R. Beckers, S. Goss, J. L. Deneubourg, and J. M. Pasteels.
Colony size, communication, and ant foraging strategy.
Psyche 96:239-256, 1989.

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COLONY SIZE, COMMUNICATION AND
ANT FORAGING STRATEGY*
BY R. DECKERS, S. Goss, J. L. DENEUBOURG, J. M. PASTEELS Unit of Behavioural Ecology, C.P. 23 1,
Universitk Libre de Bruxelles, 1050 Bruxelles, Belgium Some 12,000 ant species are known by now, with colony sizes ranging from a few individuals to 20,000,000 individuals. What con- straints does this vast range of colony sizes place on the systems of organisation that they use? Alternatively, how does this range of colony sizes reflect the different systems of organisation used? We shall examine these questions in relation to ant foraging strategy, which as well as being the most visible aspect of their activity illus- trates most clearly the roles and limits of communication in their collective behavior.
This paper aims to verify a prediction of the following hypothesis (Pasteels et al. 1985; Deneubourg et al. 1986). In theory, the organi- zation of a small insect society can rely on most individuals at any moment "knowing", principally by learning, what it must do, where it must go, etc., and the workers' behavior has a strong determinist component. In a large insect society organization by individual learning is harder to achieve (Deneubourg et al. 1987). The workers' behavior is necessarily more random and their coordination becomes a major problem. To cope with this, a completely different organisational system is added to that already in place. This sup- plementary system is based on the complex collective structures, patterns and decisions that spontaneously emerge from simple auto- catalytic interactions between numerous individuals and with the environment, mediated by essentially chemical communication (see, e.g., Pasteels et al. 1987; Goss and Deneubourg 1989; Beckers et al. in press; Deneubourg et al, 1989, in press; Goss et al. 1990). The prediction that follows from this hypothesis is that the larger the colony size, the less foraging is individually based and the more *Manuscript received by the editor April 5, 1989. 239




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Psyche [Vol. 96
the individual foragers are coordinated by mass chemical communication.
We shall use the following categories of foraging strategy that, as shall be discussed below, represent a crescendo of the integration of the individual forager into a network of communication: individual, recruitment, trunk trail and group hunting, their definitions being inspired by the work of different authors (e.g. Rosengren 1971; Wilson 1971; Leuthod 1975; Maschwitz 1975; Oster & Wilson 1978; Moffet 1988; Traniello 1989).
By individual foraging we mean foraging without systematic cooperation or communication in the discovery, capture or trans- port of prey items. Each forager leaves the nest, searches for food and transports it individually (e.g. Cataglyphis bicolor, Pachycon- dyla apicalis).
By foraging with recruitment, we mean that a scout having dis- covered a food item returns to the nest and transmits the informa- tion concerning its location to inactive foragers waiting in the nest. These recruits can become recruiters in their turn. It should be noted that recruiting species rely to a large extent on individual foraging for the discovery and exploitation of small sources. Roughly speaking, three recruitment types can be distinguished. With tandem recruitment, the scout guides one recruit to the food item, with or without trail laying (e.g. Leptothorax sp.). With mass recruitment, a trail laid by the recruiter while returning to the nest guides recruits to the food (e.g. Solenopsis invicta, Monomorium pharaonis). Invitation by the recruiter in the nest is often active. With group recruitment, the scout guides a group of nestmates, in some cases (if not all) laying a pheromone trail to the nest (e.g. Tetramorium caespitum, Camponotus socius). However, as every species that we know uses group recruitment also uses a more or less efficient mass recruitment, we shall refer to group/ mass recruit- ment. Note that some authors distinguish a fourth recruitment sys- tem, group raiding (type IV-Oster and Wilson 1978), which is characterised by a very strong invitation and recruitment trail that results in a large group of recruits leaving the nest together in a rush. We have included this system in group/ mass recruitment. With trunk trails, semi-permanent trails guide foragers to long- lasting food sources (e.g. many Formica sp.), and also serve as starting-off points for individual foragers, which may also recruit to



