Infanticide and cannibalism by male chimpanzees at Ngogo, Kibale National Park, Uganda

Researchers have documented infanticide by adult males in four wild chimpanzee populations. Males in three of these have killed infants from outside of their own communities, but most infanticides, including one from Kanyawara, in Kibale National Park, Uganda, took place within communities. Here we report two new cases of infanticide by male chimpanzees at a second Kibale site, Ngogo, where the recently habituated chimpanzee community is the largest yet known. Both infanticides happended during boundary patrols, which occur at a high frequency there. Patrolling males attacked solitary females who were unable to defend their infants successfully. The victims were almost certainly not members of the Ngogo community. Males cannibalized both infants and completely consumed their carcasses. These observations show that infanticide by males is widespread in the Kibale population and that between-community infanticide also happens there. We discuss our observations in the context of the sexual selection hypothesis and other proposed explanations for infanticide by male chimpanzees. The observations support the arguments that infanticide has been an important selective force in chimpanzee social evolution and that females with dependent infants can be at great risk near range boundaries, but why male chimpanzees kill infants is still uncertain.     

New cases of inter-community infanticide by male chimpanzees at Ngogo,Kibale National Park,Uganda

Infanticide by males has been recorded in four chimpanzee populations, including that in Kibale National Park, Uganda. Some infanticidal attacks occur during inter-community aggression. The sexual selection hypothesis does not easily explain these attacks because they may not directly increase male mating opportunities. However, females in the attackers’ community may benefit by expanding their foraging ranges and thereby improving their reproductive success; thus infanticide may increase male reproductive success indirectly. We report two new cases of infanticide by male chimpanzees at Ngogo, Kibale National Park. Like two previous cases, these occurred during a boundary patrol and were almost certainly between-community infanticides. The patrolling males attacked despite the proximity of males from the victims’ presumed community. This probably explains why, unlike the earlier cases, they did not completely cannibalize their victims. Such attacks seem to be relatively common at Ngogo and infanticide may be an important source of infant mortality in neighboring communities. Our observations cannot resolve questions about the sexual selection hypothesis. However, they are consistent with the range expansion hypothesis: the infanticides occurred during a period of frequent encounters between communities associated with a mast fruiting event, and Ngogo community members greatly increased their use of areas near the attack site during another mast fruiting event one year later. Our observations contribute to growing evidence that lethal intergroup aggression is a common characteristic of wild chimpanzee populations.     

Fine resolution of early hominin time, Beds I and II, Olduvai Gorge, Tanzania

Reconstructing paleoenvironments and landscapes within lake-centered, hominin-yielding basinal sequences requires a resolution of time-rock units finer than but complementary to that provided by the present tephrostratigraphy. Although indispensable in providing an absolute time frame at Olduvai, the average 15,000-20,000 year intervals between successive tuff units lack the time resolution to construct a sufficiently contemporary paleolandscape within sedimentary intervals away from the interleaved tuffs. Such control is essential to construct valid paleogeographies in which to contextualize contemporaneous paleoanthropological sites and the traces of hominin land use they contain. Within Beds I and II of the Olduvai Basin a Sequence Stratigraphic analysis has achieved a relative time framework in which time-rock units, “lake-parasequences,” each generated by a major advance and withdrawal of the lake system, are recognizable for average periods of about 4000 years duration. Within each of these time slices at least two paleogeographic landscapes are identifiable, reducing the time constraints of an individual landscape reconstruction to a few thousand years. Within the sedimentary succession both highly incised and less incised unconformities are identifiable to provide sequence boundaries. Within each sequence the higher frequency lake-parasequences can be identified by (1) a disconformable base, (2) accretion of sediment during lake transgression and at maximum, (3) a disconformable top caused by lake withdrawal, and (4) a soil profile generated beneath that disconformable land surface. Individual lake-parasequences can be recognized in lake marginal and fan settings, and their imprint can also be seen in the lake setting where, for example, maximum flooding might be marked by a layer of dolomite. Lower Bed II parasequences represent time intervals of <5000 years, while parasequential periods between Tuffs IB and IC in Bed I are <4300 years. Analogous Holocene lake-level changes of the same order in East Africa have a period close to 4200 years. The estimated period is close to that defined by Stadial/Interstadial Dansgaard-Oeschger Events recorded in the Greenland Ice record, which force cycles of period similar to lake-parasequences, both in the Arabian Sea and Lake Malawi. Lake-parasequences not only aid construction of landscapes, they also allow contextualization of individual paleoanthropological occurrences. For instance, a parasequence lies between Leakey’s Level 1 and her butchered Deinotherium occurrence at FLK N. However, elephantid and giraffid skeletons associated with stone artifacts at VEK, uncovered by OLAPP excavations, are situated on the same land surface as a possibly butchered rhinocerid at KK. To complement the existing absolute radiometric time framework, relative Sequence Stratigraphic techniques might be applied to any lake-centered, hominin-yielding basinal sequence, not only those found within East Africa. Because they are climatically controlled, and might plausibly be related to globally driven Dansgaard-Oeschger Events, lake-parasequences and their associated sequences might be correlatable between various East African basins in the Plio-Pleistocene in the same way as they presently are for the Holocene.     

