Christian Peelers and Robin Crewe.
Foraging and recruitment in ponerine ants: solitary hunting in the queenless Ophthalmopone berthoudi (Hymenoptera: Formicidae).
Psyche 94(1-2):201-214, 1987.

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FORAGING AND RECRUITMENT IN
PONERINE ANTS: SOLITARY HUNTING IN
THE QUEENLESS OPHTHALMOPONE BERTHOUDI
(HYMENOPTERA: FORMICIDAE)
BY CHRISTIAN PEETERS* AND ROBIN CREWE
Department of Zoology, University of the Witwatersrand, Johannesburg 2001, South Africa.
Recent studies have indicated that, in ponerine ants, a eusocial organization is viable without the presence of a queen caste. In Rhytidoponera (Haskins and Whelden, 1965; Ward, 1983; Pamilo et al., 1985) and Ophthalmopone berthoudi (Peeters and Crewe, 19851, some of the workers (= gamergates; Peeters and Crewe, 1984) engage in sexual reproduction. Although there is still reproductive division of labor within the colonies, the loss of the queen caste represents a retrogression since the replacement egg-layers lack spe- cialized ovaries. It is of interest to discover whether other aspects of the colonial organization of permanently queenless ponerines have changed with this modification in the reproductive structure. One consequence of this change is that queenless colonies have low levels of relatedness among nest inhabitants (there are many mated laying workers), and Ward (1981, 1983) has suggested that this leads to lower levels of altruistic behavior. Thus Ward would expect colony defence and foraging efficiency to be less well-developed in these species. The natural history of more species of queenless ponerines needs to be documented in order to test the validity of this opinion. The genus Ophthalmopone belongs to the tribe Ponerini (sub- tribe Poneriti), and is derived from Pachycondyla (W. L. Brown, pers. comm.). 0. berthoudi Fore1 occurs in AngolaJ ZambiaJ Zim- babwe, Mozambique, and the eastern part of South Africa (Prinz, 1978). The workers are monomorphic, and it is exclusively termito- phagous. Alfred Ilg described columns of hunting ants in 0. ilgi *Present address: School of Zoology, University of New South Wales, P.O. Box 1, Kensington N.S.W., Australia 2033.
Manuscript received by the editor August 22, 1986.



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202 Psyche [vo~. 94
(reported in Forel, 1928), and it has been assumed that group- raiding behavior occurs in the whole genus (Wheeler, 1936; Wilson, 1958). The colonies of 0. berthoudi are polydomous (Peeters, 1984). This study was carried out in one locality in Mkuzi Game Reserve (north-eastern Natal, South Africa), during 198 1- 1983. Over 100 working days were spent in the field. Mkuzi is semi-arid (Goodman, 1981), with a hot humid summer from mid-September to the end of March. The rainfall is highly seasonal, peaking in February and lowest in June. The greatest variability in the mean monthly rainfall occurs during the dry winter months. The colonies of Ophthalmo- pone berthoudi occurred in open woodland with a grass component less than 0. The micro-distribution of these ground-dwelling ants is influe ced by soil characteristics. They nest in sandy clay loam which,'^ f free-draining (i.e. no sub-surface clay pans). The surface is slightfy capped and compacted; this increases the run-off and decreases the infiltration. The ponerine community at this location also included Plectroctena mandibularis and P. conjugata, Pachy- condyla sp. (soror-complex) and P. krugeri. Various nests were selected for intensive study, and every ant coming in or out of these was colour marked. Some ants were also collected inside the nests and marked. Spots of paint were applied to 1-5 pre-selected dorsal positions and, with a choice of five colours (Humbrol enamel paint), a sufficient number of unique combina- tions was generated. The ants did not lose their marks with time because painted ants were observed for several months and unex- pected color codes were never recorded (the paint holds well to the pubescent cuticle). The activities of individual ants were monitored for varying numbers of days, with the help of a portable tape recorder. Many of the nests were excavated at the end of field visits. Nest description
Nests consisted of a number of underground chambers and galler- ies with no distinct spatial arrangement, and seldom extended deeper than SO cm. Nest entrances were simple holes in the ground with little or no accumulation of soil around the hole. Active termite



