The factors that influence food choice have implications for animal survival, reproduction and population growth. We conducted a 1-y study of food choice by four mountain gorilla groups that consumed herbs and fruit at two locations differing spatially and temporally in food availability in Bwindi Impenetrable National Park, Uganda. We collected data on 45 important foods consumed by the gorillas, the availability of those foods in each gorilla group's home range and their corresponding nutrient and phenolic concentrations. Employing a linear multiple regression, we tested three hypotheses regarding the influence of food availability and the nutritional and phenolic concentrations of food on food choice. Regardless of changes in herb availability, the choice of herbs was positively influenced by their abundance and sugar concentrations and negatively influenced by their fibre, condensed tannin and protein concentrations. Furthermore, regardless of changes in fruit availability, the choice of fruit was positively influenced by its abundance and negatively influenced by its condensed tannin concentrations. During periods of low fruit availability, the gorillas did not increase the consumption of herbs high in fibre and sugar. The choice of herbs low in fibre had less of an influence on food choice at the location with lower fruit availability than the other location. Our results underscore the importance of incorporating both availability and nutrient concentrations into studies of food choice; by doing so we found Bwindi gorillas were able to choose abundant, relatively high-quality foods year round.

Mountain gorillas Gorilla beringei beringei are Critically Endangered, with just two small populations: in Bwindi Impenetrable National Park in south-western Uganda and the nearby Virunga Volcanoes on the borders with Rwanda and Uganda. A survey of the Bwindi population was carried out in 2002 and results were compared with the previous census in 1997. Our estimate of total population size increased over that period by c. 7% to 320 individuals and the structure and distribution of the population were largely unchanged. Signs of human disturbance were more common in 2002 than 1997, and gorillas tended to be found in areas of relative low disturbance. This suggested that disturbance could be a constraint on population growth and distribution but demographic stochasticity may also be responsible for the observed level of population change over a short time period. Other potential limiting factors, including habitat availability and disease, are discussed. While conservation activities in Bwindi have probably contributed to the stability of the population, strengthening of law enforcement and continued vigilance are needed to ensure the population's long-term growth and survival.

Mountain gorilla Gorilla beringei beringei tracking tourism generates important revenue for conservation efforts but brings with it the threat of disease transmission into the gorilla population. This study quantifies for the first time aspects of encounters between gorillas and tourists at Bwindi Impenetrable National Park that are likely to contribute to the risk of disease transmission. These include how close tourists get to gorillas, how close encounters are initiated, how long they last, and the age class of gorillas involved. Tourists got significantly closer to gorillas than the park rules allow (a mean of 2.76 m, compared to the rule of 7 m), and remained close for long periods. Contacts with the gorillas most vulnerable to disease, the juveniles, were closer but of shorter duration than those with adults. Contacts initiated by gorillas were closer but shorter than those initiated by tourists. Taken together these results demonstrate that the present rules are failing, and that the risk of disease transmission may be greater than previously believed.

Small populations are particularly susceptible to disturbance. Routine censusing to monitor changes is important for understanding both population dynamics and the effectiveness of conservation strategies. Mountain gorillas Gorilla beringei beringei in the Virunga Volcanoes region of Rwanda, Uganda and the Democratic Republic of Congo have been censused five times since 1970. However, due to war and political unrest in the region since 1990, no census had been conducted since 1989, when the population was thought to number 324 gorillas. In 2000 we estimated population size using repeated observations of 17 habituated groups and information on 15 unhabituated groups obtained during patrols. The minimum population was 359 gorillas, and a best-case scenario correcting for groups that might not have been counted was 395. Using the minimum population and best-case scenario respectively, this represents a 0.9% or 1.8% annual growth rate over the last decade and 1.0% or 1.3% annual growth rate since 1972. This is lower than growth estimates made in several population viability analyses, but approximately 5% of the 1989 population is known to have died due to military activity over the last decade. Different subsets of the population exhibited different responses to disturbance caused by war. We discuss conservation strategies that are likely to have contributed to an increase in the gorilla population during this time of turmoil. While the population has grown, the results should be viewed with caution, not only because all known growth during the last decade can be attributed to one subset of the population, but also because the region is still plagued by political unrest.