Influence of chimpanzee predation on the red colobus population at Ngogo, Kibale National Park, Uganda

Frequent hunting of red colobus monkeys (Procolobus rufomitratus) takes place at all long-term chimpanzee (Pan troglodytes) study sites where both species are present. Red colobus are the most commonly selected prey of chimpanzees even when other monkey species are more abundant. In particular, the chimpanzee community at Ngogo, Kibale National Park, Uganda, preys heavily on red colobus monkeys: the chimpanzee hunting success rate is extremely high, and chimpanzees kill many individuals per successful hunt. Census data had suggested that the red colobus population is declining and that predation by chimpanzees may be contributing to this decline. In this paper, I address the impact of hunting on the red colobus population at Ngogo. To test the hypothesis that chimpanzee hunting is sustainable, I am using demographic data collected on red colobus monkeys over a period of 3 years, as well as fecundity and mortality data from previous studies of this species. I apply matrix models and vortex analyses using a sensitivity analysis approach to project future population development. Results show that current rates of hunting are not sustainable, but that chimpanzees are neither more “noble”, nor more “savage” than humans are, but that they also hunt to ensure maximum benefit without regard for the consequences for the prey population.     

Effects of male group size,parity, and cycle stage on female chimpanzee copulation rates at Ngogo,Kibale National Park,Uganda

Chimpanzees have complex and variable mating strategies, but most copulations occur when females with full sexual swellings are in parties with multiple males and mate with most or all of those males. Daily copulation rates for fully swollen females vary at different times of a female’s cycle, among females, and across communities and populations. Variation in female age, parity, and cycle stage underlie some of this variation, but possible demographic effects on copulation rates have not been systematically investigated. Demographic variation can affect many aspects of behavior and ecology, including the frequency and success of different mating tactics. Analysis of data from the unusually large chimpanzee community at Ngogo produces two results that are consistent with the hypothesis that demographic variation affects female copulation rates. Copulation rates were high compared with those reported from other research sites, where females had fewer potential mates available. Daily copulation rates of fully swollen females were also positively related to the number of males with whom they associated. Ngogo data also re-confirm results from other studies, of both wild and captive populations, showing that female copulation rates increase during periovulatory periods. This is consistent with the hypothesis that sexual swellings and extended receptivity and proceptivity help to protect females against infanticide by helping to ensure they mate with all potential sires. As at some other sites, parous females at Ngogo copulated at higher rates than nulliparous females. Possible effects of demography on sexual behavior should be considered in assessments of differences between chimpanzees and bonobos and of variation across chimpanzee populations.     

The zooarchaeology and paleoecology of early hominin scavenging

Questions about the timing, frequency, resource yield, and behavioral and biological implications of large animal carcass acquisition by early hominins have been a part of the “hunting‐scavenging debate” for decades. This article presents a brief outline of this debate, reviews the zooarchaeological and modern ecological evidence for a possible scavenging niche among the earliest animal tissue‐consuming hominins (pre‐2.0 Ma), revisits some of the questions that this debate has generated, and outlines some ways to explore answers to those questions with evidence from the archaeological record.     

The learning and use of gestural signals by young chimpanzees: A trans-generational study

Observations of chimpanzee gestural communication are reported. The observations represent the third longitudinal time point of an ongoing study of the Yerkes Primate Center Field Station chimpanzee group. In contrast to observations at the first two time points, the current observations are of a new generation of infants and juveniles. There were two questions. The first concerned how young chimpanzees used their gestures, with special focus on the flexibility or intentionality displayed. It was found that youngsters quite often used the same gesture in different contexts, and different gestures in the same context. In addition, they sometimes used gestures in combinations in a single social encounter, these combinations did not convey intentions that could not be conveyed by the component gestures, however. It was also found that individuals adjusted their choice of signals depending on the attentional state of the recipient. The second question was how chimpanzees acquired their gestural signals. In general, it was found that there was little consistency in the use of gestures among individuals, especially for non-play gestures, with much individual variability both within and across generations. There were also a number of idiosyncratic gestures used by single individuals at each time point. It was concluded from these results that youngsters were not imitatively learning their communicatory gestures from conspecifics, but rather that they were individually conventionalizing them with each other. Implications of these findings for the understanding of chimpanzee communication and social learning are discussed.     

