J . Zool., Lond. (1996) 239, 697-716

Diet and foraging behaviour of group-living meerkats, Suvicata suricatta, in the southern Kalahari

S . P. DOOLAN*~ AND D . W. MACDONALD

Wildlije Conservation Research Unit, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK

(Accepted 17 October 1995)

(With 6 figures in the text)

Slender-tailed meerkats (Suricata suvicarra) are small, diurnal, and gregarious mongooses which inhabit the semi-arid regions of southern Africa. In the south-western Kalahari, substantial fluctuations in productivity are caused by extreme seasonality in rainfall and temperatures. We observed the foraging behaviour of habituated meerkats from January to July, a period covering the entire birth season and stages of high and low prey availability. Insects were the most frequently occurring prey class (78.1 YO), of which larvae (33.4% total frequency) and adult Coleoptera (27.5% total frequency) were the most important prey items throughout the year. Reptiles were heavily utilized in terms of prey bulk-an index of volume-(19.9%), but not by frequency (9.2%). Consumption of Coleoptera was positively correlated with rainfall, and negatively with temperature. Meerkats used a mean of 6.7 & 1.1 prey categories daily, arld there were significant monthly differences in prey diversity in the diet. Dietary shifts were apparently related to fluctuations in prey availability and the presence of preferred prey. There were no differences between the sexes in dietary diversity or niche breadth, but pregnant and lactating females foraged at significantly higher rates than males. The timing of foraging activity altered over the months in response to changes in daylength and thermoregulatory constraints. Foraging behaviour and seasonality in foraging effort are described, and the implications of an insect prey base for meerkat socioecology are discussed.

Introduction

The goal of a foraging animal is to meet its metabolic requirements in an environment that can be spatially and temporally variable. Whether or not it attains this goal is dependent on prey availability, environmental conditions, and competition. A central tenet of behavioural ecology is that the spatial and temporal dispersion and quality of resources link the environment with social organization and population abundance on both an inter- and intraspecific basis. The Resource Dispersion Hypothesis (Macdonald, 1983; Carr & Macdonald, 1986; Bacon, Ball & Blackwell, 1991) suggests that the quality and distribution of resources may allow territory holders to tolerate additional animals at little foraging expense in times of resource super- abundance. Such conditions apply when, for example, there is a rapid rate of resource renewal (waser, 1981; Waser & Waser, 1985) or extreme temporal variability in resource levels (von Schantz, 1984; Moehlman, 1986), thereby facilitating the presence of supernumeraries on a territory. These, and similar ideas (von Schantz, 1984; Moehlman, 1986), may explain features of several carnivore groups (e.g. Woodroffe & Macdonald, 1992).

*To whom correspondence should be addressed 'Present address: Earthwatch Europe, Belsyre Court, 57 Woodstock Road, Oxford OX2 6HU, UK Email: EWOXFORD@VAX.OXFORD.AC.UK

691 8 1996 The Zoological Society of London

698 S. P. DOOLAN AND D. W. MACDONALD

Mongoose diets range from highly insectivorous to primarily carnivorous, but as a rule they are opportunistically generalist, incorporating a broad range of small vertebrate and invertebrate prey (Rood, 1975; Gorman, 1979; Lynch, 1980; Sadie, 1983; Baker, 1989; Hiscocks & Perrin, 1991; Avenant & Nel, 1992; Cavallini & Nel, 1992; Palomares, 1993; Cavallini & Serafini, 1995). As for other carnivores, dietary and foraging patterns are regarded as underlying the form of sociality in mongooses (Gorman, 1979; Waser, 1981; Rood, 1986). The socially solitary species typically eat more vertebrates, whereas the gregarious species are mainly insectivorous (Waser, 1981; Rood, 1986; Baker. 1989).

The slender-tailed meerkat Suricata suricatta is a small (c. 500-800 g), gregarious mongoose (Doolan & Macdonald, In press a). Its range extends throughout the arid and semi-arid regions of southern Africa, where it favours open thorn and grassland savanna (Smithers, 1983). In semi- arid environments, rainfall and temperature vary dramatically between seasons and act as primary determinants of vegetative productivity (Rutherford, 1980; van Rooyen et al., 1990), and consequent numbers, biomass, and availability of arthropods (Seely & Louw, 1980; Wrege & Emlen, 1991; Poulin, Lefebvre & McNeil, 1992; Waser et al., In press) and lizards (Pianka, 1986). Hence, meerkats face considerable seasonal fluctuation in resource levels, with food increasing in response to the summer rains, and becoming poorer and scarcer over the dry winter. Winter represents a crunch period, when the effects of environmental harshness and within-group competition are most likely to be felt.

