Gorilla Nest Research Articles

Past Connectivity but Recent Inbreeding in Cross River Gorillas Determined Using Whole Genomes from Single Hairs.

The critically endangered western gorillas (Gorilla gorilla) are divided into two subspecies: the western lowland (G. g. gorilla) and the Cross River (G. g. diehli) gorilla. Given the difficulty in sampling wild great ape populations and the small estimated size of the Cross River gorilla population, only one whole genome of a Cross River gorilla has been sequenced to date, hindering the study of this subspecies at the population level. In this study, we expand the number of whole genomes available for wild western gorillas, generating 41 new genomes (25 belonging to Cross River gorillas) using single shed hairs collected from gorilla nests. By combining these genomes with publicly available wild gorilla genomes, we confirm that Cross River gorillas form three population clusters. We also found little variation in genome-wide heterozygosity among them. Our analyses reveal long runs of homozygosity (>10 Mb), indicating recent inbreeding in Cross River gorillas. This is similar to that seen in mountain gorillas but with a much more recent bottleneck. We also detect past gene flow between two Cross River sites, Afi Mountain Wildlife Sanctuary and the Mbe Mountains. Furthermore, we observe past allele sharing between Cross River gorillas and the northern western lowland gorilla sites, as well as with the eastern gorilla species. This is the first study using single shed hairs from a wild species for whole genome sequencing to date. Taken together, our results highlight the importance of implementing conservation measures to increase connectivity among Cross River gorilla sites.

Variation in behavioral traits of two frugivorous mammals may lead to differential responses to human disturbance.

Human activities can lead to a shift in wildlife species’ spatial distribution. Understanding the specific effects of human activities on ranging behavior can improve conservation management of wildlife populations in human‐dominated landscapes. This study evaluated the effects of forest use by humans on the spatial distribution of mammal species with different behavioral adaptations, using sympatric western lowland gorilla and central chimpanzee as focal species. We collected data on great ape nest locations, ecological and physical variables (habitat distribution, permanent rivers, and topographic data), and anthropogenic variables (distance to trails, villages, and a permanent research site). Here, we show that anthropogenic variables are important predictors of the distribution of wild animals. In the resource model, the distribution of gorilla nests was predicted by nesting habitat distribution, while chimpanzee nests were predicted first by elevation followed by nesting habitat distribution. In the anthropogenic model, the major predictors of gorilla nesting changed to human features, while the major predictors of chimpanzee nesting remained elevation and the availability of their preferred nesting habitats. Animal behavioral traits (body size, terrestrial/arboreal, level of specialization/generalization, and competitive inferiority/superiority) may influence the response of mammals to human activities. Our results suggest that chimpanzees may survive in human‐encroached areas whenever the availability of their nesting habitat and preferred fruits can support their population, while a certain level of human activities may threaten gorillas. Consequently, the survival of gorillas in human‐dominated landscapes is more at risk than that of chimpanzees. Replicating our research in other sites should permit a systematic evaluation of the influence of human activity on the distribution of mammal populations. As wild animals are increasingly exposed to human disturbance, understanding the resulting consequences of shifting species distributions due to human disturbance on animal population abundance and their long‐term survival will be of growing conservation importance.

Impacts of Selective Logging and Associated Anthropogenic Disturbance on Intact Forest Landscapes and Apes of Northern Congo

