Violent encounters between social units hinder the growth of a high-density mountain gorilla population

Abstract

Density-dependent processes such as competition for resources, migration, predation, and disease outbreaks limit the growth of natural populations. The analysis of 50 years of mountain gorilla data reveals that social behavior changes observed at high group density may also affect population growth in social species. A sudden increase in social group density observed in 2007 caused a threefold increase in the rate of violent encounters between social units (groups and solitary males). A fivefold increase in the rate of infanticide and seven cases of lethal fights among mature males were subsequently recorded, and the annual subpopulation growth rate declined by half between 2000 and 2017. The increase in infanticide alone explains 57% of this decline. These findings highlight the complex relationship between population density and growth in social species and hold important implications for the management of island populations.

Fig. 1. Location of the research area in the Virunga massif at the border of Uganda, Rwanda, and the Democratic Republic of the Congo.

The transparent red area corresponds to the 95% kernel outline of the 2000–2017 study group relocation data. D.R. Congo, Democratic Republic of the Congo.

Fig. 2. Growth of the study subpopulation.

(A) Fluctuation of the annual growth rate of the study subpopulation between 1968 and 2017. Black dots: Independent estimations for each of the 50 years of the study. Blue line: Fitted values of a generalized additive model with moving average temporal autocorrelation structure of order 1. Gray band: Simultaneous 95% CIs. The statistical significance of the fitted slope can be assessed on (B). (B) First derivative of the slope of the growth rate curve derived from the generalized additive model and shown on (A). The gray band corresponds to the simultaneous 95% CIs of the estimates of the slope. Areas where the gray band does not overlap with the y = 0 line indicate periods when the growth rate varies significantly. After a period of increasing growth in the mid and late 1980s, the growth rate started declining significantly in the late 1990s. CIs for more recent slope estimates (2008–2017) become larger because of the absence of data from the future.

Fig. 3. Change in social dynamics and local density between 2000 and 2017.

(A) Variation in study group composition between 2000 and 2017. Each horizontal colored band corresponds to a gorilla group, with color indicating the number of mature males (>12 years old) in the group, and thickness indicating the number of individuals in the group. Black lines indicate individual transfers between study groups, between consecutive months. Black line thickness is proportional to the number of individuals transferring. Red asterisks: Infanticide events. Time step: 1 month. (B) Monthly variation in gorilla group and individual density in the study area. The total areas used to calculate density values were the 95% kernel densities of all relocation data available for the 12-month periods centered around each month of the timeline. gr, group; ind, individual.

Fig. 4. Relationship between the group home range overlap and the number of encounters observed between social units from 2002 to 2017.

Horizontal dashed lines indicate the average annual number of encounters for the 2002–2006 period (before group density increase) and the 2007–2017 period (after group density increase).

Fig. 5. Rate of infanticide before and after the increase in group density.

The significance of the difference between the two distributions was tested using the exact Wilcoxon-Mann-Whitney test.