Previous studies of slender-tailed meerkat diet have established that they are functionally insectivorous, but also eat a wide range of other arthropods and small vertebrates (Viljoen & Davis, 1973; Stuart, 1977; Lynch, 1980; Roberts, 1981; Rautenbach, 1982; Smithers, 1983). Reptiles are taken much less often than insects, and other vertebrate prey, such as small mammals and birds, are a negligible component. Amongst the insect prey, coleopteran larvae and adults are of primary importance, followed by lepidopteran larvae. Scorpions (Smithers, 1983) and Orthoptera (Lynch, 1980) may be seasonally significant, and other arthropod prey, such as Myriapoda and Isoptera. also appear in the diet. However, little is known of how dietary composition and foraging behaviour of meerkats relate to gregariousness or to climatic factors (Roberts, 1981). Indeed, few data exist upon rates of prey intake for any carnivore.

In the south-western Kalahari, meerkats live in bands containing 4.2 adults (S.D. 2.1) and 1.9 (S.D. 1.6) yearlings (Doolan & Macdonald, In press a). The data presented here cover the middle of the hot, wet summer and cold, dry winter, and span the entire breeding season of 1986/87 (Doolan & Macdonald, In press b). Here, we seek to: (1) describe diet quantitatively by means of direct observation; (2) assess the impact of extreme seasonality in rainfall and temperature on diet; ( 3 ) determine if diet is influenced by sex; (4) describe variation in foraging behaviour on a daily and seasonal basis; and (5) assess variation in foraging effort in relation to season.

Study area

The study was conducted in the adjoining Kalahari Gemsbok National Park (South Africa) and Gemsbok National Park (Botswana). Three bands of meerkats were observed in a 50 km2 area immediately south of Kwang pan (25"17/S, 20'32'E).

The southern Kalahari is an extremely open savanna of tufted grasses, such as Eragrostis, Arislicla, Stipagrostis, and Schnzidtia species, interrupted by the trees Acacia erioloba, A . hueniotosjlon, and Boscia albirrunca (Acocks, 1988). Leistner & Werger (1973) described 12 plant communities which correspond to the dune, river terrace, and river-bed habitats defined by

MEERKAT DIET AND FORAGING 699

van Rooyen, Bredenkamp & Theron (1991). In the study area, large trees are confined to the dry clay bed of the Nossob River which bisects the site. River terraces are dominated by the shrub Rhigozum trichotomum and by short grasses, whilst the rolling dunes are covered by long grasses.

Rainfall in the south-western Kalahari fluctuates widely and the region experiences large daily and seasonal temperature variation. A n annual mean of about 250 mm of rain falls during a medium-term (approximately 20-year) cycle (van Rooyen et al., 1990). Clearly defined seasons are lacking and the simplest distinction (Mills, 1990) is between the hot, wet summer (October to April) and the cold, dry winter (May to September). Virtually all precipitation occurs during the summer, when mean monthly temperatures are above 20 "C; at least 70% of the rains fall between January and April. Negligible amounts of rain fall in the dry season, during which mean monthly temperatures are below 20°C. November to February are the hottest months of the year and June to August the coldest. Air temperature can exceed 40°C during the summer, whereas overnight frosts are not infrequent between May and September.

Methods

The dietary habits reported here were determined by continuous dawn to dusk focal observation of individually recognizable foraging meerkats, within 3 focal bands, during January to July 1987. Animals were followed on foot at a distance of 1-3 m, and were very well habituated to observation. When a focal individual was lost from sight it was searched for until relocated. The criteria for selection of the data set were that observation periods covered the entire activity time of the focal animals, with fewer than 20 min of missing observation time within the complete meerkat day, and less than 20% unknown items. Observation sessions spanned 7.2 to 16.4 hours per day and a total of 2963 items were recorded over 35 days and 421 hours of observation.

Almost all prey taken by meerkats could be identified visually. Forty types ofprey were initially identified; these were subsequently allocated to 13 major prey categories recorded during all observations over this period: Coleoptera, Hymenoptera, insect larvae, insect pupae (dipteran and lepidopteran), Orthoptera, scorpions, spiders, solifugids, myriapods, miscellaneous arthropods (e.g. Neuroptera), reptiles, small mammals, and unknown. All 13 categories were used for calculation of all indices since this represented the entire prey spectrum being used by meerkats. However, capture of small mammals was observed only once during 1987 and this did not feature in the subset of feeding records presented here. No capture of termites was observed during this season. For clarity of presentation, Orthoptera, solifugids, spiders, and myriapods are amalgamated into the category 'Other'.

In order to estimate the volumetric contributions of prey to the diet, a rough index of size was derived from the number of chews per prey item. The volumetric importance of prey is hereafter referred to as prey bulk, since it involved a categorical approximation rather than precise determination of volume. Prey were allocated to 1 of 4 size classes: (1) tiny: ingested instantly, with up to 10 chews e.g. ants, small coccoons, very small coleoptera; (2) small: taken into the mouth and chewed 11 to 30 times, with few if any external traces visible, e.g. small coleoptera (< 2 cm), acridid Orthoptera; (3) medium: pawed for several seconds before being taken into the mouth and chewed 31 to 50 times, initially leaving some of the item protruding, e.g. large Coleoptera (carabids and tenebrionids), large locustid, and gryllid Orthoptera, smaller scorpions, barking gekkos Ptenopus garrulus, and small Chondrodactylus or Pachydactylus gekkos; and (4) large: items held down with the forepaws and chewed at length (1 51 times), with repeated tearing, e.g. large scorpions, myriapods, small snakes, amphisbaenids, Iacertid, and agamid lizards and large gekkos, particularly Chondrodactylus angulifer.