The tropical forests of Western Equatorial Africa are home to extraordinary biodiversity, including sympatric chimpanzees (Pan troglodytes troglodytes) and western lowland gorillas (Gorilla gorilla gorilla). The region is also comprised of significant stands of Intact Forest Landscapes (IFL) that are in rapid decline. As part of a regional monitoring effort, we partnered with local government officials, conservation NGOs, and the timber company working in the region to assess ape abundances in relation to habitat characteristics and anthropogenic disturbances and compare IFL and non-IFL areas in the Sangha Trinational landscape, Republic of Congo. We found that chimpanzees and gorillas occur at high densities in IFL, as well as non-IFL. To better understand how selective logging changes floristic factors, we compared herb and tree densities from botanical surveys conducted in IFL and non-IFL. IFL had higher tree stem densities and less terrestrial ground herbs than logged habitats. However, few ape resources were logged in this extraction cycle and areas with tree stems removed had higher abundances of ape preferred terrestrial herbs, which may contribute to the elevated ape abundance estimates. Floristic differences in logged forest were however identified to coincide with differences in ape resource use. The chimpanzee tree nesting niche was reduced in non-IFL as night nests were constructed significantly closer to the ground than in IFL. Whereas gorilla nest height locations did not differ significantly between IFL and non-IFL. To identify other potential anthropogenic impacts, we assessed direct and indirect impacts of road expansion and illegal hunting on wildlife in these remote areas. Increased access to IFL that facilitates illegal hunting raises concern for protecting wildlife across Western Equatorial Africa. We urge that the results of biodiversity assessments and strategic aspects of long-term protection should be taken into account when identifying conservation set-asides and maintaining diverse states of modified forests. Finally, the results of our monitoring efforts are provided as evidence of the value of long-term collaborations among local stakeholders, government officials, conservation agencies, and industrial partners to improve the implementation of certification standards and biodiversity conservation initiatives.

Inlabru: an R package for Bayesian spatial modelling from ecological survey data

Abstract Spatial processes are central to many ecological processes, but fitting models that incorporate spatial correlation to data from ecological surveys is computationally challenging. This is particularly true of point pattern data (in which the primary data are the locations at which target species are found), but also true of gridded data, and of georeferenced samples from continuous spatial fields. We describe here the R package inlabru that builds on the widely used RINLA package to provide easier access to Bayesian inference from spatial point process, spatial count, gridded, and georeferenced data, using integrated nested Laplace approximation (INLA, Rue et al., 2009). The package provides methods for fitting spatial density surfaces and estimating abundance, as well as for plotting and prediction. It accommodates data that are points, counts, georeferenced samples, or distance sampling data. This paper describes the main features of the package, illustrated by fitting models to the gorilla nest data contained in the package spatstat (Baddeley, & Turner, 2005), a line transect survey dataset contained in the package dsm (Miller, Rexstad, Burt, Bravington, & Hedley, 2018), and to a georeferenced sample from a simulated continuous spatial field.

Assessing the impact of seasonality on Cross River Gorilla Nest Construction at Kagwene Gorilla Sanctuary, North-West Cameroon

The Cross River gorilla, one of the most endangered subspecies of western gorilla (Gorilla gorilla) is endemic to 12 to 14 sites at the Cameroon-Nigeria border, where it is facing enormous threats from habitat loss, bush meat trade and minor climate changes (seasonal changes). In a strive to reduce the enormous stressors on this subspecies, this study assesses the impact of seasonality on nest construction by Cross River gorillas in the Kagwene Gorilla Sanctuary with the objective of evaluating the impact of seasonal changes on the nest construction. To achieve this goal, the study employed a hunter guided survey team, as well as guides/trackers of the Wildlife Conservation Society. This team regularly tracks and searches the forest for gorilla signs using vegetation trails, food prints, dung, and feeding signs which are subsequently followed onwards to the nesting site. Data were collected from January to December 2014. A total of 268 fresh gorilla nest sites were observed containing 1813 individual nests. Out of the 1813 nests recorded, 39.6% (N=718) of the nests were on the ground and 60.4% (N=1095) on trees. The frequency of each nest type varied significantly among months (P =0.4433). There was a significant effect of season on nest category. Ground categories (bare earth, herbaceous, woody and mixed) were more common in the dry season (74.9%) than in the wet season (25.1%). On the other hand, tree nests were more common in the wet season (77.4%) than in the dry season (22.6%). Mean nest diameter for the two seasons (dry and wet) was 1.1 and 1.3 m, respectively. The mean nest height was 12.5 m for both seasons. Cross River gorillas showed higher preferences for tree nests within the height class interval of 11 to 15 m (49.6%). The distribution of nests across gradient of altitudes did not vary significantly between seasons (P>0.05), and nests were most common on steep slopes (n=810). Seasonality did not significantly affect the choice of slope (P>0.05). There was a significant difference in the habitat types (primary forest, light gap, secondary forest and rock) used for nesting, with primary forest being most preferred. Nest site distribution was predominant in the northern section of the sanctuary. This study reveals that, both temperature and rainfall play an important role in nest construction by Cross River gorilla at Kagwene. Key words: Cross River gorilla, nest, seasonality, Kagwene Gorilla Sanctuary.

