messier 1980) reinforces this statement; group-living coyotes mark much more and differently than solitary ones, a trait interpreted as a spacing-out behavior (see also Bowen and McTaggart Cowan 1980). Although not conclusively established, evidence of coyote terri- toriality is currently accumulating in other parts of the animal's range (Gipson and Sealander 1972; Witham 1977; Althoff 1978; Andelt and Gipson 1979).
Unlike mated pairs, our solitary animals used over- lapping living areas which encompassed many terri- tories. We have presented evidence that some solitary coyotes show home-range behavior (i .e., Nos. 4-77, 8-77), while others are nomadic (i.e., Nos. 4-76, 6-77, 5-77) after dispersal. Apparently, offspring even without noticeable family bonds, can preferentially use their parents' territory for an extended period. We believe that such offspring benefit by having a relatively secure place where to forage until they disperse. This is
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possible as long as parents can differentiate their offspring from intruders and be less aggressive toward them (Bowen 1978). This discrimination can be quite adaptive where the absence of large prey prevents group foraging, forcing an offspring to forage by itself before an appropriate time to disperse outside the parents' territory.
Nomadic animals frequently change their living area and may cover vast areas in relatively short periods. This type of space use is also well documented in solitary wolves (Mech and Frenzel 1971; Van Ballenberghe et al. 1975; Scott 1979). One can view this as a conse- quence of continual aggression from resident territorial animals. It may also be advantageous for a solitary animal to continue moving and monitoring new areas in search of a vacant territory. Nomadic wanderings may increase the probability of finding a mate as well. In summary, coyote spatial organization can be viewed as a two-level system. First, breeding groups fill a suitable habitat with contiguous territories. Superimposed on that, solitary animals use mutually overlapping home ranges or living areas that ignore the existing territories. Although this pattern is likely to be the rule, we do not exclude the possibility that some variance occurs. For example, the presence of an unusual food concentration such as a feedlot carcass dump can greatly influence spatial organization (Danner and Smith 1980; see also Mac- donald 1979~) . In this study, the yearly high concentra- tion of deer in the winter yard did not alter the spatial organization. Human disturbance by trapping or hunting may also distort the general pattern.
How and why are territories held? Barrette and Messier (1980) showed that scent-
marking is an important component of coyote territorial behavior. Scent-marking by territorial coyotes is quan- titatively and qualitatively different from that of nonter- ritorial animals. However, it is not obvious why intruders would obey scent marks. Some of our observa- tions indicated that scent marks can deter potential intruders from trespassing. However, retreat is not the only response; trespassing may also occur as Mech (1977a, 1977 b) and Carbyn (198 1) have shown for wolves, and Wells and Bekoff (1981) for coyotes. We may view territoriality as a conflict between rivals for a given resource (Maynard Smith and Price 1973), where scent-marking constitutes an important signal in a game in which an intruder has to consider the costs and benefits of trespassing (Dawkins and Krebs 1978). In other words, scent marks ". . . do not represent in and of themselves, barriers to movement" (Wells and Bekoff 1981). Instead, the response of the intruder to a scent mark depends on the risk of injury if a direct encounter occurs. However, since large carnivores such as wolves and coyotes range over wide areas and live at relatively
low densities, the frequency of encounters is probably quite low. Therefore, wolves and coyotes, for example, must demonstrate a high degree of aggressiveness during encounters, thereby maintaining a high risk of injury for a trespasser and enforcing obedience to scent marks. Indeed, Mech (1 977 b) reported many instances of fatal encounters in such circumstances in wolves. However, we would expect trespassing to occur if a great benefit results, such as a vital food acquisition during periods of food scarcity (Mech 1977 b), or the use of a carcass near a territorial boundary (Bowen 1978).