Representation of prey items in the diet was expressed as: i) the percentage occurrence of a particular prey item in the total number of prey items recorded, and (ii) the percentage bulk of a prey item of the total prey bulk (i.e. the sum of the prey size indices). Proportions were arcsine transformed prior to analysis with 2-way

700 S . P. DOOLAN AND D. W. MACDONALD

ANOVAs, with sex and month as main effects. Monthly mean values were calculated by weighting all days equally, and correlations with monthly means for climatic variables were tested using the Spearman rank correlation coefficient. For presentation in the figures these were scaled to 100%.

Foraging rates were determined for each day by dividing the observed number and volume of prey captures by the hours of focal observation time spent foraging, less any time spent not foraging during the heat of the day or any period longer than 45 minutes. (The latter interval was specified on account of the duration of bouts of vigilance: Doolan, 1994). Foraging rates were also determined for individual prey categories. Rates were log transformed prior to 2-way ANOVA, with sex and month as main effects. In addition, as an index of foraging effort we noted the depth of the foraging hole for each prey capture. Hole depth was recorded as surface (0), rake (l),forepaws only ( 2 ) , headandshoulders (3), halfbody (4), entire body (5) or tunnel (6), in which the foraging hole was considerably greater than the forager’s body.

For each month we examined dietary diversity and breadth based upon the occurrence and volume of each prey category. Dietary diversity was assessed using the Shannon-Weaver index, H’, and the original Simpson’s index, A. The Shannon-Weaver index is a measure of the degree of uncertainty in predicting to what class an individual item will belong when chosen at random from a sample (Ludwig & Reynolds, 1988). This is zero when only a single class is present and reaches a maximum when all classes are equally represented. By contrast. Simpson’s index, which varies from 0 to I , measures the probability that 2 items drawn at random from a sample belong to the same class. Hence low diversity is indicated by a high value for the index.

To evaluate trophic niche breadth, we applied the standardized Levin’s index (Ludwig & Reynolds, 1988). Levin’s index is the inverse of the simple Simpson’s A, and standardization results in a scale from zero (lowest niche breadth) to one (highest niche breadth). Results were arcsin transformed prior to statistical analysis.

Results

Dietar?. composition

Meerkats fed primarily upon invertebrates (Figs l a and b), with insects comprising 78.1% of the overall diet by frequency of occurrence, and 68.9% by bulk. Insect larvae (33.4%) and Coleoptera (27.5%) were the most frequently occurring classes in the overall diet and Hyme- noptera accounted for a further 12.6% of prey occurrences. Insect pupae (1.6%), scorpions (1 .go/,). and Orthoptera (1.6%) also figured, whereas spiders, myriapods, and other miscella- neous items such as solifugids and other arthropods, occurred very infrequently. Small unknown items comprised 9.8% and reptiles 9.2% of all prey occurrences. Reptiles taken included lizards, gekkos. snakes, and amphisbaenids. Snakes and amphisbaenids were a minor prey component. Ptenopus garrulus accounted for 53.7% of vertebrate prey occurrences, Eremias and Nucrus 24.1 YO, and Chondroductj~lus angulifer 13.3 YO.

The ranking of prey categories in terms of bulk contribution to the total diet remained substantially the same as by occurrence, with the notable exception of reptiles which, by bulk, constituted the third most important prey consumed (19.9Yo). Scorpions (4.5%) and Orthoptera (3.3%) were also slightly more prominent by bulk than by frequency of occurrence. Plant material was never noted in the diet.

Insect larvae and Coleoptera were staples of the diet in all months of the year, both in terms of frequency of occurrence (Fig. la) and bulk (Fig. lb). The mean frequency of occurrence of insect larvae in a meerkat‘s diet was low in January (18.1 f 10.So/o, n = 7), with a major peak in March (60.6 k 27.0%. P I = 3) and a secondary peak in May (47.3 It 23.4%, n = 5). Pupae peaked

MEERKAT DIET AND FORAGING 701

somewhat later, in April, at only 11.5 f 5.6% (n = 4). Adult Coleoptera also had a high rate of occurrence in January (37.5 f 9.0%, n = 7) and February (46.3 & 12.5%, n = 6), which dropped sharply in March (11.8% & 8.7%, n=3) and then increased in April (24.4 f 8.9%, n = 4 ) to remain at a level above 16%. Hymenoptera showed a comparable trend, being high in January (21.6 f 11.9%, n = 7) and again in the winter months from April onwards (8.1 f 6.7% to 11.1 f 6.5%). Reptiles were most frequent in the diet in February (13.9 f 6.5%, n = 6) and March (17.6 zk 7.5%, n=3) and again in July (18.1 f 8.1%, n=5). Unknown items generally increased in frequency from April to June but then decreased in July.

