SELF-STRUCTURING IN ARTIFICIAL "CHIMPS" OFFERS

NEW HYPOTHESES FOR MALE GROUPING IN

CHIMPANZEES

by

IRENAEUSJ.A. TE BOEKHORST1,2) and PAULIEN HOGEWEG3,4)

(1Department of Comparative Physiology, Section Ethology & Socio-ecology, University of Utrecht, Padualaan 14, PO Box 80.086, 3508 TB LA Utrecht, the Netherlands; 3Bioinfor-

matics, University of Utrecht, Padualaan 8, 3584 Utrecht, the Netherlands)

(With 6 Figures) (Ace. 20-VII-1994)

Summary

Chimpanzees live in societies that are characterised both by disorder and order. On the one hand, party size fluctuates in a randomlike fashion and party membership is unpredict- able ; on the other hand, fundamental party structures are apparent; males are often in all- male parties whereas females remain mostly solitary. The customary sociobiological expla- nation is based on the assumptions that 1) competition for food favors solitariness (espe- cially in females); 2) chimpanzee males share the costs of territorial defense against rivals from neighbouring communities and 3) genetical relatedness among males within a com- munity compensates for fitness losses due to their competition for food and females. We point to some theoretical flaws in the reasoning that forms the basis of the current neo- Darwinistic model and to the lack of empirical data concerning male relatedness. Most importantly, chimpanzee-like party structures emerge by self-organisation in an artificial "world" in which "CHIMPs" do nothing more than searching for food and mates, without requiring any of the assumptions of the sociobiological model.

Introduction

Approaches to the complexity of grouping patterns.

Chimpanzee societies are characterized by two, apparently opposing features. On the one hand they show clear patterns (males are relatively

2) Current address: Institut für Informatik, Universität Zürich, Winterthurerstrasse 190, 8032 Zürich, Switzerland. 4) We should like to thank Ben HESPER and Lottie HEMELRIJK for encouraging discussions and Jan VAN HOOFF for his continuous support. Thanks are also due to David HILL and Robin DUNBAR, who critically commented upon a first version of this paper.

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social, female tend to be much more solitary), on the other hand an

unpredictable joining and leaving of parties by community-members

(GOODALL, 1986). The overall patterns of male grouping and female

solitariness have been the subjects of neo-Darwinistic interpretations. The short-term fluctuations in party structure, however, would be tradi-

tionally - and from a black box perspective - attributed to "error" (i.e. the

combination of measurement mistakes, external disturbances, inefficien-

cies during information processing within the system and other unknown

or uncontrolled factors) and are therefore a candidate for stochastic

modelling (as has been carried out for orang-utans by CoHEN, 1975, and

MITAxI et al., 1991). ).

Although the neo-Darwinistic approach seeks explicitly for functional

explanations whereas fitting statistical models is also used for estimating the effects of direct, proximate factors, both have in common a more or

less "linear" view of nature. In the logic of natural selection, organisms are often considered as a sum of fitness components (cf. DAWKINS, 1976,

p.l04;McFARLAND, 1976; SIBLY McFARLAND, 1976; RUBINSTEIN, 1982) in the same way as measurement outcomes are statistically described as a

sum of variance components. This determines the way (biological) sys- tems are normally analysed, namely by investigating features in isolation

and assuming that by putting them together, the complexity of the whole

is understood (for a critique on this approach, see LEWONTIN & LEVINS,

1987). This results in a tendency to offer separate explanations (selection

pressures, selective advantages) for even so many phenomena. It has also

lead to the notion that the unraveling of complex systems asks for

complex explanations (i.e. that involve a very large number of variables) and that the hallmark of complexity is unpredictability.

At present, an alternative way of thinking about complexity permeates

many branches of science. Instead of stressing its stochastic nature, it is

increasingly acknowledged that complexity may emerge through self-

structuring from a limited number of simple relationships. Although these relationships may themselves be wholly deterministic, their non-

linear5) interactions are the fundaments of rich behaviour: for certain

values of their control-parameters, such dynamical systems can produce

5) We refer to the non-linearity of the differential (or difference-) equations describing the dynamics of a system rather than the non-linearity of functions per se.

