J . Zool., Lond. (1996) 240, 59-73

Dispersal and extra-territorial prospecting by slender-tailed meerkats (Suvicata suricatta) in the south-western Kalahari

S . P. DOOLAN*' A N D D. W. MACDONALD

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

(Accepted 8 November 1995)

(With I figure in the text)

Dispersal in slender-tailed meerkats or suricates, Suricata suricafta, in the south-western Kalahari occurred mainly during the early breeding season, and was age- and sex-dependent. Among yearlings, more males than females were immigrants, but more females than males disappeared. There were no sex differences in dispersal among two-year-olds, but among animals aged three years or older, more males than females emigrated. Most dispersers moved into adjacent bands or joined other transients, and females apparently suffered higher rates of mortality than males. Kinship with the same-sex or opposite-sex breeder had no discernible effect on the likelihood of dispersing. Both males and females madeprospeciing~~ra~s to other groups, apparently to assess dispersal and breeding opportunities. Males made frequent and repeated forays, often in coalitions with other band members or transients, whereas prospecting by females was generally solitary, and they were not known to make multiple forays. Prospecting males successfully took over dominance of two bands, and attempted to take over hands on three other occasions. Animals attempting to join or follow a band (trailers) behaved submissively and were readily chased by residents, whereas those attempting a dominance takeover (invaders) scent-marked at a high rate and showed no submissive behaviours. Dispersing meerkats maximized reproductive success by increasing their mating opportunities, while animals which delayed dispersal received indirect benefits by helping to raise kin.

Introduction

Dispersal plays a central role in mating systems and life-history tactics (Greenwood, 1980), and has a strong influence on demography, spatial distribution, and social organization (Johnson & Gaines, 1990; Stenseth & Lidicker, 1992; Alberts & Altmann, 1995). This is particularly true among co-operative breeders like the slender-tailed meerkat, or suricate Suricuta suricatta (Doolan, 1994; Doolan & Macdonald, In press a). In such species, benefits of philopatry (Zack, 1990; Stacey & Ligon, 1991) and constraints on dispersal (Emlen, 1991; Koenig rt al., I992), when balanced against competition for reproductive opportunities, can result in the accumulation of a labour force of potential helpers. This in turn can enable dominant animals to suppress breeding in subordinates (Vehrencamp, 1983; Keller & Reeve, 1994).

Meerkats are gregarious mongooses found in the arid and semi-arid regions of southern Africa (Smithers, 1983; Doolan & Macdonald, In press b). As in other group-living mongooses and carnivores (Rood, 1990; Creel & Waser, 1994; Creel & Macdonald, 1995; Waser, In press), meerkats experience an interplay between the chances of inheriting a breeding position versus the

'Address for correspondence 'Present address: Earthwatch Europe, Belsyre Court, 57 Woodstock Road, Oxford OX2 6HJ, UK

59 C 1996 The Zoological Society of London

(10 S. P DOOLAN . 4 N D D. W . M A C D O N A L D

chances o f obtaining 21 breeding position through dispersal. and the costs and benefits of raising young versus helping close kin to raise young. This interplay is likely to be important in determining thc rates and consequences of dispersal. Here we document dispersal and inter- group movements of meerkats in relation to age. sex. season. and social context. We then consider these data i n the light of fitness costs and benefits that might affect the decision to dispme.

The study was conducted in the south-western Kalahari. in the Kalahari Gemsbok National Park o f South Africa. and i n the adjoining Genisbok National Park of Botswana. Observations were concentrated on three bands of tneerkats in a 50 km' area immediately south of Kwang pan (25 17's. 20 32'E).

'The habitats of the southern Kalahari are characterized by an extremely open savanna of tufted grasses. interrupted by A i w i t i and Boscirr trees (Bothma & de Graaf. 1973; Acocks, 1988). Leistner 8L Wergcr (1973) described 12 plant communities which correspond to the dune, river terrace. and river bed habitats detined by Van Rooyen. Bredenkatnp & Theron (1991).

The Kalahari is subject to erratic rainfall and wide daily and seasonal temperature variation. An annual mean of about 250 mm of rain falls during a medium-term (approximately 20 year) cycle ( V a n Rooyen et u1., 1990). Over the duration of the study, above average rains fell in 1987/ 88. whilst 1985 86 was exceptionally dry. Clearly defined seasons are lacking and the simplest distinction (Mills. 1990) is betwren the hot, wet summer (October to April) and the cold, dry winter (May t o September). Mean monthly temperatures in the summer are above 20"C, and at least 70'1lt of the rains Fall betwcen January and April. Negligible amounts of rain fall in the dry season. during \vhich mean monthly temperatures are below 20-C (Leistner. 1967). November to February are the hottest months of the year and June to August the coldest. Sand surface tcmpewtrrre can exceed 70 C when the air temperature i s 40 C, and there may be a 3O'C dilTcrencc between open and shade temperatiires at ground level (Leistner. 1967). Conversely, cloudless skies and nocturnal reradiation result in occasional ground frosts between May and September. with night temperatures plummeting a s low as - 10 C.

