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

Authors:
David Morgan at Thomas Edison State University
David Morgan
Crickette Marie Sanz at Washington University in St. Louis
Crickette Marie Sanz
Jean Robert Onononga
Jean Robert Onononga
Jean Robert Onononga
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Samantha Strindberg at Wildlife Conservation Society
Samantha Strindberg
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Abstract and Figures

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.
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Factors Influencing the Survival of Sympatric
Gorilla (Gorilla gorilla gorilla)andChimpanzee
(Pan troglodytes troglodytes)Nests
David Morgan
1
&Crickette Sanz
2,3
&
Jean Robert Onononga
2
&Samantha Strindberg
4
Received: 28 March 2016 /Accepted: 7 October 2016 / Published online: 12 December 2016
#Springer Science+Business Media New York 2016
Abstract 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 character-
istics 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
Int J Primatol (2016) 37:718–737
DOI 10.1007/s10764-016-9934-9
Handling Editor: Joanna M. Setchell
*David Morgan
dmorgan@lpzoo.org
1
Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, Chicago, IL 60614, USA
2
Wildlife Conservation Society, Congo Program, B.P. 14537, Brazzaville, Republic of Congo
3
Department of Anthropology, Washington University in Saint Louis, Saint Louis, MO 63130, USA
4
Global Conservation Program, Wildlife Conservation Society, Bronx, NY 10460, USA
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... The use of this method, despite its practicality, has some disadvantages. Because rates of chimpanzee nest production decay vary significantly between populations and depend on a complex array of ecological and seasonal factors (Morgan et al., 2016), locallyacquired nest decay and production rates should be used whenever possible (Kühl et al., 2008;Laing et al., 2003;Wessling & Surbeck, 2022). These data are, however, often not available, in which case a range of values from other sites is typically used to provide an estimate (e.g., Hicks et al., 2014). ...
... Frequent ground nesting in some populations may complicate this approach, particularly if ground nests are ignored or missed by observers or if decay rates differ between terrestrial and arboreal nests. Morgan et al. (2016) reported that the terrestrial nests of Western lowland gorillas (Gorilla gorilla gorilla) decayed more rapidly than arboreal ones, which they argued was influenced by nest construction type, canopy cover, rainfall, and forest structure. To our knowledge, no study has compared decay rates between ground and tree nests in chimpanzees; thus between 2020 and 2021, we conducted such a study in the Bugoma Central Forest Reserve (BCFR), where the chimpanzees are known to regularly build nighttime ground nests (Hobaiter et al., 2022). ...
... Valley, Stewart et al. (2011) observed average nest survival times of 210 versus 336 days in the dry versus wet season in forested habitat, respectively. Only one study, on sympatric gorilla and chimpanzee nests, has analyzed the effects of habitat type, rainfall, nest type, and construction style on nest decay (Morgan et al., 2016). Vegetation type used in nest construction also likely plays a role: as we found in this study, the nests built by the BCFR chimpanzee using the Omusetera tree (Maerua sp.) lasted longer than a year (see below). ...
Decay rates of arboreal and terrestrial nests of Eastern chimpanzees (Pan troglodytes schweinfurthii) in the Bugoma Central Forest Reserve, Uganda: Implications for population size estimates
Article
Full-text available
  • Jul 2023
  • AM J PRIMATOL
Chimpanzees were once thought to sleep primarily in the trees, but recent studies indicate that some populations also construct terrestrial night nests. This behavior has relevance not only to understanding the behavioral diversity of Pan troglodytes, but also to the conservation of the species, given that nest encounter rates are often used to estimate great ape population densities. A proper estimate of decay rates for ground nests is necessary for converting the encounter rate of nests to the density of weaned chimpanzees. Here we present the results of the first systematic comparative study between the decay rates of arboreal and terrestrial chimpanzee nests, from the Bugoma Central Forest Reserve in western Uganda. We followed the decay of 56 ground and 51 tree nests in eight nest groups between April 2020 and October 2021. For 15 of the ground and 19 of the tree nests, we collected detailed information on the condition of the nests every two weeks; we checked the remaining 73 nests only twice. On average, ground nests lasted 238 days versus 276 days for tree nests (p = 0.05). Of the 107 total nests surveyed, 51% of tree and 64% of ground nests had disappeared after six months. Based on our results, we propose a modification of the formula used to convert nest density into chimpanzee density. Our results highlight the importance of taking into account potential differences in decay rates between ground versus tree nests, which will likely influence our understanding of the distribution of ground nesting behavior in chimpanzee across tropical Africa, as well as our estimations of the densities of ground nesting populations.