Factors Influencing the Survival of Sympatric Gorilla (Gorilla gorilla gorilla) and Chimpanzee (Pan troglodytes troglodytes) Nests

Accurate and precise surveys of primate abundance provide the basis for understanding species ecology and essential information for conservation assessments. Owing to the elusive nature of wild apes and the vast region of dense forest they inhabit, population estimates of central chimpanzees (Pan troglodytes troglodytes) and western lowland gorillas (Gorilla gorilla gorilla) have largely relied on surveys of their nests. Specific information about the nesting behavior of apes permits the estimation of the number of nests built (nest creation rate). Similarly, information on nest characteristics and environmental factors can be used to estimate the time it takes nests to decay (nest decay rate). Nest creation and decay rates are then used to convert nest density estimates to absolute ape densities. Population estimates that use site-specific estimates of nest creation and decay rates are more accurate and precise. However, it is common practice to generalize these conversion factors across sites because of the additional cost of studies required to gather the information to estimate them. Over a 9-mo study period, we detected and monitored the time to decay of gorilla nests (N = 514) and chimpanzee nests (N = 521) in northern Republic of Congo. We investigated the influence of nest characteristics and environmental factors on nest survivorship and estimated the mean time to nest decay (or equivalently survival) using MARK. Key factors influencing nest decay rate included ape species, forest type, nest height, mean rainfall, nest structure, nest type, and primary aspects of nest construction. Our findings highlight the synergistic effect of behavior and environment on great ape nest degradation, as well as providing practical insights for improving measures to monitor remaining populations of these endangered species.

Catastrophic Decline of World's Largest Primate: 80% Loss of Grauer's Gorilla (Gorilla beringei graueri) Population Justifies Critically Endangered Status.

Grauer’s gorilla (Gorilla beringei graueri), the World’s largest primate, is confined to eastern Democratic Republic of Congo (DRC) and is threatened by civil war and insecurity. During the war, armed groups in mining camps relied on hunting bushmeat, including gorillas. Insecurity and the presence of several militia groups across Grauer’s gorilla’s range made it very difficult to assess their population size. Here we use a novel method that enables rigorous assessment of local community and ranger-collected data on gorilla occupancy to evaluate the impacts of civil war on Grauer’s gorilla, which prior to the war was estimated to number 16,900 individuals. We show that gorilla numbers in their stronghold of Kahuzi-Biega National Park have declined by 87%. Encounter rate data of gorilla nests at 10 sites across its range indicate declines of 82–100% at six of these sites. Spatial occupancy analysis identifies three key areas as the most critical sites for the remaining populations of this ape and that the range of this taxon is around 19,700 km2. We estimate that only 3,800 Grauer’s gorillas remain in the wild, a 77% decline in one generation, justifying its elevation to Critically Endangered status on the IUCN Red List of Threatened Species.

Spatial and Temporal Dynamics of a Mortality Event among Central African Great Apes.

