Ngogo Chimpanzee Project

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David Watts (Yale University) and John Mitani (University of Michigan) started the Ngogo Chimpanzee Project in 1995 and, with the help of Ph.D. and Post-doctoral students, Ugandan Research Assistants, and Dr. Jeremiah Lwanga (Project Manager and now also Director of MUBFS), have maintained a continuous research presence at the site since then.
The chimpanzee community (social group) at Uganda, which has over 160 members, is Chimp36by far the largest ever known.  Ngogo is particularly favorable habitat for chimpanzees, which helps to explain why such a large community can exist and why population density there is three times as high as at the nearby site of Kanyawara, the second long-term research site in Kibale.

The Ngogo chimpanzees are well habituated to the presence of observers, which has allowed researchers to collect extensive and detailed information on many topics.  Among others, these have included male social relationships and social networks; female social relationships; mating behavior and influences on male reproductive success; effects of kinship on social behavior; influences on hunting frequency and success and on meat sharing; the impact of predation by chimpanzees on the local red colobus monkey population; and the function of inter-group aggression in chimpanzees.  The large community size means that social dynamics there are particularly complex at Ngogo; correspondingly, research there has greatly expanded our knowledge of the range of variation in chimpanzee social behavior and ecology.

Ngogo researchers contribute importantly to protection of the chimpanzees and their habitat both by their presence and by their financial support for local and Uganda Government  protection and conservation efforts.

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Perhaps the most stunning research result of the Ngogo Chimpanzee Project has been simply documentation of the extraordinary size and the composition of the chimpanzee community there and of the concomitant extent of variation in community size and population density within Kibale.  As of September, 2011, the Ngogo community had 166 members, including 32 adult males and 50 adult females; these numbers are unprecedented.  In contrast, the Kanyawara community has fluctuated between 44-52 during the course of research there, and average community size at all research sites is 50-60 (including about 8 adult males), while the largest previously documented community, Mahlae “M Group”, reached 114 before declining to 70-80 individuals.  The Ngogo community’s home range is about the same size as that of the Kayawara community, which means that population density is three times higher at Ngogo.

Ficus mucuso

Long-term data on chimpanzee feeding ecology and on the density and phenology (schedules of flowering, fruiting, and leaf production) go far toward explaining the astonishing differences in community size and population density between Kanyawara and Ngogo.  Much of the diet at Ngogo comes from tree species that are rare or absent at Kanyawara despite the close proximity of the sites.  Most important among these is the giant fig Ficus mucuso, which reaches relatively high density at Ngogo and is the top food in the chimpanzees’ diet there, but does not occur at Kanyawara.  Fruit availability varies less at Ngogo, and the chimpanzees there have abundant  fruit for a greater proportion of the time than those at Kanyawara.  Part of the reason for this is again F. mucuso, which does not fruit seasonally and which produces enormous fruit crops, some of which are available most of the time.

Other notable Ngogo research findings, some new and others of which extend the known range of variation in chimpanzee behavior and/or help to answer important evolutionary questions, include:

1) Hunting and meat sharing

Current alpha male Miles (right) sharing meat with another male, Lofty.

We have long known that chimpanzees hunt a variety of vertebrate prey and that they particularly hunt red colobus monkeys wherever the two species co-exist.  Ngogo is no exception: we have documented predation on 11 species, including all the other diurnal primates, but the great majority of hunts and over 80% of kills are of red colobus.  Most hunts are by groups of chimpanzees (mostly males) pursuing groups of monkeys.  Success rates in such hunts are unusually high compared to those of large mammalian carnivores (they are similar or higher only for African wild dogs): chimpanzees catch at least one prey in 50% of hunts at well-documented sites other than Ngogo.  Ngogo again stands out, however: chimpanzees there succeed in over 80% of red colobus hunts, and also capture prey in the majority of hunts of other species.  In most successful hunts of red colobus at other sites, the chimpanzees capture a single monkey; Ngogo chimpanzees capture an average of four monkeys per successful hunt.  The high success rate and high number of kills per hunt are among the many demographic effects on behavior that we have discovered at Ngogo: hunting parties there are typically far larger than at other sites, which makes it easier for the hunters to overwhelm the monkeys’ defenses (the number of male chimpanzees present when chimpanzees encounter red colobus is the best predictor of whether they will hunt).  However, we have also shown that the chimpanzees selectively pursue red colobus where the forest canopy is low and/or broken, which hinders the monkeys’ ability to escape; the hypothesis that male hunting strategies depend partly on how canopy structure influences their assessment of whether they can succeed had been proposed in the 1980s, but we were the first to test the hypothesis formally.

