The numbers and distribution of buffalo in the Ruwenzori National Park*, Uganda

The buffalo (Syncerus coffer (Sparrman)) in the Ruwenzori National Park, Uganda were censused by means of aerial survey. Details of earlier censuses of buffalo from the air are given. Herds were located by flying along flight lines 1–6 km apart at a height of 300 m above ground. Buffalo in small herds and in bachelor groups containing less than thirty animals were counted individually but most herds were photographed and the number of buffalo counted from the prints. Two full photographic counts were made in November 1968 and December 1969 and subsequently, counts of herds only were made four times a year during each wet and dry season. The total numbers of buffalo were estimated from the herd counts on the basis of the mean herd size and percentage of bachelors recorded earlier. A total of 17 835 buffalo, comprising 16 749 in 165 herds and 1086 bachelor bulls, was recorded in 1968 and 18 040 buffalo made up of 17 141 in 162 herds and 899 bachelors were counted in 1969. The mean herd size was 101-5 in 1968 and 105-8 in 1969 with bachelors representing 6-1% and 5-0% of the totals in 1968 and 1969 respectively. The mean size of the bachelor groups was 4–7 and 3-3 in 1968 and 1969 respectively. The density of the buffalo was the same each year at twelve animals per sq. km. If only the preferred habitat is considered, the density becomes thirty-eight buffalo per sq. km. The mean areas occupied by a single herd were 9-4 and 9-6 sq. km in 1968 and 1969 respectively. The mean area of preferred habitat used by one herd was 2–9 sq. km each year. It is concluded that herd counts are not a satisfactory method for assessing the total buffalo population although they have value as indices, provided the same observer is used. However, they do suggest in the present work that there is a slight but definite seasonal movement of buffalo into forested areas or towards permanent water during the dry season.     

Current population estimate and distribution of the African buffalo in Chebera Churchura National Park,Ethiopia

An investigation into the population status and distribution of the African buffalo (Syncerus caffer Sparrman, 1779) in Chebera Churchura National Park, Ethiopia, was carried out during the wet and dry seasons of 2012–2015. This study tested the hypothesis that buffalo would demonstrate seasonal habitat preferences and changes in population density. Sample counts were carried out in an area of 1215 km². The estimated buffalo population was 5193 individuals, with the population density of 4.3/km². The population showed an increase from 2617 to 5194 individuals during 2006–2015. Males comprised 42.6%, while females 46.7% of the population. Age structure was dominated by adults, which constituted 52.5% of the total population. Subadults comprised 24.3% and young 12.4% of the population. Larger herds of up to 30 individuals were observed during the wet season, and smaller herds of a minimum of four individuals were seen during the dry season. The mean herd sizes during the wet and dry seasons were 29.59 and 16.95, respectively. They were observed more in the riverine vegetation types during the dry season. Of the total, 57.6% utilized riverine habitat during the dry season, whereas 39.8% used this habitat during the wet season. Relative abundance of food sources, green vegetation cover and availability of water were the major factors governing their distribution in the present study area.     

The numbers and distribution of large mammals in Ruaha National Park,Tanzania*

Two aerial sample counts were carried out in Ruaha National Park, Tanzania, in the dry season of 1972 and the wet season of 1973. Population estimates of elephant, buffalo and other large mammals were made, and trend surface analysis was used to identify the basic gradients and contours of density, diversity and biomass within the Park. A cluster analysis, based on similarities between principal component scores, and validated by a multiple discriminant analysis, produced a classification of the Park into three Regions which differed significantly in densities, diversity and biomass. Ruaha Park carries year round a high density of elephants (1–7 km^-8). Seasonal movements into and out of the Park were not found, although movements within and between Regions could be detected.     

Genetic structure of African buffalo herds based on variation at the mitochondrial D-loop and autosomal microsatellite loci: Evidence for male-biased gene flow

Sexual differences in herding behaviour ofAfrican buffalo (Syncerus caffer) werestudied by analysing at the herd levelmitochondrial D-loop hypervariable region I andfourteen autosomal microsatellites. Three herdsfrom Arusha National Park in Tanzania wereanalysed with mtDNA and five herds from KrugerNational Park in South Africa with mtDNA andmicrosatellites. Significant mtDNAdifferentiation was observed among herds inArusha NP (F_ST = 0.12, based on haplotypefrequencies). Assignment tests withmicrosatellite data from Kruger NP showed thatmost frequent migration between herds is bymales two years. This was confirmed bytests for herd differentiation and analyseswith Lynch and Ritland's relatedness estimator.Within a herd, males younger than two years andfemales showed a higher relationship through acommon father rather than a common mother,indicating that female herd members mate withonly a few dominant males. This in turnsuggests a female:male sex ratio larger than5:1. The migration rate per generation betweenherds was estimated to be 5–2% for femalesand close to 100% for males. Finally, theimplications for the management of buffalopopulations are being discussed.     