Analysis of the bulk contributions of prey categories revealed similar trends, but emphasized the contributions of reptiles and scorpions in all months.

Correlation with climatic factors

Consumption of Coleoptera was strongly positively correlated with rainfall, both by frequency of occurrence (r,=0.82, P < 0.03, n=7) and by bulk (r,=0.90, P < 0.01, n=7), but was unrelated to other prey categories. Usage of Coleoptera and Hymenoptera together was high during the relatively dry summer month of January (Fig. la and b) in the wake of heavy November rains (Fig. lc). Large Coleoptera were more heavily represented in the diet in February as they increased their activity following intense thunderstorms, whilst the smaller Hymenoptera diminished in importance. Consumption of insect larvae subsequently increased sharply in March, whereas the use of Coleoptera and Hymenoptera declined precipitously. Intake of Hymenoptera was highest in January when there was little rain, and again from May to June when there was no rain (Fig. la and b).

Coleopteran bulk in the diet was significantly negatively correlated with mean temperature (r, = -0.79, P < 0.03, II = 7), but coleopteran frequency was not (rs = -0.68, P < 0.09, n = 7). No other correlations between any prey category and minimum, maximum or mean tempera- tures proved significant.

Frequency of occurrence of Coleoptera and insect larvae in the diet were significantly negatively correlated (rs= -0.83, P < 0.03, n=7), but not by bulk (rs= -0.72, P < 0.07, n = 7). Occurrence of Hymenoptera was not significantly related to occurrence of Coleoptera (frequency: rs = 0.51, P > 0.20, n = 7; bulk: r, = 0.31, P > 0.50, n = 7), but was negatively related to frequency of insect larvae (r, = -0.81, P < 0.03, n = 7). The relationship between consumption of Hymenoptera and reptiles did not attain significance (frequency: rs = -0.71, P < 0.08, n = 7; bulk: rs= -0.72, P < 0.07, n=7).

Prey size

Differences across months in the mean prey size index were not significant (Fig. 2a). However, mean prey size tended to increase in February and March, when more large insect larvae, Coleoptera, and Ptenopus gekkos were taken following the February rains. Unlike January, small Coleoptera and Hymenoptera were not taken during this period. Prey size then tended to fall during the dry months of April to June, because of a decrease in frequency of reptilian prey and an increase in Hymenoptera, and smaller insect larvae and Coleoptera (Fig. la and b). A subsequent rise in the consumption of large ChondrodactyZus gekkos tended to shift the mean prey size upwards in July.

702 S . P. DOOLAN AND D . W . MACDONALD

MEERKAT DIET A N D FORAGING 703

When averaged across all months, no significant variation was evident in mean prey size throughout the day (Fig. 2b). However, within most months the size of prey captured tended to rise over the course of the morning to a peak around midday. This trend was most evident in the dry summer month of January and in the winter months of May to July, when meerkats concentrated upon small prey such as Hymenoptera and Coleoptera during their initial foraging bouts after leaving the den, but later switched to larger and more profitable prey. No such trend was evident for February when larger Coleoptera were prominent in the diet, or in April when small larvae were present.

Dietary diversity and niche breadth

Meerkats used a mean of 6.7 i 1.1 prey categories each day, with increases evident in April and July (Fig. 3a). Dietary diversity (by bulk and by frequency of occurrence), as measured by the Shannon-Weaver index (H’), displayed a similar pattern, decreasing from January to March, and then increasing from April to July, with an early peak in April (Fig. 3b). Mean monthly dietary diversity (H’), by frequency and by bulk, respectively, ranged from 1.16 k 0.51 to 1.64 -+ 0.17, and from 1.16 & 0.52 to 1.64 h 0.08. There was significant monthly variation in diversity by prey bulk (F(6, 33) = 3.60, P < 0.02) but not by frequency of occurrence (F(6, 33) = 2.24, P < 0.08).

Mean monthly dietary diversity as measured by Simpson’s index (A) ranged from 0.23 i 0.08 to 0.46 i 0.25 for frequency data, and from 0.23 i 0.04 to 0.45 i 0.26 for bulk data (Fig. 3c). Differences between months were also significant for bulk (F(6, 33) = 2.92, P < 0.04) but not for frequency of occurrence (F(6, 33) = 1.56, P > 0.20). Complementing the Shannon-Weaver index, the probability of a forager selecting two items from the same prey category was highest in March and lowest in July.