Methods

Thrcc I'ocal band3 o f meerkats (Raiders. Woodlanders. and Outliers) were monitored in detail over 10 hand years during t h t follo\\irig periods: Raiders: October I984 to July 1988: Woodlanders: January 1986 to Jul> IWS: Outliers: Ma) 19x6 to Ju ly 1988. Focal bands Lvere \veil habituated and were watched between da\\ 11 ;tiid dusk on foot from ;I distance of 1 5 i n . All deaths. injuries, immigrations, emigrations and unexplained dis;ippcarances \\ere recorded: the presence of floaters and transient groups was noted ad /ihi/wi/. .Animuls ~ - e r c indi\tdually identifiable by scarring and by coat colour and pattern. and individuals were considered to be present for monitoring purposes if the! \yere present within bands for more than 4 \vecks during the breeding >ear (October to September). Individual fcmales produced up to 3 litters each durins .I hrceding se;iwn \\ hich lasted from October through J L I I ~ . hut the exact tiiiiing and duration of hreecling :rcti\ity varied bet\veen years (Doolaii & Macdonald. In press ( I ) .

Meerkits are scxuall~ monomorphic and animals \yere divided into 3 age classes (juveniles, yearlings, and adults) &lined i n terms o f the number of drq seasons (May to September) following an individual's birth (Rood. IWO). Juveniles had not yet survived a dry season. yearlings were between their first and second dry \easoiis. iind adults had sunivcd 2 or nioredry seasons. Adults were further categorized as 2 years old, or 3 o r Inore !cars old. Age clasws \\'ere identifiable 011 the basis of body size and the relative development of mitsctil~~r rtct9c.s on thc c r m n o f the head and abo1.e the brows (Sadie, 1983).

MEERKAT PROSPECTING A N D DISPERSAL 61

Emigrations were recorded when individuals disappeared and were subsequently noted in other groups or seen outside their own band, often in association with transients or other dispersers from the same band. Death was recorded when a dominant female disappeared (dominant females are extremely unlikely to disperse: Rood, 1987, 1990; Waser, Creel & Lucas, 1994), when predation was directly observed or inferred from the presence of a predator, or when an animal failed to emerge from the den into which i t had descended the previous evening (meerkats are strictly diurnal). Other losses were classed as unexplained disappearances. Immigration occurred when a band was joined by an outsider for a t least 4 weeks. Members of both sexes made occasional forays from their own ranges, and these animals were defined as prospc~c~or.s (McGrew & McLuckie, 1986) if they temporarily trailed after or joined a group and subsequently left to rejoin their original group. Animals which originated outside the core bands and briefly trailed them were also classified as prospectors. We refer to immigrations, emigrations, and disappearances collectively as dispersal events. In calculating rates of dispersal, events that occurred in the absence of observers were allocated equally among months between those for which definite information was available.

Relationships between dispersers and opposite-sex breeders were classed as: close kin (offspring and f~ill sibs), moderate kin (half sibs), and unknown. Paternity was assigned to the dominant male, except when takeovers occurred, in which case the previous dominant was designated as the father. When joint litters were produced (Doolan & Macdonald, In press cc), observations on the order of births and subsequent size discrepancies between the young on emergence from the den permitted identification of the mothers.

All statistical tests followed Sokal & Rohlf (1981) and Kanji (1993). Williams’ correction was applied to the G-tests. Statistical tests were 2-tailed and P < 0.05 was regarded as significant. Means are expressed &one standard deviation.

Results

Dispersal

Records for individual dispersal events were available over 104 focal group months, during which 51 individuals were monitored as juveniles, 36 as yearlings, 21 as two-year-olds, and 40 as three-year-olds and above. The known dispersers (emigrants and immigrants) consisted of 27 males and 11 females (Gad, = 6.18, 1 d.J, P < 0.05), of which 63.2% were adults, 34.2% were yearlings, and 2.6% were juveniles. There were 19 emigrations, 19 disappearances, and 19 immigrations (Table I; for simplicity the single juvenile male immigrant is not included).