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... However, our estimates must be considered with caution: the two conversion factors, nest production rate and decomposition time, needed in nest counts distance sampling analyses (used in this study) remain a source of concern when estimating great ape densities, as they affect the reliability of abundance estimates (Aebischer et al., 2017). For example, if the real mean nest decay time was shorter than the applied value, which may be the case in areas located close to the Equator where higher precipitation may have accelerated nest decay time (Morgan et al., 2016), nest counts could have underestimated bonobo density. Conversely, if nest construction rates in Salonga were higher than the value used in our analysis (1.37; Mohneke & Fruth, 2008), nest counts would have overestimated density. ...
Bonobo (Pan paniscus) Density and Distribution in Central Africa's Largest Rainforest Reserve: Long-term Survey Data Show Pitfalls in Methodological Approaches and Call for Vigilance
Article
Full-text available
  • Dec 2024
  • INT J PRIMATOL
Conservation measures require accurate estimates of density and abundance and population trend assessments. The bonobo (Panpaniscus) is considered Endangered in the IUCN Red List. This classification assumes that available population data are representative. However, with only 30% of the bonobo’s historic geographical range surveyed, reliable information is needed to assess the species' population status. Here, we use information from 13 surveys conducted between 2002 and 2018 in an area of 42,000 km², representing ~27% of bonobo-suitable habitat: Salonga National Park and its corridor, Democratic Republic of the Congo (DRC). Using 8310 km of reconnaissance and transect walks and 27,045 days of camera trapping, we: (1) provide updated estimates of bonobo population density and distribution (42,000 km²; ~5,000 km² of which, to the best of our knowledge, have not been surveyed before by scientists), (2) assess population trends (15,758 km²; 2002–2008 vs 2012–2018), (3) compare estimates obtained with different methods, and (4) assess the factors driving bonobo density and distribution. Although we detected a non-significant population decline, our study suggests that Salonga is a bonobo stronghold, with a population ranging between 8244 and 18,308 mature individuals (density: 0.31 individuals/km²). Standing crop nest counts returned non-significantly lower density estimates than camera trap distance sampling. Nest count-estimates were higher in areas with Marantaceae understorey and those farther away from rivers, while camera trap-estimates were higher in areas with lower human presence. Regardless of the method, bonobos were rarer in proximity to villages. They occurred more often in areas of dense forest cover and in proximity to ranger posts. Our results point towards a declining bonobo population in Salonga, but do not provide sufficient evidence to confirm this statistically. Consequently, the continued monitoring of the bonobo population and preservation of the integrity of Salonga, considering its biological and cultural heritage, will be crucial in the preservation of this stronghold of wild bonobos.
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... Direct observations were also taken when travelling between transects. Note that dung and nest decay rates vary according to many factors (Morgan et al., 2016) and may thus generate density-biased estimates between forest types. ...