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19891 Beckers et al. - Ant foraging strategy 24 1 the trunk-trails. Finally, group hunting foragers (sensu Moffet 1988, including army ants) leave the nest and forage collectively in a swarm along a well-defined trail system that is constructed as the swarm progresses (e.g. Dorylinae sp.).
These descriptions are by no means meant to be definitive, and there are of course species whose foraging does not fall neatly into one or indeed any of these categories. Nevertheless, as shall be discussed below, they represent a crescendo of the integration of the individual forager into a network of communication. Other recruitment systems are known to exist (such as short-distance recruitment or non-directional recruitment), but for lack of data have been omitted. Similarly, the colony sizes given are average figures, obtained by different techniques, and generally with rather small sample sizes. Polycalic societies pose a special problem. For these reasons, the values quoted must be considered only as first- order approximations.
Table 1 presents the colony size and foraging system of 98 differ- ent species. Fig. 1 presents the foraging system as a function of the colony size. Although that data base is small compared to the number of known ant species, a distinct trend is clear (note the logarithmic scale). The smaller societies rely on individual foragers that do not transmit their discoveries. The largest societies rely on permanent chemical communication between the individuals. Be- tween these two limits, one finds the different types of recruitment. Again, the smaller recruiting societies rely on a slow, individual recruitment, where a recruiter interacts directly with one or a few individuals. The larger recruiting societies rely on the faster mass recruitment, where one recruiter can interact via a chemical trail with a large number of potential recruits. The trail transmits both the position of the source and that of the nest to the recruits. Note that we have listed the species in Table 1 by alphabetical order for facility, and that the same tendency appears in each sub-family. Taking these results into consideration we propose two extreme blueprints for the way in which ants organise their foraging. The first blueprint consists of small societies which rely on the capacity for learning of its members to exploit the foraging area efficiently. Individual foragers, for example, develop fidelity to



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242 Psyche [Val. 96
parts of their foraging area and can orient themselves over large distances (Wehner et al. 1983; Fresneau 1985). They do not interact directly with each other, nor do they communicate their food dis- coveries, yet they are capable of dividing the foraging area amongst themselves (Deneubourg et al. 1987). The society may be considered to have placed its complexity at the level of the individual. The second blueprint consists of large societies whose individual behavior may be considered as simple. They rely on a highly deve- loped network of chemical communication based on permanent trail-laying behavior to coordinate the foragers' activity and to aid their orientation. Their capacity for individual orientation is limited not only because it is not so needed as the trails are there, but also because too much individuality could prevent collective foraging from functioning efficiently. It is surely no coincidence that the largest and most chemically integrated societies, i.e. the different army ants and termites, are practically blind. The colony size is large, not simply to ensure that their "inefficient" workers manage to perform the necessary tasks by sheer weight of numbers (Oster and Wilson 1978; Herbers 1981), but because they need a large reserve of individuals for the amplifying mechanisms (e.g. recruit- ment) by which they structure their foraging to work (e.g. Pasteels et al. 1987). The society may be said to have placed its complexity more at the level of the interactions between individuals. Between these two extremes, we find intermediate sized societies which rely on individual scouts to forage small food items and on recruitment to amplify the information relating to important food sources. The sequence tandem/ group-mass/ mass is characterised by an increasing number of individuals that react to the recruiters' signals, and is associated with an increasing colony size. In mass recruiting species there is a tendency in the largest societies to lay trail pheromone not only when returning with food but also when leaving the nest and more or less continually in the foraging area (e.g. Pheidole militicida - Holldobler and Moglich 1980; fridomyr- mex humilis - Van Vorhis Key and Baker 1986; Aron et al. 1989). There is of course a large degree of overlapping between the different categories in fig. 1. This is to be expected whenever one tries to categorize nature, but is also the result of imprecision in our knowledge of colony size, which is anyway highly variable for a



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Deckers et al. - Ant foraging strategy
243
Group Hunting
Trunk Trail
Mass
GroupIMass
Tandem
Individual
0 0%009<f9P0000~ w 0
A A A
Colony Size
Fig. I.
Foraging strategy as a function of colony size for 98 ant species (see Table 1). The arrows mark the 25,50 (median) and 75 percentiles. given species. Furthermore, others factors such as the size, distribu- tion and type of food exploited intervene, and ant foraging strategy and food type are obviously connected (e.g. Carrol and Janzen 1973; Traniello 1989).
Other less precise data confirm the tendency seen in fig. 1. For example we know that Crematogaster ashmeadi colonies are very large and that they use mass recruitment (Leuthold 1968a, b), whereas Bothroponera tesserinoda colonies are small and use tandem recruitment foraging.
The same overall tendency as shown here for ant species, also noted by Buschinger (1980) for dulotic ants, is well known in the Apidae. Species with small colonies, such as bumblebees, use an individual foraging strategy. Those with large colonies, such as honey bees, melipones and trigones, use recruitment (Lindauer and Kerr 1958; Seeley 1985). The tendency is also observed in terns (Erwin 1978).
We would like to end this paper with an appeal to readers to help us increase the size of our data base. We would welcome any infor- mation about colony size and foraging system, whether for species already in Table 1 or for any other ant species.



