Socioecological factors influencing population structure of gorillas and chimpanzees

Differences in distribution and density between gorillas and chimpanzees ar e reconsidered with special reference to population structure. Both ecologi cal and social factors influencing population structure are compared betwee n species and between habitats within species. Gorillas and chimpanzees res pond differently to a decline in food quality, such as fruit scarcity: gori llas change diet and decrease range, while chimpanzees do not change diet b ut may expand range. These responses result in different effects on their g rouping patterns. For gorillas the dispersed distribution and reduction of range size decreases the rate of inter-unit encounters and female transfer. The concentration of social units increases the rate of aggressive contact between units and stimulates female transfer. Social units of gorillas may crowd or disperse in order to attain the optimal density. This tendency ma y result in similar densities of gorillas across habitats. By contrast, the distribution patterns or range size may not affect inter-unit relationship s of chimpanzees. Within a single unit-group, various reproductive strategi es are adopted by both sexes. Independent travel of females and flexible gr ouping patterns enable them to survive at very low density in extraordinary large ranges. Density and inter-unit relationships are good criteria for a healthy population of gorillas, while the size of unit-group and inter-ind ividual relationships are good criteria for chimpanzees. Conservation plann ers should consider these differences for sympatric and allopatric survival in these species.