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"chaotic" time series that cannot be distinguished from random fluctua-

tions by standard statistical methods, whereas for other parameter values

the same system may adopt various forms of ordered behaviour such as

(quasi-) periodicity and stable equilibria (GLEICK, 1987; CAMPBELL, 1988).

Hence, to explain the (sometimes unavoidable erratic) behaviour of such

processes, there is less need to postulate external factors as in the usual

approach (STEWART, 1989). The theory of chaos - and of non-linear dynamics in general - can be

formulated within the framework of cybernetics, but its implications are

not limited to systems theory. From a structural perspective, based on

individual entities situated in a spatial environment instead of a set of

coupled non-linear differential equations, similar unexpected and intri-

cated patterns may originate as a consequence of direct interactions

founded on local information and simple "rules". Formulated in this way, a non-linear dynamic approach aids in the development of synthetic models. The importance of these models is their ability to generate heuristic patterns rather than making precise "predictions" of a particu- lar set of observations. As such they aim at producing hypotheses that

may help explaining qualitative features rather than providing realistic,

detailed descriptions (SIGMUND, 1993; VILLA, 1992). Since these models

have a strong tendency to produce conspicuous and persistent patterns that are expressions of "just" epiphenomena (which do not require sepa- rate explanations), the type of hypotheses they generate are typically about parsimonious explanations.

In this paper, we apply these principles to the study of party formation

by setting up a computer model that generates patterns by virtue of its

self-structuring disposition. We have chosen chimpanzees as an example because of the intriguing combination of clear long-term patterns on the

one hand and the bewildering short-term fluctuations in party size on the

other hand. Stricktly speaking, our interest is therefore not a primatologi- cal topic, but an exercise in complexity theory. However, we hope to

show that the rules specified in our model have heuristic value. To

emphasize the latter, we stress that the presented model is not meant to

simulate chimpanzees, but to stimulate the formulation of new hypoth- eses concerning their social organization. To justify the need for a fresh

perspective, we first summarize the current explanations for the social

structure of chimpanzees and point to some of its weaknesses.

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The conventional interpretation.

The possible function of being in parties for chimpanzee males has been

indicated at a between- and within-community level (WRANGHAM, 1986;

BoEHM, 1992). At the between-community level, the assumed function of male parties

is to built up "macro-coalitions" (BOEHM, 1992) for the joint defence of

the territory against rival males from a neighbouring community (WRAN-

GHAM, 1986, 1987). The theoretical foundation is TRIVER'S (1972) asser-

tion that food intake influences the reproductive success of females more

than that of males, but males benefit more than females by maximising succesful matings. Chimpanzees are mainly frugivorous, and because of

the resulting competition for food, especially females spread out. This

makes it impossible for a male to monopolize females. Therefore, a male

is better off defending these females against intruders together with males

from his own community. Observations on lone chimpansees being attacked and killed by groups of neighbouring males (GOODALL, 1986) are

cited as evidence and form the basis Of WRANGHAM'S (1979a, b, 1987) model. According to this model, chimpanzee communities have evolved

by escalation from a hypothetical solitary male system in which males

joined each other to defend themselves against attacks from paired

conspecifics.

By sharing the costs of defense, males benefit through increased indi-

vidual and inclusive fitness (GHIGLIERI, 1984) since they are assumed to

be closely related (WRANGHAM, 1979a; GHICLIERI, 1984). The latter is

concluded from the observation that females tend to migrate to other

communities when becoming adult (PUSEY, 1979); in due time this should

have resulted to an ever closer degree of relatedness among the phi-

lopatric males (GHIGLIERI, 1984, quoting GLASS, 1953). An even closer degree of kinship, namely on the level of brotherhood,

has been proposed for within-community alliances (Riss & GOODALL,

1977; GOUZOULES & GouzouLES, 1987). Here, male aggregations are

believed to be a key factor in the formation of alliances by which males

assist each other during competition for status and resources. The case of

an older brother assisting his younger brother to the position of alpha- male (Riss & GOODALL, 1977) is used to exemplify this suggestion.