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19871 Peelers & Crewe-Ophthalmopone berthoudi 203 tunnels were sometimes intimately associated with the ant nests, and we suggest that the ants modify abandoned termite tunnels and use them as nests.
Since adults and brood were frequently carried between nests, we soon determined that a single colony occupies more than one nest site (= polydomy). Colony units were made up of 2-7 nests, and these nests were separated by distances varying from 30 cm to 75 m. None of the nests were connected underground? even those with entrances very close to each other.
Individual nests contained variable numbers of adults (mean = 186 k 151 s.d., range = 20 - 840, n = 34) and brood. Four colonies had all their component nests excavated, and the following esti- mates of total population were obtained (mean = 517): January 1981: 3 nests, 453 adults (84, 142, 227). April 1981: 3 nests, 838 adults (445, 75, 318). December 198 1: 2 nests, 3 1 1 adults (261, 50). February 1982: 5 nests, 464 adults (168, 124, 72, 80, 20). Patterns of activity
The ants were observed outside their nests throughout the year? although the level of above ground activity decreased during the dry winter months. A subjective impression was that foragers then spent more time away on single hunting expeditions, and that they returned with fewer termites. This is probably linked to decreased termite availability; the dry soils and the low temperatures at night lead to a reduction in the nocturnal activity of the termites* Diurnal patterns of activity varied during the year. The ants remained outside the nests throughout the day during winter, but during the rest of the year, above ground activities stopped during the middle of the day. The biphasic summer pattern (04h30-1OhOO; 17hOO-sunset) appeared to be regulated by soil temperatures. Indeed, foragers returning to their nests towards the end of the morning period of activity repeatedly climbed up short grass stems. They remained there for short periods before they ventured on the ground again and ran across to the next plant. When the ground surface became very hot (50å¡C) the ants stayed inside their nests. On cool, cloudy or rainy summer days, foraging continued for longer periods and even for the whole day.



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204 Psyche [VOI. 94
Activities around the nest entrances
Excavation occurred frequently after rain, with soil being brought out of the nests to the surface. Nest sanitation was a distinct activity, whereby workers came up to the surface and discarded pellets of termite remains, as well as empty cocoons. The former consisted of jaws, limbs, and other bits of exocuticle, which were all packed into a distinctive rough-looking ball. These pellets were carried for 1-5 m before being left on the ground; they were never discarded close to the nest entrances. Individual cleaners repeatedly dropped their pellets in the same place.
No natural instances of predation on 0. berthoudi were recorded, but it is suspected that lizards feed on the ants. A number of arthropods (beetles, spiders, ant lion larvae) were observed trying to steal the termites retrieved by foragers. Carrying behavior: adult and brood transfer Adult carrying occurred daily and was preceded by a stereotyped "invitation behavior". This involved vigorous antennation and was similar to that described by Moglich and Holldobler (1974) for Rhytidoponera metallica. However, the carrying posture in 0. ber- thoudi is completely different (Fig. I), with the recruit carried on its side underneath the other ant, Not all invitation interactions were followed by carrying, and the outcome seemed influenced by the age and motivation of the participants.
Adults were carried between existing nests of the same colony. Single cocoons, single larvae of all sizes and packets of 3- 14 eggs were also frequently transported above ground. Males were carried between nests during January-February. Different recruiters that were active between the same pairs of nests seldom followed the same route, and the paths were sometimes strikingly different. This is further evidence that the nests are not connected together by chemical trails.
Nest emigration is a frequent occurrence in ponerine ants. Species which do not lay chemical trails for social coordination use social carrying or tandem running (e.g. Moglich and Holldobler, 1974; Fukumoto and Abe, 1983; Holldobler, 1984; Traniello and Holl- dobler, 1984; Fresneau, 1985). Both these primitive recruitment techniques are preceded by the same invitation behavior, and in Pachycondyla obscuricornis they are used alternatively to recruit