There were no sex differences in the occurrence of all dispersal events combined (emigration, immigration, and disappearances) within any of the age classes, although among meerkats aged three years or older a non-significantly greater proportion of males (54.2%) than females (25%) dispersed (Table 1). Among yearlings, a significantly higher proportion of males (36.8%) than females (5.9%) were immigrants into focal bands. They also emigrated at a higher rate (males, 21.1 YO; females, 5.9%), but more females (52.9%) than males (15.8%) disappeared (Table I). No sex differentiation was apparent amongst two-year-olds. A significantly higher proportion of males (33.3%) than females (6.3%) aged three years and above emigrated (Table I).

Focal bands received immigrants at a rate of 1.3 5 1.3 males and 0.6 i 0.8 females per band year. Bands lost males at a rate of 1.4 IIZ 1.5 per band year to emigration, and 0.7 & 0.8 per band year to disappearances. Females were lost by emigration at a rate of 0.5 i 1.1 per band year and by disappearances at 1.2 i 1.9 per band year. Differences between the sexes in dispersal rates per band year were not statistically significant (Wilcoxon signed ranks tests; P > 0.05).

The incidence of dispersal events fluctuated throughout the year (Fig. 1). Most emigrations and immigrations took place in November, early in the breeding season, prior to, o r coinciding with, the onset of mating. Combining all age-sex classes, dispersal events were significantly

seasonal: cmigration and immigration occurred mostly in the first quarter of the breeding year (emigration: Kolmogorov-Smirnov D = 0.54, P < 0.002: immigration: Kolmogorov-Smirnov D = 0.56. P < 0.002). while disappearances began somewhat earlier (Kolmogorov-Smirnov D = 0.45. P < 0.003). Emigration of adult males was most pronounced in November (Fig. la), while immigration of adult and yearling males was highest in November and December (Fig. Ib). Dispersal frequency was higher in males than in females for most months of the year. A secondary, although minor. peak of yearling immigration occurred during the dry season. Disappearances, in which yearling females were prominent, began late in the dry season in August and continued at a high level until January (Fig. Ic).

The large number of animals of unknown relatedness confounded any patterns in the relatedness of dispersers to either the opposite-sex (Table 11) or same-sex dominant breeders (Table I l l ) . Among animals of known kinship, females were more likely than males to emigrate (66.7% vs. 27.3%) or disappear (50.0"/0 vs. 20.0%) when they were related to the opposite-sex dominant. However, females were less likely to disperse than males when they were kin to the same-sex dominant (emigration: 33.30/0 vs. 55.6%: disappearance: 00/0 vs. 16.6%).

Ftirr f ' dispwsei~s

Of the 19 emigration events recorded (14 males and five females), the destinations of 13 were known. Six of eight males moved to adjacent groups. as did all five females. Another two males moved to more distant ranges and two joined transients. Although some dispersers moved out of the study site and werc not resighted, those that were relocated travelled distances of2.25-5.5 km after dispersal, and traversed up to two ranges. One yearling male travelled at least 4.3 km in just two days and prospectors were followed while covering distances of up to 4.7 km per day.

M E E R K A T PROSPECTING A N D DISPERSAL 63

1 .o

0.8 r, s E 2 0.6 b f 5 0.4 E E

Q

Q. v )

.- c 0 .-

0.2

0.0

1 .o

2 0.8 E" n 2

0.6 f a

O N D J F M A M J J A S Month

U A d f E B Y m

Ad m

8

B $ fi 0.2

C

m a,

2 0.4

0.0 O N D J F M A M J J A S

Month

Month

m y f O A d f EZlYm I Ad m

0.8 q 0.6

Q 3

OI 2

k 0.4 9 v)

2 0.2

0.0 O N D J F M A M J J A S

Month

FIG. I . Seasonal occurrence of dispersal events per group observation month for three meerkat bands in the south- western Kalahari. Months follow the breeding year from October to September. Ad m: adult male; Y m: yearling male; Ad f: adult female; Y f yearling female. (a) Seasonal occiirrence of emigration among adult and yearling males and females. (b) Seasonal occurrence of immigration among adult and yearling males and females. (c) Seasonal occurrence of disappearances among adult and yearling males and females. (d) Seasonal occurrence of forays by male and femalc meerkats.