One may still ask: Why do breeding coyotes defend a territory? Davies (1978) argued that territoriality origi- nated from numerous evolutionary forces. Thus, it is not surprising to find many possible functions of territorial behavior. In general, the adaptive nature of territoriality can be related to (1) increased foraging efficiency (e.g., in fish and nectar-feeding birds, Dill 1978; Gass 1978, 1979; Pyke 1979), (2) controlled access to mates (Emlen and Oring 1977; Owen-Smith 1977), (3) defence of a food hoard vital in periods of food shortage (Smith 1968), and (4) improved offspring survival (e.g., by reducing the risks of predation; Tinbergen et al, 1967; Davies 1978). In coyotes, territoriality is restricted to breeding groups; this suggests that territoriality is related to reproduction, in particular to offspring sur- vival. Food availability does appear to be critical during the rearing season; breeding males and females captured in summer were always emaciated (F. Messier, personal observation), and pup malnutrition was reported in coyotes (Gier 1968), and is well known in wolves (Van Ballenberghe and Mech 1975). The fact that pups are fixed to a small area around a den for many months after birth greatly restricts adult wandering (Andelt et al. 1979). Hamilton and Watt (1970) stressed that anything which forces animals to forage radially from some more or less fixed point will have a great influence on social structure. Pup survival primarily depends on the ability of the parents to obtain food from the immediate surroundings of the den or the area where the pups are left during hunting. Therefore, exclusion of conspecific competitors from that area is probably advantageous. In addition, territoriality probably enhances food acquisi- tion during other periods of the year as well, but appears to be less critical.
One can argue that territoriality is not observed in solitary coyotes because they cannot economically defend (Brown 1964) an area against breeding pairs. However, we believe that territoriality in coyotes is not a prerequisite for individual survival since solitary coyotes, even nomadic ones, seem to meet their energy requirements. Three of the four solitary coyotes we necropsied had important subcutaneous fat deposits (the exception was a juvenile of eight months which is a normal situation). However, the burden of rearing
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sedentary pups makes a territory a prerequisite of breed- ing. In this view, territoriality in coyotes should be considered as a form of year-round parental investment to insure higher pup survival.
Coyote social structure Coyote group-living varies widely throughout its
range. The breeding pair constitutes the basic group as in most other canids (Kleiman and Eisenberg 1973; Kleiman 1977). However, the length of time that offspring remain with their parents influences social group size (Kleiman and Brady 1978). For example, in Rocky Mountain National Park (Colorado), young dispersed before or during their first winter, leaving the pair alone (Bekoff and Wells 1980). In contrast, Bowen (198 1) observed in Jasper National Park (Alberta) that nearly 60% of the coyotes lived in packs of three to eight related adults, yearlings, and nondependent young. Such pack formation was also reported near Jackson Hole, Wyoming (Camenzind 1978; Bekoff and Wells 1980). Our coyote population exhibited inter- mediate characteristics.
From a consideration of the current literature (e.g., Bekoff and Wells 1980; Bowen 1981), we are left with the impression that pack formation in coyotes evolved as an adaptation allowing more efficient capture and (or) defense of ungulate prey. This cooperative foraging hypothesis was seen by Kruuk (1975) as the main driving force of sociality in carnivores (see also Fox 1975; Wilson 1975; Zimen 1976; Mills 1978). This may be valid for some carnivores, particularly the Hyaenidae, but seems to us a weak evolutionary hypothesis to explain intraspecific variation of group size in Canidae (see Lamprecht 198 1).
First, no study to our knowledge was able to demon- strate that an increase in numbers of the social group leads to an increase in food intake per capita, a prerequisite for supporting the hypothesis of cooperative foraging. In fact, Nudds (1978) showed that the optimal foraging group size in wolves is far smaller than the observed social group size (see also Caraco and Wolf 1975). In coyotes, it is true that where packs form they rely primarily on large prey such as ungulates. But this can be an effect rather than a cause: an already existing group using the most appropriate prey size to ensure group cohesion.
A second flaw in the cooperative foraging hypothesis is that even if group living results in tangible gains in terms of food intake, nonbreeders cannot translate such gains directly into improved fitness since the breeding pair alone reproduces in the group (Bowen 1978; Camenzind 1978; Bekoff and Wells 1980; see also Packard and Mech 1980). They may, however, obtain some genetic benefits by assisting their parents in rearing pups of the subsequent litters. Although helping
has been observed in coyotes (Camenzind 1978; Bekoff and Wells 1980), it remains to be shown that it increases the number of mature offspring produced (see Bertram 1976; Rood 1978; Moehlman 1979; Frame et al. 1979; and Macdonald 1979b). But it may not be that simple either. Helping in canids may also have another func- tion.
