Seasonality in the Ecology and Life Histories of Mountain Gorillas (Gorilla gorilla beringei)

The abundance of food, especially that of fruit and often that of young leaves, varies considerably over time for most primates. This variation can depend on or be independent of seasonality in rainfall. Mountain gorillas (Gorilla gorilla beringei) in the Virungas are exceptional: their habitat contains almost no edible fruit, and they mostly eat perennially available herbs and vines that are densely and evenly distributed in much of their habitat. Earlier studies documented little consistent temporal variation in mountain gorilla diets and habitat use, except for seasonal use of bamboo by some groups, and documented no birth seasonality. Long-term data (≤ 7 years) on 6 mountain gorilla groups confirm these results for habitat use, except that they show some seasonality in use of the upper altitudinal extremes of the gorillas' home ranges for unclear reasons. Relatively low and inconsistent variation in habitat quality over time should lower the costs of grouping for gorillas compared to other apes. Long-term data also confirm the absence of seasonality in births and conceptions. However, they show that mortality rates and risk of respiratory infections vary directly with rainfall. These relationships are probably causal and may be mediated through thermoregulatory stress.