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Figure I.
Worker of Ophtha1mopone berthoudi carrying a male ktwwn two nests-workers are carried in an identical way. sexuals or workers (Traniello and HfiHdobler, 1984). In 0. ber- thoudi, social carrying is not only used during the evacuation of old nests and the settlement of new ones, but is a habitual, routine event between established nests. An ant carried to another nest in a colony remains associated with it; if it was active above ground, it will return to this new nest after future performanee of its task, While being carried, an ant is also provided with visual navigation cues which enable her to return to the nest of origin. When marked recruiters and recruits came apart outside the nests, the latter were unable to proceed but could walk back to the nest from which they originated. A detailed analysis of the pattern of recruitment between nests will be present& elsewhere. While the proximate adaptive significance of inter-nest transfers is unclear, they have the effect of maintaining contact between the nests of polydomous colonies, Foraging behavior
0. berfhmdi feeds exc~usively on termites. The ants only hunted those species which foraged in accessible places, and termite nest# were never raided. Cooperative hunting was never obsemed. Three termite specks seemed to make up most of the ants' diet. Macro- iermes nalalemis and Odontorermes badius forage principally on



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206 Psyche [vo~. 94
wood-, grass-, or leaf-litter on the surface, covering these with a thin layer of soil and feeding beneath this protective cover. Chemical cues contained in fresh soil sheeting of these two termite genera are detected by scout ants of Megaponera foetens (Longhurst and Howse, 1978); this effect was not investigated in the present study. Foragers of 0. berthoudi broke open the soil sheeting or looked for natural openings. Termites were captured one by one; the prey was held in the ants' mandibles, impaled on the sting and flung under- neath the body to the rear. After catching up to 15 termites in this way, the foragers gathered most of the semi-immobilized bodies and stung them once again. They packed the termites between their mandibles and brought them back to the nest. 0. berthoudi also preyed on Hodotermes mossambicus, which are grass-harvesters on the surface, and which sporadically exit in small groups from underground galleries. The ants located these visually, and captured single termite workers which were immobilized with the sting. Suc- cessful foragers only remained inside their nest for a short while before they reappeared (in laboratory nests they leave their prey in the entrance chambers), and revisited the same locations. Foragers were often found with missing limbs, and such injuries are presuma- bly sustained during hunting.
Recruitment never occurred, and single foragers independently exploited a food source until it was exhausted. Social facilitation was observed in laboratory nests however; the return of successful foragers induced others to go out and hunt, but to no particular location. The lack of cooperation between hunters is associated with the absence of trails in this species. The distinctive trail-laying gait was never observed, and simple experiments showed that foragers do not become disoriented when soil ahead of them is disturbed. Instances of tandem running were never seen. Some kind of discrete marking was sometimes observed however. Workers stood momen- tarily still and rubbed the ventral tip of their gaster sideways over objects lying on the ground (cg. pebbles, dead leaves, sticks,. . .); marking was never done directly onto the bare ground. This behav- ior was especially conspicuous around nest entrances after rain, and also occurred when some foragers walked away from their nests. We suggest that ants which are unfamiliar with a new area outside their nests lay these marks and then use them as personal orientation cues



