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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.
References
BOEHM, C. (1992). Segmentary "warfare" and the management of conflict: comparison of East-African chimpanzees and patrilineal-patrilocal humans. - In: Coalitions and alliances in humans and other animals (A.H. HARCOURT & F.B.M. DE WAAL, eds). Oxford Scientific Publications, Oxford, p. 138-173.
TE BOEKHORST, I.J.A. (1991). Social structure of three great ape species: an approach based on field data and individual oriented models. - Ph. D. thesis, University of Utrecht.
_ _, SCHÜRMANN, C.L. & SUGARDJITO, J. (1990). Residential status and seasonal move- ments of wild orang-utans in the Gunung Leuser Reserve (Sumatra, Indonesia). _ Anim. Behav. 39, p. 1098-1109.
_ _ & HOGEWEG, P. (1994). Effect of tree size on travel band formation in orang-utans: data analysis suggested by a model study. - In: Artificial life IV (R. BROOKS & P. MAES, eds). MIT Press, Cambridge (MA).
CAMPBELL, D. (1988). Introduction to nonlinear phenomena. - In: Lectures in the sciences of complexity, Vol.I of Lecture Volumes of the Santa Fe Institute studies in the sciences of complexity (D.L. STEIN, ed). Addison-Wesley, Redwood City, California, p. 5-105.
CLUTTON-BROCK, T.H. & ALBON, S.D. (1982). Parental investment in male and female offspring in mammals. - In: Current problems in sociobiology (King's College Sociobiology group, eds). Cambridge University Press, Cambridge, p. 223-247.
COHEN,J.E. (1975). The size and demographic composition of social groups of wild orang- utans. - Anim. Behav. 23, p. 53- 550.
DAWKINS, R. (1976). The selfish gene. _ Oxford University Press, Oxford. DUNBAR, R.I.M. (1988). Primate social systems. - Croom Helm, London. GALDIKAS, B.F. (1979). Orangutan adaptation at Tanjung Puting Reserve: mating and
ecology. - In: The great apes (D.A. HAMBURG & E.R. McCOWN, eds). Benjamin/ Cummings, Menlo Park, California, p. 195-233.
GHIGLIERI, M.P. (1984). The chimpanzees of Kibale forest. - Columbia University Press, New York.
GLASS, H. B. (1953). The genetics of the dunkers. _ Sci.Am. 189 (2), p. 76-81. GLEICK,J. (1987). Chaos: the making of a new science. - Viking Press, New York. GOODALL, J. (1986). The chimpanzees of Gombe: patterns of behavior. --- Belknap press of
Harvard University Press, Cambridge, Massachuetts & London. GOUZOULES, S. & GOUZOULES, H. (1987). Kinship. - In: Primate societies (B.B. SMUTS, D.L.
CHENEY, R.M. SEYFARTH, R.W. WRANGHAM & T.T. STRUHSAKER, eds). Chicago Uni- versity Press, Chicago, p. 299-305.
HALPERIN, S.D. (1979). Temporary association patterns in free-ranging chimpanzees. _ In : The great apes (D.A. HAMBURG & E.R. McCOWN, eds). Benjamin/Cummings, Menlo Park, California, p. 490-499.
251
HOGEWEG, P. (1988). MIRROR beyond MIRROR, puddles of life. - In: Artificial life I. Santa Fe Institute studies in the sciences of complexity (C. LANGTON, ed.). Addison Wesley, Redwood City (CA), p. 297-315.
_ _ (1989). Simplicity and complexity in MIRROR universes. - Biosystems 23, p. 231-246.
_ _ & HESPER, B.(1985). Socioinformatic processes, a MIRROR modelling methodology. - J. theor. Biol. 113, p. 311-330.
_ _ & _ _ (1986). Knowledge seeking in variable structure models. - In: Modelling & simulation in the artificial intelligence era (ELZAS, OREN & KLING, eds). North Hol- land, the Netherlands, p. 227-315.
_ _ & _ _ (1991). Evolution as pattern processing: TODO as substrate for evolution. - In : From animals to animats (J.A. MEYER & S. WILSON, eds). MIT Bradford Books, Boston, p. 492-497.
VAN HOOFF, J.A.R.A.M. & VAN SCHAIK, C.P. (1992). Cooperation in competition: the ecology of primate bonds.- In: Coalitions and alliances in humans and other animals (A.H. HARCOURT & F.B.M. DE WAAL, eds). Oxford Scientific Publications, Oxford, p. 357-389.
MACKINNON, J.R. (1974). The ecology and behaviour of wild orangutans (Pongo pygmaeus). - Anim. Behav. 22, p. 3-74.
LEWONTIN, R.C. & LEVINS, R. (1987). Aspects of wholes and parts in population biology. _ In : Evolution of social behavior and integrated levels (G. GREENBERG & E. TOBACH, eds). Erlbaum, Hillsdale, New Jersey, p. 31-52.