Buffalo Herd Structure and its Repercussions for Condition of Individual African Buffalo Cows

The African buffalo Syncerus caffer was studied in Lake Manyara National Park, Tanzania. Emphasis was placed on the study of (individual) buffalo cows, which live in mixed herds. Buffalo herds are discrete social units and females were never observed in another herd than their own. The herd showed a structure with respect to the distribution of sex-age classes. Individual cows generally kept the same location within the herd. The location in the herd appeared to be coupled to food intake and was strongly related to physical condition. The best location (highest intake and best condition) was between the front and the centre of the herd, the worst location was the rear of the herd (when moving or grazing). Females with calves appeared to have the highest position in the hierarchy as determined from the rate of displacement over food; adult bulls did not interact with cows. Few births were observed during the late dry season and it appears that there is a calving peak at the end of the long rains. Conception rate increased when cows increased in condition and dropped when cows lost condition. Cows showed a strong seasonality in condition but bulls on average hardly changed in condition, except for a loss in condition during the inferred conception peak. Buffalo herds in Manyara showed a fusion-fission pattern independent of season but strongly influenced by the size of the herd: large herds split more often than smaller ones. In large herds, buffalo grazed closer together than in small herds and it appeared likely that competition was more severe in large herds. Animals in the rear of a large herd lost condition faster during the dry season than animals in the best location in the herd, and especially cows in the rear split off most frequently from the herd to graze in a smaller fragment. From the literature on cattle, it is inferred that the reproductive success of cows in the rear of the herd will be lower than of cows in the best location, and this differential is confirmed by the behaviour of adult bulls. It is as yet unclear what the advantage is for adult cows in the rear of a large herd to stay in that herd but the sharing of information with more successful individuals seems a good candidate.     

Population structure and ecology of the African buffalo (Syncerus caffer Sparrman, 1779) in Chebera Churchura National Park,Ethiopia

An investigation of population structure and ecology of the African buffalo (Syncerus caffer Sparrman, 1779) was carried out in the Chebera Churchura National Park, Ethiopia during the wet and dry seasons of 2005–2006. Sample counts of African buffaloes were carried out in an area of 1,215 km². The estimated population of buffalo was 2,617 individuals. Males comprised 35.0%, while females 52.8% of the population. The remaining 12% of the population was young of both sexes and of all ages of unknown sex. It was difficult to categorize the young into male and female in the field, as their primary sexual characteristics were not easily visible. Male to female sex ratio was 1.00 : 1.51. Age structure was dominated by adults, which constituted 72.06% of the total population. Subadults comprised 22.02%, and young accounted for 5.9% of the population. Larger herds of up to 27 individuals were observed during the wet season and smaller herds of eight individuals during the dry season. The mean herd size during wet and dry seasons was 24.81 and 7.77, respectively. The African buffaloes were distributed in four habitat types such as grasslands, woodlands, montane forests and riverine habitat in the study area. They were observed more in the riverine vegetation types during the dry season. Relative abundance of food resources, green vegetation cover and water availability in the area were the major factors governing their distribution in the present study area. They spent a greater proportion of the time in feeding and resting/ruminating activities. On the average, 49.7% of the daytime was spent in feeding, and 38.75% of the day on resting (lying down and standing). Morning and the late afternoon activity peaks were more pronounced during the dry season than the wet season.     