Combining satellite and field data reveals Congo's forest types structure, functioning and composition
Article
Full-text available
  • Oct 2024
Tropical moist forests are not the homogeneous green carpet often illustrated in maps or considered by global models. They harbour a complex mixture of forest types organized at different spatial scales that can now be more accurately mapped thanks to remote sensing products and artificial intelligence. In this study, we built a large‐scale vegetation map of the North of Congo and assessed the environmental drivers of the main forest types, their forest structure, their floristic and functional compositions and their faunistic composition. To build the map, we used Sentinel‐2 satellite images and recent deep learning architectures. We tested the effect of topographically determined water availability on vegetation type distribution by linking the map with a water drainage depth proxy (HAND, height above the nearest drainage index). We also described vegetation type structure and composition (floristic, functional and associated fauna) by linking the map with data from large inventories and derived from satellite images. We found that water drainage depth is a major driver of forest type distribution and that the different forest types are characterized by different structure, composition and functions, bringing new insights about their origins and successional dynamics. We discuss not only the crucial role of soil–water depth, but also the importance of consistently reproducing such maps through time to develop an accurate monitoring of tropical forest types and functions, and we provide insights on peculiar forest types (Marantaceae forests and monodominant Gilbertiodendron forests) on which future studies should focus more. Under the current context of global change, expected to trigger major forest structural and compositional changes in the tropics, an appropriate monitoring strategy of the spatio‐temporal dynamics of forest types and their associated floristic and faunistic composition would considerably help anticipate detrimental shifts.
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... Therefore, here, we provide the following nesting data: GPS location, approximate age (1 day, 2 days, under a week, over a week), the nest type (ground, tree), and the number of ground and tree nests in the nesting site from August 2019 to June 2022. Nest decay rates vary substantially between populations and are shaped by local ecological and seasonal factors (Morgan et al., 2016;Wessling & Surbeck, 2021). We followed (n = 4); similarly, the majority of nest sites were located in mixedsecondary forest (n = 121) rather than disturbed (n = 8) or savannah (n = 1; unknown n = 8). ...
Habitual ground nesting in the Bugoma Forest chimpanzees (Pan troglodytes schweinfurthii), Uganda
Article
Full-text available
  • Nov 2023
  • AM J PRIMATOL
We report the presence of habitual ground nesting in a newly studied East African chimpanzee (Pan troglodytes schweinfurthii) population in the Bugoma Central Forest Reserve, Uganda. Across a 2‐year period, we encountered 891 night nests, 189 of which were classified as ground nests, a rate of ~21%. We find no preliminary evidence of socio‐ecological factors that would promote its use and highlight local factors, such as high incidence of forest disturbance due to poaching and logging, which appear to make its use disadvantageous. While further study is required to establish whether this behavior meets the strict criteria for nonhuman animal culture, we support the argument that the wider use of population and group‐specific behavioral repertoires in flagship species, such as chimpanzees, offers a tool to promote the urgent conservation action needed to protect threatened ecosystems, including the Bugoma forest.
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... Consumption rate estimates are based on single population densities across a species range, as new studies show that there is no tendency for systematic increases in abundance towards either climatic or geographic centres (Morgan et al., 2016;Santini et al., 2018). ...
Late‐Quaternary megafauna extinctions have strongly reduced mammalian vegetation consumption
Article
  • Jun 2023
  • GLOBAL ECOL BIOGEOGR
Aim How much stronger would the effects of herbivorous mammals be in natural ecosystems if human‐linked extinctions and extirpations had not occurred? Many mammal species have experienced range contractions, and numerous species have gone extinct in the late Quaternary, completely or in large part linked to human pressures. Therefore, herbivore consumption rates in seemingly natural ecosystems will deviate from their pre‐anthropogenic state. Here, we estimate the size of this deviation. Location Terrestrial systems, globally. Time period Current. Major taxa studied All late‐Quaternary terrestrial mammals . Methods We estimated and mapped vegetation consumption rate by all late‐Quaternary terrestrial mammals. We did this through the estimation of natural densities and dietary needs. We mapped their consumption rate in both current ranges and present‐natural ranges, that is estimated ranges in the absence of human‐linked range contractions and extinctions. We compared these estimated consumption rates to current net primary productivity (NPP). We summarized the results across ecosystem types everywhere as well as for only the last remaining wilderness areas. Results We estimate that wild mammals consume a median of 11% of NPP (at the scale of 96.5 km × 96.5 km grid cells) in current natural areas and that this would have been much higher in the absence of extinctions and extirpations, namely 21%. Looking at the change per grid cell, the mammal losses result in a median 42% reduction in consumption rate. Importantly, we estimate very similar declines in herbivory in what are considered the last remaining wilderness areas. Main conclusions Our results suggest that the natural interaction of mammalian herbivores with vegetation in ecosystems across the world is strongly reduced by prehistoric and historic to recent species losses, even in the last remaining wilderness areas, likely with major effects on ecosystem structure and functioning.
