A population estimate of chimpanzees (Pan troglodytes schweinfurthii) in the Ugalla region using standard and spatially explicit genetic capture-recapture methods.

Population parameters such as size, density, and distribution of a species across a landscape are important metrics that inform conservation science and are key to management strategies. In this study, we used genetic capture-recapture methods to estimate the population size and density of the little-studied chimpanzees in the Ugalla region of western Tanzania. From 237 fecal samples collected non-invasively over a 10-month period, we identified a minimum of 113 individuals. Based on the two-innate rate method (TIRM) modeled in the software capwire, we obtained a maximum-likelihood estimate of 322 (CI 227-373) individuals over the 624 km(2) area surveyed. Using a spatially explicit capture-recapture (SECR) method, we estimated a population density of 0.25 (CI 0.16-0.38) individuals/km(2) . Observations of nests and search effort data revealed areas of more intense usage. The findings of this study are an important step in the characterization of the Ugalla chimpanzees, and substantially improve our understanding of the number of chimpanzees that occupy this savanna-woodland region at the easternmost extent of the geographic range of this endangered subspecies.

High prevalence of simian immunodeficiency virus infection in a community of savanna chimpanzees.

Simian immunodeficiency virus of chimpanzees (SIVcpz) has a significant negative impact on the health, reproduction, and life span of chimpanzees, yet the prevalence and distribution of this virus in wild-living populations are still only poorly understood. Here, we show that savanna chimpanzees, who live in ecologically marginal habitats at 10- to 50-fold lower population densities than forest chimpanzees, can be infected with SIVcpz at high prevalence rates. Fecal samples were collected from nonhabituated eastern chimpanzees (Pan troglodytes schweinfurthii) in the Issa Valley (n = 375) and Shangwa River (n = 6) areas of the Masito-Ugalla region in western Tanzania, genotyped to determine the number of sampled individuals, and tested for SIVcpz-specific antibodies and nucleic acids. None of 5 Shangwa River apes tested positive for SIVcpz; however, 21 of 67 Issa Valley chimpanzees were SIVcpz infected, indicating a prevalence rate of 31% (95% confidence interval, 21% to 44%). Two individuals became infected during the 14-month observation period, documenting continuing virus spread in this community. To characterize the newly identified SIVcpz strains, partial and full-length viral sequences were amplified from fecal RNA of 10 infected chimpanzees. Phylogenetic analyses showed that the Ugalla viruses formed a monophyletic lineage most closely related to viruses endemic in Gombe National Park, also located in Tanzania, indicating a connection between these now separated communities at some time in the past. These findings document that SIVcpz is more widespread in Tanzania than previously thought and that even very low-density chimpanzee populations can be infected with SIVcpz at high prevalence rates. Determining whether savanna chimpanzees, who face much more extreme environmental conditions than forest chimpanzees, are more susceptible to SIVcpz-associated morbidity and mortality will have important scientific and conservation implications.