Factors influencing the hunting success of an African wild dog pack

Transcription

1 Anim. Behay., 1993, 45, Factors influencing the hunting success of an African wild dog pack JOHN H. FANSHAWE* & CLARE D. FITZGIBBONt Serengeti Wildlife Research Centre, Box 661 Arusha, Tanzania (Received 18 October 1991; initial acceptance 29 November 1991; final acceptance 27 July 1992; MS. number: 3971 ) Abstract. The success of a pack of African wild dogs, Lycaon pictus, hunting Thomson's gazelles, Gazella thomsoni, and blue wildebeest, Connochaetes taurinus, in Serengeti National Park, Tanzania, was influenced by the age of the prey and the number of dogs hunting together, but not by the amount of cover available, the size of prey groups, or the distance at which prey groups fled. The study suggested two ways in which wild dogs may benefit from communal hunting. First, it increased the range of prey species available to the pack. Although single dogs regularly killed both immature and adult Thomson's gazelles, they were not observed to kill wildebeest calves, and groups of two did not hunt adult wildebeest successfully. Larger groups were more successful than smaller ones. Second, hunting in groups reduced in terspecific competition from spotted hyaenas, Crocuta crocuta, through improved defence of carcasses. Studies of carnivore hunting behaviour have tended to concentrate on determining the number and type of prey killed by such predators, and the degree to which predators preferentially select the old and weak members of prey populations (e.g. Hornocker 197; Mech 197; Fritz & Selander 1978; FitzGibbon & Fanshawe 1989). Relatively little information is available on the factors influencing hunting success, principally because of the difficulty of observing hunting behaviour under natural conditions. However, in the open plains habitat of Eastern and Southern Africa observations are comparatively easy and a number of predator-prey studies have been undertaken. For example, studies of lions, Panthera leo, and cheetahs, Acinonyxjubatus, both stalking predators which tend to rely on surprise for their success, have emphasized the importance of vegetation cover and moonlight, the type of prey and its level of alertness, and how close predators approach to prey groups before starting their final chases (Schaller 1967, 1972; Van Orsdal 1984; FitzGibbon 1988, 1989; Stander & Albon, in press). Although bunting behaviour and prey selection by the African wild dog, Lycaon pietus, a coursing predator which hunts communally, have been described (Estes & *Present address: Wildlife Conservation Research Unit, Department of Zoology, University of Oxford, South Parks Road, Oxford OX I 3PS, U.K. t Present address: Large Animal Research Group, Department of Zoology, University of Cambridge, Cambridge CB2 3E3, U.K. Goddard 1967; Kruuk & Turner 1967; Reich 1981; Fuller & Kat 199), few quantitative data examining the factors influencing its success are available. Interest in the hunting success of social predators, such as wild dogs and lions, in part stems from the theoretical question of whether the benefits of communal hunting could be an important evolutionary cause of sociality. While the improved ability of larger groups to capture and subdue their prey, and to defend kills from competitors, have been suggested as factors favouring the evolution of group-living (Estes & Goddard 1967; Kruuk 1972, 1975; Curio 1976; Lamprecht 1978, 1981; Cooper 1991; Stander & Albon, in press), a number of studies have concluded that communal hunting is a consequence of gregariousness rather than its evolutionary cause (Macdonald 1983; Packer 1986; Packer & Ruttan 1988). Further data are required on the influence of predator group size on prey detection, hunting success and the defence of carcasses from competitors before the issue can be settled. African wild dogs are one of the most social canids, forming packs of 2-5 animals (Frame et al. 1979; Malcolm 1979). In the Serengeti National Park, Tanzania, where this study was carried out, their main prey are Thomson's gazelles, Gazella thomsoni, and wildebeest, Connochaetes taurinus. In contrast to stalking predators, wild dogs tend to run towards their prey in full view and appear to rely on stamina rather than surprise and great speed for their success (Estes & Goddard 1967; Kruuk /93/ $8./ 1993 The Association for the Study of Animal Behaviour 479