Dietary niche breadth measured by Levin’s standardized index (Lev,,,B) ranged from 0.14 i 0.14 to 0.27 i 0.08 for frequency and 0.36 f 0.18 to 0.56 * 0.07 for bulk. This also declined from January to March and increased from April to July (Fig. 3d), but monthly variation was significant for neither prey bulk (F(6, 33) = 1.56, P > 0.20) nor frequency (F(6, 3 3 ) = 2.38, P < 0.07). Sex had no influence upon any of these composite indices.

Foraging behaviour

Meerkats foraged in bands throughout the day, and foraging activity occupied most of the active period. Seasonal shifts in timing of foraging activity (Fig. 4a) suggest that meerkats were limited by thermoregulatory costs, and avoided both very high and very low temperatures. Animals avoided the extremely high midday temperatures during January, February, and March by retreating into burrows for a ‘siesta’ period (Fig. 4b). This thermoregulatory avoidance behaviour changed, and siestas ceased, as temperatures dropped in April. As temperatures continued to fall and day length shortened in the winter, meerkats emerged from the den later, commenced foraging later, and finished their activity before sunset.

FIG. 1. Diet of slender-tailed meerkats, based on 421 hours of focal observations at close quarters of habituated individuals within three bands in the south-western Kalahari, January to July 1987. Diet is expressed on a monthly basis in terms of (a) percentage frequency of major prey items (n = 29631, and (b) percentage contribution by bulk (prey size units) of major prey items in the diet in the context of (c) monthly variation in rainfall and mean temperatures at Nossob, Kalahari Gemsbok National Park, October 1986 to September 1987 (error bars indicate mean minimum and maximum temperatures).

704 S . P . DOOLAN AND D. W. MACDONALD

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F I G . 2 . Variation in the size of prey eaten by meerkats in the south-westem Kalahari in terms of (a) monthly variation in mean size index i S.D.. and (b) hourly variation in mean size index f S.D. Prey were allocated to size categories on the basis of the number of chews taken to swallow an item, with a scale of 1 ('tiny') to 4 ('large'). Further details ofindex given in Methods. Observation details as in Fig. 1.

MEERKAT DIET AND FORAGING 705

Meerkats foraged throughout the day in tightly cohesive bands. Individuals generally main- tained nearest neighbour distances of less than 5 m and were often within 1 m of each other. Overall, the foragers typically remained along an axis of 20-50 m, sometimes extending up to 100 m in the dry season (Doolan & Macdonald, unpubl.).

Foraging by meerkats was a highly active, continuous process, accompanied by incessant contact vocalizations (‘contact vurruks’). Individuals carefully explored patches of abundant prey, such as ungulate middens, by walking slowly, scratching, and raking at the soil surface with virtually every step and pausing frequently to sniff vegetation, deadwood, and crevices. Good foraging success frequently resulted in intensive quartering of the area. Poorer habitat patches were traversed more rapidly, with animals running between foraging sites.

The extent to which different hunting strategies were used varied with the size of prey and the hunter’s individual skills. Most food items were snapped up in the jaws or hooked up with a paw and swallowed after chewing. The tongue and incisors were used to pick up small items such as ants or cocoons. Potentially noxious items, such as millipedes and lepidopteran larvae with urticating hairs, were rolled with the forepaws and repeatedly bitten whilst the meerkat shuffled backwards for several metres. In general, meerkats did not actively pursue prey, but gave chase over distances of several metres when large items such as lizards, gekkos or orthopterans were disturbed by other foragers or flushed from cover during digging. Foragers easily lost sight of prey once it became motionless. Reptiles were usually caught with the mouth, while pouncing was mostly employed to catch Orthoptera; juveniles and yearlings were more likely to chase orthopterans than adults.

Foraging interference was rare, and largely occurred during the dry season, with dominant animals forcing subordinates away from particular foraging holes, accompanied by growling and occasional side-barging. Prey caching was never observed and does not appear to be an option when foraging for small, widely dispersed items requiring intensive search (see Macdonald, 1976).

Variation in foraging efort and rate

Once a site was located, the effort spent in foraging varied from a few scratches at the soil surface (rake) to the total destruction of a set of prey burrows leaving mounds of spoil and a tunnel up to 30 cm deep. Holes were dug using swift alternate movements of the forepaws, and during excavations animals periodically used both forepaws together to drag out loose sand before inspecting the spoil. Deeper holes were dug for larger and better quality items such as scorpions or reptiles.

Overall, most foraging efforts were holes of forepaw (48.2%), head and shoulders (20.2%) or rake (15.4%) depth (Fig. 5). Prey was less frequently dug out of holes of half body (7.1%) or whole body depth (4.2%). Capture of items on the surface (3.4%) or in deep tunnels (1.4%) was rare. The occurrence of reptiles in the diet was greatest in March and July, when there was a correspondingly high incidence of foraging holes of head and shoulder, or half body depth. Similarly, most rakes occurred during January and February, a time when Coleoptera and Hymenoptera at the bases of clumps of vegetation were the most frequently eaten prey items.

