Census of gorillas in northern Republic of Congo

We censused gorilla populations in northern Congo from February to April 1989 and June 1990. The objective was to provide the first quantitative data from a variety of sites on gorilla populations from a country that had unknown but potentially high populations. The method used was a census of nests along strip transects. A total of 401.0 km of transects was sampled in four different study areas. The overall density calculated for all transects was 0.4 nesting gorillas/km². The highest density, 1.2 nesting gorillas/km², was found in the vast Likouala swamp area of north central Congo. The two northern sites showed low densities of 0.1 and 0.2 nesting gorillas/km², respectively. The northwestern site showed an intermediate density of 0.6 nesting gorillas/km². The vegetation type with the highest density was swamp forest with 2.4 nesting gorillas/km². The limited sample presented shows that gorillas are widespread and common in northern Congo, even in the swamp forests previously considered unsuitable as gorilla habitat. It is probable that Congo holds the largest population of gorillas in Africa after Gabon. © 1992 Wiley-Liss, Inc.     

Survey of Grauer's Gorillas (Gorilla gorilla graueri) and Eastern Chimpanzees (Pan troglodytes schweinfurthi) in the Kahuzi-Biega National Park Lowland Sector and Adjacent Forest in Eastern Democratic Republic of Congo

We describe the distribution and estimate densities of Grauer's gorillas (Gorilla gorilla graueri) and eastern chimpanzees (Pan troglodytes schweinfurthi) in a 12,770-km ² area of lowland forest between the Lowa, Luka, Lugulu, and Oku rivers in eastern Democratic Republic of Congo, the site of the largest continuous population of Grauer's gorillas. The survey included a total of 480 km of transects completed within seven sampling zones in the Kahuzi-Biega National Park lowland sector and adjacent Kasese region and approximately 1100 km of footpath and forest reconnaissance. We estimate total populations of 7670 (4180–10,830) weaned gorillas within the Kahuzi-Biega lowland sector and 3350 (1420–5950) individuals in the Kasese survey areas. Within the same area, we estimate a population of 2600 (1620–4500) chimpanzees. Ape nest site densities are significantly higher within the Kahuzi-Biega lowland sector than in the more remote Kasese survey area in spite of a significantly higher encounter rate of human sign within the lowland sector of the park. Comparison of our data with information obtained by Emlen and Schaller during the first rangewide survey of Grauer's gorillas in 1959 suggests that gorilla populations have remained stable in protected areas but declined in adjacent forest. These findings underscore the key role played by national parks in protecting biological resources in spite of the recent political and economic turmoil in the region. We also show that forest reconnaissance is a reliable and cost-effective method to assess gorilla densities in remote forested areas.     

Population Density of Chimpanzees and Gorillas in the Petit Loango Reserve,Gabon: Employing a New Method to Distinguish Between Nests of the Two Species

We studied the vegetation and population density of chimpanzees (Pan troglodytes troglodytes) and gorillas (Gorilla gorilla gorilla) in the Petit Loango Reserve, Gabon. The study site consists mainly of primary and secondary forest, and it also includes patches of savanna, inundated forest, and coastal scrub. The density of herbaceous plants of Marantaceae and Zingiberaceae, which serve as important foods for great apes at other study sites, is extremely low. It was very difficult to distinguish between the nests of chimpanzees and gorillas because most gorilla nests were made in trees. We developed a new method to identify the builder of nests via a combination of field observation of nests and fecal samples, and examination of hair samples under a scanning electron microscope. Nests of chimpanzees were mainly in primary forest, and the estimated density (0.78 individual per km ² ) is higher than the average density in Gabon and comparable to the nest density in the Lopé Reserve, Gabon. Nests of gorillas were mainly in secondary forest, and their estimated density (0.21) is similar to the average next density in Gabon but lower than the gorilla nest density in the Lopé Reserve. The low density of herbaceous foods seemed to influence the density of gorillas but not the density of chimpanzees.     