2 48 Animal Behaviour, 45, ). In East African safari-lore, their hunting success and level of cooperation are legendary, although, in reality, the degree to which different individuals cooperate to bring down their prey has never been closely studied. We examine factors that influence the ability of wild dogs to capture their prey. In this paper we have two main aims: (1) to determine whether the factors influencing the hunting success of wild dogs differ from those influencing the success of other predator species, particularly the stalking cheetahs and lions; and (2) to determine the advantages of communal hunting for this species, in terms of increased hunting success, food intake and defence of carcasses from spotted hyaenas, Crocuta crocuta. METHODS We observed a wild dog pack on the short, intermediate and long-grass plains of the Serengeti National Park, Tanzania between March 1985 and April During this time 229 hunts of Thomson's gazelles, 13 hunts of Grant's gazelles, Gazella grand, and 1 hunts of wildebeest were observed. Nearly all the data were collected during daylight hours (63-19 hours), usually at dusk and dawn (when hunts were most common, Kiihme 1965; Estes & Goddard 1967; Frame 1986; Fuller & Kat 199), although a small amount were collected on moonlit nights. Observations were made through 1 x 5 binoculars from a Landrover and recorded onto tape for later transcription to data sheets. Because they occupy large home ranges (averaging 15 km' in the Serengeti: Frame et al. 1979), wild dogs are extremely difficult to follow, often moving long distances overnight (Fuller & Kat 199). They remain in one place for any significant period, for 8-1 weeks, only when the pack is denning. The majority of data presented here were collected during this sedentary period 2 years running ( ). The pack varied in size during this time, consisting of 9-15 dogs of at least 1 year of age. Whenever one or more members of the pack were observed hunting gazelles or wildebeest (defined as chasing a prey group) the following data were collected: how the animals approached a prey group, either quickly, without masking their advance in any way, or slowly, often bunched together with their ears lowered (see picture in Estes & Goddard 1967); the number of individuals involved in the hunt (a dog was said to have been involved if at any time during the chase it was the pack member closest to the prey, or if it helped to bring the captured animal to a standstill or onto the ground); the duration of the chase (measured in seconds from when the first dog started to run towards the prey to when the dogs either succeeded in catching hold and bringing the animal to a standstill, or when the last dog abandoned the hunt) and the hunt's outcome (successful chases resulted in a kill). When calculating hunting success, only hunts in which the dogs had selected a particular prey animal from the fleeing group (i.e. they followed its movements, ignoring other prey animals) were used, since we wanted to control for the effects of prey age and sex. We collected the following data with respect to the prey animals selected by the dogs: (1) sex and age (Thomson's gazelles, on the basis of external physical characteristics, were classed as fawns, half-grown, adolescents and adults: Walther 1973; FitzGibbon & Fanshawe 1989; while wildebeest were classed as calves, yearlings and adults); (2) the number of prey animals in the target group (an animal was defined as being in a group if it was within 5 m of a conspecific); and (3) the distance at which the first group member detected the approaching dogs (indicated by the gazelle adopting a stare posture while looking in the direction of the pack) and at which the group fled from the dogs, measured from the nearest wild dog to the nearest prey group member. Towards the end of a chase, when the dogs were closing on their prey, the gazelle often made a series of sharp turns, zig-zagging from side to side, apparently in an attempt to outmanoeuvre its pursuers. To determine the effectiveness of this tactic, the predator-prey distance at the start and end of a period of zigzagging was recorded, as was the duration (in s), enabling the rate at which the dogs gained on the prey animal during a period of zig-zagging to be determined. Distances were estimated by eye to the nearest metre if less than 1 m, to the nearest 1 m if less than 1 m and to 5 m if greater than 1 m and were regularly checked against distances of known length. Finally, the grass height, being either more or less than 3 cm, in which the hunt took place, was also recorded (3 cm was chosen because it was this height at which the vegetation started to conceal the dogs to any extent). Once wild dogs had made a kill, the duration (in min) of time for which the dogs fed at the carcass