As proximate mechanisms, attraction to common resources (WRAN-

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GHAM & SMUTS, 1980; GHIGLIERI, 1984) and oestrus females (REYNOLDS &

REYNOLDS, 1965; NISHIDA, 1968; GOODALL, 1986) have been mentioned.

However, when close relatedness is to be crucial in the formation and

maintenance of male parties, some kind of kin selected affiliative mecha-

nism must be postulated.

Problems and research question.

Confusion arises because 1) chimpanzee communities are entirely defined

in terms of (the home range of) male ranges (VRANGHAM, 1979b; but see

GHIGLIERI, 1984, for critical comments) and 2) the two levels at which

male parties may function differ concerning the assumed minimal degree of male relatedness but are easily mixed up (cf. GHIGLIERI, 1984, p.187),

especially because parties fulfill functions on both the between- and

within-community level (BOEHM, 1992). Consequently, in WRANGHAM'S

model the function of chimpanzee communities cannot be separated from

those of male parties. There is also a problem concerning the origin of chimpanzee commu-

nities : if pair formation developed as an answer against attacks from

other, larger groups of males what then has brought the latter together in

the first place? And what could aggressive pairs hope to gain from

attacking solitary males? Certainly not a territory containing females, because this could never have been guarded by a single male. In short, the

model tries to explain how male associations evolved as an adaptation to

the sort of conflicts that are believed to occur nowadays between commu-

nities although at that time communities themselves did not yet exist.

Furthermore, the hypothetical solitary stage is questionable in itself. It

is unlikely to have been a phylogenetic precursory phase in the gorilla and

the semi-solitariness of orang-utans is probably a derived rather than an

original feature (e.g. RIJKSEN, 1978). Whatever the phylogenetical details, one could always claim that the

benefits of staying together for males are just to be such that male

philopatry was selected for. In order to avoid the deleterious effects of

inbreeding, the females was left no other choice than to become the

migrating gender and this resulted in high relatedness among males

(PUSEY & PACKER, 1987). In this view high male relatedness emerges as a

consequence of the community structure rather than as its starting point.

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Relatedness might thus be no more than a reinforcing factor promoting the maintenance of macro-coalitions and brotherhood is by no means a

conditio sine qua non for the development of communities. Of course, brotherhood may still be the fundament of within-community alliances.

Yet, only under a rather specific condition males can profit from their

brothers' help. Because of the long birth-intervals in chimpansees (about four to five years, see TuTm, 1980), a younger brother can only be of use

when he is the next offspring born. If dominance rank (and hence repro- ductive success) of males would depend on fraternal assistence, natural

selection would either favour females that produce sons in an uninter-

rupted sequence or, if not, females that produce offspring at short inter-

vals. Both of these conditions are refuted by empirical data (actually, in

GOODALL'S study area a significant tendency for consecutive offspring to

alternate in sex has been found. CLUTTON-BROCK & ALBON, 1982, quoting

unpublished results from TUTIN). Furthermore, research has shown that

alliances do not necessarily depend on brotherhood (NISHIDA, 1979.

Captive chimpansees: DE WAAL, 1982). The assumed fraternal basis of

male parties is therefore anecdotical.

Finally, there is even no conclusive empirical evidence for the claim

that chimpanzee males are close kin at the level of communities. High relatedness among males is at present no more than a hypothesis and

seems doubtful considering a number of "diluting" mechanisms. Matings between peripheral females and males from a neighbouring community, females giving birth in their natal community after having mated else-

where and females immigrating in company of their sons are all described

by GOODALL (1986) and especially the impact of the first two phenomena is not to be underestimated.