In 2006–2007 we observed an unusual mortality event among apes in northern Republic of Congo that, although not diagnostically confirmed, we believe to have been a disease outbreak. In 2007–2011 we conducted ape nest surveys in the region, recording 11,835 G. g. gorilla nests (2,262 groups) and 5,548 P. t. troglodytes nests (2,139 groups). We developed a statistical model to determine likely points of origin of the outbreak to help identify variables associated with disease emergence and spread. We modeled disease spread across the study area, using suitable habitat conditions for apes as proxy for local ape densities. Infectious status outputs from that spread model were then used alongside vegetation, temperature, precipitation and human impact factors as explanatory variables in a Generalized Linear Model framework to explain observed 2007–2011 ape nest trends in the region. The best models predicted emergence in the western region of Odzala-Kokoua National Park and north of the last confirmed Ebola virus disease epizootics. Roads were consistently associated with attenuation of modeled virus spread. As disease is amongst the leading threats to great apes, gaining a better understanding of disease transmission dynamics in these species is imperative. Identifying ecological drivers underpinning a disease emergence event and transmission dynamics in apes is critical to creating better predictive models to guide wildlife management, develop potential protective measures for wildlife and to reduce potential zoonotic transmission to humans. The results of our model represent an important step in understanding variables related to great ape disease ecology in Central Africa.

Conservation research presence protects: a case study of great ape abundance in theDja region,Cameroon

AbstractResearch programmes have been shown to exert a positive impact on faunal communities, but few studies provide empirical evidence. In this study, we surveyed three neighbouring sites, differing in terms of official protection status and level of active management, in theDja Conservation Complex, southeastCameroon. We recorded signs of human activity and anthropogenic pressures and found that they significantly differed by site. We assessed chimpanzee and gorilla relative abundance using nest count surveys. We found that chimpanzee nest abundance was related to the presence of active management, but both active management and availability of suitable habitat affected gorilla nest abundance. Our results suggest that gorillas are more tolerant of human activity. We also provide evidence that the presence and activities of the conservation research projectProjetGrandsSinges served to actively deter poachers and limit hunting of great apes as a result of researcher presence, community sensitization and of valuing living apes and intact forests by local people. Such empirical evidence for the positive effect of research activity on biodiversity preservation should encourage continued investment in such programmes as part of a landscape‐wide, multi‐stakeholder conservation management of great ape habitats.

Habitat Suitability Modeling of Endangered Primates in Nigeria: Integrating Satellite Remote Sensing and Spatial Modeling Techniques

This paper investigates the impacts of forest cover and spatial structure changes on the forest landscape across Afi-Mbe-Okwangwo protected area of Cross River State, Nigeria and its corresponding implication on two endangered primates (Cross River Gorilla and Nigeria-Cameroon Chimpanzee) habitat using satellite remote sensing and modeling techniques. Using remote sensing change detection analysis, the spatial extent and annual rate of deforestation for the study area was determined as 34,620 hectares and 1.5% respectively (from 2000 to 2014). The protected areas with highest annual deforestation rates were Afi Mountain Wildlife Sanctuary (2.6%) and Mbe Mountains (2.2%), both prominent for gorilla and chimpanzee sightings and nests. Further investigations on changes to the forest landscape structure revealed high levels of forest fragmentation across the study area for the 14-year period investigated. As a means of further understanding effects of forest landscapes changes across the study area, a 14-year forward simulation was performed using the Markov model as to determine the spatial extent of futuristic forest cover changes. The results showed that if this current trend of forest cover change continued, 28,121 hectares of forests would be lost to deforestation in 2028 (approximately 16% of the total landmass of the entire study area). Using Maxent modeling, suitable primate habitats were predicted and the total coverage determined as 30,940 hectares (54.4% situated in CRNP—Okwangwo division, 29.4% in AMWS, 14.3% in Mbe Mountains and 1.9% in ARFR). Further analysis revealed 6468 hectares of predicted primate habitat were affected by deforestation in 2014 (21% of the predicted primate habitats). These results indicate that suitable primate habitats (particularly for gorillas and chimpanzees) are under immense pressure from deforestation and forest fragmentation. This paper presents a cost effective and time saving approach for determining suitable primate habitats and understanding the effects of forest transition on primate habitat suitability.