Ant dipping and meat eating by wild chimpanzees in the kalinzu forest, uganda

New evidence of ant dipping and meat eating by chimpanzees was recorded in the Kalinzu Forest, Uganda. We found stems and branches at the nests of driver ants,Dorylus molestus, just after chimpanzees had left the spot. Fecal samples also revealed that chimpanzees sometimes ate driver ants. The configuration of stems and branches and the condition of holes at the driver ant's nests suggested that chimpanzees used them as wands to dip for ants. The frequency of ant dipping and length of wands may be more related to culturel rather than ecological factors. Although hunting was not seen, we found chim-panzees eating a blue monkey and a redtail monkey. In both cases, they ate meat and leaves alternatively, and shared meat with each other.     

Gombe chimpanzee sex differences in the pelvis and observations of pubic and preauricular areas

The skeletons of Gombe chimpanzees provide an opportunity for analysis of bony tissue with reference to known sex and parity in combination with observations on other life history variables and behavior. Measurements of the pelvic bones show a mosaic of sex differences. Well-defined resorption areas on the dorsomedial aspect of the pubis and the preauricular area of the ilium have been associated with sex and parity in humans and other species. However, these are not present in either females or males in this chimpanzee skeletal series.     

On the ecological scenario of the first hominin dispersal out of Africa

The archaeopaleontological site of Dmanisi in Georgia, dated to ～1.8 Ma, provides evidence on the first hominin dispersal out of Africa, while the sites of Barranco León and Fuente Nueva-3 in Spain, dated to ～1.4 Ma, record the earliest hominin settlements in Europe. However, a number of issues related to the dispersal route, the climatic conditions and the ecological scenario of this dispersal event are subject to debate. In a recent paper in L’anthropologie, Agustí and Lordkipanidze (2019) proposed an alternative scenario for the arrival of hominins in the Caucasus, which they conceived as a forest refugium area during the Early Pleistocene, and discarded that their dispersal coincided with that of other members of the Ethiopian and Asian faunas, like the sabertooth Megantereon whitei or the giant hyena Pachycrocuta brevirostris. Our review of these issues suggests that: (i) the elongated sabers and reduced postcanine teeth of African M. whitei limited the ability of this predator to process the prey carcass, which resulted in scavengeable resources for the Dmanisi hominins; (ii) the mass estimate in excess of 100 kg obtained for the trochlear perimeter of the distal humerus of the hyena from Dmanisi shows that it can be confidently ascribed to the genus Pachycrocuta; (iii) the postcranial anatomy of the Dmanisi hominins was not advantageous for scavenging tree-stored prey; (iv) the laterally flattened upper canines of M. whitei could not withstand the loads that would result from climbing a prey carcass into a tree; (v) paleobotanical analyses suggest a temperate grassland ecosystem in Dmanisi, not dominant forest conditions, with enhanced aridity in the level of hominin occupation; (vi) similarly, the low frequency of arboreal pollen in the Levantine Corridor at ～1.8 Ma points to more arid conditions than today in this area; (vii) many archaeopaleontological sites of the Rift Valley and its extension towards the Red Sea, the Levant and the Caucasus show evidence of tectonic, volcanic and/or hydrothermal events; and (viii) the delay of 400 ka in the arrival of hominins in Western Europe did not result from a lower availability of scavengeable resources.     

A contribution to tropical rain forest taphonomy: retrieval and documentation of chimpanzee remains from Kibale Forest, Uganda

The absence of Pan and Gorilla fossils from Africa has led some to suggest that African rain forests are not conducive to bone preservation. The absence of fossils is unfortunate as it hampers phylogenetic and socioecological interpretations on the divergence of the earliest hominids. For the most part, taphonomic studies have been restricted to cave and open country contexts. With this in mind, we have initiated a taphonomic project in a tropical rain forest, the Kibale Forest of western Uganda. In the course of bone gathering activities over the past 4 years, we have documented the retrieval of skeletal remains representing nine chimpanzees (Pan troglodytes schweinfurthii). Crania and mandibles are most commonly recovered, followed by elements of the axial skeleton, and finally, appendages. Vertical segregation of small compact bones can occur in areas with a soft substrate. Scavenging activity suggests the role of suids, but this has not been proven. Geochemical tests suggest that Kibale soils are neutral and may be conducive to bone preservation. Our independent preliminary data from bone weathering/survival experiments indicate that bones appear undamaged after several years and are able to accumulate on the forest floor. These results contrast with popular assumptions on the potential of African rain forests as potential fossil reservoirs.     