Clearly, the situations under which male bondedness evolves are not

well understood (VAN HOOFF & VAN SCHAIK, 1992) and a thorough investi-

gation of the importance of kinship is needed. If, for instance, it could be

demonstrated that male parties can arise without any considerations

about relatedness and the existence of neighbouring communities, we are

one step closer in solving the dilemma about initial evolutionary condi-

tions. We therefore set ourselves to answer the following research

question: "Can chimpanzee-like party structures emerge from simple rules that

specify nothing more than direct interactions between the behavior of

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individuals on the one hand and the structure of their local environment

on the other hand?"

With "local environment" we refer exclusively to phenomena that are

within the detection distance of individuals. We do neither consider

neighbouring communities, nor attribute individuals with global knowl-

edge or special cognitive capacities of any sort (i.e. ours is not a computa- tional model). Simplicity of rules means we make no a priori specific

assumptions about costs, benefits, competition, or relatedness.

This is a different kind of explanation than the usual functional one.

Instead of starting from an explicitly formulated optimality approach, we

investigate the generative power of the direct environment concerning the emergence of parties by means of a computer model that will be

outlined next.

The model

General considerations.

The model should be able to generate male grouping, female solitariness but at the same time complex fission-fusion dynamics (that are not just the result of a random generator). This calls for a deterministic model that generates both pattern and stochastic behaviour. Our aim to get complexity out of simplicity determined the choice of the model formalism: the design of the model should be such that it allows for a high degree of self-structu ring.

W therefore set up a so-called MIRROR model. In this type of model, developed by HOGEWEG & HESPER (1985, 1986, 1988, 1989, 1991), an artificial (MIRROR-) world is created that consists of a SPACE containing PATCHes (names of entities in the MIRROR world are written in capitals to distinguish them from those in our own world). In this world DWELLERs roam. A DWELLER is a local information processing entity that behaves according to simple TODO rules. The TODO principle entails the following. If a DWELLER in a PATCH finds itself in a certain situation ("state"), say x, it is specified TODO only action y. This action changes the states of both PATCH and D?1'ELLER, but eventually also that of another DWELLER, into a NEXTSTATE. Brought in this new situation, another TODO is activated etcetera. For example, a "hungry" D?NELLER is in a food PATCH: it is then instructed to eat and consequently empties the PATCH. In this way it changes its own state (hungry-not hungry), that of the PATCH (full empty) and also determines the (next) state of a second DWELLER (who is now unable to use the same resource and is therefore, when hungry, directed to search for another PATCH).

Because of these interactions and the resulting ever changing STATES, even with a small number of DWELLERS, a MIRROR model in a sense "leads a life of its own". Therefore, its output may be complex and is often surprising. Note, that we strive to obtain an output that is of a higher level of complexity than the input and self-structuring dynamics may allows for this. Hereto, direct interactions are crucial; thus, we are not concerned with what the "average CHINIP" should do "on average" in order to optimize certain "goals".

The "CHIMP" world.

In the artificial world presented here, the DWELLERs are called ``CHIMPS" because they have a few things in common with chimpanzees. However, we stress that CHIMPs are by

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no means identical to chimpanzees: whereas the latter are complex, intelligent beings with information processing capacities similar in several respects to those of humans, CHIMPs are extremely simple and "stupid" entities. It is not our purpose to diminish this difference, but to show that kin selected affiliative mechanisms, the presence of neighbouring commu- nities and complex cognitive capacities are not prerequisites for party structures as observed in chimpanzees. When the party structure of CHIMPs in some aspects reflects that of chimpanzees, the responsible underlying TODOs offer interesting hypotheses for further investigations.

The things CHIMPs have in common with (a certain community of)6) chimpanzees are: 1) the size of their habitat (= 4 km2) 2) their community composition, consisting of 14 MALEs and 1 1 FEMALEs. About two FEMALEs are synchronously in oestrus for ten successive days per month.

In the CHIMP world there are 480,000 TREEs of which some 1200 bear fruit at the same time. They do this once a year for ten successive days, if not already emptied by CHIMPs before that time. TREEs have an average crown diameter of about 10 meter7) and crop size is expressed as the time one CHIMP can feed from it. This varies from 0.1 - 7.0 hours. In addition, the CHIMP world contains 250 PROT sources which are directly renewed when eaten. These represent a further undefined food item other than FRUI'T and are only eaten by FEMALEs (if. protein sources, such as insects, leaves etcetera).