64

Status of oppmitc-sex doinin'int breeder

close hin i i o i i - k i n t in k now n __

~- ~ - ~-

3 (21.4) 8 (57.1) 3 (11.4) I (14.3) 4 (57.1) 2 (28.6) 4 (19.0) 12 (57.1) 5 (73.8)

2 (40) I (20) 3 (40) 6 (50) 6 (50) 0 8 (47.1) 7 (41.1) 3 (11.8)

5 (26.3) 9 (47.4) 5 (26.1) 7 (36.8) 10 (51 .6 ) 2 (10.5)

I? (31.6) I9 (50.0) 7 (18.4)

Several features suggest that unexplained disappearances were emigrations rather than deaths: in 10 band ycars only two of nine dominant females and none of the nine dominant males disappeared so that within band mortality of breeding adults was low; disappearances showed a broadly similar scassnal pattern to emigration and immigration, beginning late in the dry season and continuing through until January: of 19 disappearances seven coincided with periods of flux and eight occurred synchronously with at least one other individual, suggesting that coalition dispersal occurred. I f all disappearances are considered to be emigrations, then the greater proportion of females ( 2 7 . 3 % ) than males (13.2%) involved suggests that females either travelled

. _ . ~. -. .

Male.; Emigratioil Disuppcat-cince E + D

Emigration Disappearmcc E i D

E i n igrn ti o n Di sit ppeara nee k t- D

~ ~ c l n a l r s

Both sC'\es

Statti\ of same-sex dominant breeder

4 (28.6)

9 (42.0)

? ( d o )

7 (41.2)

6 (31.6) 10 (52.6) 16 (42.1)

5 (71.4)

5 (11.7)

tinknown -

5 (35.7) I (14.1) 6 (28.6)

2 (40) l ( 5 8 . 3 ) 9 (52.9)

7 (36.8) 8 (42. I )

15 (39.5)

MEERKAT PROSPECTING A N D DISPERSAL 65

further than males or suffered higher mortality during their travels, particularly at the yearling stage when discrepancies between the sexes were largest. When all disappearances and known deaths were ascribed to mortality, 22.7% of all yearlings and adults monitored died during the study; fewer males (13.2%) than females (34.1 %) died. Amongst adults only, the estimates were 11.8% for males and 22.2% for females, still indicating lower survivorship in females. Juveniles which survived their first three months of life were present throughout the remainder of the breeding season (Doolan & Macdonald, In press a) and probably experienced the same mortality rate as older age classes.

Mortality and serious injury from conspecific attack was a considerable risk among both philopatric and dispersing meerkats in the Kalahari. Episodic targeting of aggression (sensu Vick & Pereira, 1989) within groups resulted in severe injuries among animals being ousted. Many prospectors also showed signs of wounding and at least one adult male died from the injuries that he sustained from resident males while attempting to immigrate into the Outliers.

Prospecting forays

Prospecting meerkats foraged very little and spent most of their time travelling. They generally followed meandering routes and spent long periods inspecting burrow systems and sites used as vantage points for vigilance. They intensively investigated fresh scent-marks and clearly followed these to locate bands. Rather than retreating, prospectors made straight towards groups they encountered and then began cautiously to trail the other animals. When chased by residents they behaved submissively or retreated a short distance before returning.

Seven males and five females from the focal bands were known to make a total of 23 individual forays to other groups, sometimes in association with other group members. Males accounted for 78.3% of the forays made by individuals and females for 21.7%. Of 17 meerkats observed actually leaving their bands, only six (35.3%) left after an increase in agonistic interactions with other band members (Gad, = 1.45, 1 d$, P < 0.05). Twelve of 18 forays (66.7%) took place after encounters with other bands or intruders, while six (33.3%) were made in the absence of such encounters: of these six, three coincided with aggressive targeting and another with the loss of a litter to predation. The immediate circumstances surrounding five forays were unknown. Unlike dispersal events, forays took place throughout the breeding season (Fig. Id). The peak in prospecting recorded for July resulted from a series of forays and attempted takeovers by coalitions of focal males in 1988.

Prospecting males made up to four excursions apiece over extended periods of time. They often returned repeatedly to trail the same bands and their forays lasted for up to two months. Females were never observed to make more than one foray each. These lasted up to a month and were generally made alone, but following a series of encounters with a band of prospecting males, three females formed a prospecting coalition and joined the males. Of 13 forays made by males, three were coalitions of two male band mates and one involved three male band mates. The same four males were repeatedly involved in coalition forays: one male joined three coalitions and three males joined two coalitions each. Males immigrated into their target groups on only one occasion. Encounters between prospecting males from different groups were cautiously amicable: they were observed allogrooming, resting, and denning together, and relationships were known to last for 1- 16 days. Including both sexes, 30.4% of the 23 forays made by individuals were carried out solitarily. 52.2% in the company of band mates, and 30.4% with other prospectors (this figure exceeds 100% since it includes the merging of the female coalition with prospecting males).