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244 Psyche [vo~. 96
Table 1. Average colony size and foraging strategy of 98 ant species. The subfam- ilies (in brackets) are: 1 = Aneuretinae, 2 = Cerapachyinae, 3 = Dolichoderinae, 4 = Dorylinae, 5 = Formicinae, 6 = Leptanillinae, 7 = Myrmeciinae, 8 = Myrmicinae, 9 = Ponerinae, 10 = Pseudomyrmecinae. The foraging types are: I = Individual, TR = Tandem recruitment, GM = Group/Mass recruitment, MR = Mass recruitment, TT = Trunk trail, GH = Group hunting. Frg.
Species (subfamily) Nest Size Type References Acromyrmex landolti (8)
octospinosus
Aenictus laeviceps (4)
Amblyopone pallipes (9)
Aneuretis simoni (1)
Anomma nigricans (4)
wilverthi
Atta cephalotes (8)
sexdens
texana
Azteca foreli (3)
Camponotus aethiops (5)
pennsylvanicus
sericeus
truncatus
Cataglyphis bicolor (9)
cursor
Conomyrma bicornis (3)
Crematogaster sumicrasti (8)
Cyphomyrmex rimosus (8)
Daceton armigerum (8)
Diacamma rugosum (9)
Dinoponera australis (9)
quadriceps
Eciton burchelli (4)
hamatum
rapax
Ectatomma ruidum (9)
Jaffe pers. comm.
Blum et al. 1964;
Jaffe pers. comm.
Schneirla 1965
Traniello 1978; Lachaud
pers. comm.
Traniello and Jayasuriya 198 1 ;
Jayasuria and Traniello I985
Vosseler 1905
Raignier and van Boven 1955
Jaffe and Howse 1979;
Jaffe pers. comm.
Riley et al. 1974;
Jaffe pers. comm.
Moser and Blum 1963; Riley
et al. 1974; Jaffe pers. comm.
Jaffe pers. comm.;
Suzzoni pers. comm.
Pricer 1908; Traniello 1977
Holldobler et al. 1974
Suzzoni pers. comm.
Wehner et al. 1983
Cagniant 1983;
Lenoir pers. comm.
Jaffe pers. comm.
Jaffe pers. comm.
Blum et al. 1964;
Jaffe pers. comm.
Blum and Portocarrero 1965;
Jaffe pers. comm.
Fukumoto and Abe 1983
Fowler 1985
Dantas de Araujo pers. comm.
Schneirla 1957
Schneirla 1957; Rettenmeyer 1963
Sudd and Franks 1987
Lachaud et al. 1984
Erebomyrma nevermanni (8) 180 TT Wilson 1986