3 Fanshawe & FitzGibbon: Hunting success of wild dogs 481 was noted, starting when the first dog started to feed and ending when the last dog abandoned the main carcass. The maximum number of hyaenas that approached within 1 m during this period was also noted. We estimated the profitability of hunting in different-sized groups by dividing the weight of meat available on a carcass by the number of dogs in a hunting group and multiplying by the probability that a hunt involving that number of dogs would be successful. This produced a meat yield index. For these analyses we assumed that some 6% of total body weight was edible (Fuller & Kat 199), i.e. 182 kg for adult wildebeest, 6 kg for wildebeest calves, and 18 kg for Thomson's gazelle (using the mean weight of males and females given in Sachs 1967). RESULTS Hunts of Thomson's Gazelles Age and sex of prey Wild dogs were far more successful hunting immature than adult gazelles (x , df=3, P<.1) and their success rate exceeded 9% when hunting fawns and half-grown animals (Fig. la). On the whole, younger prey could also be caught more quickly (ANOVA, F235 =16.7, P<.1), reducing the time and energy required for the hunt (Fig. 1b), and compensating at least in part for the smaller amount of meat available. Considering adult gazelles only, hunts were just as likely to be successful if the gazelle being chased was male (31.6% of hunts successful, N=95) or female (35.8%, N=38; x2 =.28, df=1, Ns), although the wild dogs selected to hunt nearly twice as many male gazelles as females (sex ratio of gazelles hunted: 1-8:1). Since the sex ratio of the population is strongly biased to females (63% female: Bradley 1977; 71% female: FitzGibbon 199a), this indicates a strong preference for males. This may have resulted from the tendency of females to flee from approaching dogs at greater distances than males (X+ se flight distances= 169± 12 m, N=73 and m, 98, respectively; t=3.38, P < -1). As a result, when the dogs caught up with the prey group, males were usually concentrated at the back, and perhaps were more likely to be selected as a result. Prey group size and predator detection To determine the effect of gazelle group size on hunting success, group size was categorized so each size class included roughly the same number of hunts (Fig. 2). Only hunts of adults were used in the analysis. Gazelle group size had no effect on wild dog hunting success (x2 = 2.3, df= 5, NS; Fig. 2), and larger groups did not flee at greater distances than smaller ones (correlation between gazelle group size and flight distance, r=.41, N = 171, Ns). In nearly all the hunts we observed, all members of the prey group had detected the approaching wild dogs long before the start of the chase and often a few individual gazelles returned to feeding before fleeing. Vegetation The height of the surrounding vegetation had no effect on the probability of the dogs making a successful kill (% hunting success in low ( 3 cm) and high (> 3 cm) vegetation = 24.1%, N=87 and 34-5%, N = 116, respectively; ^,c2 = 1-18, df =1, Ns). We did not see the dogs in areas with dense cover, although they do occur in such places in Serengeti and in Kruger National Park, South Africa (Reich 1981). Wild dog hunting method Wild dogs employed two main ways of approaching prey: either running straight up in full view (77% of occasions) or approaching slowly, with group members bunched together with their heads lowered (on the remaining 23% of hunts). Approaching slowly enabled dogs to get closer to the prey before it fled; the mean predator prey distance at which the first gazelle fled was greater when dogs ran towards their prey (X + se -= m, N =132) than when they approached more slowly (97± 11 m, N=39; t = 3-98, P <.1). The type of approach did not, however, influence the dogs' probability of making a kill. Hunts were as likely to be successful if the dogs ran (15.6% success, N= 64) or walked towards their prey (1.5%, N= 19; x 2 df= 1, Ns). In fact, the distance to which the dogs managed to approach before the prey fled appeared to have little influence on their success. If anything, there was a non-significant tendency for flight at greater distances when hunts were successful (3ir+ SE = m, N =18) than when they were not (X+ se = tri, N=74; t=1.87, P= -7).