We also provided the first formal test of the hypothesis that chimpanzees hunt more often when they experience nutritional shortfalls because fruit is scarce.  Instead of supporting this hypothesis, our data strongly support the alternative hypothesis that chimpanzees hunt more often when fruit abundance is high.  This makes sense, because at such times they can easily meet their energy needs by eating fruit, and thus can afford to invest time and energy in searching for groups of red colobus despite having no guarantee of finding any or of succeeding in capturing any if they hunt, and they can also afford to form large parties that include many males with facing unduly high levels of feeding competition.

Chimpanzees sometimes aggressively compete over control of and access to meat, but more remarkable is that individuals who lack meat usually beg for it from those who possess it instead of trying to appropriate it aggressively and that meat is commonly transferred non-aggressively to beggars from possessors, who either allow beggars to take some or actively give them shares.  Various hypotheses have been proposed to explain why such transfers occur.  One that has received considerable popular attention is that males (most often the possessors) preferentially share meat with sexually receptive females in exchange for mating access.  This “meat for sex” hypothesis, while catchy, has now been rejected with data from two sites, the first of which was Ngogo.  We cannot reject the hypothesis that some transfers at Ngogo occur because meat possessors simply give in to “harassment” by beggars so that they experience fewer interruptions while eating meat.  However, we find strong support for the hypothesis that males use meat as a “social tool” to develop and maintain strong social bonds with each other: they transfer meat preferentially to their main grooming partners and their allies, most such transfers involve active donation of meat, and these important social partners disproportionately receive relative large shares instead of just small pieces.

One unexpected consequence of the high rate of predation by chimpanzees on red colobus at Ngogo has been documentation of a major decline in the local red colobus population.  We have combined our observations with long term census data on monkey populations (starting with Struhsaker’s work in the 1970s) to show that the red colobus population at Ngogo has declined steeply since 1975 (whereas populations of other monkey species have either remained more or less stable or have increased) and that the rate at which chimpanzees encounter red colobus has declined steeply since the mid-1990s (although the decline has not been linear: it accelerated sharply after a prolonged period of intense predation pressure in 2002).  The only other known cases of unsustainable predation on primate populations are ones in which humans are the predators.  Unsurprisingly, red colobus hunts occur less often at Ngogo now than in the past, but the chimpanzees continue to exert considerable pressure on the remaining population.  We do not expect the red colobus to suffer a local extinction, if only because the chimpanzees are not obligatory carnivores and red colobus may move into the Ngogo area from surrounding parts of the forest where predation pressure is lower, but whether the red colobus population will recover or remain low is an important focus of ongoing research.

2) Intergroup Aggression:

Chimp24Aggression between neighboring communities, boundary patrols, and occasional lethal attacks on strangers (especially adult males and unweaned infants) have now been documented at all long-term chimpanzee research sites except Boussou (where the study community is isolated in a small area of forest).  Acceptance that events are part of the evolved repertoire of chimpanzee behavior is now widespread, if still not universal, and our observations at Ngogo have been crucially important for winning this acceptance.  We were the first to document patrolling and lethal aggression at a site where researchers had not provisioned the chimpanzees with food to habituate them and/or where human disturbance to the animals’ habitat (e.g., by logging) was minimal.  The Ngogo Chimpanzee Project now has an invaluably extensive data base on the frequency and location of encounters and patrols and on variation in participation in patrolling (which is mostly a male activity).  Moreover, the majority of known intergroup killings by chimpanzees throughout Africa have been documented at Ngogo despite the fact that chimpanzees have been studied far loner at several other sites; this is largely a consequence of the intensity of our research effort and of the extraordinary size and composition of the community (males can easily assemble large parties to patrol or to respond to outsiders when they hear or encounter them during the course of their usual activities).  Ngogo data provide strong support for the hypothesis that the propensity for males to cooperate with each other, combined with fission-fusion sociality, allows groups from one community to make potentially lethal attacks on competitors from neighboring communities at minimal risk to themselves.  More importantly, we have documented a major expansion of the Ngogo territory to the northeast into areas where patrols, encounters, and lethal attacks have been disproportionately concentrated and where attacks by Ngogo chimpanzees have been a major source of mortality for their neighbors.  This is strong support for the hypothesis that inter-community aggression ultimately functions to defend and, if successful enough, to improve access to food resources and thereby to maintain improve survival prospects for community members, notably the infants born to resident females and sired by males who engage in the aggression.