Mitochondrial DNA phylogeography of western lowland gorillas (Gorilla gorilla gorilla)

The geographical distribution of genetic variation within western lowland gorillas (Gorilla gorilla gorilla) was examined to clarify the population genetic structure and recent evolutionary history of this group. DNA was amplified from shed hair collected from sites across the range of the three traditionally recognized gorilla subspecies: western lowland (G. g. gorilla), eastern lowland (G. g. graueri) and mountain (G. g. beringei) gorillas. Nucleotide sequence variation was examined in the first hypervariable domain of the mitochondrial control region and was much higher in western lowland gorillas than in either of the other two subspecies. In addition to recapitulating the major evolutionary split between eastern and western lowland gorillas, phylogenetic analysis indicates a phylogeographical division within western lowland gorillas, one haplogroup comprising gorilla populations from eastern Nigeria through to southeast Cameroon and a second comprising all other western lowland gorillas. Within this second haplogroup, haplotypes appear to be partitioned geographically into three subgroups: (i) Equatorial Guinea, (ii) Central African Republic, and (iii) Gabon and adjacent Congo. There is also evidence of limited haplotype admixture in northeastern Gabon and southeast Cameroon. The phylogeographical patterns are broadly consistent with those predicted by current Pleistocene refuge hypotheses for the region and suggest that historical events have played an important role in shaping the population structure of this subspecies.     

Foods consumed by sympatric populations ofGorilla g. gorilla andPan t. troglodytes in Gabon: Some preliminary data

Data on foods consumed by gorillas and chimpanzees living in primary forest in Gabon were collected, mainly by examination of the contents of feces. Gorillas ate fruit very regularly (some fruit remains were present in 97.6% of 246 fecal samples examined), in addition to leaves, stems, pith, and bark. Some fruit remains were present in all chimpanzee fecal samples examined. Mean numbers of fruit species per fecal sample were 2.5 for gorillas and 2.1 for chimpanzees. Sixty percent of all identified foods recorded for gorillas were recorded for chimpanzees as well. Our results indicate that important differences in diet exist between western lowland gorillas and the eastern gorilla populations of Kahuzi-Biega and the Virunga Volcanoes. It is now clear that western gorillas cannot be accurately classed as folivores.     

Insect-eating by sympatric Lowland gorillas (Gorilla g. gorilla) and chimpanzees (Pan t. troglodytes) in the Lopé Reserve,Gabon

Sympatric populations of lowland gorillas (Gorilla gorilla gorilla) and chimpanzees (Pan troglodytes troglodytes) in the Lopé Reserve in central Gabon consumed insects at similar average frequencies over a 7-year period (30% versus 31% feces contained insect remains). Data came mostly from fecal analysis supplemented by observation and trail evidence. The weaver ant (Oecophylla longinoda) was the species eaten most frequently by both gorillas and chimpanzees. Other species of insects wore eaten but there was virtually no overlap: Chimpanzees used tools to eat Apis bees (and their honey) and two large species of ants; gorillas ate three species of small ants. Thus, despite their shared habitat, the esources utilized were not identical as gorillas do not show the tool-use “technology” of chimpanzees. The frequency of insect-eating by both species of ape varied seasonally and between years but in different ways. This variation did not seem to be related to the ratio of fruit to foliage in their diets. Gorillas of all age-classes ate insects at similar rates. Comparisons with insectivory by other populations of gorillas indicate differences exist. Mountain gorillas (Gorilla g. beringei) in the Virunga Volcanoes, Rwanda, consume thousands of invertebrates daily, eating them inadvertently with handfuls of herbaceous foods but they deliberately ingest insect-foods only rarely. Lowland gorillas at Lopé habitually ate social insects, and their selective processing of herbaceous foods probably minimizes inadvertent consumption of other invertebrates. Gorillas at Belinga in northeastern Gabon, 250 km from Lop6, ate social insects at similar rates but ignored weaver ants in favor of Cubitermes sulcifrons, a small species of termite that occurs at Lopé but was not eaten by gorillas. This indicates that local traditions similar to those reported for chimpanzees also exist amongst populations of gorillas. © 1992 Wiley-Liss, Inc.     