4 482 Animal Behaviour, 45, (a 1 II 24 8 _ 64 % Hunting success I I I I Adult Adolescent Half-grown Fawn Adult Yearling Calf Thomson's gazelles Wildebeest IL 2 ( b) 18 Duration of successful chases (s ) Adult T Adolescent/ Hal f-grown 1 12 T Fawn Figure 1. (a) The effect of prey age on the hunting success of wild dog packs. (b) The effect of gazelle age on the duration of successful chases (X+ st). The number of hunts is shown above each histogram. Onepossible advantage of the slow approach may the dogs more likely to stalk when approaching have been a reduction, although not quite signifi- larger gazelle groups (comparing mean gazelle cant, in the length of subsequent chases (mean +SE group size when dogs adopted each approach tacchase duration after slow and fast approaches = tic; t =1.15, N= 63,19, Ns) or when fewer dogs were s, N--=17 and 147± 15.4 s, N= 57, involved (1=1.39, N = 132,39, Ns), respectively; t = 1-87, P=-6). In addition, a slow approach may have been more useful in certain Number of dogs situations, although it was no more likely to be employed in high than in low vegetation (% of Although there were always more than eight hunts employing slow approach in low ( 3 cm) dogs in the pack we followed, they often split up and high vegetation (> 3 cm) = 24.4% and 15.%, during the course of hunting forays, usually respectively, N=98, x2 =.8, df= 1, NS). Nor were because members started to chase different prey

5 Fanshawe & FitzGibbon: Hunting success of wild dogs % Hunt ing success > Gazelle group size Figure 2. The effect of gazelle group size on wild dog hunting success. The number of hunts is shown above each histogram. animals and did not always reunite later in the hunt to concentrate on one particular prey. Individual dogs regularly hunted alone (3% of gazelle hunts were by single animals, Fig. 3a). The number of dogs involved in a hunt varied with the age of the gazelle being hunted (Kruskal Wallis one-way ANOVA, H-= 16., P <.1): fawns were regularly hunted by just one or two dogs, while more dogs were usually involved in hunts of adults (Fig. 3b). Considering only hunts of adult gazelles, hunting success varied significantly according to the number of dogs involved (V 16-34, df= 6, P <. I), but the only real difference was between groups of four and those of other sizes (Fig. 4a). If the group size of four was excluded there was no effect of group size on hunting success and, altogether, groups were no more successful than singletons (x2 =1.82, df= 1, Ns). Three adult males and a yearling hunted as a group regularly and they may have been more successful than when hunting separately or with other pack members. There was no suggestion that an increase in group size reduced the amount of time required for a hunt (correlation between chase duration and group size for successful chases only = -128, N= 21, Ns) and successful hunts by groups lasted as long as hunts by singletons (1=.21, N= 14,7, Ns). Nor were larger groups better at outmanoeuvring gazelles as they zig-zagged from side to side towards the end of a chase; the rate at which dogs gained on prey during this period did not increase when more dogs were involved (correlation between rate of gain and hunting group size=.167, N=33, Ns), and groups were no more successful than singletons (comparing rate of gain, t =,38, N= 6,27, Ns). Overall, it seemed that group size had little effect on the ease with which wild dogs hunted Thomson's gazelles. Hunts of Wildebeest With wildebeest, wild dogs were far more successful when hunting calves than adults (x2 =6.4, df= 2, P < -5; Fig. la), although the number of dogs involved was no different (Mann Whitney U- test, Ns; Fig. 3b). On average, more dogs were involved in wildebeest than gazelle hunts (Mann Whitney U-test, z= 3.71, N =63,131, P <.1 using data from adult gazelles, but both adult and immature wildebeest, Fig. 3a). The success against wildebeest varied with hunting group size, with four or more dogs being most successful (x' 12.4, df= 6, P=.5; Fig. 4b; hunts of calves, yearlings and adults were combined in the analysis owing to small sample sizes). When hunting wildebeest, one dog would usually grab hold of the muzzle, preventing it from running away and wheeling around, while the other group members disembowelled the immobilized animal. Single dogs and groups of two dogs never attempted to hunt and subdue adult wildebeest.