KANO, T. (1982). The social group of pygmy chimpanzees (Pan paniscus) of Wamba. _ Primates 23, p. 171-188.
McFARLAND, D.J. (1976). Form and function in the temporal organisation of behaviour. - In : Growing points in ethology (P.P.G. BATESON & R.A. HINDE, eds). Cambridge University Press, Cambridge (UK), p. 55-93.
MITANI, J.C., GRETHER, G.F., RODMAN, P.S. & PRIADNA, D. (1991). Associations among wild orang-utans: sociality, passive aggregations or chance?. - Anim. Behav. 42, p. 33-46.
NISHIDA, T. (1968). The social group of wild chimpanzees in the Mahali Mountains. - Primates 9, p. 167-224.
PUSEY, A. (1979). Intercommunity transfer of chimpanzees in Gombe National Park. - In: The great apes (D.A. HAMBURG & E.R. McCOWN, eds). Benjamin/Cummings, Menlo Park, California, p. 465-480.
_ _ & PACKER, C. (1987). Dispersal and philopatry. - In: Primate societies (B.B. SMUTS, D.L. CHENEY, R.M. SEYFARTH, R.W. WRANGHAM & T.T. STRUHSAKER, eds). Chicago University Press, Chicago, p. 250-266.
REYNOLDS, V. & REYNOLDS, F. (1965). Chimpanzees of the Budongo forest. - In: Primate behavior (I. DEVORE, ed.). Holt, Rinehart & Winston, New York, p. 368-424.
RIJKSEN, H.D. (1978). A field study of sumatran orang-utans (Pongo pygmaeus abelli, Lesson 1827): ecology, behaviour, and conservation. - H. Veenman & Zonen, Wageningen, the Netherlands.
RISS, D.C. & GOODALL,J. (1977). The recent rise to alpha rank in a population offree living chimpanzees. - Folia Primatol. 28, p. 134-151.
RODMAN, P.S. (1973). Population composition and adaptive organization among orang- utans of the Kutai Reserve. - In: Comparative ecology and behaviour of primates (R.P. MICHAEL & J.H. CROOK, eds). Academic Press, London, p. 171-209.
RUBINSTEIN, D.I. (1982). Risk, uncertainty and evolutionary strategies. - In: Current problems in sociobiology (King's College Sociobiology group, eds). Cambridge Uni- versity Press, Cambridge, p. 91-111.
SIBLY, R.M. & McFARLAND, D.J. (1976). On the fitness of behaviour sequences. - Am. Nat. 110, p. 601-617.
252
SIGMUND, K. (1993). Games of life: explorations in ecology, evolution and behaviour. -
Oxford University Press, Oxford. STEWART, I. (1989). Does God play dice? The new mathematics of chaos. - Blackwell,
Oxford. SUGARDJITO, J., TE BOEKHORST, I.J.A. & VAN HOOFF, J.A.R.A.M. (1987). Ecological con-
straints on the grouping of wild orang-utans (Pongo pygmaeus) in the Gunung Leuser National Park, Sumatera, Indonesia. - Int. J. Primatol. 8, p. 17-41.
TRIVERS, R.L. (1972). Parental investment and sexual selection. - In: Sexual selection and the descent of man (B. CAMPBELL, ed.). Aldine, Chicago, p. 136-179.
TUTIN, C.E.G. (1980). Reproductive behavior of wild chimpanzees in the Gombe National Park. -J. Reprod. Fert. Suppl. 28, p. 43-57.
VILLA, F. (1992). New computer architectures as tools for ecological thought. - TREE 7 (6), p. 179-183.
DE WAAL, F.B.M. (1982). Chimpanzee politics. -Jonathan Cape, London. WRANGHAM, R.W. (1979a). On the evolution of ape social systems. - Soc. Sci. Inform. 18,
p. 335-368. _ _ (1979b). Sex differences in chimpanzee dispersion. - In: The great apes (D.A.
HAMBURG & E.R. McCOWN, eds). Benjamin/Cummings, Menlo Park, California, p. 481-490.
_ _ (1982). Mutualism, kinship and social evolution. - In: Current problems in socio- biology (King's College Sociobiology group, eds). Cambridge University Press, Cam- bridge, p. 269-289.
_ _ (1986). Ecology and social relationships in two species of chimpanzee. - In: Ecologi- cal aspects of social evolution. Birds and mammals (D.I. RUBINSTEIN & R.W. WRAN- GHAM, eds). Princeton University Press, Princeton, p. 352-378.
_ _ (1987). Evolution of social structure. - In: Primate societies (B.B. SMUTS, D.L. CHENEY, R.M. SEYFARTH, R.W. WRANGHAM & T.T. STRUHSAKER, eds). Chicago Uni- versity Press, Chicago, p. 282-296.
_ _ & SMUTS, B.B. (1980). Sex differences in the behavioural ecology of chimpanzees in the Gombe National Park, Tanzania. - J. Reprod. Fert. Suppl. 28, p. 13-31.