Habitat utilization and conservation of the Cross River gorilla (<i>Gorilla gorilla diehli</i.) in Afi Mountain Wildlife Sanctuary of Cross River State, Nigeria

The Cross River gorilla is one of Africa’s most critically endangered species of primates. There is scanty information on the conservation status of this population of gorillas. This work was undertaken to ascertain the seasonal migration pattern, the feeding habits and the population status of this species of gorilla with a view to recommending the best strategies for their conservation. Two patrol teams each consisting of twelve (12) rangers were used in the study. Six sample plots (240×5 km each, totaling 7.2 km2) were selected and used simultaneously in the northern and southern parts of the sanctuary in the rainy season and repeated in the dry season in 2006 and 2007. Data collected included the number of sites of gorilla nests, number of nests per site and plant species utilized for food by the gorillas. Student’s t-test and x2 test were used to analyse data. Results indicated that the total number of nests and therefore the population of gorillas decreased significantly (p < 0.05) from 616 ±138 in 2006 to 405 ±131 in 2007 while the mean number of gorilla nests per site also decreased significantly (p < 0.05) from 10.14 ± 2.6 in 2006 to 6.6 ± 2.14 in 2007 There was no significant reduction (p > 0.05) in poaching activities and the number of poachers apprehended during the study period. Thirteen poachers were apprehended in 2006 as against 12 in 2007. The gorillas migrated to the northern part of the sanctuary in the dry season and moved to the south in the rainy season. A total of 32 plant species were found to have been utilized by the gorillas for food. The gorillas were found to be frugivorous but resorted to herbaceous plants when fruits were scarce. It was recommended that consultations be held with the members of the surrounding villages to dissuade them from gorilla hunting and habitat destruction.Keywords: Habitat utilization, conservation, Cross River gorilla, Afi Mountain, Nigeria.

The Influence of Transect Use by Local People and Reuse of Transects for Repeated Surveys on Nesting in Western Lowland Gorillas (Gorilla gorilla gorilla) and Central Chimpanzees (Pan troglodytes troglodytes) in Southeast Cameroon

Monitoring populations of endangered species over time is necessary to guide and evaluate conservation efforts. This is particularly important for nonprotected areas that ensure connectivity between protected populations but are prone to uncontrolled hunting pressure. We investigated whether use of transects by local people and transect reuse for repeated surveys influence great ape nesting and bias results. We conducted simultaneous marked nest count surveys over 12 mo on established and newly opened transects in a nonprotected area subject to traditional heavy use by local people and recorded forest composition and signs of human activity. Chimpanzee and gorilla density estimates and encounter rates per kilometer were lower on established transects than on new ones. A generalized linear model indicated that hunting activity, distance to a regularly used forest trail, and transect type (old or new) predicted chimpanzee nest abundance, and distance to the trail and transect type predicted gorilla nest abundance, with no effect of habitat type (percentage suitable habitat) for either species. We, therefore, suggest that the difference in great ape nesting is a result of high levels of hunting by local people on established transects and forest trails. Our results support the use of repeated line transect surveys for monitoring great ape populations in many circumstances, although we advocate taking precautions in nonprotected areas, to avoid the bias imposed by use of established transects for hunting.

Density of herbaceous plants and distribution of western gorillas in different habitat types in south‐east Cameroon

AbstractThe study of forest herb availability improves knowledge of ecology and conservation of gorillas that depend on such herbs. Density patterns of herbs and location of western gorilla nest sites were studied in different habitat types at a site in south‐east Cameroon to assess their relationship. Herb stems of the families Marantaceae and Zingiberaceae were identified and counted in 10,713 1‐m2 plots distributed within six habitat types. Stem density correlated with light availability and ranged from 2.38 stems m−² in near primary forest to 4.66 stems m−² in light gaps. Gorillas showed marked preferences for habitats with high herb densities such as light gaps, swamps and young secondary forest. However, no clear relationship exists between terrestrial herbaceous vegetation and gorilla densities across Central Africa. It is suggested that differences in ecological factors and land use history within and between sites may explain differences in herb density and diversity which partly account for variations in the historical and present population distribution and density of western gorillas. Formerly logged and swamp forests, which are characterized by an abundance of herbs, may prove to be of great value in the conservation of western gorillas given appropriate forest management practices, adequate protection from poaching and limited human encroachment.