6 484 Animal Behaviour, 45, 3 % Of hunts Hunting group size Mean hunting group size (b) _ Adult Adolescent Half-grown Fawn Adult Calf Thomson's gaze4les Wildebeest Figure 3. (a) The distribution of hunting group size according to the type of prey (1: Thomson's gazelles; ; wildebeest; data from hunts of adult gazelles only but of both adult and immature wildebeest since hunting group size did not vary with age in this situation), (b) The influence of prey age on the number of dogs involved in a hunt. The number of hunts is shown above each histogram. Single dogs were seen hunting wildebeest calves, but were never successful. Pairs managed to kill unaccompanied calves (i.e. without their mothers nearby to defend them) on three occasions. Larger groups were more successful because some dogs could chase and distract the mother, while the other group members focused on catching the calf itself. For the same reason groups of hyaenas were also reported to be more successful than singletons when hunting wildebeest calves (Kruuk 1972). Calculating the profitability of hunting in different group sizes (Fig. 5) indicated that dogs hunting wildebeest maximize their food intake in groups of three or four, while dogs hunting gazelles do best alone. Improved carcass defence may, however, make larger groups more advantageous (see below). Loss of Carcasses to Hyaenas Hyaenas were present at virtually all wild dog kills of both Thomson's gazelles and wildebeest (85.5%, N=62, excluding kills of half-grown and fawn gazelles which were consumed so quickly that hyaenas rarely had time to approach), One to 18 hyaenas would congregate at a kill, but the most

7 Fanshawe & FitzGibbon: Hunting success of wild dogs % Hunting success 4 _ ( b 1 14 _ % Hunting succes s l 3 I I I I Size of hunting group Figure 4. The influence of hunting group size on success rates when hunting (a) adult Thomson's gazelles and (b) wildebeest (adults and calves combined due to small sample sizes). The five hunts by pairs of dogs were all of wildebeest calves rather than adults. The number of hunts is shown above each histogram. common number was two. The number did not depend on prey type (comparing adult gazelles, adult/yearling wildebeest, adult Grant's gazelles and wildebeest calves, F3, 58= 1.3, Ns). Although hyaenas ultimately took over all kills of adult prey, their attempts to appropriate them frequently only succeeded when the dogs had nearly finished feeding while small prey were normally consumed before the hyaenas arrived. The presence of up to four hyaenas did not significantly reduce the time for which dogs were able to feed, compared with situations when no hyaenas were present, but groups of more than four did discourage the dogs (ANOVA, F4, =5'41, P <.1; Fig. 6). Where hyaenas were present, lower numbers meant the dogs maintained access to carcasses for longer (correlation between number of hyaenas and delay to abandoning kill =.447, N= 53, r 2 =., P <.1), especially when the number of dogs was high (correlation between dog number and delay = -422, N=53, r , P<,1). When only one or two dogs were present, the hyaenas sometimes succeeded in stealing the kill almost as

8 486 Animal Behaviour, 45, 3 Meat yield index Is =7 Size of hunting group Figure 5. The influence of hunting group size on the profitability of hunting adult wildebeest, wildebeest calves and adult Thomson's gazelles. See Methods for explanation of calculation of the meat yield index. : Wildebeest calves; a Wildebeest adults; : Thomson's gazelles Du ra t ion of feed ing (min ) I >6 Number of hyaenas at 1I Figure 6. The effect of hyaena numbers on the duration (X± se) for which wild dogs fed at kills (mean duration is significantly different in the following cases as tested by the Schaffe F-test: versus 5-6, versus >6,1-2 versus 3-4,1-2 versus >6). The number of hunts is shown above each histogram. soon as the dogs had started to feed. The length of hyaenas were particularly numerous around a kill time for which dogs fed also depended on the rela- site. tive numbers of hyaenas and dogs, being longer when the ratio of dogs to hyaenas was high (corre- DISCUSSION lation between delay and dog/hyaena ratio=.63, N= 53, r2 --=.39, P <.1, Fig. 7). Thus, even large Previous studies of the stalking predators, cheetahs groups lost control of carcasses relatively quickly if and lions, have shown that hunting success tends to