3. Influence of kinship on male social relationships

Ngogo males

Because male chimpanzees remain in their natal communities for life, thus with their mothers, maternal brothers, and paternal male relatives, it has long been thought that high relatedness helps to account for cooperation among males.  Once non-invasive genetic sampling (using DNA extracted from hair or feces) became available, however, researchers discovered that average relatedness among males is lower than was generally assumed.  Nevertheless, careful analysis of social relationships among males, combined with genetic data, has shown convincing evidence of maternal kinship effects: on average, maternal brothers are more likely than other pairs of males to associate, groom, and form alliances with each other than are other pairs of males, and that maternal brothers also are particularly likely to share meat with each other and to participate jointly in boundary patrols.  Still, Ngogo data also show convincing evidence that males groom, share meat, and support each other in coalitions independently of kinship, and also that much cooperation among males – especially formation of coalitions, in which one male helps another direct aggression at a third party – qualifies as “mutualism”, in that the cooperators gain immediate net advantages from their actions, instead of paying costs that must be repaid in some form if the behavior is to be in their own long-term interest.  These findings, and others from detailed study of male-male relationships and social networks at Ngogo, are producing a much more sophisticated and nuanced understanding of social dynamics; again, this is possible in large part because of the extraordinarily large community size and the long-term nature of data collection.

4. Variation in male mating strategies and reproductive success

Chimpanzees have a complex and varied mating system.  Most matings occur in group settings, where females copulate with most or all of the males in their communities during “estrus” periods that can last more than two weeks per menstrual cycle.  Data from Ngogo accord with those from Tai and Gombe in showing that achieving high dominance rank indeed confers advantages in terms of access to estrous females that in turn to real reproductive advantages, although the positive relationship between male rank and actual paternity success becomes weaker as the number of males in a community increases.  But Ngogo data have revealed another male mating tactic, independent of rank, previously unknown for chimpanzees.  Males generally use the entire community home range, whereas females, including those at Ngogo, tend to concentrate their activities in relatively small parts of the home range (“core areas”) except when they are in estrous.  Males at Chimp21Ngogo also show some spatial substructuring, however – that is, many of them also tend to spend disproportionate amounts of time in certain areas, where they associate disproportionately often with females whose core areas are in those areas.  The extent to which particular males and females associate with each other turns out to be as good a predictor of the probability that a male fathers a female’s infant as is the male’s dominance rank.  This result that was quite unexpected for chimpanzees, but again makes sense in view of the enormous community size: accurately tracking the reproductive status of so many females dispersed over so large an area is extremely challenging for males, and it seems to be an effective alternative tactic for a male to restrict his ranging somewhat and to maximize the amount of information he has about a subset of the females.  In one extreme example, Garrison, a fairly high-ranking male about 30 years old who spends most of his time in the western of the community range, is tied for the current lead in number of assigned paternities (six); all of his offspring are with “western” females.  Interestingly, the “western” subgroup often enters or is close to areas where the Ngogo home range overlaps that of a neighboring community and often hears or meets those chimpanzees; such encounters can be highly dangerous, and hormonal data derived from non-invasively collected samples (urine or feces) during one of several studies that have focused on aspects of behavioral endocrinology show that he typically has high cortisol levels; this seems to be a consequence of his high risk/high payoff reproductive strategy.  The number of paternity determinations for Ngogo now far exceeds that from any other chimpanzee research site, and we intend to continue to collect such data; this gives us rich opportunities for investigating influences on male reproductive success and for making inferences about how human mating and social systems might have evolved from a rather chimpanzee-like ancestral condition.

