Fine-scale genetic structure analyses suggest further male than female dispersal in mountain gorillas

Abstract

Background: Molecular studies in social mammals rarely compare the inferences gained from genetic analyses with field information, especially in the context of dispersal. In this study, we used genetic data to elucidate sex-specific dispersal dynamics in the Virunga Massif mountain gorilla population (Gorilla beringei beringei), a primate species characterized by routine male and female dispersal from stable mixed-sex social groups. Specifically, we conducted spatial genetic structure analyses for each sex and linked our genetically-based observations with some key demographic and behavioural data from this population.

Results: To investigate the spatial genetic structure of mountain gorillas, we analysed the genotypes of 193 mature individuals at 11 microsatellite loci by means of isolation-by-distance and spatial autocorrelation analyses. Although not all males and females disperse, female gorillas displayed an isolation-by-distance pattern among groups and a signal of dispersal at short distances from their natal group based on spatial autocorrelation analyses. In contrast, male genotypes were not correlated with spatial distance, thus suggesting a larger mean dispersal distance for males as compared to females. Both within sex and mixed-sex pairs were on average genetically more related within groups than among groups.

Conclusions: Our study provides evidence for an intersexual difference in dispersal distance in the mountain gorilla. Overall, it stresses the importance of investigating spatial genetic structure patterns on a sex-specific basis to better understand the dispersal dynamics of the species under investigation. It is currently poorly understood why some male and female gorillas disperse while others remain in the natal group. Our results on average relatedness within and across groups confirm that groups often contain close relatives. While inbreeding avoidance may play a role in driving female dispersal, we note that more detailed dyadic genetic analyses are needed to shed light on the role of inbreeding avoidance as an ultimate cause of female dispersal in mountain gorillas.

Figure 1

Map of the Virunga Massif within the African continent. The average GPS nesting locations of both habituated and unhabituated gorilla groups (n = 32 groups) used in this study are shown. Figure modified from Figure One in [56].

Figure 2

Linear regression between pairwise F_ST/(1 – F_ST) ratios and the natural logarithm of geographic distance separating two groups. Only groups containing two or more individuals were used in the analyses. (A) All mature individuals (n = 32 groups); (B) Adult females only (n = 25 groups); (C) Silverback males only (n = 17 groups). Regression lines are displayed for each plot. The regression equation, the coefficient of determination (R²), and the probability (P) to obtain a regression slope higher than the one observed are also reported. Note the identical scale for all plots.

Figure 3

Correlogram plots of the genetic correlation coefficient (r) as a function of geographic distance between individuals. The 95% confidence interval about the null hypothesis of a random distribution of genotypes (dashed lines) and the bootstrapped 95% confidence error bars are also shown. The number of pairwise comparisons within each distance class is presented above the plotted values. (A) All mature individuals (n = 158); (B) Adult females only (n = 109); (C) Silverback males only (n = 49). All individuals of the same group fall within the 0.5 km distance class. Asterisks denote significantly positive r values at α = 0.05.

Figure 4

Influence of modifying the second distance class on the spatial autocorrelation analyses. Only the second distance class is shown, for increasing distance class sizes from 3 to 10 km. The thicker line denotes the genetic correlation coefficient (r), and the thinner lines indicate lower and upper bounds of the 95% confidence interval about the null hypothesis of a random distribution of genotypes. Bootstrapped 95% confidence error bars are also shown. The number of pairwise comparisons within each distance class size is presented above the plotted values. (A) All mature individuals (n = 158); (B) Adult females only (n = 109); (C) Silverback males only (n = 49). Asterisks denote r values significant at α = 0.05.