Understanding the nesting spatial behaviour of gorillas in the Kagwene Sanctuary, Cameroon

We use spatial point pattern methods to analyse gorilla nest site data, and to enhance our understanding of the nesting behaviour of the Gorilla gorilla diehli in the Kagwene Sanctuary, Cameroon. Data were split into different seasons and different gorilla groups to better understand gorilla nesting behaviour at these different scales. Gorilla nest site distribution was found to be inhomogeneous and clustered, as a result of the inhomogeneity in the distribution of the environmental factors (such as elevation, slope, vegetation and aspect), and because of the interaction between nest sites. The proposed models reflected therefore a combination of the effect of environmental factors and interaction between nest sites. Predictions from these models showed that there is less space available for gorilla nest site location in the dry season than in the rainy season. It also showed that the Minor gorilla group has a bigger niche than the Major group, suggesting a nesting disadvantage in the larger size group. We also found that nest site locations of Major gorilla groups attract Minor groups, and vice versa.

Distinguishing between the nests of sympatric chimpanzees and gorillas

SummaryOur current inability to estimate precisely the population sizes of chimpanzees and gorillas across much of the Congo Basin has been detrimental to the development of conservation strategies for the preservation of these endangered apes. Systematic counts of nests are currently the most commonly used method to estimate ape abundance, but distinguishing between the nests of sympatric chimpanzees and gorillas has proven to be an enduring obstacle to estimating species‐specific abundance. In general, the builder of more than 75% of nests recorded during surveys is undetermined. We hypothesized that sleeping habits and nest building patterns would allow us to differentiate between the nests of these apes.We constructed a predictive model using stepwise discriminant function analysis to determine characteristics that accurately distinguished between chimpanzee and gorilla nests. We analysed 13 variables associated with 3425 ape nests from three independent surveys conducted in the Goualougo Triangle of the Nouabalé‐Ndoki National Park, Republic of Congo.The model correctly classified more than 90% of nests in our validation subsample. Nest height, nest type, forest type and understorey closure were identified as important variables for distinguishing between chimpanzee and gorilla nests at this site. Attributing nests to either species increased the precision of resulting density estimates, which enhanced the statistical power to detect trends in population fluctuation.Although specific variables may differ between study sites, we have demonstrated that predictive models to distinguish between the nests of sympatric chimpanzee and gorillas provide a promising approach to improving the quality of ape survey data.Synthesis and applications. Our study introduces an innovative solution to the dilemma of discriminating between the nests of sympatric chimpanzees and gorillas, which increases the specificity and precision of resulting ape abundance estimates. There is an urgent need to improve methods to evaluate and monitor remaining ape populations across western and central Africa that are experiencing the imminent threats of emergent diseases, poaching and expanding human development. Increasing the quality of density estimates from field survey data will aid in the development of local conservation initiatives, national strategies and international policies on behalf of remaining ape populations.

EVALUATING THE STEADY STATE ASSUMPTION: SIMULATIONS OF GORILLA NEST DECAY

Large mammal surveys are often based on indices of animal abundance such as dung or, for great apes, sleeping nests. They also tend to rely on the “steady state” assumption that the rate at which an index is deposited in the environment is exactly balanced by the rate at which it disappears (decays). Here we use a data set on western gorilla (Gorilla gorilla) sleeping nest decay rates to show that this assumption is likely to be strongly violated often and in ways that have serious implications for our ability to accurately survey and monitor a wide range of endangered species, particularly in tropical forests. We first fit the data using a model describing the daily probability of nest decay as a function of nest age and monthly rainfall. We then use this decay model to simulate time series of gorilla nest standing stock, given observed rainfall regimes from a series of sites in the equatorial African country of Gabon. These simulations suggest that, within a given site, the relationship between nest standing stock and true gorilla density fluctuates wildly and is, on average, tens of percent away from the mean nest decay time for the empirical data set. The behavior of nest standing stock is extremely sensitive to variability in rainfall, not just annual mean rainfall. Differences between sites in rainfall variability produce counterintuitive differences between sites in nest standing stock. Multiyear trends in the mean and variance of rainfall produce the spurious impression of trends in animal abundance. Furthermore, heterogeneity in rainfall exists at all spatial and temporal scales, so that attempts to use regression models based on rainfall measurements taken at one location or time are not likely to accurately predict the nest decay regime at other places or times. We close with some suggestions on alternative estimation methods that do not rely on extrapolations of environmental conditions from one time or place to another.