A survey of the apes in the Dzanga-Ndoki National Park, Central African Republic: a comparison between the census and survey methods of estimating the gorilla (Gorilla gorilla gorilla) and chimpanzee (Pan troglodytes) nest group density

A survey of apes was carried out between October 1996 and May 1997 in the Dzanga sector of the Dzanga‐Ndoki National Park, Central African Republic (CAR), to estimate gorilla (Gorilla gorilla gorilla) and chimpanzee (Pan troglodytes) densities. The density estimates were based on nest counts. The strip transect census and the line transect survey method (Standing Crop Nest Count) were used to estimate the gorilla nest group density. The strip transect has been most commonly used to date. It assumes that all nest groups within the width of the strip are detected, but as this assumption is easily violated in the dense tropical rain forest, the line transect survey was also used. In this method, only the nest groups on the transect line itself should be detected. This method proved to be an adequate and easy technique for estimating animal densities in dense vegetation. The gorilla density of 1.6 individuals km^?2 (line transect survey method) found for the Dzanga sector is one of the highest densities ever reported in the literature for the Western lowland gorilla. The density estimate for chimpanzees was 0.16 individuals km^?2 (census method). The results of this study confirm the importance of the Dzanga‐Ndoki National Park for primate conservation.     

Potential for Ebola transmission between gorilla and chimpanzee social groups

Over the past decade Ebola hemorrhagic fever has emerged repeatedly in Gabon and Congo, causing numerous human outbreaks and massive die-offs of gorillas and chimpanzees. Why Ebola has emerged so explosively remains poorly understood. Previous studies have tended to focus on exogenous factors such as habitat disturbance and climate change as drivers of Ebola emergence while downplaying the contribution of transmission between gorilla or chimpanzee social groups. Here we report recent observations on behaviors that pose a risk of transmission among gorilla groups and between gorillas and chimpanzees. These observations support a reassessment of ape-to-ape transmission as an amplifier of Ebola outbreaks.     

Electroejaculation in chimpanzees and gorillas and artificial insemination in chimpanzees

Electroejaculation was performed in 3 chimpanzees, 1 pygmy chimpanzee, and 2 gorillas with an instrument that delivers a modified sine wave current with a frequency of 24 Hz. The current stimuli were applied by a rectal probe with longitudinal electrodes. The electrical parameters varied from 6 to 12 V and from 30 to 40 mA for response of erection and lay between 8 and 18 V and between 40 and 145 mA during semen emission. Eleven chimpanzee semen samples showed the following data (x ± SD): total volume 1.9 ± 1.3 ml, volume of the liquid fraction 0.3 ± 0.2 ml, spermatozoa per ejaculate 743 ± 376 × 10⁶, sperm motility 52.7 ± 9.6%, morphologically abnormal spermatozoa 12.2 ± 7.5%. From an adult gorilla, three semen samples were collected, in each case without spermatozoa. The electrostimulation of a 6-year-old gorilla led to an erection, but not to semen emission. Three female chimpanzees were inseminated with fresh or frozen semen, each of them within three different estrous cycles. None of these inseminations led to a pregnancy.     

Ranging and grouping patterns of a western lowland gorilla group at Bai Hokou,Central African Republic

The ranging and grouping patterns of a gorilla group were studied during 27 months from 1990–1992 at the Bai Hokou study site, Central African Republic. The study group ranged far daily (average = 2.3 km/day) and had a large home range (22.9 km²), relative to mountain gorillas, and ranging patterns differed between years. During 1990–1992, the bimale study group foraged less cohesively and had more flexible grouping patterns than mountain gorillas. The study group sometimes split into two distinct foraging subgroups, each led by a silverback, and these subgroups occasionally slept apart (mean = 950 m apart). Lowland gorillas rely on many of the same fruit resources as sympatric chimpanzees, and under certain demographic situations gorillas, like sympatric chimpanzees, may adapt their foraging group size to reduce intragroup feeding competition. However, the fiber content of the lowland gorilla diet likely relaxes constraints on foraging party size and facilitates group cohesion relative to chimpanzees. Am. J. Primatol. 43:111–133, 1997. © 1997 Wiley-Liss, Inc.     