Variation of group size among African buffalo herds in a forest-savanna mosaic landscape

Documenting within species group size variation is important to completely understand social organization within species and to interpret variation among species. Here, I investigated group size of African buffalo Syncerus caffer over 2 years in a heterogeneous landscape. African buffalo use closed continuous forest and vast open savannas, and anecdotal observations suggest that habitat type influences their social structure. While the Cape buffalo Syncerus caffer caffer is well studied, few data exist for the forest-dwelling Syncerus caffer nanus . I observed forest buffalo at Lopé National Park, Gabon, and examined variation of group size. Eighteen forest buffalo herds used the study area with an estimated population of 342 individuals (∼5 buffalo km⁻²). The mean group size for the 18 herds was 12 ± 2 (range of means=3–24), considerably smaller than Cape buffalo herds. For eight radiocollared forest buffalo, the mean group size was stable, varying little with time of day, across seasons, or between savanna and marsh habitat. However, herd size varied widely across herds, from fewer than 10 individuals in the smallest herds to more than 20 buffalo in the largest. Large herd size is associated with home ranges that contain substantial areas of open habitat, and thus more food resources than forested habitats.     

Double-observer approach to estimating egg mass abundance of pool-breeding amphibians

Interest in seasonally flooded pools, and the status of associated amphibian populations, has initiated programs in the northeastern United States to document and monitor these habitats. Counting egg masses is an effective way to determine the population size of pool-breeding amphibians, such as wood frogs (Rana sylvatica) and spotted salamanders (Ambystoma maculatum). However, bias is associated with counts if egg masses are missed. Counts unadjusted for the proportion missed (i.e., without adjustment for detection probability) could lead to false assessments of population trends. We used a dependent double-observer method in 2002–2003 to estimate numbers of wood frog and spotted salamander egg masses at seasonal forest pools in 13 National Wildlife Refuges, 1 National Park, 1 National Seashore, and 1 State Park in the northeastern United States. We calculated detection probabilities for egg masses and examined whether detection probabilities varied by species, observers, pools, and in relation to pool characteristics (pool area, pool maximum depth, within-pool vegetation). For the 2 years, model selection indicated that no consistent set of variables explained the variation in data sets from individual Refuges and Parks. Because our results indicated that egg mass detection probabilities vary spatially and temporally, we conclude that it is essential to use estimation procedures, such as double-observer methods with egg mass surveys, to determine population sizes and trends of these species.     

A comparison of methods used to count the elephant population of the Addo Elephant National Park, South Africa

Evaluation of alternative techniques used to estimate elephant population sizes is important in order to assess the accuracy of the results obtained, upon which management decisions may be based. Data from annual helicopter surveys carried out in the Addo Elephant National Park (1978–97) are compared with registration counts obtained from intensive ground surveys in which all elephants within the park are individually known. On average, total population size estimated in aerial surveys is 8.0% lower than registration counts (n₁₉₉₇ = 251), aerial calf (< 1 year) counts are 48.8% lower than registration counts, and aerial carcass counts are 50% below the total number of deaths documented in ground surveys. Registration counts provide more accurate demographic data than aerial surveys, the results of which are shown to vary widely and unpredictably, thus compromising their value. Where population estimates rely exclusively on aerial surveys, replicate counts are recommended in order to generate confidence intervals.     

Diet selection and density estimates of forest buffalo in Campo‐Ma’an National Park,Cameroon

We studied diet selection and density of forest buffalo in the Campo Ma’an National Park of southern Cameroon. The buffalo’s diet in this rainforest comprised 43% grass, including 15%Leptochloa caerulecens. Other species eaten were non‐graminoid monocots (21.3%), mainly Commelinaceae (18.2%), including Palisota spp. and dicotyledons (32.7%), mainly leaves (26.5%). This diet revealed that buffalo collect their food on road verges, logging tracks and along large rivers. This agrees well with the distribution of buffalo tracks in the study area. Mean buffalo density in the forest was only 0.01 buffalo km^?2, whereas the density on the road together with its direct surroundings was 0.4 buffalo per km of road. We estimated the total number of buffalo in the 650 km² southern part of National Park Campo‐Ma’an of only twenty individuals. We suggest that the buffalo’s ancestral niche was not a primeval type of rainforest, as suggested by Kingdon, but an interface between savanna and rainforest. The Campo‐Ma’an buffalo most likely depend to a very large extent on anthropogenic vegetation types. Maintaining some grass cover by annual clear‐cutting along roads and on logging tracks might be necessary for the survival of the buffalo population within the National Park.     