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19871 Peelers & Crewe- Ophthalmopone berthoudi 207 on their return. Similarly, scouts in Pachycondyla tesserinoda de- posit and use scattered chemical orientation marks which have no recruiting effects (Jessen and Maschwitz, 1985). Foraging originated from every nest and marked hunters were always recorded as returning to their nest of origin in a colony. Some foragers did not hunt around their own nests, and travelled long distances to hunt in areas nearer other nests in the colony, where they had been observed to be previously active. On consecu- tive days, the same individuals returned to the same part of their colony's home range.
Ophthalmopone berthoudi invariably hunts alone; this is in conflict with previous references to this genus in the literature. There is no cooperation among foragers, either through the transfer of information about the location of new sources of prey, or through direct assistance during the killing and retrieving of prey. Many other species of ponerines are also solitary predators (Table l), although they are in many cases more opportunistic in their choice of prey. Group retrieving (involving a small number of workers) is occasionally seen in some species of solitary hunters (when prey is too large or numerous to carry), but this cooperation is not always a consequence of recruitment, i.e. a huntress can attract nestmates in her immediate vicinity through the release of alarm pheromones (Amblyoponepallipes; Traniello, 1982), which is not equivalent to returning to her nest to recruit one or more nest- mates to the prey (e.g. Rhytidoponera purpurea; Ward, 1981). In Plectroctena conjugata (Peeters, unpublished), several foragers are sometimes led together to a hunting area, but they capture prey and return to the nest independently.
Solitary predation contrasts with the elaborate systems of recruitment and cooperative hunting displayed by other ponerine species. Fletcher (1 973) has critically reviewed column-raiding in the Ponerinae. The comparative overview in Table 1 reveals that simple and complex hunting strategies occur in different species irrespec- tive of phylogenetic relationships. Thus 0. berthoudi and Mega- ponera foetens are closely related (W. L, Brown, pers. comm.) but



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w
Table 1. Overview of the foraging techniques exhibited in the Ponerinae (only a few species of Leptogenys have been included). The process of hunting living prey (=predation) is broken up into raiding (getting to the prey) and retrieving, a distinction made by Wilson (1958). "Group" refers to a small number of participants (less than 10-15), while "column" involves many ants walking together in one of several distinct files. The species mentioned are all predatory, and they belong to the following tribes and sub-tribes (Brown, 1958, 1960, 1975, pers. comm.): (1) Amblyoponini; (2) Cerapachyini; (3) Ectatommini; (4) Platythyreini; (5) Ponerini, a: Odontomachiti, b: Poneriti, c: Leptogenyiti. Recruitment technique
associated with
Species recorded in the
foraging literature (and tribe) Hunting strategy References no chemical trails, Amblyopone pluto
no recruitment A. pallipes
Rhytidoponera sp. 12
Ectatomma ruidum
Platythyrea conradti
Odontomachus bauri
Ophthalmopone berthoudi
Diacamma rugosum
Dinoponera gigantea
Pachycondyla obscuricornis
P. (= Neoponera) apicalis
recruitment by Pachycondyla tesserinoda
tandem-running
Mesoponera caffraria
Hypoponera sp.
solitary
solitary
solitary*
solitary
solitary
solitary
solitary
solitary
solitary
solitary
solitary*
solitary, group retrieving
large prey
solitary, group raiding
and retrieving
solitary, group retrieving
(large prey)
Gotwald & Lkvieux 1972
Traniello 1982
Pamilo et al. 1985
Lachaud 1985
Lkvieux 1976
Jaffe & Marcuse 1983
this study
Fukumoto & Abe 1983
Haskins & Zahl 1971
Traniello & Holldobler 1984
Fresneau 1985
Maschwitz et al. 1974
Agbogba 1984
Agbogba 1984