Mean rates of overall prey capture, as items per hour and as bulk per hour, varied across the months (Fig. 6a and b). Overall prey intake during the summer months of January and April was significantly higher than during the cold winter months of June and July (frequency: F(6,33) = 4.48, P < 0.005; bulk: F(6,33) = 4.17, P < 0.007). In addition, adult females foraged at higher rates than

706 S. P DOOLAN AND D W MACDONALD

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Jan. Feb. Mar. Apr. Month

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FIG. 3. Measures of monthly variation in the dietary niche of meerkats in the south-western Kalahari (with 13 prey categories in total). Measures are presented as means of daily means i S.D. Observation details as in Fig.1. (a) Monthly variation in the number of prey categories used: (b) monthly dietary diversity. measured using thc Shannon-Weaver index (H'). for frequency and bulk contributed by prey items: (c) monthly dietary diversity. measured using the Simpson's index (A). for frequency and bulk Contributed by prey items eaten. and (d) monthly dietary niche breadth, measured using Levin'\ standardized index. for frequency and bulk contributed by prey items.

MEERKAT DIET AND FORAGING

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FIG. 3 (c, d)

males during the reproductively active months from January to April (Fig. 6a and b). At this season, the relative rates of consumption of prey items per hour were males 11.9 (S.D. 2.7, M = 12) and females 17.4 (S.D. 7.1, M = 8)(log transform, t = -2.65, P < 0.02). This difference was even more pronounced in terms of the bulk of prey intake (males: 19.7, S.D. 25.2, n = 12; females: 28.1, S.D. 7.5, n = 8, t = -3.1, P < 0.006). No such difference between the sexes was apparent between May-June, however, the pooled rates of intake for both sexes, according to both measures, were significantly higher in summer (January-April) than in winter (May-June): items in Jan.-April:

708 S. P. DOOLAN AND D. W. MACDONALD

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n Jan. Feb. Mar. Apr. May Jun. Jul.

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Fici. 4. Variation in the daily distribution of foraging activity by meerkats in the south-westem Kalahari during full days of observation. Observation details as in Fig. 1. (a) Shifts in the the daily distribution of the foraging activity period (m) within months. (b) Monthly shifts in the percentage of foraging records ('foraging activity') throughout the day.

MEERKAT DIET A N D FORAGING 709

FIG. 5. Monthly variation in foraging effort by meerkats in the south-western Kalahari. Relative effort measured as percentage of foraging records falling into seven categories, classified by the depth of hole dug to obtain prey. Details of depth categories given in Methods. Observation details as in Fig. 1.

14.1,S.D. 5.5,n=20;May-June:7.9,S.D. 1.5,n=15;logtransformed,t=5.9,P<0.0001.Bulk in Jan.-April: 23.1, S.D. 7.4; May-June: 12.1, S.D. 2.5, t-6.61, P<O.OOOl).

Discussion

The data presented here confirm the generalist, largely insectivorous foraging habits reported for meerkats by previous workers (Lynch, 1980; Roberts, 1981; Smithers, 1983). Dispersion, abundance, and availability of food resources underlie the dietary habits and foraging behaviour of many carnivores (e.g. Corbett & Newsome, 1987; Doncaster, Dickman & Macdonald, 1990; Mills, 1990; Maas, 1993; Lucherini & Crema, 1994; see Creel & Macdonald, 1995). Hence, it is not surprising that meerkats primarily feed upon the high populations of insects and other arthropods present in the semi-arid savanna of the Kalahari. In the period reported here, insects accounted for 78.1% of all prey captured, and 68.9% of the diet by bulk. Dietary requirements were largely met by insect larvae and adult Coleoptera, with small reptiles providing a valuable supplement in all months.

In contrast to Lynch (1980), we found no evidence of termites in the diet, although an outbreak of termite alates the following year was utilized heavily (Doolan, unpubl.). Harvester termites Hodotermes mossambicus are a staple of bat-eared foxes Otocyon megalotis in the study area, particularly during the winter (Nel, 1978, 1990). During a drought in the north-western Cape, termites accounted for less than 1 YO of prey remains in scats (Roberts, 198 l), but they may be a more significant dietary component in wetter habitats elsewhere. For instance, they occurred in

710 S. P . DOOLAN AND D. W. MACDONALD

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0 ' Jan. Feb. Mar. Apr. May Jun. Jul.

Month

FIG. 6 . Mean monthly foraging rates for male and female meerkats in the south-western Kalahari expressed as (a) items per hour, and (b) bulk ingested per hour. Observation details as in Fig. 1. Points (& S.D.) are monthly mean rates calculated from daily means. Bulk calculated from prey size indices (Fig. 2).