6 h S . P. DOOLAN AND D. W . M A C D O N A L D

When prospectors and immigrants attenipted to join groups they met with gang ~ n r responses, involving aggressive chirr . r i i ig vocalizations and a ritualized rock i~ ig gnii (Ewer, 1963; Macdonald, 1992; see Rasa. 1987 for ;I description of similar behaviour in dwarf mongooses). The intensity of residents' reactions curicd: potential immigrants encountered most aggression from residents of the same sex. whcrcas opposite sex residents. particularly younger animals, sometimes tolerated, or initiated. contacts wit11 prospectors during recurrent encounters. In general, repeated chasing by residents occurred. invariably accompanied by increased rates of scent-marking by the dominant male: burrows and vantage points used by the intruders were vigorously marked. When trailers \+ere persistent these displays were reinforced by threat scratching and frenzies of scent-marking involving chin-cheek marking and threshing of vegetation (Doolan & Macdonald, In press ( I ) . For example. during one of a series of encounters between the Outliers and a transient trio ofniales. therc were 66 chases over 7 hours and 29 minutes (8.82 h-I), and the dominant male deposited 220 scent-marks (29.41 h-l): by contrast. once the intruders had left, the dominant marked a t ;I significantly lolver rate of only 0.99 h - ' over the remaining three hours of the afternoon ( i2 r 81.1. I c1.f : . P 0.005). The notable exception to this pattern occurred in December 1986 LA lirn t\$ o yearling male prospectors were sniffed, allomarked, and allogroonicd by the dominant male Lt-ithin hours of first trailing the Raiders. and joined the band on the day after their initial appeat-ancc. Just prior to this. three subordinate males had emigrated to take over the Woodlanders. but the remaining dominant male and two heavily pregnant females had resisted the attempted immigration of the two adult males ousted from the adjacent band.

Don I i m i i I w t c r l i cw w r s

Prospecting males appeared t o asses dispersal and takeover opportunities within groups they repeatcdlq visited. Male 03 from the Raiders made at least two forays to the Woodlanders over a period of 19 months. and on one occasion was accepted into the band for several hours. During a subsequent mating period (November 1986) he was ousted from the Raiders in a series of highly aggressive attacks initiated by the dominant male and a lower ranking subordinate male 04. He as thcn reaccepted following two separate forays to the Woodlanders (during which he associated aith tnu other prospectors) and a presumed foray to the contiguous HV band range. Some days later he led the Raiders into the Woodlanders range. moving between the two bands over the course of an hour until they caught sight of each other. After a series of running battles. the two inales in the Woodlanders were ousted and male 03 and two subordinates (males 04 and 16) transferred to the band. Unlike trailing prospectors, the three invaders, particularly male 03. scent-marked at a high rate during the takeover and repeatedly overmarked the resident Woodlander males' deposits.

Two adult malcs trom the Outliers band (46 and 47) attempted to take over the Raiders in July 1988. This attempt M ~ S again preceded by a visit by male 47. during which time he associated with a female ( 2 3 ) who had herself been temporarily ousted from the Raiders during a mating period. During their takeover bid. both males conspicuously scent-marked. They repeatedly tried to associale with the females and. in the only observed copulation, male 47 managed to mate with female 23 (8.4-3 mounts fi-' over two hours). The conflict then escalated: the dominant resident male seriously injured male 46's scrotum during one of 11 physical fights which ensued before the invaders returned to the Outliers.

Folloiving ;L brief encounter 1 1 days later. males 46 and 47 and the yearling male 79 again left the Outliers to trail the neighbouring (and smaller) KFI band. The invading males scent-marked

M E E R K A T P R O S P E C T I N G A N D D I S P E R S A L 61

within nietrcs of the band before being rebuffed by the resident male and, again, the intruders' attentions were directed towards the females. One of these (female 42) had emigrated from the Outliers 19 months earlier. On four occasions, invaders allogroomed with this female and twice the dominant female allowed herself to be sniffed carefully by them, but encounters were invariably broken up by a charge from the resident male. After nearly six hours of trailing and fighting over a distance of 1.4 km, the band fled from the area and the invading males returned to the Outliers.