5. Variation in chimpanzee female social relationships

Research on social behavior at Ngogo has not been restricted to adult (and adolescent) males: four Ph.D. projects, two each by graduate students at Yale and Michigan, have focused primarily on females.  Perhaps the most important finding from this work has been the documentation of great variation in gregariousness and highly differentiated social networks among females.  Females form “spatial” neighborhoods (e.g., the “western” females), which is not unusual.  However, within -- and to some extent, independently of -- these, they also form social neighborhoods: certain pairs (and sometimes larger subgroups) of females associate with each other as much or more than most males do and also regularly groom with each other, and these close social ties can persist for years.  Similarly strong social bonds between some females have been documented at Tai (western chimpanzees), where females are relatively gregarious compared to those at Gombe, Mahale, and Kanyawara, but were thought to be absent in eastern chimpanzees.  Research at Ngogo shows that no such eastern/western dichotomy exists.  Indeed, female-female social relationships at Ngogo are more like those at Tai than those at Kanyawara, in the same forest, presumably because the much better food supply at Ngogo means that females there can afford to associate with each other much more, without experiencing undue competition for food, than can their counterparts at Kanyawara.

Continued investigation of female social worlds and ecology is an important priority for future Ngogo research.  Now that most females and their dependent infant and juvenile offspring are well habituated to humans, research on mother-infant relationships and on behavioral development is feasible and has also assumed high priority.

6. Variation in chimpanzee life histories

Chimp4Comparisons of data on demography and fertility in recent populations of human hunter-gatherers to long-term data on chimpanzee birth rates and survival from Gombe, Mahale, Kanyawara, Tai, and Bossou have generated a list of differences that highlight the distinctiveness of human life histories.  These differences include higher infant and adult survivorship and greater longevity in humans; slower maturation in humans, as expected given our larger brains; and, paradoxically, higher female fertility (faster reproductive rates) in humans once they reach maturity.  The paradox occurs because female reproductive rates also are inversely related to brain size in primates generally, but those in humans are much faster than expected from our extremely large brains.  The realization that humans are really “cooperative breeders” resolves the paradoxes: human females can meet the associated extremely high energetic and nutritional costs of reproducing because their efforts are subsidized by others, especially by men, but also by older offspring.

We now must add data from Ngogo to the comparisons.  Ngogo data do not challenge this general picture of human/chimpanzee contrasts, but they indicate that the magnitude of the differences has probably been exaggerated because all of the previous chimpanzee data comes from sites and populations that have experienced serious disturbance by humans, including disease outbreaks and poaching (Tai, Gombe, Mahale), and/or are in relatively poor habitats (Gombe, Bossou, Kanyawara).  Ngogo thus assumes great importance because it is highly favorable habitat where human disturbance has been minimal, and is thus more likely to represent typical conditions for chimpanzees throughout their evolutionary history.  Our preliminary findings are that infant survival there is not unusually high despite favorable habitat conditions (although survival is disproportionately low for first-born infants, a common finding for wild primates; infants born to older, parous mothers survive better), but that adult female survival is considerably higher than reported elsewhere.  Also, the average interval between a surviving birth and the birth of the mother’s next offspring is the lowest found for a wild chimpanzee population, at just over five years.  Continuing to collect data on survival and birth rates at Ngogo can give us a much better picture of what is typical for chimpanzees (and of the possible range of variation) and thus help to pinpoint exactly how humans differ and clarify how the differences might have occurred during our own evolutionary history; obviously, collecting the data requires continuing the long-term research effort there.

Finally, the importance of research for conservation deserves another brief mention.  The Uganda Wildlife Authority is in charge of park management, which includes coordination of efforts to curtail poaching and other illicit activities within the protected area.  They are grateful for the logistic and financial support that we have provided for Park Guards and for our willingness to hire three highly skilled “civilian” anti-poaching experts who work with the Guards; the financial support comes from both Project funds and personal contributions.  Park officials would welcome an expansion of the non-governmental protection team, which would help with protecting a much larger part of the chimpanzees’ habitat, if we could afford to employ more workers (whom we would find with the help and advice of our dedicated current staff).

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