The behaviour of CHIMPS. (Fig. 1)

CHIMPs look for FRUITING TREES and when they detect one, approach, enter and eat from it. When satiated, they leavc the trec and rest (during a proportion of time spent eating in the FRUITING TREE) in a nearby TREE. When the FRUITING TREE is empty before their stomach is filled, FEMALEs start looking around for a next FRUITING TREE. When FEMALEs (but not MALEs!) are unable to find one, they scan for PROT sources. MALEs differ in one other, very important aspect: in precedence to foraging, they seek FEMALES. Hereto, they are instructed to move forward in the direction of any CHIMP in sight and inspect it. If the other CHIMP is a FEMALE and in oestrus (which is detectable from a distance of 15 m), a MALE follows her until she is no longer in oestrus. She than represents an ELSE, as would another MALE, in which case the inspecting MALE falls back to the foraging routine as described above. CHIMPs travel with an average speed of I km/hour and detect other CHIMPs within 100 m.

Results

Protocols of individual focal CHIMPS.

CHIMPs can in principle be investigated by the same techniques as

applied in field studies of any other animal. For instance, in the MIR-

ROR world, individual ("focal") CHIMPs were followed continuously for

35 days during which encounters with other individuals, the FRUITING

6) The community composition in the CHIMP world reflects the one at Gombe as studied by HALPERIN (1979). The dynamics of cycling females were modelled on the basis of data from TUTIN (1980). 7) GHIGLlERI (1984) considers trees with a diameter of about ten meter as the smallest latge trees he used as "vigils" (posts to observe wild chimpanzees).

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Fig. 3. Frequency distribution of the mean proportion of time FEMALE and MALE CHIMPs spend in parties of increasing size.

TREEs visited, time spent feeding, walking and in parties were recorded.

Examples of such protocols are summarized in Fig. 2.

From this figure, we see that MALE 10 (Fig. 2a) is often surrounded by a number of other MALEs but hardly by FEMALEs - expect the two that

were in oestrus at the time of following. Note the outspoken aggregation of MALEs in the presence of these FEMALEs.

In contrast, MALE 5 (Fig. 2b) did not encounter cycling FEMALEs and

is rather alone. Still, he does partake in MALE grouping several times

whereas meetings with more than one FEMALE are always of a short

duration.

Figure 2c and d show the results from two FEMALEs. Like MALEs,

they do not encounter (other) FEMALEs frequently. FEMALE 8 (Fig. 2c) is in company with MALEs, but only when she is in oestrus. FEMALE 9

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(Fig. 2d) is not cycling and relatively solitary. To sum up, the results of the

protocols:

1) are in accordance with GOODALL'S (1986) notion that male parties often

originate around an oestrus female,

2) show extensive fluctuations in party size, thus reflecting the typical

dynamics of the chimpanzees' fission-fusion society and

3) reveal male aggregation and relative solitariness in females (see also

Fig. 3).

Party composition of CHIMPS and chimpanzees.

The last point was studied in more detail by comparing party composi- tions of CHIMPS and chimpanzees (data from HALPERIN, 1979) (Fig. 4). The similarities are obvious: if anything, MALE CHIMPs spend some-

what more time in all-male parties than reported by Halperin for chim-

panzees. Note that in the CHIMP world male grouping is no more than a

consequence of searching for food and mates, and - in order to make the

outcomes more "realistic" - we even would have to impose an extra rule

to surpress this side-effect!