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... The drivers of sleeping site selection by great apes have been the subject of long debate among primatologists, in particular for chimpanzees and bonobos (Pan paniscus) that are known to build sleeping platforms (often referred to as nests) in trees (Fruth & Hohmann, 1993;Hakizimana et al., 2015;Morgan et al., 2016). In addition to tree sleeping platforms, chimpanzees also build their sleeping platforms on the ground (Koops et al., 2007;Tagg et al., 2013). ...
The influence of vegetation structure on sleeping site selection by chimpanzees (Pan troglodytes troglodytes)
Article
  • May 2023
  • AM J PRIMATOL
Sleep is an important aspect of great ape life; these animals build sleeping platforms every night. In a community of chimpanzees, each subgroup selects a sleeping site where each individual builds a sleeping platform, mostly on a tree. Previous studies have measured the heights of sleeping platforms and sleeping trees to test the predation avoidance and thermoregulation hypotheses of sleeping site selection. However, it remains unclear how components of vegetation structure (vertical and horizontal) together determine the selection of sleeping sites by chimpanzees. Using botanical inventories around sleeping sites in a tropical rainforest of Cameroon, we found that chimpanzees preferentially sleep in trees measuring 40-50 cm in diameter. Regarding height, on average, sleeping trees measured 26 m and sleeping platforms were built at 16 m. To build sleeping platforms, chimpanzees preferred four tree species, which represent less than 3% of tree species in the study area. We demonstrate that the variation in abundance of tree species and the vertical and horizontal structure of the vegetation drive chimpanzee sleeping site selection. It was previously thought that preference for vegetation types was the driver of sleeping site selection in chimpanzees. However, results from this study indicate that the importance of vegetation types in sleeping site selection depends on their botanical characteristics including the variation in tree size, the abundance of all trees, the abundance of sleeping trees, and the occurrence of preferred sleeping tree species, which predict sleeping site selection. The height and diameter of trees are considered by chimpanzees when selecting a particular tree for sleeping and when selecting a site with a specific vertical structure. In addition to tree height, the abundance of smaller neighboring trees may also play a role in the chimpanzee antipredation strategy. Our results demonstrate that chimpanzees consider several vegetation parameters to establish sleeping sites.
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... For example, the conditions of the local environment are a commonly acknowledged influence on the probability of sign encounter, and heterogeneity is common in metrics of sign decay rates across contexts (e.g., Walsh & White, 2005;K€ uhl et al., 2007;Bessone et al., 2021). Sign decay has been linked to a number of variables such as climatic seasonality, construction material or dung matrix, storm frequency, and sun exposure (e.g., Plumptre, 2000;Nchanji & Plumptre, 2001;Laing et al., 2003;Kouakou, Boesch & K€ uhl, 2009;Morgan et al., 2016;Kamgang et al., 2020;Bessone et al., 2021). Therefore, it is commonly recommended that decay rates are measured locally during surveying, as failure to do so may result in imprecise measurement and hinder the validity of inter-site comparisons (e.g., Laing et al., 2003;K€ uhl, 2008;Mohneke & Fruth, 2008;Bessone et al., 2021). ...