The traditional view of helping in carnivores, best illustrated by Moehlman ( 1979, 198 l), is that by staying in the family a "helper" increases the survival of its younger siblings. The only alternative considered being that helpers have no effect on their younger siblings' survival. A likely alternative is that yearlings that stay with their parents without helping have a deleterious effect on pup survival since they compete with their parents for food. The advantage of helping would then be to compensate for that effect (see Rowley 1981).
Prey size and distribution (both spatially and tem- porally) undoubtedly have a profound influence on how a pack splits in a functional way to forage. Camenzind (1978), Bekoff and Wells (1980), and Bowen (1981) have presented convincing evidence and arguments on this. What we want to discuss here is why a social group gets larger than the optimal size for foraging.
Kleiman and Brady (1978) suggested that variation in coyote group size resulted primarily from differences in age at dispersal. Moreover, recent studies suggest that age at sexual maturity affects the timing of offspring dispersal (Gadgil 197 1 ; S tom and Montgomery 1975; Storm et al. 1976; Bekoff 1977b). In Kansas, Gier (1968) reported that up to 80% of the yearling females may breed. But that seems to be exceptional. Hilton (1978) in Maine, and Berg and Chesness (1978) in Minnesota found that none of the yearling females they autopsied had given birth in their first spring. Moreover, they presented evidence that the females did not show signs of oestrus during the breeding season. In Alberta, Nellis and Keith (1976) observed that the pregnancy rate of yearling females was only 14%. Although food supply can induce annual variations (Gier 1968; Knowl- ton 1972), we suspect that fewer females reproduce in the northern part of the coyotes' distribution. More data are needed, but if this is the case, group living in coyotes will be favored at higher latitudes since delayed maturity can mean delayed dispersal (Barash 1974).
Most importantly, population density may also result in delayed dispersal. At high densities the probability of finding a suitable and unoccupied territory for reproduc- tion is greatly reduced. It may be advantageous for an animal to stay within the family group for a longer period, thereby benefiting from a "safe place to live" and probably acquiring greater experience and competi- tive ability (Bekoff 1977b). Mech (1977b) found that wolf pack size can be correlated with wolf density. Rausch (1967) reported that pack size in Alaska in-
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creased steadily during a recovery of a wolf population to saturation level. The highest coyote sociality was reported in wildlife reserves where coyote densities were relatively high (Camenzind 1978; Bowen 1978). This suggests that a density-dependent process underlies dispersal strategies in juvenile coyotes, and therefore population density must be considered as an important variable to comprehend group living in coyotes (see also Brown 1974, 1978; Stacey 1979; Rowley 1981 for a discussion on habitat saturation and group living in birds).
In conclusion, we hypothesize that group living in coyotes (i.e., pack formation) results from delayed dispersal of juveniles which benefit individually from a secure place to forage and live, and probably from a higher survival rate until they disperse and initiate their own reproductive effort, or attain a breeding status within the group if one parent dies for example. This delayed dispersal can result from a lack of vacant territories (i.e., habitat saturation) or a higher age at sexual maturity. Large prey are considered to play only a secondary role by allowing or, more precisely, facilitat- ing group living. If the group can forage as one unit on large prey, it is likely that stronger bonds will persist between members of the group. Thus, large prey allow offspring to maintain bonds with their parents. Ob- viously, pack formation may result from these lengthy bonds, but we think it unlikely that the existence of groups larger than a breeding pair of coyotes evolved as a result of cooperative foraging benefits.
Acknowledgements Funding for this study was provided by a grant from
the Ministkre du Loisir, de la Chasse et de la Peche of Quebec. We are grateful to Domtar Inc., for allowing us to study coyotes on their private land. F.M. wishes to thank the Natural Sciences and Engineering Research Council of Canada for financial support. D. M. Shack- leton, F. L. Bunnell, D. Seip, and an anonymous reviewer provided useful comments on the manuscript. Finally, thanks are expressed to R. St-Pierre for helping with data processing.
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