Female Chimpanzee (Pan troglodytes schweinfurthii) Dispersal and Grouping Behavior in Forest Fragments in Rwanda
Open AccessHuman landscape use has led to fragmentation and deforestation in over 50% of tropical and subtropical forests (Haddad et al., 2015). Habitat fragmentation can alter ecological processes (Spielman et al., 2004) and have dire consequences for species living in fragments. Species in fragmented landscapes may face restricted dispersal that can lead to inbreeding depression, increased risk of disease, and edge effects, all of which threaten population viability (Hockings et al., 2007, 2012). Non-human primates are particularly vulnerable to fragmentation since most species are forest specialists (Benchimol & Peres, 2014; Estrada, 2013). Researchers estimate that 60% of primate species are now threatened by extinction (Estrada et al., 2017; Oates, 2006). This conservation crisis has larger implications for global biodiversity since primates are crucial to seed dispersal and ecosystem functions (Beaune et al., 2013; Bourlière, 1985). In this thesis, I investigated female chimpanzee (Pan troglodytes) behavior and dispersal in two forest fragments in Rwanda: Gishwati Forest Fragment (GFF) and Cyamudongo Forest Fragment (CFF). A female perspective on how primates cope with fragmentation is critical given their central role in population growth and viability. Chimpanzees are an interesting system in which to investigate this topic for several reasons. First, they have a longer developmental period and lower fertility than most other primates (Watts & Pusey, 2002) which may limit the extent to which this species can rebound from deleterious effects of fragmentation. Second, chimpanzees are one of the few mammal species characterized by female-biased dispersal, meaning that females born into forest fragments may face limited dispersal opportunities. Third, chimpanzees occupy large home ranges, but their terrestrial locomotion may allow them to exploit human-modified landscapes as evidenced by crop-raiding (Krief et al., 2014) and recent reports indicate that females can disperse over large geographic areas occupied by humans (McCarthy et al., 2018; Gombe National Park, Tanzania: unpublished data). Finally, chimpanzees are characterized by a fission-fusion (FF) social organization, in which communities temporally split into smaller groups for several hours to days in order to reduce feeding competition (Aureli et al., 2008; Goodall, 1986). This social heterogeneity may allow females more flexibility to offset costs associated with fragmentation and reduced food availability. In this dissertation, I investigated how fragmentation relates to female behavior in terms of grouping pattern and dispersal. Specifically: In Chapter 1, I characterized chimpanzee habitat quality in the two fragments under investigation by integrating vegetation surveys and remote sensing data. By combining vegetation plots and monthly phenology data, I generated monthly food availability indices for fruit trees and flowering trees that are consumed by chimpanzees at the two study sites. I then performed MaxEnt modeling to generate habitat quality surfaces based on species presence and key environmental predictors that predict food species distribution and abundance. My results demonstrates that CFF has a higher habitat quality in general but there are key differences in the overall higher habitat quality in CFF. GFF has a higher prevalence of flowers and higher habitat quality in the edge than in the core forest, compared to CFF. These habitat differences likely have consequences on chimpanzee ranging and subgrouping patterns, in a manner that presents differing risks to females living in the two fragments (Chapter 2). In Chapter 2, I investigated the relationship between habitat quality, subgrouping, and ranging in the two fragments to test the following predictions: (1) Subgroups are larger in general at CFF than at GFF; (2) Subgroups are larger in the edge than in the core at GFF; (3) Subgroups are larger in the core than in the edge for CFF; (4) GFF females spend more time feeding in the edge than in CFF; and (5) GFF females spend more time in the edge and in human-modified landscapes (HML) than CFF females. All of these predictions were supported except #5; I did not detect any difference in the amount of time females spend in HML between the two fragments. These patterns suggest that GFF female chimpanzees may face increased risks of zoonotic diseases as they spend a relatively larger proportion of time in the edge than CFF. On the other hand, I conclude that CFF females may face increased within-community disease transmission and contest competition, resulting from being forced to range in larger groups (Table 1). In Chapter 3, my coauthors (senior author, Dr. Emily Wroblewski) and I analyzed mtDNA to test our hypothesis that female dispersal would be compromised at GFF based on its further distance from a larger chimpanzee population (Nyungwe National Forest) which would result in lower mtDNA diversity. Contrary to our predictions, CFF had the fewer haplotypes (2), both of which were present among the 10 haplotypes identified in NNP. By comparison, we identified six haplotypes at GFF, four were not present among the other two populations. These results suggest that female dispersal from GFF is limited, but diversity may be maintained (in the short-term) through alternate female mating strategies. The low haplotype diversity in CFF suggests that there is female dispersal to nearby NNP without replacement of immigrants coming to CFF, potentially leaving CFF as a sort of population sink without more connectivity to the larger forest (Table 1). In sum, the results of this dissertation highlight that finer-scale consideration of local ecology, and behavior, can reveal specific risk factors associated for individuals in fragments. It is our hope that these results provide data that can shape and inform future conservation efforts.
- All rights reserved
Notice to Authors
If you are the author of this work and you have any questions about the information on this page, please use the Contact form to get in touch with us.