Newly Discovered Gorilla Population in the Ebo Forest, Littoral Province, Cameroon

We report on a new population of gorillas discovered in November 2002 in the Ebo Forest, Littoral Province, Cameroon. We observed A group of ≥q7 gorillas directly for 83 min, and they were in auditory range for 155 min. Further evidence of gorilla presence included 8 nest groups totaling 38 nests, distinctive feeding signs accompanied by footprints, and a gorilla cranium collected from the nearby village of Iboti. This newly discovered gorilla population is geographically intermediate between the 2 extant populations of western gorillas: Gorilla gorilla gorilla, the most populous gorilla subspecies living in Gabon, Equatorial Guinea, Congo-Brazzaville, Central African Republic and Cameroon to the south of the Sanaga River, and G. g. diehli or the Cross River gorilla, a small population of ca. 250 individuals on the Cameroon-Nigeria border. It is not possible to assign the new gorilla population to either subspecies on the basis of measurements of the single male cranium. Genetic analyses of freshly shed hairs, collected from gorilla nests, may help to resolve the taxonomic status of the Ebo gorillas.

Nest Building in Captive Gorilla gorilla gorilla

Although various aspects of gorilla nest building have been described in wild populations, nest-building behavior of captive gorillas has not been subject to much scientific review. We observed nest building in 17 gorillas during three periods: summer baseline, winter baseline, and winter treatment, in which the amount of available nesting material was doubled. We conducted observations exclusively in the indoor holding area in the hour following evening departure of animal care staff. During baseline, gorillas engaged in nest-building on 3.1% of scans and were on a constructed nest on 27.9% of scans. Overall, gorillas spent significantly more time on elevated nests than on floor nests. There were no statistically significant sex, age class, or rearing history differences in nest building, time spent on a nest, or nest location preference. Nest building consisted of both gathering and manipulating materials. The gorillas spent significantly more time building nests in winter than in summer. Additionally, they were more often on elevated nests during winter than summer. Doubling the amount of nesting material did not increase nest building, but it increased the number of floor nests. Our results suggest that providing adequate materials to captive gorillas for nest building may facilitate performance of species-typical nest-building, minimize competition among individuals for nesting sites and materials, and permit individual thermoregulation.

Population density and nesting behaviour of lowland gorillas (Gorilla gorilla gorilla) in the Ngotto forest, Central African Republic

AbstractThe population density and nesting behaviour of gorillas (Gorilla gorilla gorilla) was studied in the Ngotto forest, Central African Republic. We carried out a nest count along 94 km of transects. Also, the herbaceous vegetation of the understory was quantified and we attempted to assess the decay rate of nest sites. A total of 145 gorilla nests in 27 nest sites was found. We found 82.1% of gorilla nests > 2 m above ground. There was a significant correlation between the height of nests and the diameter of trees with nests. The density was estimated at 0.34–0.40 weaned gorillas per km2 depending on the value of nest site decay rate used. The herbaceous vegetation was scarce in the study site (42 out of 50 quadrats had no herbaceous stems). Compared with the gorilla population of the nearby Dzanga‐Sangha forest, the population density of gorillas at Ngotto was lower but the proportion of nests found above ground was much higher. These differences are discussed and we suggest that they were the result of the scarcity of herbaceous vegetation in the Ngotto forest.

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

AbstractA 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.