9 Fanshawe & FitzGibbon: Hunting success of wild dogs E 7: e e - lee 8 OS" e 4! I _I Ratio number of dogs/hyaenas Figure 7. The influence of the relative numbers of hyaenas and wild dogs on the duration for which wild dog packs maintained access to kills. be lower in areas with little cover, when the prey group is more vigilant, or when hunts are initiated further from the prey (Elliot et al. 1977; Van Orsdal 1984; FitzGibbon 1988). The hunting success of the wild dog pack we studied was, in contrast, independent of the amount of cover available and the ability of the Thomson's gazelles to detect the approaching dogs. Ali prey groups detected the approaching dogs before they started to chase and there was no effect of prey group size on hunting success even though large groups of gazelles detect approaching predators at greater distances than small ones (FitzGibbon 199b). In addition, chases started closer to the prey were no more successful than those started further away. These differences appear to be related to the hunting techniques adopted by the predator species. While cheetahs and lions stalk their prey, relying on surprise and short fast chases for success, wild dogs generally run towards their prey in full view and their chases were relatively long (average 97 s for hunts of adult gazelles versus a maximum of 35 s for lions, Elliot et al. 1977). Wild dogs did sometimes adopt a type of stalking approach, advancing slowly with heads lowered, but this did not appear to function as a means of avoiding detection. Instead it may have disguised the fact that the dogs were actually hunting. Gazelles usually detected approaching predators long before they fled, but when the dogs stalked or walked, the gazelles delayed fleeing until the predators were closer. Prey Selection Since younger gazelles and wildebeest were so much easier to catch than mature animals, it was not surprising that they were selected in preference to adults (Schaller 1972; FitzGibbon & Fanshawe 1989), even though they provided a smaller meal. In common with the stalking predators, cheetahs and lions (Schaller 1972; FitzGibbon 199b), the wild dogs selected to hunt more male gazelles than females. Males tend to be less vigilant, to be found in smaller groups, and to concentrate on the periphery of groups which probably predispose them to selection by stalking predators, such as cheetahs (FitzGibbon 199a). However, since wild dogs do not rely on such an element of surprise for their success, do not preferentially hunt smaller prey groups and do not generally select their prey animal until after the chase has started, these factors are unlikely to favour the selection of males by wild dogs. It is more likely to be the reduced flight distance of males, perhaps resulting from a reluctance to leave their territories (Estes & Goddard 1967), that causes them to be selected so often, Although flight distance does not influence a gazelle's probability of escaping, the fact that males tended to delay fleeing for longer meant that they

10 488 Animal Behaviour, 45, 3 were concentrated at the rear of the fleeing group and were therefore more readily available. The poor condition of males relative to females is also likely to be a contributory factor (FitzGibbon & Fanshawe 1989). In support of this, males stott at lower rates than females when fleeing (X= 1.8 and 2. stotts/s, respectively) and, in an earlier paper (FitzGibbon & Fanshawe 1988), we showed that stott rate is likely to be a reliable signal of a gazelle's physical condition which dogs appear to take into account when selecting their prey. Group Size and Foraging Success The increased ability of larger groups to hunt prey has often been considered a major benefit of communal living for social carnivores, although the available data are often inconclusive (reviewed in Packer & Ruttan 1988; Caro, in press). For wild dogs, communal hunting was beneficial in that it increased the range of prey species that could be hunted. While lone dogs were able to kill adult gazelles, the individual members of our study pack, at least, appeared unwilling to tackle wildebeest on their own and adult wildebeest were never killed by hunting packs