Another takeover attempt in November 1986, by five males trailing the Outliers, coincided with an outbreak of targeted aggression directed by the dominant female (06) at the other adult females. Over several days the subject of most of these attacks, female 40, repeatedly approached and temporarily joined the invading males. The dominant female and the resident males tried to prevent this, and on each occasion female 40 was forced to submit to female 06. When the invaders were chased more than 3 km away, they were finally joined by female 40 and two other adult females (41 and 42). Band membership was dynamic over the following two months, with hostile sub-groups composed of varying numbers of the original females combined with both original and immigrant males. At the end of this period the Outliers stabilized at the original dominant female, two of the original males and two immigrant males.

Discussion

Rood (1990) outlined five reproductive strategies available to dwarf mongooses, of which four involve dispersal: (1) remain within the natal group and wait to inherit the breeding position; ( 2 ) emigrate and join a group containing a breeder of the same sex, thereby entering a reproductive queue; (3) disperse and attain breeding status in a group remnant lacking a same-sex breeder; (4) disperse and join with floaters of the opposite sex to form a new group; and (5) emigrate in a coalition, take over a group and oust resident adults of the same sex. Amongst those animals for which the route to breeding status was known, philopatry and inheritance occurred in 43% of females but in only 8 % of males (Rood, 1987, 1990).

Kalahari meerkats adopted essentially similar strategies to dwarf mongooses but the duration of our study meant that the probability of detecting breeding position inheritance by philopatric animals was low. No two-year-olds remained on the natal territory throughout the breeding season. In the females this may have reflected the fact that bands contained young dominants, but the apparently higher survival rates of meerkats over dwarf mongooses may also have contributed. Meerkats of both sexes transferred into bands as subordinates and as new dominants, and many of the dispersers and prospectors departed with kin, particularly with siblings or with animals that they had previously helped to rear. Coalition forays and dispersal, sometimes culminating in takeover bids, were frequent among males but less prevalent among females, and males were more likely than females to join up with other transient males in temporary associations. Among those dispersers for which both source and destination were known, most meerkats moved to adjacent bands, a pattern typical of many carnivores and primates (Pusey & Packer, 1 9 8 7 ~ ; Waser, In press). Data upon dispersal distances have been criticized as reflecting sampling effort rather than biological reality (Barrowclough, 1978; W. Koenig, pers. conim.). Nevertheless, the patterns of prospecting and transfer indicate that the ties of relatedness and familiarity did not end at the end of the iiatal home range (S. Creel, pers. comni.), a fact which should be recognized i n future studies of dispersal.

6X S. I’ D O O L A N A N D D . W . M A C D O N A L D

C’o.rt.v rirrcl benefits qf‘ tlisprrstil tit id pliilopcitrj-

Dispersal is oftcn connected ivith the duration of tenure, and continued presence, of the opposite sex parent on the natal territory, indicating the importance of inbreeding avoidance (Clutton-Brock. 1989: Holekamp & Sherman. 1989: Hooglund, 1992). Intra-sexual competition also exerts an influence. i n monogamous species, or when adults of both sexes remain in the natal group. generally no skew is evident i n the sex ratio of dispersers (Rood, 1987; Pusey, 1987; Pusey & Packer. 1 9 8 7 ~ . 1): Clutton-Brock. 1989; Gese & Mech. 1991; Fuller et ul., 1992; Wolff, 1993). Our data were insufficient to detect any effects of kinship with the opposite-sex breeder on dispersal in meerkats. However. the role of intra-sexual competition was highlighted by the fact that trailers. particularly adults. were most vigorously attacked by same-sexed residents. Yearlings and young adults encountered less aggression and resistance, as in dwarf mongooses (Creel. Wildt & Monfort. 1993: S. Creel, pers. coinin.).

Most analyses of dispersal i n co-operatively breeding species have emphasized the part played by direct fitness for dispersers. with philopatric, non-reproductive individuals benefiting from increases in the indirect component of inclusive fitness (Brown. 1987; Stacey & Koenig, 1990; Koenig C I d.. 1992). Delaying dispersal creates the opportunity to gain indirect fitness benefits through helping kin rear offspring. particularly if those young can later reciprocate aid. However, indirect benefits of helping may also accrue to dispersers that emigrate with kin or towards previously dispersed kin (see Cheney & Seyfarth. 1983: van Noordwijk & van Schaik, 1985; Creel & Waser. 1994: Rasa & Lloyd. 1994). Some dispersing meerkat coalitions were composed of siblings: for example inale 03 dispersed with two males that he helped to rear, as did female 06 in a heterosexual coalition. Male 04 subsequently re-emigrated and was joined in his new band by his younger sibling male 16. although i t was unclear whether they combined before or after the transfer .