MEERKAT DIET AND FORAGING 71 1

40% of 98 stomachs examined in the Orange Free State by Lynch (1980) and were more frequent in the summer (61%) than the winter (27%). Termites also commonly occurred in the diets of yellow mongooses Cynictis penicillata (74%: Lynch, 1980; 37%: Smithers, 1983); dwarf mongooses Helogale parvula (33.1%: Smithers, 1983; 8.1%: Hiscocks & Perrin, 1991) and banded mongooses Mungos nzungu (33%: Rood, 1975; 32.7%: Hiscocks & Perrin, 1991). However, it is worth noting that in Sadie’s (1983) analysis of banded mongoose diet, the biomass contribution of termites (0.2%) was minimal in comparison to their numerical contribution (25.8%) or their overall occurrence in scats (95%).

Reptiles were taken in all months of the year, and accounted for a greater proportion of the daily dietary bulk (22.0 & 13.1 %) than suggested by their frequency of occurrence (1 1.1 f 7.8%). Nocturnal species such as Nucrus tesselata and Ptenopus garrulus were frequent in the diet, highlighting the meerkat’s reliance upon foraging for prey in subterranean refuges. This has also been commented upon by Smithers (1983) with respect to capture of scorpions. Consumption of reptiles was high in March and again in July. Small Ptenopus garrulus in shallow tunnels were the focus of the earlier peak, which followed the heavy rains in February, when these gekkos were most active, In July, much more effort was expended upon digging deep foraging holes to capture larger Chondrodactyius anguiifer.

Reptiles occurred at a comparable level in the diet of yellow mongooses at a site in the semi-arid Coastal Strandveld of South Africa, tending to increase during the warmer summer months, but were largely absent from scats collected at another site in the Karoo (Avenant & Nel, 1992). However, in the same study, rodents accounted for the bulk of vertebrate consumption in the Strandveld (40.8% total relative occurrence vs. 13.3% for reptiles) but were rare in the Karoo. At a wetter site in Natal, reptiles were not recorded in the diets of either dwarf or banded mongooses, whereas rodents occurred in 7.1 YO of banded mongoose scats but only 0.6% of dwarf mongoose scats (Hiscocks & Perrin, 1991). Reptiles were also relatively less frequent (0.2%) in the diet of small Indian mongooses at a Mediterranean site in Europe than either mammals (26%) or birds (13%). These observations suggest that meerkats are more reliant upon reptiles than other mongooses but that they are not likely to take larger and more active vertebrate prey.

Seasonal differences in meerkat dietary composition arose due to differences in the relative contributions by insect larvae, Coleoptera, and reptiles. There was a greater dependence upon ants and small insect prey in the dry season as general prey abundance declined. This was probably due to larger, more energetically rewarding items such as the large scarabaeid, tenebrionid, and carabid larvae descending deeper into the soil during the dry season, leaving ants and small Coleoptera readily available with little effort. Further investigation of this relationship would require detailed seasonal monitoring of prey abundance (see Hiscocks & Perrin, 1991; Avenant & Nel, 1992; Waser et al., In press).

Qualitative descriptions suggest that meerkat foraging behaviour is more comparable to that of banded mongooses than to dwarf mongooses, which tend to invest less effort in digging activity (Hiscocks & Perrin, 1991). In the present study, there was an overall seasonal shift in prey availability, partly compounded by the effects of shifts in meerkat foraging activity in response to changing thermal constraints. These factors combined to negate the prediction that meerkat foraging effort, as measured by depth of foraging hole, would be greater during the dry season. By contrast, marked seasonal changes in the relative types of digging activity have been recorded for foraging echidnas Tachyglossus acuieatus (Smith, Wellham & Green, 1989). This would indicate that better indices of foraging effort among meerkats would examine the ratio of failed and successful foraging activity, as well as their depth distributions.

712 S . P. DOOLAN AND D. W. MACDONALD

In contrast to the banded mongooses observed by Sadie (1983) in the more temperate climate of the Transvaal, meerkats captured fewer myriapods in the winter, paralleling the decreased activity of these invertebrates in the cold dry period of the southern Kalahari (Dangerfield, Milner & Matthews, 1992). Indeed. myriapods were not a favoured prey: they were mainly captured during intervals of low foraging success and were more heavily utilized by yearlings and juveniles than by adults (Doolan, unpubl.).

As with all other mongoose species, gregarious (e.g. Rood, 1975; Rasa, 1987), semi-gregarious (e.g. Albignac, 1976; Rasa et ul., 1992) and functionally solitary (e.g. Stuart, 1983; Baker, 1989; Palomares, 1993). meerkats were very much opportunistic feeders. Animals selected a wide range of prey items and classes throughout the year but niche breadth was greater in terms of bulk than frequency of occurrence. Changes in diet composition reflected invertebrate responses to seasonal pulses in rainfall. However, even when these items were heavily foraged, no single category dominated the diet. This indicates that meerkats are never entirely dependant upon any of these categories, a sound strategy in the light of the variability in abundance and availability of resources in the Kalahari.