of two dogs. Larger hunting groups of two other coursing predators, spotted hyaenas and wolves, Canis lupus, have also been reported to prefer large prey (Mech 197; Kruuk 1972; Mills 199). It is often difficult to determine whether this results from the greater food requirements of these groups or because larger hunting groups can indeed capture large prey more easily. In the case of the wild dogs followed in this study, the pack was feeding puppies so meat was always in demand and it is unlikely that smaller groups were not hunting wildebeest because the additional food was not required. In addition to the wildebeest described here, packs of wild dogs have been reported to hunt regularly hartebeest, A lcelaphus lichiensteini (approximately 15 kg, Mitchell et al. 1965), kudu, Tragelaphus strepsiceros (approximately 17 kg; Reich 1981), and even zebra, Equus burchelli ( kg; Malcolm & van Lawick 1975; weights from Sachs 1967). Profitability estimates for different-sized hunting groups indicated that those of three or four dogs would yield most meat/hunt/dog when wildebeest were taken, while dogs would do best by hunting gazelles alone. This did not take into account, however, the fact that larger groups can defend carcasses for longer so larger packs are likely to be more profitable in areas with high hyaena densities. The fact that communal hunting enabled the dogs to take a greater range of prey species increased the likelihood of their regularly finding suitable prey in an ecosystem where the migratory habits of Thomson's gazelles means that the supply of this prey species is irregular (Durant et al, 1988). The benefits are likely to apply only during the denning period when the pack is tied to one locality and unable to follow the gazelle migration. During the two denning seasons we observed, there were often long periods when the gazelles moved away and the dogs would have had to travel considerable distances if wildebeest had not been suitable prey. Cheetah females, which feed almost exclusively on gazelles, are sometimes forced to abandon their cubs because the herds have moved far from denning sites (K. Laurenson, personal communication). All the data we collected were from one pack and to determine fully the benefits of grouping, further studies are required of the effects of packs of different sizes on relative hunting success and the reliability of obtaining food (i.e. the variance in foraging success). Since small groups are less able to kill larger animals, our data suggest that they may find it more difficult to find suitable prey on a regular basis. Although wild dogs usually hunted wildebeest in groups, they regularly hunted gazelles alone, supporting Packer & Ruttan's (1988) prediction that hunting cooperatively is likely to be more common when individual hunting success is low. In general, communal hunting of gazelles appeared to confer few benefits: groups were no more successful hunting gazelles than were single dogs. Chase duration did not shorten with increasing group size, nor did larger groups cope more effectively with zigzagging (as suggested by Kruuk & Turner 1967) or run in relays (with one dog taking over when another tired). The benefits of grouping for wild dogs extended to defence of kills from their main competitor, the spotted hyaena. As found in many other studies (Estes & Goddard 1967; Kruuk 1972; Schaller 1972; Fuller & Kat 199), hyaenas were present at a large percentage of kills and were capable of stealing them before the dogs had finished feeding. The data presented in this paper show that loss of carcasses to hyaenas is particularly severe for small groups or when the ratio of hyaenas to dogs is high. Larger groups of lions are also reported to have a