Dispersal in meerkats \vas strongly age-dependent. Yearling inales dispersed at higher rates than tctiiales (although when unexplained disappearances were taken into account similar proportions remained philopatric). while dispersal among two-year-olds occurred at similar levels in both sexes. However. inales aged three years or more dispersed at over three times the rate of fernales and a lower proportion remained as residents throughout the year. A similar pattern is evident among dwarf mongooses (Rood, 1987, 1990) where the likelihood of dispersal is linked to age and sex. and is strongly influenced by the relative sizes of the direct and indirect fitness returns from dispersing and philopatric tactics (Creel & Waser, 1994).

In our study. female meerkats immigrated at a lower rate than males, were less likely to form coalitions. and probably suffered higher mortality during dispersal. Among dwarf mongooses, the probability of survival for dispersing feniales is lower than that for males (Waser et al., 1994), perhaps because the larger dispersing male coalitions reduce predation and increase the like- lihood of successful transfers or takeovers. Female dwarf mongooses tend to disperse singly or in m a l l parties. and are niore discriminating in their selection of target group. Consequently, they may be forced to float for longer periods. exposing them to greater mortality risks.

FOIYI,I~.S t i r id cis.w.sst?icrit

One \vaq in m.hicl-1 meerkats might assess the costs and benefits of dispersal, while retaining the advant:tges of mcmbership of a proven band and territory, is by making prospecting forays (also see Rood. 19x7: L i c k . 1990: Thompson, Rose & Kok, 1992: Holekamp et ( I / . , 1993; Waser, In

MEERKAT PROSPECTING A N D DISPERSAL 69

press). Prospectors initiated forays both voluntarily and following within-group aggression. Making extra-territorial excursions would increase familiarity with other areas, thereby dimin- ishing the risk of predation whilst away from the home range (Isbell, Cheney & Seyfarth, 1990; Larsen & Boutin, 1994): individual meerkats were clearly familiar with the dens and vantage points that they encountered on their repeated excursions. By investigating scent-posts and trailing bands, prospectors may also be able to assess band composition, the competitive abilities and sexual status of residents, and the likelihood of successful immigration (Rood, 1983, 1987; Wenhold & Rasa, 1994). Attempted immigration and takeover bids by meerkats often followed forays or encounters between groups. Dwarf mongooses also frequently transfer after grotlp II~CW.Y

(Rasa, 1987). In this study, seven male meerkats re-emigrated or disappeared after successful immigration

into groups. At least two of these males successfully transferred into another band containing more females (although it was unclear whether they had attained breeding rank), while another immigrant yearling male was last seen trailing a group containing only a single adult male and no other yearling males. Thus, even after dispersal, the animals apparently continued to evaluate the relative costs and benefits of membership of another group. Repeated visits back to a group which the individual previously left have been recorded in a range of species including wedge- capped capuchin monkeys Cebus olivacrus (Robinson, 1988), wolves Cunis lupus (van Ballen- berghe, 1983), spotted hyenas Crucutu crocutu (Holekamp et al., 1993), Pinyon jays Gymnorkmus cyanocephulus (Marzluff & Balda, 1989), and groove-billed anis Crotuphugu sulcirostris (Bowen, Koford & Vehrencamp, 1989). Subordinate dwarf mongooses have also been documented returning to their original packs some time after successful takeovers of other packs (Rood. 1987). Among African wild dogs, unsuccessful dispersers may return to their natal packs and this can even result in accession to the breeding position (S. Creel, pers. comm.).

The role of dispersal in attaining breeding status

During the course of this study there was only one definite change of dominant male, which occurred when a male coalition ousted the resident males. Observation of a previous band takeover by a male coalition (DWM, pers. obs.) and of takeover bids, suggests that this route to dominance is not uncommon for older males.

In some instances, the precise way in which breeding positions were filled by females remains unknown; immigrants may have joined the groups after the breeding position became vacant, females may have immigrated and subsequently competed over positions, or males may have recruited females from elsewhere (see Rowley, 1981; Rood, 1990; Ligon, Ligon & Ford, 1991 for descriptions of wije-hunting). Our observations of meerkats suggest that females could become breeders by inheritance, by aggressive competition, and by immigration into a group without a same-sex breeder. For example, the vacancy created in the Woodlanders by the death of the dominant female over the dry season of 1985/86 was filled by the oldest female resident, although it is unknown whether she was natal philopatric or a previous immigrant. Changes in group membership of the Raiders following the dominant female’s death during the dry season of 1984/ 85 were suggestive of animals leaving a disintegruting band, with subsequent immigration of two young adult females rrjuvenuting it (sensw Rood, 1990).