Food intake, as measured by the hourly number of items and hourly bulk of items, was lower for winter foraging than for summer. Meerkats were apparently unable to compensate for this by increasing the length of time spent foraging when harvesting rate and prey size were diminished. This was presumably due to the exigencies of thermoregulation and the necessity to minimize exposure to adverse temperature conditions below the thermoneutral zone (Muller & Lojewski, 1986).

In addition, breeding females foraged at higher rates than males throughout the period during which they were reproductively active from January to April (Doolan, 1994; Doolan & Macdonald, In press b). This was partly due to the fact that males typically allocated more time to vigilance and were stationary more often than females (Doolan, 1994), a feature also invoked to account for sex differences in behaviour among primates such as squirrel monkeys, Sainiiri oerstecii (Boinski, 1988). However, once the energetic demands of gestation and lactation had ceased, foraging rates of female meerkats returned to levels comparable to those of males, indicating that a primary reason lay in the increased energetic demands of reproduction. Since meerkats are monomorphic, the contrast was not imposed by sex-related differences in body size which could affect food requirements or prey-handling strategies (Rose, 1994). Dwarf mongoose females have been observed to respond in a similar manner, by increasing the amount of time that they spend foraging whilst breeding (Creel & Creel, 199 I). Co-operatively breeding carnivores have significantly higher rates of pre- and post-natal investment in their young than other carnivores (Creel & Creel, 1991; Creel & Macdonald, 1995). By accounting for the bulk of babysitting and provisioning behaviour, helpers can make major contributions to offsetting the energetic and temporal demands of breeding (Rasa, 1987; Creel & Creel, 1991).

Foraging Competition is often regarded as a major factor in the evolution of gregariousness (Macdonald, 1983; Kruuk & Macdonald, 1985; Janson, 1988; Barton & Whiten, 1993; Wrang- ham, Gittleman & Chapman, 1993). Direct aggressive interactions, decreased harvesting rates or increased day range lengths are a virtually unavoidable consequence of group-living (Rasa 1986a, h: Janson, 1988; van Schaik & van Noordwijk, 1988; Packer, Scheel & Pusey, 1990; Henzi, Byrne & Whiten, 1992; Byrne ef ul., 1993). At the minimum, individuals may have to reschedule their activities to take account of those of others, and end up with more or less than their desired foraging time (Dunbar, 1988, 1992).

If they were to concentrate upon vertebrate feeding, one would predict considerable foraging competition between individual mongooses in large groups, both in terms of mutual interference

MEERKAT DIET A N D FORAGING 713

and indirect scramble competition. As noted by Gorman (1979), groups of mongooses “in constant vocal communication, are not likely to be conducive to the successful exploitation of small vertebrates”. Observations of yellow mongooses, Cynictis penicillata, which are sympatric with and similar in size to meerkats, support this suggestion. They are more reliant upon small to medium-sized vertebrates than meerkats (Lynch, 1980; Shepherd, Leman & Hartwig, 1983; Avenant & Nel, 1992), and tend to forage solitarily (Earle, 198 l), coming together to interact and sleep at the dens.

It has been argued that the high renewal rate of insect prey p a s e r , 1981; Waser & Waser, 1985; Waser et al., In press), combined with its relative predictability and dispersed distribution, facilitates group-living in insectivorous mongooses. This may hold true for meerkats since animals in the present study typically foraged within several metres of each other, but overt food- related aggression was rarely witnessed (Doolan, pers. obs.). However, a definitive test of this hypothesis requires a detailed examination of spatial relationships within foraging bands and the accompanying patterns of approaches, departures, and displacements among individuals (see Robinson, 1981; Barton, 1993).

The nature of their prey base is consistent with the suggestion that insectivory sets the stage for the evolution of more complex social behaviour among meerkats. However, this also has implications for the sort of assistance that helpers can provide during co-operative breeding efforts and the way in which the young are provisioned (Rasa, 1987; Doolan, 1994). Other co- operatively breeding carnivores, such as many canids (Moehlman, 1986, 1989) and the semi- gregarious yellow mongoose (Rasa et al., 1992), can extend the safer period of development for the young by provisioning them with vertebrate prey at the den. However, this option is not available to meerkats or the other predominantly insectivorous mongooses since it is energeti- cally inefficient to bring small items to the den.

This work forms part of a D.Phil. thesis by SPD who was funded by the Royal Commission for the Exhibition of 1851, an Anglo-Irish Scientific Exchange Scholarship, the Prendergast Trust and Wolfson College of the University of Oxford. DWM held a visiting fellowship from the CSIR, hospitably sponsored by Prof. John Skinner of the Mammal Research Institute at the University of Pretoria. We gratefully thank the Trustees of the National Parks Board of South Africa and, in particular, Mnr. Elias LeRiche, warden of the KGNP, for permission to undertake this research. Earlier drafts of this paper benefited greatly from input by Fran Tattersall, Emilio Herrera, and an anonymous referee.

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