11 Fanshawe & FitzGibbon: Hunting success of wild dogs 489 greater chance of defending carcasses from hyaenas in the Chobe National Park, Botswana (Cooper 1991). For wild dogs living in Serengeti National Park, Tanzania, interspecific competition with hyaenas is one factor likely to favour maintenance of large packs. Although competition is not thought to be a major factor controlling wild dog population numbers in Africa (Fanshawe et al. 1991), it is possible that hyaenas in the Serengeti (where hyaena numbers have been increasing in the past -3 years, Borner et al. 1987) may have a localized effect, especially when wild dog pack sizes are small and prey is in short supply (Malcolm 1979; Frame & Frame 1981). ACKNOWLEDGMENTS We thank the Government of Tanzania and Tanzania National Parks for permission to conduct research, the Serengeti Wildlife Research Institute for facilities, Frankfurt Zoological Society, Mr and Mrs Neil Silverman and the Science and Engineering Research Council, U.K. for financial support, and Mr 13ernard Maregesi, Chief Warden of Serengeti National Park, Professor K. N. Hirji, Co-ordinator of the Serengeti Wildlife Research Institute, Mr H. M. Nkya, Director of the Serengeti Wildlife Research Centre, and Dr Markus Borner, FZS for their help and support throughout the study. We are very grateful to Tim Caro, Joshua Ginsberg, James Malcolm, Gus Mills and Tim Roper whose comments on previous drafts greatly improved the original manuscript. REFERENCES Horner, M., FitzGibbon, C. D., Homer, Mo., Caro, T. M., Lindsay, W. K., Collins, D. A. & Holt, M. E The decline in the Serengeti Thomson's gazelle population. Oecologia BerL ), 73, Bradley, R. M Aspects of the ecology of Thomson's gazelles in the Serengeti National Park, Tanzania. Ph.D. thesis, Texas A & M University. Caro, T. M. In press. Cheetahs of the Serengeti: Patterns of Grouping in an Asocial Fetid. Chicago: University of Chicago Press. Cooper, S. M Optimal hunting group size: the need for lions to defend their kills against loss to spotted hyaenas. Air. J. Ecol., 29, Curio, F The Ethology of Predation. Berlin: Springer Verlag. Durant, S. M., Caro, T. M., Collins, D. A., Alawi, R. M. & FitzGibbon, C. D Migration patterns of Thomson's gazelles and cheetahs on the Serengeti Plains. Afr, J. Ecol., 26, Elliot, J. P., McTaggart Cowan, I. & Holling, C. S Prey capture by the African lion. Con. J. Zoo!., 55, Estes, R. D. & Goddard, J Prey selection and hunting behavior of the African wild dog. J. Wildl. Mgmt, 31, Fanshawe, J. Pl., Frame, L. H. & Ginsberg, 3. R The wild dog: Africa's vanishing carnivore. Oryx, 25, FitzGibbon, C. D The antipredator behaviour of Thomson's gazelles. Ph.D. thesis, University of Cambridge. FitzGibbon, C A cost to individuals with reduced vigilance in groups of Thomson's gazelles hunted by cheetahs. Anim. Behar., 37, FitzGibbon, C, D. 199a. Why do hunting cheetahs prefer male gazelles? Anim. Behay., 4, , FitzGibbon, C. D. 199b. Mixed-species grouping in Thomson's and Grant's gazelles: the anti-predator benefits. Anim. Behay., 39, FitzGibbon, C. D. & Fanshawe, 3. H Stotting in Thomson's gazelles: an honest signal of condition. Behay. Ecol. Sociohiol., 23, FitzGibbon, C. D. & Fanshawe, J. H The condition and age of Thomson's gazelles killed by cheetahs and wild dogs. J. Zool. Land., 218, Frame, G. W Carnivore competition and resource use in the Serengeti ecosystem of Tanzania. Ph.D. thesis. Utah State University, Logan. Frame, L. H. & Frame, G. W Swift and Enduring: Cheetahs and Wild Dogs of the Serengeti. New York: E. P. Dutton. Frame, L. H., Malcolm, J. R., Frame, G. W. & van Lawick, L. H The social organisation of African wild dogs (Lycaon pictus) on the Serengeti Plains, Tanzania, Z. Tierpsychol., 5, , Fritz, S. H. & Selander, 1 A Diets of bobcats in Arkansas with special reference to age and sex differences. J. Wildl. Mgmt, 42, Fuller, T. K. &Kat, P. W Movements, activity and prey relationships of African wild dogs (Lycaon pictus) near Aitong, southwestern Kenya. Af r. J. Ecol., 28, Homocker, M. G An analysis of mountain lion predation upon mule deer and elk in the Idaho Primitive Area. Wild. Monogr., 21, Kruuk, H The Spotted Hyena. Chicago: Chicago University Press. Kruuk, H. (975. Functional aspects of social hunting in carnivores. In: Function and Evolution in Behaviour (Ed. by G. Baerends, C. Beer & A. Manning), pp Oxford: Oxford University Press. Kruuk, H. & Turner, M Comparative notes on predation by lion, leopard, cheetah arid wild dog in the Serengeti area, East Africa. Marnmalia, 31, Kiihme, W. D Freildandstudien zur soziologie des hyaenahundes (Lycaonpicruslupinus Thomas 192). Z. Tierpsychol., 22, Lamprecht, J The relationship between food competition and foraging group size in some larger carnivores: a hypothesis. Z. Tierpsychol., 46,

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