By contrast, flux in membership of the Outliers band between May 1986 and January 1987 strongly suggested inter-female competition over reproductive opportunities. The degree of aggression between the two oldest females was paralleled only by the hostility between resident

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

males and adult males attempting to immigrate or takeover a band, and between the dominant feinale and an aduit female immigrating into the Raiders. Episodic targeting of aggression initiated by the dominant feinale occurred over a three-month period, and eifectively established her dominance position (Doolan. 1994). Thc band also attracted both male and female irnmigrants. m a n y of which only stayed for short periods before disappearing, emigrating or being evicted.

Aggrcssion has been suggested to promote dispersal in inany studies (Holekamp, 1986; Hanby & Bygott. 1987: Vick & Pereira. 1989; but see Wolff. 1993). and eviction from the band was important in precipitating dispersal events in meerkats, with both males and females being aggressively targeted or forcibly ousted. The most pronounced within-group hostility was between members of the same sex during oestrus, indicating that a primary reason for outbreaks of aggression was rcproductive competition. For example, two outbreaks of aggression by dominant females towards yearlings apparently corresponded to the yearling females' initial oestrous periods. whilst male 03 was ousted from the Raiders by other males during a mating period and subsequently reaccepted. Similarly, band takeovers resulted in the expulsion of the original males.

By contrast, intrasexual competition in females is likely to be dictated by the costs of reproduction, rather than simply by the occurrence of mating by other females. Simultaneous breeding by ii subordinate would not affect the allocation of guards to young at the den, although it might detract from the eKectiveness of defence against predators. Predation of juvenile dwarf mongooses away from the den is reduced in larger groups (Rasa, 1986, 1989; Rood, 1990), although the relationship is not as direct for meerkats (Doolan & Macdonald, In press a). The rate of development o f young mongooses is also strongly affected by the number of animals which help in provisioning (Creel & Creel. 1991). Hence division of the provisioning effort between t w o or more litters n.ould probably slow down the rate of growth of the young, and indircctly increase the burden of lactation borne by thc mother. The advantage to a dominant fcmale of reproductive suppression of subordinates, even to the extremity of infanticide (Sillero- Zubiri. 1994: Rasa. 1994: Doolan & Macdonald. In press (1. h ) , is that it guarantees the undivided attcntions of helpers for her offspring. even if it may entail the loss of a female helper through dispersal.

In conclusion. meerkats in the Kalahari dispersed as a means of maximizing probable reproductive success. Conipetition over reproductive opportunities was high and individuals of both sexes made periodic extra-territorial excursions to other groups, and sometimes followed these by transferring to the visited band. Dispersal among adults was highest early in the breeding season n4ien most mating opportunities were available. Most transfers for which both source and destination were known involved moves to bands containing fewer older animals of the same sex. Coalitions frequently involved kin. and repeated contacts were made between animals originat- ing from the same bands. In common with inany primates and other co-operative breeders, meerkats evaluate and adopt a variety of flexible behavioural tactics in response to the opportunities af'forded by physiological maturation. environmental conditions, group composi- tion. and social rclationships. Long-term studies arc required to document the fitness payoffs gained by individuals employing these tactics in ii complex co-operative, but competitive, society.

Our t h a n k s to ihc Tnistees of the National Parks Board of South Africa, and especially Mnr Elias LeRiche, warden of the KGNP. for permission to work in the KCNP. Karen Wiltshire helped collect some of the data. and M aans D r e y r . Mike Knight. Colin Sapsford. David Payntrr. Richard Goss, Steve Giddings, and

M E E R K A T P R O S P E C T I N G A N D DISPERSAL 71

Barry Lovegrove contributed additional observations. Earlier versions of this manuscript were substantially improved by discussion with, and comments by, Claudio Sillero-Zubiri, Fran Tattersall, Walt Koenig, Scott and Nancy Creel, Kevin McGowan, Rosie Woodroffe, Einilio Herrera, and Robin Dunbar. SPD was supported by scholarships and awards from the Royal Commission for the Exhibition of 1851, the Anglo- Irish scientific exchange scheme, Wolfson College, Oxford, and the Prendergast Trust, University of Oxford, not to mention a long-suffering family. Part of this work was conducted while DWM was a visiting research fellow at the Mammal Research Institute of the University of Pretoria, where the hospitality of Prof. J. R. Skinner and the support of the CSIR are warmly acknowledged. We particularly appreciate the provision of vehicles by Maurice Calvert-Evers and John Elford of Nissan S.A. during critical periods of fieldwork.

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