Urinary immunoreactive estrogen and pregnanediol-3-glucuronide during the normal menstrual cycle of the female lowland gorilla (Gorilla gorilla)

Urine samples were taken daily during 22 menstrual cycles of six normal adult female gorillas. Urine was analyzed for total immunoreactive estrogens (E_t) and pregnanediol-3-glucuronide (PdG) and indexed by creatinine (Cr). An average cycle length of 32 ± 1 days (mean ± SE) with a range of 25–42 days is reported. Estrogen values range from 4 to 128 ng/mg Cr and show a midcycle peak and a midluteal rise. PdG values range from 0.01 to 2.4 μg/mg Cr and display a low, flat follicular phase followed by a luteal elevation. The follicular phase is 19.5 ± 1 days in length (range 11–30 days) and accounts for the variation in cycle length. The luteal phase is 12.3 ± 0.3 days long (range 10–14 days). In contrast to previous studies, PdG levels rise two days before the estrogen peak. The results from the present study are compared with information available on the gorilla, chimpanzee, and human. The accuracy of various alignment methods is discussed, as well as the importance of the methods presented in this study for the captive propagation of gorillas.     

Ape Abundance and Habitat Use in the Goualougo Triangle, Republic of Congo

Chimpanzee (Pan troglodytes troglodytes) and western gorilla (Gorilla gorilla gorilla) populations in central Africa are rapidly declining as a result of disease epidemics, commercial bushmeat hunting, and habitat destruction. Our main objective was to estimate the absolute abundance and habitat utilization of chimpanzees and gorillas in the intact forests of the Goualougo Triangle in the Republic of Congo, and in an adjacent area in which selective logging will take place in the near future. The estimates provide a unique baseline for apes inhabiting an undisturbed environment. A second objective was to compare estimates of abundance and patterns of habitat utilization generated by different techniques: 1) distance sampling of individual ape nests and nest sites along line transects, 2) direct observations of apes during reconnaissance surveys, and 3) observations of ape traces during reconnaissance surveys. We completed a total of 222 km of line transect surveys in 4 sampling areas, resulting in overall density estimates of 1.53 chimpanzees/km² and 2.34 gorillas/km² from nest sites. We generated a density estimate of 2.23 chimpanzees/km² from direct observations during reconnaissance surveys of a semihabituated community in 1 of the 4 sampling areas. Habitat use profiles that nest surveys depicted on transects differed from those of direct observations and traces we encountered on reconnaissance surveys. We found the highest overall abundance of chimpanzee nests in monodominant Gilbertiodendron forest, whereas our direct observations showed that chimpanzees preferred mixed species forest. Transects that traversed the core area of the community range had the highest encounter rates of chimpanzee nests and nest sites. Gorilla nests on transects showed a preference only for mixed species forest with an open canopy, but direct observations and traces on reconnaissance surveys clearly indicated that gorillas use several habitat types. We conclude by evaluating the precision of these nest surveys and our ability to detect future trends in ape densities in the Goualougo Triangle.

Social structure and life-history patterns in western gorillas (Gorilla gorilla gorilla)

Life-history traits and ecological conditions have an important influence on primate social systems. Most of what we know about the life-history patterns and social structure of gorillas comes from studies of eastern gorillas (Gorilla beringei sp.), which live under dramatically different ecological conditions compared to western gorillas (Gorilla gorilla sp.). In this paper we present new data on western gorilla social structure and life histories from four study sites, and make comparisons with eastern gorilla populations. Data were obtained from two study sites with gorilla groups undergoing the habituation process (Lossi, Democratic Republic of Congo and Bai Hokou, Central African Republic) and two "bai" studies (Maya Nord and Mbeli Bai, Republic of Congo). The size and structure of these groups were similar to those seen in eastern gorillas. However, differences in the occurrence of various group transitions (group formations, changes between one-male and multimale composition, and group disintegrations) exist, and western gorillas notably exhibit much higher rates of male emigration and correspondingly fewer multimale groups compared to mountain gorillas. Certain phenomena have been observed only rarely, including predation by leopards. The preliminary data show no significant differences in birth rates between western gorillas and mountain gorillas. The ecological variability across gorilla habitats likely explains the flexibility in the social system of gorillas, but we need more information on the social relationships and ecology of western gorillas to elucidate the causes for the similarities and differences between western and eastern gorillas on the levels of individuals, social groups, and population dynamics.     