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19891 Beckers et al. - Ant foraging strategy 245 Table 1. Continued
Frg.
Species (subfamily) Nest Size Type References Formica aquilonia (5)
bruni
cunicularia
fusca
lugubris
polyctena
pratensis
rufa
yessensis
Iridomyrmex humilis (3)
Lubidus praedator (4)
Lasius fuliginosus (5)
niger
Leptogenys chinensis (9)
ocellifera
Leptothorux acervorum (8)
am biguus
curvispinosus
duloticus
longispinosus
muscorum
nylunderi
unifasciatus
Messor barbara (8)
sancta
Monomorium pharaonis
Mvrmecia gulosa (7)
Myrmica rubra (8)
ruginodis
sabuleti
Zakharov 1978
Cherix and Maddalena-Feller
I987
Deffernez pers. comm.
Wallis 1964;
Moglich and Holldobler 1975
Rosengren 197 1; Breen 1979
Rosengren 197 1 ; Kruk-de-Bruin
et al. 1977; Horstmann I982
Rosengren 197 1; Jensen 1977
Gosswald 195 1 ; Rosengren 197 1
Ito 1973; Cherix 1987
Keller pers. comm.
Rettenmeyer 1963
Hainaut-Riche et al. 1980;
Quinet et Pasteels 1987.
Stradling 1970; Brian 1977
Maschwitz and Schonegge 1983
Maschwitz and Muhlenberg 1975
Dobrzanski 1966; Buschinger
197 1 ; Moglich et al. 1974;
Moglich 1979
Talbot 1965; Moglich 1979
Headley 1943; Talbot 1965;
Moglich 1979
Talbot 1957; Alloway 1979;
Moglich 1979
Headley 1943
Buschinger 1966; Moglich et al.
1974
Plateau pers. comm.
Lane 1977; Plateau pers. comm.
Delye pers. comm.
Delye pers. comm.;
Suzzoni pers. comm.
Peacock et al. 1955; Sudd 1960
Haskins and Haskins 1950;
Robertson 1971
Stradling 1970; Petal 1972;
Cammaerts and Cammaerts 1980
Stradling 1970, Brian 1972;
Cammaerts and Cammaerts 1980
Brian 1972;
Cammaerts and Cammaerts 1980




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246
Table 1. Continued
Psyche [Vol. 96
Species (subfamily)
Myrmicaria eumenoides (8)
Myrmoteras barbouri (5)
tor0
Neivamyrex nigrescens (4)
Novomessor cockerelli (8)
albicetosus
Ocymyrmex barbiger (8)
Odontomachus bauri (9)
haematoda
Nest Size
troglodytes 240
Oecophylla longinoda (5) 480,000
Ophthalmopone berthoudi (9) 400
Ologomyrmex overbecki (8)
Pachycondyla apicalis (9)
caffraria
commutata
obscuricornis
villosa
Pheidole embolopyx (8)
fallax
pallidula
Pheidologeton diversus (8)
silenus
Pogonomyrmex badius (8)
occidentalis
Ponera eduardi (9)
Proatta butteli (8)
Pseudomyrmex termitarius ( 10) 75
triplarinus 10,000
Serrastruma lujae (8) 57
serrula 78
Frg.
Tvve References
Levieux 1983
Moffet l986a
Moffet 1986a
Topoff et al. 1980
Holldobler et al. 1978.
Holldobler et al. 1978.
Marsh 1985
Jaffe and Marcuse 1983
Holldobler and Engel 1978;
Jaffe pen. comm.
Dejean 1982;
Dejean and Bashingwa 1985
Way 1954;
Holldobler and Wilson 1978
Peeters and Crewe 1987
Moffet 1986b
Lachaud et al. 1984;
Fresneau 1985
Lkvieux 1967; Agbogba 1981
Mill 1982, 1984
Traniello and Holldobler 1984;
Fresneau 1984
Lachaud et al. 1984;
Lachaud pers. comm.
Wilson and Holldobler 1985
Law et al. 1965;
Jaffe pers. comm.
Detrain pers. comm.
Moffet 1988
Moffet I988
Brian et al. 1967;
Holldobler and Wilson 1970
Holldobler and Wilson 1970;
Erickson 1972
Lavigne 1969;
Holldobler and Wilson 1970
Le Masne 1952; Bernard 1968
Moffet 1986c
Jaffe pers. comm.
Jaffe pers. comm.
Dejean 1982
Dejean 1982




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19891
Deckers et al. - Ant foraging strategy
Table 1. Continued
Species (subfamily)
Smithistruma emarginata (8)
truncatidens
Solenopsis invicta (8)
Tapinoma erraticum (3)
Tetramorium caespitum (8)
Trachymyrmex urichi (8)
Zacryptocerus varians (8)
Nest Size
Frg.
Type References
I Dejean 1982
1 Dejean 1982
MR Wilson 1962; Tschinkel 1987
MR Meudec 1979;
Verhaeghe et al. 1980
GM Brian et al. 1967;
Pasteels et al. 1987
MR Jaffe and Villegas 1985;
Jaffe pers. comm.
MR Wilson 1976; Jaffe pers. comm.
Drs. G. Delye, D. Fresneau, K. Jaffe, L. Keller, J. P. Lachaud, L. Plateau and J.-P. Suzzoni kindly sent us unpublished informa- tion. This work is supported in part by the Belgian program on interuniversity attraction poles and Les Instituts Internationaux de