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rudimentary chemical
recruitment
(short-lived trails)
advanced chemical
recruitment (more
persistent trails,
larger groups)
Rhytidoponera chalybaea
R. purpurea
Plectroctena mandibularis
Paltothyreus tarsatus
Leptogenys attenuata
Cerapachys turner!
Paraponera clavata
Pachycondyla laevigata
P. commutata
Megaponera foetens
Simopelta oculata
Leptogenys chinensis
L. nitida
L. kitteli
Onychomyrmex
solitary, group retrieving*
same, with long-range
trails to aphids
solitary, group retrieving
(large prey)
solitary, group raiding
and retrieving*
solitary, also group
raiding (2-5 ants)
column raiding (on
ant nests)
solitaryt, group foraging to
nectar
column raiding, solitary
retrieving?
column raiding and
retrieving
column raiding (complex)
column raiding (on ants)
group raiding (2-6),
solitary retrieving
group raiding (few ants)
and retrieving
column raiding (40-60)
and retrieving
swarm raiding (destination
of column is not
Ward 1981
Ward 1981
Fletcher 1973
Holldobler 1984
Fletcher 197 1
S-1
Holldobler 1982
Breed & Bennett 1985 $
^1
Holldobler & Traniello 1980
Mill 1984
Longhurst & Howse 1979
Gotwald & Brown 1966
Maschwitz & Schonegge 1983
Fletcher 197 1
Baroni Urbani 1973
Brown 1960,
Holldobler et al. 1982
predetermined) ~-a
* also scavengers
t along established trunk trails




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210 Psyche [vo~. 94
exhibit solitary hunting and column raiding respectively, and members of the genus Pachycondyla exhibit a range of hunting strategies. Furthermore, representatives of each of four tribes hunt in well-coordinated raids (Table 1). These data emphasize that foraging characteristics are the product of the unique selective pres- sures facing each species. The evolution of these characteristics is not governed by anatomical constraints, e.g. all members of the sub-tribe Poneriti seem to have the exocrine glands necessary for recruitment, but only some of them hunt in groups. In addition, some solitary-hunting species have recruitment capabilities which they do not use during foraging, e.g. Diacamma rugosum, Dino- ponera gigantea and Pachycondyla (=Neoponera) apicalis use tandem-running during nest emigration only (Fukumoto and Abe, 1983; Overal, 1980; Fresneau, 1985).
Wilson (1958) suggested that group hunting only becomes selec- tively advantageous with respect to certain prey preferences. This is evident for species which prey on large arthropods or on the brood of other ants. However, strictly termitophagous species include both solitary and column hunters, i.e. a number of strategies are approp- riate to exploit this strongly-clumped prey. Column raiding appears to have evolved a number of times and from different behavioral antecedents (Furthermore, different exocrine glands are involved in recruitment.), and this term thus describes a range of "variations on one theme". Indeed in some species, group raiding is followed by solitary retrieving of the prey. The occurrence of group hunting is unlikely to be related to one ecological factor only. There has not been a phylogenetic trend towards the elaboration of patterns of cooperation and recruitment during predation in the Ponerinae, and these characteristics can vary from species to species regardless of the nature of the reproductive system. Indeed, group foraging is characteristic of the genus Leptogenys, and this behav- iour is unaffected by the change to gamergate breeding which has occurred in L. schwabi (M. Zini, in prep.). Thus we reject the possi- bility that the simple hunting strategy seen in 0. berthoudi is a secondary modification caused by a reduction in altruism in colo- nies with numerous matrilines, We note that solitary-hunting spe- cies with a highly-specific diet are more vulnerable to seasonal fluctuations in prey availability. Rather than implying that elimina- tion of the queen caste would result in a simplification of social



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19871 Peelers & Crewe-Ophthalmopone berthoudi 21 1 relationships, attempts should be made to understand how it might have arisen as an effect of various attributes of the life histories. We are grateful to William L. Brown Jr. for his information on the phylogenetic relationships among the Ponerinae, and to Barry Bolton for ant identifications. Peter Goodman provided logistic assistance in the field, and the Natal Parks Board gave permission to work in Mkuzi Reserve. Anthony Bannister took the photograph in Figure 1. We thank R. H. Crozier, W. L. Brown, and M. W. Moffett for comments on this manuscript. This study was funded by grants from the University of the Witwatersrand and the C.S.I.R., and the manuscript was prepared while CP was supported by the Australian Research Grants Scheme.
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2 12 Psyche [vo~. 94
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