Another striking similarity concerns travel distance. As in chimpanzees MALE CHIMPs travel further than FEMALEs (CHIMPS: 3.9 vs 2.7 km.

per day; chimpanzees: 3.8 vs 2.8 km. per day as estimated from Table 3 in

WRANGHAM & SMUTS, 1980). Note that this effect was not a priori specified: MALEs walk just as fast as FEMALEs, but since they spend more time in

larger parties (Fig. 3) FRUITING TREEs are depleted faster and

MALES are therefore speeded up. Also, because of the larger distance

travelled, MALEs have a higher probability to encounter others which

reinforces the process. A corrolary, in accordance with observations on

chimpanzees (WRANGHAM & SMUTS, 1980), is that CHIMPs, in particular

MALEs, spend less time feeding in parties than when alone.

Robustness of the model.

We varied the number of TREEs, PROT sources, size of TREEs and

community composition (the parameter values and associated party com-

positions are presented in Table 1). The most striking result is that despite considerable changes in these parameters, the typical pattern of MALE

aggregation and relative solitariness of the FEMALEs remains intact

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Fig. 5. Box-and-whiskers plot for the proportion of time MALE and FEMALE CHIMPs spend in various party types. In the plot, central boxes cover the middle 50% of the data values, between the upper and the lower quartiles. The "whiskers" extend out to the extremes, while the central line is at the median. The whiskers extend only to those points that are within 1.5 times the interquartile range beyond the central box. Any values beyond that distancp above or below the box are plotted as individual points and considered as outliers. Letters for extreme values and outliers correspond with those indicating the settings

in Table 1.

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(Table 1, Fig.5). Some conspicuous variations, revealing a number of

interesting side-effects, however did emerge and are visualized in the box-

and-whiskers plot of Figure 5. From this figure it can be seen that the only real outliers in party composition are from the entries I and J. All the

other extreme values but two are due to the parameter values of entries I

and K.

I and J are the only settings with extreme parameter values for the

PROT resources: in I no PROTs are available at all, in J PROTs have the

highest density of all entries relative to the total number of resources.

When PROTs are at their maximum relative density we find the highest values for solitariness in MALEs. Without PROTs, CHIMPs (especially

FEMALEs) are less often in same sex-parties and mixed groups of both

sexes prevail. In other words, the absence of a specific FEMALE food

source appears to depress the sexual difference in party composition. The solely conspicuous feature of condition K is its low density of

CHIMPs (with a sex ratio more or less the same as in the standard setting, 0.70 vs 0.78) and this results to roughly the opposite outcome as condition

I, i.e. an exaggeration of the typical chimpanzee party structure (MALEs often in all-male parties, FEMALEs often solitary and an infrequent occurrence of mixed groups in both sexes). Because a lower number of

CHIMPs implies a lower total consumption of food, the effect of K is due

to an increased density of resources. The influence of a change in

resource density can be gauged from comparing entries D, E and H (same

average crop size, density of food sources in H twice that of D, E): in a

habitat with more resources, the CHIMPs spent more time in same sex

parties and less time solitary or in mixed parties (Fig. 6a). A similar

comparison for different crop sizes at the same resource density (settings B,C versus H) reveals no clear effects (Fig. 6b).

We propose the following interrelations between these features:

1. A higher number of FRUITING TREES. Leads to as well an increased

proportion of time spent in parties (see above) as an increase in party size,

probably because it results to a higher degree of "consensus" between

CHIMPs as to where to go. As can be seen from entries N-O in Table 1, the increase in party size with resource density holds especially for

FEMALEs (for whom food, in particular PROTs, is the only source of

directional consensus). Absence of PROTs, therefore, leads to a higher

degree of consensus between sexes and hence to a prevalence of mixed

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parties. It is important to keep in mind that the consensus not necessarily concerns only those CHIMPs that occupy the same FRUITING TREE.

2. A lower number of CHIMPS. As explained above, at lower population

densities, less FRUITING TREEs are exploited in total and the number

of fruit bearing TREES is therefore higher. Following (1) this results in a

higher degree of sociality which offsets the decrease of chance encounters.

