Effects of forest fragmentation on brown spider monkeys (Ateles hybridus) and red howler monkeys (Alouatta seniculus)

Abstract

The increasing intensity of anthropogenic land use and conversion has immense impacts on ecosystems worldwide and often results in habitat fragmentation. Fragmentation and other anthropogenic disturbances (e.g. hunting and logging activities) pose major threats to numerous animal species. Species vary greatly in their resilience to these disturbances and in their ability to survive in forest fragments. Animals might have to adjust their behavior, their grouping patters and/or dietary strategies to survive. Human-induced alterations can also result in long-term elevations of ‘stress’ hormones (glucocorticoids), which can have deleterious effects on growth, reproduction and immune system activity of animals. In this thesis I applied an interdisciplinary approach and combined behavioral observations with data on fecal glucocorticoid metabolite (FGCM) levels. To broaden our understanding of how animals cope physiologically and behaviorally with anthropogenic disturbances I studied two Neotropical primate species, brown spider monkeys (Ateles hybridus) and red howler monkeys (Alouatta seniculus). I collected fecal samples in several fragments in Colombia that differed in size (4.21 ha - 500 ha) as well as the level of human impact (determined through the occurrence and/or absence of hunting and logging activities). Using a previously validated enzymeimmunoassay I examined species-specific differences in the physiological responsiveness to both factors. Fragment size did not influence FGCM levels of either species. But spider monkeys showed elevated FGCM levels in fragments where both hunting and logging occurred, whereas howler monkeys did not show such a response. This suggests that hunting and logging activities might create long-term elevations of GC levels in spider monkeys and emphasizes why they are at a higher extinction risk than howler monkeys when living in anthropogenically altered habitats. To better understand how spider monkeys cope with fragmentation, I investigated how two groups modify their flexible grouping patterns and social behavior in a small fragment (65 ha). I collected data on subgroup sizes, aggression, habitat-wide fruit availability and collected fecal samples to determine FGCM levels. Both groups ranged in smaller subgroups and showed higher FGCM levels when fruit availability was high compared to when it was low. This was unexpected because (1) primate species that exhibit fission-fusion dynamics typically show the opposite grouping pattern and (2) a major function of glucocorticoids (GC) is the release of energy during the stress response. Consequently, GC levels typically increase in times of low resource availability. Spider monkeys are generally considered to be ripe fruit specialists. However, both groups have a more folivorous diet than other spider monkey populations. This seems to be a way to reduce the level of competition for fruit in times of low availability. When fruit availability is relatively high in this fragment it appears that the intra-group feeding competition for fruit is also high. This would also explain why FGCM levels were higher and subgroup size smaller in times of high fruit availability. This is further reinforced by unusually high levels, for spider monkeys, of female-female aggression, which was also more often observed when fruit availability was high. This illustrates how fragmentation can change the ability of spider monkeys to decrease intra-group feeding competition via flexible grouping patterns and that the relationship between resource availability, grouping patterns, aggression rates and stress levels can be more complex than assumed so far. In conclusion, this study demonstrates species-specific differences in the ability to cope with anthropogenic disturbances and that these differences can be, at least partly, attributed to different levels of physiological responsiveness. Importantly, this thesis illustrates the potential use of GC measurements as a tool to monitor populations in disturbed areas and it demonstrates that interdisciplinary research could improve conservation efforts, which will be necessary to ensure the conservation of the global biodiversity.