Homo floresiensis and the evolution of the hominin shoulder

The holotype of Homo floresiensis, diminutive hominins with tiny brains living until 12,000 years ago on the island of Flores, is a partial skeleton (LB1) that includes a partial clavicle (LB1/5) and a nearly complete right humerus (LB1/50). Although the humerus appears fairly modern in most regards, it is remarkable in displaying only 110 degrees of humeral torsion, well below modern human average values. Assuming a modern human shoulder configuration, such a low degree of humeral torsion would result in a lateral set to the elbow. Such an elbow joint would function more nearly in a frontal than in a sagittal plane, and this is certainly not what anyone would have predicted for a tool-making Pleistocene hominin. We argue that Homo floresiensis probably did not have a modern human shoulder configuration: the clavicle was relatively short, and we suggest that the scapula was more protracted, resulting in a glenoid fossa that faced anteriorly rather than laterally. A posteriorly directed humeral head was therefore appropriate for maintaining a normally functioning elbow joint. Similar morphology in the Homo erectus Nariokotome boy (KNM-WT 15000) suggests that this shoulder configuration may represent a transitional stage in pectoral girdle evolution in the human lineage.     

Tool use in insect foraging by the chimpanzees of Ngogo, Kibale National Park, Uganda

Chimpanzee tool use for resource acquisition has been reported at numerous research sites. The chimpanzees of the Kibale Forest, western Uganda, have not previously been observed to use tools in foraging for insects. Here I report the first observation of tool use by the chimpanzees of the Ngogo community of Kibale National Park, Uganda, in insect foraging. Three adult females, one adolescent male, and one juvenile male were observed making and using tools to probe into a fallen dead tree to collect insect and wood pieces. I discuss the importance of this observation, and the behavioral similarities with chimpanzees from other sites.     

Use of exclusion by a chimpanzee (Pan troglodytes) during speech perception and auditory-visual matching-to-sample

An adult female chimpanzee showed responding through use of exclusion in an auditory to visual matching-to-sample procedure. The chimpanzee had previously learned to associate specific visuographic symbols called lexigrams with real world referents and the spoken English words and photographs for those referents. On some trials, an unknown spoken English word was presented as the sample, and the match choices could consist of photographs or lexigrams that already were associated with known English words as well as unknown lexigrams or photos of objects without associated lexigrams. The chimpanzee reliably avoided choosing known comparisons for these unknown samples, instead relying on exclusion to choose comparisons that were of unknown lexigrams or photographs of items without associated lexigram symbols.     

A new procedure to study the perceptual world of animals with sensory reinforcement: Recognition of humans by a chimpanzee

A female chimpanzee touched a button to produce colored slides of pictures. Slides were present as long as she kept touching the button. Repeated touch within 10 sec after the previous release produced the same slides again. The slide was changed when 10 sec passed after she released the button. The duration of a touching response and the interval between the responses were calculated for each of 100 slides. The data for each slide were plotted on the two-dimensional space constructed with response duration and response interval. A clear differentiation of distribution on this space was found between slides with humans and those without humans. The result demonstrated that the chimpanzee recognized humans as a category, as well as that this procedure is effective for the study of the perceptual world of animals based on the reinforcing function of stimuli.     

Parasitological analyses of the male chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda

Numerous intestinal parasites identified in populations of wild nonhuman primates can be pathogenic to humans. Furthermore, nonhuman primates are susceptible to a variety of human pathogens. Because of increasing human encroachment into previously nonimpacted forests, and the potential for disease transmission between human and nonhuman primate populations, further detailed investigations of primate ecological parasitology are warranted. For meaningful comparisons to be made, it is important for methods to be standardized across study sites. One aspect of methodological standardization is providing reliable estimates of parasite prevalence and knowing how many samples are needed to adequately estimate an individual's parasite prevalence. In this study the parasitic fauna of 37 adult, adolescent, and juvenile male chimpanzees from the Ngogo group, Kibale National Park, Uganda, were assessed from 121 fecal samples collected over a 3-month period. Twelve taxa of intestinal species (five helminth and seven protozoan) were recovered from the samples. The four most prevalent species were Troglodytella abrassarti (97.3%), Oesophagostomum sp. (81.1%), Strongyloides sp. (83.8%), and Entamoeba chattoni (70.3%). No one species was found in all samples from any one animal, and Troglodytella abrassarti, the most common intestinal organism, was found in all of the serial samples of only 69.4% of the chimpanzees. The cumulative species richness for individuals significantly increased for every sequential sample (up to three to four samples) taken per animal during this study. The results indicate that to accurately diagnose total intestinal infection and evaluate group prevalence, three to four sequential samples from each individual must be collected on nonconsecutive days. This conclusion applies only to short study periods in which possible seasonal effects are not taken into consideration. Validation of these results at different study sites in different regions with different climatic patterns is needed.