3. A low crop size ofFRUITING TREES. Crop size itself appears to influence

day range distance (Table 1, cf. entries B,C versus H) but not party

composition (Fig. 6a). However, in combination with a change in

FRUITING TREE density, it has a considerable impact on party size:

party size increases especially when both the number of FRUITING

TREES is high and their size is small (entries 0 and R, Table 1). We

explain this as follows: the smaller the FRUITING TREEs, the greater the chance that food is exhausted before the CHIMPs are satiated. In that

case the CHIMPs look for another food source and this has two effects:

(a) since the next nearest food FRUITING TREE is the same for all co-

feeding party members, they do so together. In other words, decreasing FRUITING TREE size increases synchronization of movement, and

together with a larger number of FRUITING TREEs (1) this results in

increased sociality.

(b) increased travel distance, which reinforces (2a) because it leads to a

higher probability of encountering other CHIMPs.

Discussion

The model shows clearly that male relatedness and hostile neighbouring communities are not necessary for the formation of party structures that

in some aspects cannot be distinghuished from those of the chimpanzees at Gombe. We therefore conclude that it is worthwile to derive hypoth- eses from the model and test them in chimpanzees.

In the CHIMP world, the chimpanzee-like features are a direct conse-

quence of a dietary difference between the sexes and the mate-seeking behaviour of the MALEs. If a MALE goes toward another CHIMP to

inspect it, and the other is a MALE himself, each approaches the other;

when in addition MALEs happen to encounter each other in a small

TREE, a reinforcing process sets in and a travel band emerges. Of course,

the presence of an estrus FEMALE acts as an extra focal point that brings

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MALEs together. Thus, the foundations for a party are laid very much

sooner in MALEs than in FEMALEs and are themselves rooted in simple

reproductive behaviour.

How a MIRROR model can generate testable hypotheses is demon-

strated by the case of orang-utans. Field work has shown that these apes are relatively solitary, although occasionally they come together (ROD-

MAN, 1973; MACKINNON, 1974; RIJKSEN, 1978; GALDIKAS, 1979). At least

in the Ketambe Research Area (Sumatra, Indonesia), this occurs espe-

cially during periods of increased fruit production (TE BOEKHORST et al.,

1990). It has therefore been suggested that lack of competition allows for

party formation, although it is still puzzling in what way orang-utans benefit from aggregating (SUGARDJITO et al., 1987). Alternatively, consid-

erations about benefits may be void when travel bands originate in the

same fashion as in CHIMPs (interestingly, the fruit season in Ketambe is

indeed typified by the simultaneous fruit production of especially the

smaller tree species; a situation that would favor travel band formation in

the CHIMP world). We checked the merits of this proposal by designing an artificial ORANG world (that resembles Ketambe concerning the

distribution of food sources and the population composition of its inhabi-

tants) and found that the CHIMP mechanism for travel band formation

operated also under orang-utan like conditions. A number of hypotheses derived from the model, among others that the probability of remaining in a travel band depends on the size of the former tree in which the party members fed together, were confirmed with field data (TE BOEKHORST &

HOGEWEG, 1994). This example also shows that the CHIMP model can be fruitfully

extended to study other species, and an obvious candidate is the bonobo.

In the CHIMP world we found a party structure reminiscent to that of

bonobos (a preponderance of mixed parties) in the absence of an exclu-

sively FEMALE resource (PROTs). It is therefore interesting to investi-

gate whether chimpanzees are characterized by a more pronounced sexual difference in the choice of food items than bonobos. Undoubtly, the prolonged presence of estrous females in bonobos as compared to

chimpanzees (KANO, 1982; FURUICHI, this volume) must have a consider-

able influence on the party structure; in the CHIMP world it would

almost certainly lead to extensive gatherings of MALEs around

FEMALEs. This effect probably overrides that of possible dietary sim-

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ilarities between the sexes, and in that case a typical female resource may even be a stronger necessity in bonobo's than in chimpanzees for keeping females sufficiently together.

Apart from the extensive possibilities to investigate a range of (artifi-

cial) species by adapting the basic rules, what clearly makes the model

attractive is its interactive, self-structuring nature and its therefore unex-

pected, heuristic results. Because of the large number of interactions (with one another and the environment) even a small number of CHIMPs are

engaged in and the resulting alterations these bring about, what appear at

first sight to be inconsequentially simple rules that operate only on a local

scale may lead to conspicuous patterns on a larger scale. An example is

the positive feedback in the CHIMP and ORANG world by which the

formation of travel bands reinforces itself.