Population status and feeding habits of the translocated sub-population of Hunter's antelope or hirola (Beatragus hunteri,Sclater, 1889) in Tsavo East National Park,Kenya

The population status and feeding habits of Hunter's antelope or hirola (Beatragus hunteri) in Tsavo East National Park were determined in this study in 1995. This is the first study that has ever been undertaken on the hirola population of about 20 animals that were translocated to Tsavo East National Park from South Garissa District in 1963. Relevant data and information were collected using different ecological research techniques. A total of eight nursery herds (5–11 individuals per herd) comprising 60 individuals, five lone bulls and 11 lone sub-adults was observed. This gave a total population estimate of 76 animals in the entire study area. Adult animals dominated the population (60.53%), indicating a population skew towards adults. The adult sex ratio was biased towards females (43.42%). Hirola were observed to feed mainly on short, green grasses. The dominant feed species were Panicum infestum, Digitaria rivae, Latipes senegalensis and Cenchrus ciliaris. Occasionally, hirola was observed to feed on forbs (e.g. Portulaca oleraceae, Tephrosia subtriglora and Commelina erecta). Results of this study show that the population growth of the hirola sub-population in the Park has been low (1.8 individuals per year). Factors thought to contribute to this low growth include predation and low genetic variability.     

Emigration and Density Dependence in Yellowstone Bison

Abstract: Understanding the relative importance of density-dependent and density-independent feedback on population growth is essential for developing management strategies to conserve wildlife. We examined a 99-year time series of annual counts and removals for 2 bison (Bison bison) herds occupying northern and central Yellowstone National Park in the western United States. Yellowstone's aggressive management intervention effectively recovered bison from 46 animals in 1902 to > 1,500 animals in 1954. Supplemental feeding of the northern herd facilitated rapid growth (r = 0.16) during 1902 to 1952. Augmentation of the central herd with 71 animals also led to rapid growth over 1936 to 1954 (r = 0.10). In 1969, manipulative management ceased in the park, and we detected evidence of density-dependent changes in population growth rates for both herds during 1970 to 2000 as numbers increased to >3,000 animals. The central herd showed evidence of a constant density-dependent response over 1970 to 2000. In contrast, density dependence had a stronger effect on the northern herd's growth rate during 1970 to 1981 than during 1982 to 2000. We found evidence to suggest that these trends resulted from pulses of emigration from the central herd to the northern range beginning in 1982 in response to resource limitation generated by an interaction between density and severe snow pack. Corroborative evidence supporting this interpretation included 1) the annual growth of the central herd was negatively correlated with snow pack but that of the northern herd was not, 2) growth rates of the central and northern herds were uncorrelated during 1970 to 1981 but significantly and negatively correlated during 1982 to 2000, and 3) the northern herd could not have sustained the high removals experienced during 1984 to 2000 without immigration. Density-related emigration from the central herd to the northern range may be fueling bison emigration onto private and public lands where large-scale removals occur, exacerbating the brucellosis controversy for natural resource managers.     

Hunting differentially affects mixed-sex and bachelor-herds in a gregarious ungulate, the impala (Aepyceros melampus: Bovidae)

We investigated herd-sizes and herd-compositions of Impala ( Aepyceros melampus ) inside a protected area [Lake Mburo National Park (LMNP) in western Uganda] and the unprotected adjacent ranchland [the Ankole Ranching Scheme (ARS)]. Impala experience intense hunting and poaching in the study area, and poaching is especially strong on the ARS. We found evidence for changes in overall group-sizes in both mixed-sex and pure bachelor herds between areas in and outside LMNP. Mixed-sex herds strongly decreased in size outside the National Park, but bachelor herds even slightly increased in size. While the group-composition of mixed-sex herds was very similar in areas in and outside LMNP, bachelor herds comprised more yearlings and subadult males on the ARS. Our study suggests that effects of hunting and other human nuisance may differ between herd types: mixed herds probably decrease in size because females are more strongly hunted. Around LMNP, impala are usually hunted using nets and spears, thereby increasing the hunters' chance of being injured. Poachers therefore prefer hornless females (and their calves), as it is less dangerous to handle net-caught females than males. As a result, males are less hunted, but increased vigilance and, therefore, reduced aggression among the members of a bachelor herd, may account for the observed increase in herd sizes and changes in group-compositions.     