Failure to account for behavioral variability significantly compromises accuracy in indirect population monitoring
Article
Full-text available
Indirect wildlife population surveying largely depends upon counts of artifacts of behavior (e.g., nests or dung). Likelihood to encounter these artifacts is derived from both artifact production and decay, and variability in production behavior is considered to contribute minimally to inaccuracy in wildlife estimation. Here, we demonstrate how ignoring behavioral variability leads to significant population misestimation, using an example of an endangered ape, the bonobo (Pan paniscus). Until now, a single estimate of nest construction rate has been used to extrapolate bonobo densities, assumed to be broadly representative of bonobo sign production behavior. We estimated nest construction rates across seasons and social groups at the Kokolopori Bonobo Reserve, Democratic Republic of the Congo, and find nest construction rates in bonobos to be highly variable across populations as well as seasonal within populations. Failure to account for this variability led to degradation in the accuracy of bonobo population density estimates, accounting for a likely overestimation of bonobo numbers by 34%, and at worst as high as 80%. With this example, we demonstrate that failure to account for inter‐ and intrapopulation behavioral variation compromises the ability to estimate both relative and absolute wildlife abundances. We argue that variation in sign production is but one of the several potential ways that behavioral variability can affect conservation monitoring, should be measured across contexts whenever possible, and must be considered in population estimation confidence intervals. With increasing attention to behavioral variability as a potential tool for conservation, conservationists must also account for the impact that behavioral variability can have upon wildlife population estimation. Our results underline the importance of observational research to wildlife monitoring schemes as a critical component of conservation management. We discuss the avenues through which behavioral variability is likely to impact wildlife monitoring accuracy and precision and propose potential approaches for accounting for behavioral variability in wildlife monitoring.
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... For example, conditions of the local environment are a commonly acknowledged influence on probability of sign encounter, and heterogeneity is common in metrics of sign de-cay rates across locations (e.g., (Bessone et al., 2021;Kuehl et al., 2007;Walsh White, 2005). In both dung and nestcount surveying, sign decay is affected by climatic seasonality, especially in rainfall, as well as other contextual factors such as construction material or dung matrix, storm frequency, and sun exposure (Bessone et al., 2021;Kamgang et al., 2020;Kouakou et al., 2009;Laing et al., 2003;Morgan et al., 2016;Nchanji Plumptre, 2001;Plumptre, 2000). Because of these environmental influences, it is commonly recommended that local measures of decay rates must accompany surveying, as failure to do so may result in imprecise measurement and hinder validity of inter-site comparisons (e.g., (Bessone et al., 2021;Kühl, 2008;Laing et al., 2003;Mohneke Fruth, 2008). ...
Failure to account for behavioral variability overestimates bonobo populations and compromises accuracy in population monitoring
Preprint
  • Dec 2021
Wildlife population monitoring depends on accurate counts of individual animals or artefacts of behavior (e.g., nests or dung), but also must account for potential biases in the likelihood to encounter these animals or artefacts. In indirect surveying, which depends largely upon artefacts of behavior, likelihood to encounter indirect signs of a species is derived from both artefact production and decay. Although environmental context as well as behavior contribute to artefact abundance, variability in behaviors relevant to artefact abundance is rarely considered in population estimation. Here we demonstrate how ignoring behavioral variability contributes to overestimation of population size of a highly endangered great ape endemic only to the Democratic Republic of the Congo, the bonobo (Pan paniscus). Variability in decay of signs of bonobo presence (i.e., nests) is well documented and linked to environmental determinants. Conversely, a single metric of sign production (i.e., nest construction) is commonly used to estimate bonobo density, assumed to be representative of bonobo nest behavior across all contexts. We estimated nest construction rates from three bonobo groups within the Kokolopori Bonobo Reserve and found that nest construction rates in bonobos to be highly variable across populations as well as seasonal within populations. Failure to account for behavioral variability in nest construction leads to potentially severe degradation in accuracy of bonobo population estimates of abundance, accounting for a likely overestimation of bonobo numbers by 34%, and in the worst cases as high as 80% overestimation. Using bonobo nesting as an example, we demonstrate that failure to account for inter- and intra-population behavioral variation compromises our ability to monitor population change or reliably compare contributors to population decline or persistence. We argue that variation in sign production is but one of several potential ways that behavioral variability can affect conservation monitoring, should be measured across contexts whenever possible, and must be considered in population estimation confidence intervals. With increasing attention to behavioral variability as a potential tool for conservation, conservationists must also account for the impact that behavioral variability across time, space, individuals, and populations can play upon precision and accuracy of wildlife population estimation.