Survey of gorillas (<Emphasis Type="Italic">Gorilla gorilla gorilla</Emphasis>) and chimpanzees (<Emphasis Type="Italic">Pan troglodytes troglodytes</Emphasis>) in Southwestern Cameroon

A study on the distribution and population density of the western lowland gorilla (Gorilla g. gorilla) and the central chimpanzee (Pan t. troglodytes) was undertaken between December 1997 and August 2000 in the Campo and Maan Forests in southwestern Cameroon. The aim of this survey was to estimate the densities of the apes in different parts of the area, to assess the importance of the region for the conservation of these endangered species and to determine the influence of human activities such as logging and hunting. The survey was based on night nest counts on a total of 665.5 km of line transects. The overall density in the Campo Forest was estimated at 0.2 gorillas/km² and at 0.63–0.78 chimpanzees/km². The overall density of chimpanzees in the Maan Forest was estimated at 0.8–1 individuals/km². Gorilla density in this area was too low to allow an estimation. The highest gorilla nest density was found in secondary forest. The gorilla density in unlogged forest was significantly lower. Chimpanzees showed a clear preference for less disturbed areas. In unlogged forest, old secondary forests (logging more than 23 years ago) and areas of recent logging with large remaining patches of primary forest, significantly higher densities were calculated than inside the more heavily exploited logging concession. In areas with both logging and high hunting pressure both species were rare or even absent. The Campo Maan area is considered a very important area for the conservation of gorillas and chimpanzees. Conservation measures are urgently required to reduce the impact of logging and hunting. The creation of the Campo Maan National Park in January 2000 is an important measure to preserve the unique biodiversity in this so far hardly protected area.     

Urinary estrogen excretion during pregnancy in the gorilla (Gorilla gorilla), orangutan (Pongo pygmaeus) and the human (Homo sapiens)

Urinary estrogen components were separated, identified and quantified throughout the pregnancy of the gorilla (Gorilla gorilla) and orangutan (Pongo pygmaeus) and compared to estrogen levels in normal human pregnancies. Fetal and neonatal adrenals from each species were also compared in terms of weight and relative amounts of fetal zone. The results demonstrate that gorillas and chimpanzees excrete 4- to 5-fold less estrogen during pregnancy than the human and orangutan which are similar to each other. The lower estrogen excretion appears to be related to a smaller fetal adrenal in both the gorilla and chimpanzee which reveal both a reduced adrenal weight and increased definitive to fetal zone ratio when compared to either the human or orangutan.     

Densities,Distributions, and Seasonal Movements of Gorillas and Chimpanzees in Swamp Forest in Northern Congo

The conservation of gorillas (Gorilla spp.) and chimpanzees (Pan troglodytes) depends upon knowledge of their densities and distribution throughout their ranges. However, information about ape populations in swamp forests is scarce. Here we build on current knowledge of ape populations by conducting line transect surveys of nests throughout a reserve dominated by swamp forest: the Lac Télé Community Reserve in northern Congo. We estimated gorilla and chimpanzee densities, distributions across habitats, and seasonal changes in abundance. Gorilla density was 2.9 gorillas km^–2, but densities varied by habitat (0.3–5.4 gorillas km^–2) with highest densities in swamp forest and terra firma mixed forest. Average chimpanzee density is 0.7 chimpanzees km^–2 (0.1–1.3 chimpanzees km^–2), with highest densities in swamp forest. Habitat was the best predictor of ape nest abundance, as neither the number of human indices nor the distance from the nearest village predicted nest abundance. We recorded significantly greater numbers of apes in terra firma forest during the high-water season than the low-water season, indicating that many gorillas and chimpanzees are at times concentrated in terra firma forest amid a matrix of swamp forest. Seasonally high numbers of apes on terra firma forest islands easily accessible to local people may expose them to substantial hunting pressure. Conversely, the nearly impenetrable nature of swamp forests and their low value for logging makes them promising sites for the conservation of apes.     