Microsatellite variation in the reintroduced Pennsylvania elk herd

Relocation programs have restored elk (Cervus elaphus) to portions of its vast historical range. We examine the consequences of these relocation programs by assessing variation at 10 microsatellite loci in three elk herds, a source herd (Yellowstone National Park), a large herd reintroduced from Yellowstone (Custer State Park) and a bottlenecked herd reintroduced from both Yellowstone and Custer (the Pennsylvania herd). Observed single locus heterozygosities ranged from 0.000 to 0.739. Multi-locus heterozygosities ranged from 0.222 to 0.589. Although significant differences were detected among all three herds, the Yellowstone National Park and Custer State Park herds possessed similar levels of variation and heterozygosity, and the genetic distance between these two herds was small. The Pennsylvania herd, on the other hand, experienced a 61.5% decrease in heterozygosity relative to its source herds, possessed no unique and few rare alleles, and the genetic distances between the Pennsylvania herd and its sources were large. Simulations were performed to identify bottleneck scenarios in agreement with levels of variation in the Pennsylvania herd. Our data confirm that the rate of population growth post-relocation may have important genetic consequences and indicate that theoretical predictions regarding the maintenance of genetic variation during relocation events must be viewed with caution when small numbers of a polygynous species are released.     

Divergent estimates of herd‐wide caribou calf survival: Ecological factors and methodological biases

Population monitoring is a critical part of effective wildlife management, but methods are prone to biases that can hinder our ability to accurately track changes in populations through time. Calf survival plays an important role in ungulate population dynamics and can be monitored using telemetry and herd composition surveys. These methods, however, are susceptible to unrepresentative sampling and violations of the assumption of equal detectability, respectively. Here, we capitalized on 55 herd‐wide estimates of woodland caribou (Rangifer tarandus caribou) calf survival in Newfoundland, Canada, using telemetry (n = 1,175 calves) and 249 herd‐wide estimates of calf:cow ratios (C:C) using herd composition surveys to investigate these potential biases. These data included 17 herd‐wide estimates replicated from both methods concurrently (n = 448 calves and n = 17 surveys) which we used to understand which processes and sampling biases contributed to disagreement between estimates of herd‐wide calf survival. We used Cox proportional hazards models to determine whether estimates of calf mortality risk were biased by the date a calf was collared. We also used linear mixed‐effects models to determine whether estimates of C:C ratios were biased by survey date and herd size. We found that calves collared later in the calving season had a higher mortality risk and that C:C tended to be higher for surveys conducted later in the autumn. When we used these relationships to modify estimates of herd‐wide calf survival derived from telemetry and herd composition surveys concurrently, we found that formerly disparate estimates of woodland caribou calf survival now overlapped (within a 95% confidence interval) in a majority of cases. Our case study highlights the potential of under‐appreciated biases to impact our understanding of population dynamics and suggests ways that managers can limit the influence of these biases in the two widely applied methods for estimating herd‐wide survival.     

Genetic diversity and isolation in African Buffalo (Syncerus caffer)

We studied genetic diversity in 58 buffalo from the Kruger National Park (KNP) and Willem Pretorius Nature Reserve (WPNR). Thirty-three protein-encoding loci were resolved; three were polymorphic. Average heterozygosity (H) values did not differ substantially between adult and sub-adult animals from the KNP (2.65 and 2.89%, respectively), but were lower in animals from the isolated WPNR herd (H = 1.48% and only 3% polymorphic loci compared to 9.1%). Representative levels of genetic diversity exist in the large but disease-carrying herd, whereas the smaller disease-free herds available for translocations appear less polymorphic.     

Effects of a recent founding event and intrinsic population dynamics on genetic diversity in an ungulate population

Maintenance of genetic diversity has recently become a management goal for a number of species, due to its importance for present and future population viability. Genetic drift, primarily through differential reproductive success and inbreeding, can accelerate the loss of genetic diversity in recently recovered populations. We attempt to quantify the consequences of these factors on the genetic diversity contained in a small, recently founded wood bison (Bison bison athabascae) population by examining the genetic variation in this conservation herd, the calves born therein, and its large source population. The Hook Lake Wood Bison Recovery Project was initiated to found a disease-free herd of wood bison containing a representative amount of the genetic diversity present in the Wood Buffalo National Park metapopulation. Levels of diversity in the Hook Lake Wood Bison Recovery Project founders are higher than in previous salvage attempts. To examine the effects of differential reproductive success on this population, we monitored parentage of the calves born in the Hook Lake Wood Bison Recovery Project for 3 years since the founders reached sexual maturity. Two of the male founders sired over 90% of the offspring born in this population, which has led to a reduction in diversity in their calves. Monitoring of reproductive success, and incorporation of selective breeding strategies will be required to reduce the rate at which genetic diversity is lost from this small, isolated population. These steps should occur in other recovery projects, particularly when a small number of individuals are capable of dominating reproduction.     