In our model the relative invariance of the "social" structure of

CHIMPs is caused by a subtle interplay between the effect of number and

size of food sources and the community composition. This shows that

environmentally generated patterns can be self-stabilizing and thus be

fairly robust to (small) changes in the environment, without representing a "homeostasis" that is selected for because of its beneficial effects. This

does not imply that selection plays no role: once a social structure is

stabilized in this way it may offer a substrate for selection to act upon. In

the case of chimpanzees, a CHIMP like mechanism might have set the

stage for what is perhaps the most conspicuous behavioral aspect of the

species at present: especially in the vicinity of estrous females, males

almost continuously keep an eye on each other. Because this is also

evident in captive colonies (TE BOEKHORST, 1991), mutual control might have become the major force leading to male aggregations and may have

taken over the original environmental structuring (as proposed by our

model). If this is true, male aggregation in chimpanzees should not be

considered as a reflection of affiliative relationships, but rather as the

expression of a "dear enemy" strategy in a species with scramble poly-

gyny and sperm competition (a description that anyhow fits better with

the "political" manoevres that are described for chimpanzees, than a

supposed "altruistic nature"). And of course, being brought together in

this way is in turn useful when it comes to territorial clashes. These

secondary benefits might have become ever more important during the

evolution of chimpanzee communities and therefore increased the spread

249

of alleles in the gene-pool that code for our hypothetical mate and food

searching rules.

Some of the patterns yielded by the model, such as the increased

emergence of mixed parties in the absence of PROTs, were not foreseen

despite their afterwards almost trivial explanation. Apparently, our

human mind is not very good in detecting self-structuring and we need a

tool such as a MIRROR model to put us on the track. Another result, the

origination of parties especially when there are many small trees, is

counter-intuitive in view of conventional considerations about competi- tion. To put aside the dictate of "economic decisions" once in a while

might be very refreshing: although thinking in terms of cost-benefit

balances has certainly helped to categorize our knowledge of animal

behaviour, it should not become dogmatic. Often, functionalists give the

impression that by logic alone the set of relevant possible alternatives to

test against a given null-hypothesis can be deduced. However, we feel it is

unwise to decide for an organism what should be "biologically meaning- ful behaviour" and what is not since we have hardly any idea about the

number and nature of the alternative solutions and neither about the

dynamics of their associated costs and benefits in an ever changing world.

Counter-intuitive outcomes are therefore not to be abhorred (because

they may destroy a logically elegant derivation) but should be welcomed

for they force us to explore new areas and therefore might bring new

insights.

Despite these comments, we stress that our approach is not an alterna-

tive to the postulate of natural selection, but merely a parsimonious use of

it. As a matter of fact, one might even maintain that in our model we

apply TRIVER'S (1972) scenario, with males being selected to increase

succesful matings and females to increase food intake, even more rig-

orously than most other authors. For instance, WRANGHAM (1987) and

DUNBAR (1988) follow TRIVER's argument up to the explanation for

solitariness in females and the resulting inability of males to monopolize

females; for the remaining feature (male grouping) an additional adapta- tion is sought and kin selection is invoked. In our case no extra evolution-

ary explanations for separate phenomena are needed beyond that of the

fundamental rules about sex and food. We feel this is to be preferred, since (1) arguments based explicitly and only on natural selection are

generally more assumption loaden than those that are not and (2) the

250

tendency to endow each part of a pattern with its own selection pressure

represents a naive "linear" view of nature. Instead of giving separate ultimate explanations for each observed feature, we advocate to move

away from such an evolutionary phenomenology to an hierarchical

approach: the observed features are the end points of a branched tree of

self-structuring effects, their selective origin is to be found in the roots of

the tree. Knowledge of the roots provides us with an opportunity to map the relevant selection pressures.

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