Genetic diversity of North American captive‐born gorillas (Gorilla gorilla gorilla)

Western lowland gorillas (Gorilla gorilla gorilla) are designated as critically endangered and wild populations are dramatically declining as a result of habitat destruction, fragmentation, diseases (e.g., Ebola) and the illegal bushmeat trade. As wild populations continue to decline, the genetic management of the North American captive western lowland gorilla population will be an important component of the long‐term conservation of the species. We genotyped 26 individuals from the North American captive gorilla collection at 11 autosomal microsatellite loci in order to compare levels of genetic diversity to wild populations, investigate genetic signatures of a population bottleneck and identify the genetic structure of the captive‐born population. Captive gorillas had significantly higher levels of allelic diversity (t₇ = 4.49, P = 0.002) and heterozygosity (t₇ = 4.15, P = 0.004) than comparative wild populations, yet the population has lost significant allelic diversity while in captivity when compared to founders (t₇ = 2.44, P = 0.04). Analyses suggested no genetic evidence for a population bottleneck of the captive population. Genetic structure results supported the management of North American captive gorillas as a single population. Our results highlight the utility of genetic management approaches for endangered nonhuman primate species.     

Gorillas (Gorilla gorilla gorilla) in the Likouala swamp forests of north central Congo: Preliminary data on populations and ecology

Until recently it was generally believed that gorillas avoid water. In two recent, independent faunal surveys in the Likouala swamp of north central Congo, we have found that gorillas occur at high densities, extending the known range of Gorilla gorilla gorillavery close to the Oubangui River. The gorillas in this area fed on the abundant ground vegetation of herbaceous monocotyledons in the swamp forest. In adjacent islands of terra firmathere was little evidence of gorilla nesting or feeding, indicating that they spend at least part of the year almost exclusively in the swamp forests. Recent reports from several sites in West Africa indicate that swamp forests may generally provide abundant food resources that are used regularly by gorillas.     

Species association of hepatitis B virus (HBV) in non-human apes; evidence for recombination between gorilla and chimpanzee variants

Hepatitis B virus (HBV) infections are widely distributed in humans, infecting approximately one third of the world's population. HBV variants have also been detected and genetically characterised from Old World apes; Gorilla gorilla (gorilla), Pan troglodytes (chimpanzee), Pongo pygmaeus (orang-utan), Nomascus nastusus and Hylobates pileatus (gibbons) and from the New World monkey, Lagothrix lagotricha (woolly monkey). To investigate species-specificity and potential for cross species transmission of HBV between sympatric species of apes (such as gorillas and chimpanzees in Central Africa) or between humans and chimpanzees or gorillas, variants of HBV infecting captive wild-born non-human primates were genetically characterised. 9 of 62 chimpanzees (11.3%) and two from 11 gorillas (18%) were HBV-infected (15% combined frequency), while other Old world monkey species were negative. Complete genome sequences were obtained from six of the infected chimpanzee and both gorillas; those from P. t .ellioti grouped with previously characterised variants from this subspecies. However, variants recovered from P. t. troglodytes HBV variants also grouped within this clade, indicative of transmission between sub-species, forming a paraphyletic clade. The two gorilla viruses were phylogenetically distinct from chimpanzee and human variants although one showed evidence for a recombination event with a P.t.e.-derived HBV variant in the partial X and core gene region. Both of these observations provide evidence for circulation of HBV between different species and sub-species of non-human primates, a conclusion that differs from the hypothesis if of strict host specificity of HBV genotypes.     

Seed dispersal effectiveness of the western lowland gorilla (Gorilla gorilla gorilla) in Gabon

The quantitative and qualitative aspects of seed dispersal by the western lowland gorilla (Gorilla gorilla gorilla) were investigated in Gabon. Fresh faeces were collected and washed to identify and count the seeds. Seed germinability after gut passage was estimated with trials in a nursery at the study site. To assess the impact of gut passage on germination success and delay, comparative trials were run with four treatments: (i) gut passed seeds cleaned of faeces, (ii) gut passed seeds within a faecal matrix, (iii) seeds from fresh fruits surrounded by pulp, and (iv) seeds from fresh fruits cleaned of pulp. The analysis of 180 faecal units resulted in the identification of 58 species of seed. Germination trials were realized for 55 species and the mean germination success reached 46%. The impact of gut passage was investigated for Santiria trimera and Chrysophyllum lacourtianum; both species displayed higher germination success after ingestion. This study shows that gorillas effectively disperse seeds of numerous plant species, many of which provide timber or nontimber forest products or are typical of Gabonese forests. Considering the high‐quality of gorilla deposition sites, gorillas is thought to play a unique role in the dynamics of Central African forest.