Factors affecting vigilance in springbok: importance of vegetative cover, location in herd, and herd size

Vigilance in vertebrates is often inversely related to group size. We present evidence that distance to bushes and location within the herd are also critical factors in vigilance in springbok (Antidorcas marsupialis) in Etosha National Park, Namibia, where they are the preferred prey of cheetahs (Acinonyx jubatus). Most springbok feed in heterospecific herds, both by grazing on grass and browsing on bushes. We studied 1245 animals; variations in vigilance (time alert) were explained by location within the herd, distance to bushes and roads, number of springbok in each herd, and gender and age. Vigilance time decreased with increasing herd size, with increasing distance to bushes and roads, and with density. Springbok on the edge of herds devoted significantly more time to vigilance than did those in other locations, and vigilance in edge animals decreased with group size. Adults were more vigilant than young, and males were more vigilant than females. Position within the herd, and distance from bushes, were the most important variables influencing vigilance. Location in the herd and gender/age affected both browsing and grazing springbok, although other factors accounted for the differences in vigilance between browsing and grazing springbok: 1) group size was not significant for browsers, but it was for grazers, and 2) distances to bushes and road were not significant for browsers, but they were for grazers. These data relate to the risk from predators and the benefits from other group members. Springbok in bushes cannot see all members of the herd, cannot derive early warning from many group members, and are more at risk from predators because the latter can hide in the bushes. Received: 17 May 1999 / Received in revised form: 30 August 1999 / Accepted: 29 November 1999     

Population viability analysis to identify management priorities for reintroduced Elk in the Cumberland Mountains,Tennessee

We used an individual-based population model to perform a viability analysis to simulate population growth (λ) of 167 elk (Cervus elaphus manitobensis; 71 male and 96 female) released in the Cumberland Mountains, Tennessee, to estimate sustainability (i.e., λ > 1.0) and identify the most appropriate options for managing elk restoration. We transported elk from Elk Island National Park, Alberta, Canada, and from Land Between the Lakes, Kentucky, and reintroduced them beginning in December 2000 and ending in February 2003. We estimated annual survival rates for 156 radio-collared elk from December 2000 until November 2004. We used data from a nearby elk herd in Great Smoky Mountains National Park to simulate pessimistic and optimistic recruitment and performed population viability analyses to evaluate sustainability over a 25-year period. Annual survival averaged 0.799 (Total SE = 0.023). The primary identifiable sources of mortality were poaching, disease from meningeal worm (Parelaphostrongylus tenuis), and accidents (environmental causes and unintentional harvest). Population growth given pessimistic recruitment rates averaged 0.895 over 25 years (0.955 in year 1 to 0.880 in year 25); population growth was not sustainable in 100% of the runs. With the most optimistic estimates of recruitment, mean λ increased to 0.967 (1.038 in year 1 to 0.956 in year 25) with 99.6% of the runs failing to be sustainable. We suggest that further translocation efforts to increase herd size will be ineffective unless survival rates are increased in the Cumberland Mountains. © 2011 The Wildlife Society.     

Predation risk, gender and the group size effect: does elk vigilance depend upon the behaviour of conspecifics?

Many animals benefit from the presence of conspecifics by reducing their rate of scanning for predators while increasing their time spent foraging. This group size effect could arise from a decreased perception of individual risk (dilution hypothesis) and/or an increased ability to detect predators (detection hypothesis). We compared individual and group vigilance of Rocky Mountain elk, Cervus elaphus, in three regions of Yellowstone National Park, Wyoming, U.S.A. that varied in their encounter frequency with coyote, Canis latrans, grizzly bear, Ursus arctos, and grey wolf,Canis lupus , predators. Adult females without calves increased scanning and decreased foraging with high encounter risk and small herd size. Adult females with calves increased scanning and decreased foraging with high encounter risk, but showed no decrease in scanning with large herd size. Yearlings increased scanning and decreased feeding with small herd size, but not with high encounter risk. Adult males were least vigilant, fed most and were not influenced by encounter risk or herd size. These age-sex class differences led to significant differences in group vigilance depending on the composition of the herd. Herds with a majority of mothers were significantly more vigilant than herds with a majority of adult males. However, these differences in group vigilance had no influence on the individual scanning of females without calves. Thus, the decrease in individual scanning with herd size may depend more on changes in individual risk than on cooperative detection of predators. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.