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Great ape surveys and the implications of long-term monitoring in the Djéké Triangle, Republic of Congo
Article
  • Oct 2024
Existing protected areas are anchors for conservation. Safeguarding flora and fauna within their peripheral areas is essential to maintaining their integrity and to potential increases to the area under effective conservation. With the decline in tropical forests, initiatives to increase the area of undisturbed forests under strict protection, particularly those neighboring protected areas, is of critical importance. Applied research has informed such land-management decisions for areas surrounding the Nouabalé-Ndoki National Park (NNNP) in Republic of Congo since the park’s inception three decades ago. Here, we present results of the first systematic line transect survey of great ape nests conducted in the Djéké Triangle, a 100 km2 unlogged continuous forest in the Kabo Forestry Management Unit adjacent to the NNNP. Distance sampling methods applied along 26 line transects on two different occasions (2016 and 2018, with total effort of 69.4 km) provided density estimates of 0.75 (95% confidence interval (CI) 0.52–1.09) and 0.61 (95% CI 0.40–0.92) chimpanzees/km2 and 2.15 (95% CI 1.36–3.40) and 1.19 (95% CI 0.78–1.82) gorillas/km2 for each of the two surveys, respectively. Estimated ape densities were compared to others across the landscape. The findings provide a unique baseline in an area that supports ongoing behavioral research and future gorilla tourism opportunities in the Djéké Triangle. More importantly, results provided empirical evidence of the environmental value and strategic conservation importance supporting inclusion of the Djéké Triangle into the NNNP in 2023. These long-term monitoring results inform best-practice standards and ape tourism certification.
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A brief history of primate research in the Ndoki forest
Article
  • Oct 2024
  • PRIMATES
The Nouabalé–Ndoki National Park (NNNP) in Republic of Congo has become a beacon for conservation in Central Africa. This manuscript documents the arrival of primatologists, the establishment of field stations and major discoveries in primate behavior and ecology. Field stations were strategically established to study primate behavior in a variety of different contexts from stationary platforms to forest follows of habituated groups. The implementation of new technologies and analyses have also been a hallmark of research at Ndoki. Scientists are shaping a new era in primatology at NNNP by building on past successes and promoting the next generation of Congolese conservationists to address environmental challenges. Results have proven crucial in discussions with government and industry and led to conservation gains such as the inclusion of the intact forests of the Goualougo and Djéké Triangles into the NNNP. The research stations have also become essential for developing a long-term certified sustainable international gorilla tourism program. Despite the many advancements for conservation such as increased protection of forests, development of internationally recognized protocols and large-scale capacity building initiatives, there are reasons for considerable concern in the near- and long-term for primates and their forest habitats in the Ndoki landscape. To address these concerns, we emphasize the long history of forming partnerships with local communities. We also discuss shared overlap featuring multicultural and environmental use of forest resources that is likely to be crucial in championing the conservation of the Ndoki forests for the next 25 years and beyond.
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Chimpanzees Preferentially Select Sleeping Platform Construction Tree Species with Biomechanical Properties that Yield Stable, Firm, but Compliant Nests
Article
Full-text available
The daily construction of a sleeping platform or "nest" is a universal behavior among large-bodied hominoids. Among chimpanzees, most populations consistently select particular tree species for nesting, yet the principles that guide species preferences are poorly understood. At Semliki, Cynometra alexandri constitutes only 9.6% of all trees in the gallery forest in which the study populations ranges, but it was selected for 73.6% of the 1,844 chimpanzee night beds we sampled. To determine whether physical properties influence nesting site selection, we measured the physical characteristics of seven common tree species at the Toro-Semliki Wildlife Reserve, Uganda. We determined stiffness and bending strength for a sample of 326 branches from the seven most commonly used tree species. We selected test-branches with diameters typically used for nest construction. We measured internode distance, calculated mean leaf surface area (cm2) and assigned a tree architecture category to each of the seven species. C. alexandri fell at the extreme of the sample for all four variables and shared a tree architecture with only one other of the most commonly selected species. C. alexandri was the stiffest and had the greatest bending strength; it had the smallest internode distance and the smallest leaf surface area. C. alexandri and the second most commonly selected species, Cola gigantea, share a 'Model of Koriba' tree architecture. We conclude that chimpanzees are aware of the structural properties of C. alexandri branches and choose it because its properties afford chimpanzees sleeping platforms that are firm, stable and resilient.
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Nest Grouping Patterns of Bonobos (Pan paniscus) in Relation to Fruit Availability in a Forest-Savannah Mosaic
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Full-text available
A topic of major interest in socio-ecology is the comparison of chimpanzees and bonobos' grouping patterns. Numerous studies have highlighted the impact of social and environmental factors on the different evolution in group cohesion seen in these sister species. We are still lacking, however, key information about bonobo social traits across their habitat range, in order to make accurate inter-species comparisons. In this study we investigated bonobo social cohesiveness at nesting sites depending on fruit availability in the forest-savannah mosaic of western Democratic Republic of Congo (DRC), a bonobo habitat which has received little attention from researchers and is characterized by high food resource variation within years. We collected data on two bonobo communities. Nest counts at nesting sites were used as a proxy for night grouping patterns and were analysed with regard to fruit availability. We also modelled bonobo population density at the site in order to investigate yearly variation. We found that one community density varied across the three years of surveys, suggesting that this bonobo community has significant variability in use of its home range. This finding highlights the importance of forest connectivity, a likely prerequisite for the ability of bonobos to adapt their ranging patterns to fruit availability changes. We found no influence of overall fruit availability on bonobo cohesiveness. Only fruit availability at the nesting sites showed a positive influence, indicating that bonobos favour food 'hot spots' as sleeping sites. Our findings have confirmed the results obtained from previous studies carried out in the dense tropical forests of DRC. Nevertheless, in order to clarify the impact of environmental variability on bonobo social cohesiveness, we will need to make direct observations of the apes in the forest-savannah mosaic as well as make comparisons across the entirety of the bonobos' range using systematic methodology.
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Introduction to advanced distance sampling
Chapter
  • Aug 2004
This advanced text focuses on the uses of distance sampling to estimate the density and abundance of biological populations. It addresses new methodologies, new technologies and recent developments in statistical theory and is the follow up companion to Introduction to Distance Sampling (OUP, 2001). In this text, a general theoretical basis is established for methods of estimating animal abundance from sightings surveys, and a wide range of approaches to analysis of sightings data is explored. These approaches include: modelling animal detectability as a function of covariates, where the effects of habitat, observer, weather, etc. on detectability can be assessed; estimating animal density as a function of location, allowing for example animal density to be related to habitat and other locational covariates; estimating change over time in populations, a necessary aspect of any monitoring programme; estimation when detection of animals on the line or at the point is uncertain, as often occurs for marine populations, or when the survey region has dense cover; survey design and automated design algorithms, allowing rapid generation of sound survey designs using geographic information systems; adaptive distance sampling methods, which concentrate survey effort in areas of high animal density; passive distance sampling methods, which extend the application of distance sampling to species that cannot be readily detected in sightings surveys, but can be trapped; and testing of methods by simulation, so that performance of the approach in varying circumstances can be assessed. Authored by a leading team this text is aimed at professionals in government and environment agencies, statisticians, biologists, wildlife managers, conservation biologists and ecologists, as well as graduate students, studying the density and abundance of biological populations.
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Survival Analysis Using S: Analysis of Time-to-Event Data
Book
  • Jul 2003
Survival Analysis Using S: Analysis of Time-to-Event Data is designed as a text for a one-semester or one-quarter course in survival analysis for upper-level or graduate students in statistics, biostatistics, and epidemiology. Prerequisites are a standard pre-calculus first course in probability and statistics, and a course in applied linear regression models. No prior knowledge of S or R is assumed. A wide choice of exercises is included, some intended for more advanced students with a first course in mathematical statistics. The authors emphasize parametric log-linear models, while also detailing nonparametric procedures along with model building and data diagnostics. Medical and public health researchers will find the discussion of cut point analysis with bootstrap validation, competing risks and the cumulative incidence estimator, and the analysis of left-truncated and right-censored data invaluable. The bootstrap procedure checks robustness of cut point analysis and determines cut point(s). In a chapter written by Stephen Portnoy, censored regression quantiles - a new nonparametric regression methodology (2003) - is developed to identify important forms of population heterogeneity and to detect departures from traditional Cox models. By generalizing the Kaplan-Meier estimator to regression models for conditional quantiles, this methods provides a valuable complement to traditional Cox proportional hazards approaches.
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Model Selection and Multimodel Inference
Article
  • Jan 2004
This chapter gives results from some illustrative exploration of the performance of information-theoretic criteria for model selection and methods to quantify precision when there is model selection uncertainty. The methods given in Chapter 4 are illustrated and additional insights are provided based on simulation and real data. Section 5.2 utilizes a chain binomial survival model for some Monte Carlo evaluation of unconditional sampling variance estimation, confidence intervals, and model averaging. For this simulation the generating process is known and can be of relatively high dimension. The generating model and the models used for data analysis in this chain binomial simulation are easy to understand and have no nuisance parameters. We give some comparisons of AIC versus BIC selection and use achieved confidence interval coverage as an integrating metric to judge the success of various approaches to inference.
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Nationwide census of gorilla (gorilla g. gorilla) and chimpanzee (Pan t. troglodytes) populations in Gabon
Article
  • Jan 1984
  • AM J PRIMATOL
A census was made of gorilla and chimpanzee populations throughout Gabon between December 1980 and February 1983. The aim of the census was to estimate the total numbers of both species and describe their distributions. The method was based on nest counts from line transects which allowed the calculation of population densities of all individuals except suckling infants. Fifteen types of habitat were recognized and defined in terms of their structural features. In the initial phase of the study we did transects in each habitat-type and computed mean densities for each species in each habitat-type. In the second phase of the study we estimated the sizes of gorilla and chimpanzee populations throughout the country by extrapolation from these population density values. We did transects in all areas of the country and conducted interviews to check the accuracy of the population totals obtained by extrapolation. Corrections were made to the extrapolated totals to take into account different levels of hunting pressure and other human activities found to modify ape population densities. Total populations of 34,764 gorillas and 64,173 chimpanzees were estimated. An error of ± 20% was associated with the estimated population totals, which allows the conclusion that Gabon contains 35,000 ± 7,000 gorillas and 64,000 ± 13,000 chimpanzees. The figure for gorillas is much larger than previous estimates. This seems to be because (1) gorillas occur in almost all types of forest and are not restricted to man-made secondary forest as had been though; and (2) the geographical distribution of gorillas in Gabon is wider than previously believed. Gabon's large areas of undisturbed primary forest offer exceptional potential for conservation, not only of gorillas and chimpanzees, but also of the intact tropical rain forest ecosystems which they inhabit.
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Influencing Western Gorilla Nest Construction at Mondika Research Center
Article
  • Dec 2002
We collected nesting data from 512 fresh nest sites, including 3725 individual nests, of western gorillas at the Mondika Research Site, Central African Republic and Republic of Congo from 1996 through mid-1999. The mean count of nests of weaned individuals is 7.4 per nest site. Nest types included bare earth with no construction (45% of total), partial to full ground construction (34%), and arboreal (21%). Females, blackbacks, and juveniles as a combined age-sex class built significantly more arboreal nests (21% of total) than silverbacks did (2%). Proximate rainfall (independent of temperature) is significantly correlated with nest construction, i.e., as rainfall increased, silverbacks built more ground nests, and non-silverbacks built more ground and arboreal nests. Maximum daily temperature (independent of rainfall) is significantly negatively correlated with nest construction, i.e., as temperature increased, gorillas slept more often on bare earth without constructing a nest. Accordingly, we conclude that although nest building in gorillas may have innate components shared with other great apes, it is a flexible behavioral pattern that in some western populations is often not exhibited. It appears that when gorillas in this population build nests, they do so in response to both wet and cool conditions, and independently of diet, ranging, or group size.
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