Impact of food availability on ranging behaviour of the Sumatran orangutans (Pongo abelii) at the Suaq Balimbing population

Abstract

Orangutans are the largest arboreal mammals on Earth, one of human’s closest relatives, and a keystone species for biodiversity conservation, but they are threatened by extinction. Factors such as slow life history, large body size, frugivory, arboreal lifestyle, and low population density make them particularly vulnerable to population decline, especially when they face fragmentation and loss of habitat and wildlife trade. Therefore, scientists and conservationists alike are interested in their spatial and temporal ranging behavior and their ecological needs. Since they face different food availability in different tropical forests, orangutans likely adopt different strategies to ensure sufficient energy intake. Sumatran orangutans (Pongo abelii) in Suaq Balimbing spend most of their day searching for fruit. Further, they live in a habitat with higher fruit availability, in higher densities, and are more sociable than orangutans at other research sites. However, past studies found very little to no evidence that habitat fruit availability, represented as the percentage of fruit-bearing trees, has an effect on their ranging behavior. In contrast, several studies showed that the fluctuation of fruit availability in Bornean rain forests impacted the orangutans’ range behavior in Tuanan on Borneo. In the first part, this thesis aims to find an alternative way to represent food availability by looking at the actual feeding behavior of orangutans. I introduced a new food availability index called ‘experienced FAI’ (eFAI) and two sub-indices, which are based on this eFAI but split into a fruit (fruit eFAI) and non-fruit component (non-fruit eFAI). I analyzed and compared behavioral and GPS data from 2007 to 2021, collected in 1743 focal follow days and in accordance with standardized field methods by the Department of Anthropology of the University of Zurich and the Development and Evolution of Cognition Research Group at the Max Planck Institute of Animal Behavior in Konstanz, Germany. The eFAI was calculated based on a ratio of GPS points taken at fixed intervals throughout the focal follow to GPS points taken whenever the focal individual was feeding. I divided orangutans into four age-sex classes: adult females, flanged males, unflanged males, and independent immatures. The results show that the comparison of behavioral and GPS data was needed to validate the accuracy of GPS points, complement feeding locations, and improve the eFAI calculation. A bootstrapping analysis revealed a minimum number of 10 follows per month to be an appropriate threshold. Regarding the traditional, habitat-centered fruit availability (termed hFAI in this study), I confirmed that Sumatran orangutans at Suaq do not face fruit scarcity throughout the year. Nevertheless, experienced food availability (eFAI) varied depending on the age-sex classes. Results suggest that flanged males spend extended time exploiting one resource rather than switching between resources. Independent immatures are most likely less competent at locating big food patches, and thus they end up switching between food patches faster than other individuals. However, I noticed that orangutans of different age-sex classes have similar diet compositions, except for adult females that showed broader diets, which may relate to maintaining lactation and pregnancy by consuming nutrient- and protein-rich food. The spatial and temporal analysis of the eFAI suggests that fruit availability patterns may be associated with the layout of the research site and vary between forest types. In contrast, non-fruit food items can be found everywhere. In the second part, I investigated the effect of food availability on socio-spatial movement patterns. Therefore, I derived the Day Journey Length (DJL), Total Displacement Distance (TDD), and two tortuosity indices, the Straightness Index (SI) and the sinuosity index for the different age-sex classes and individuals. Fruit eFAI and non-fruit eFAI showed contradicting effects on the DJLs of orangutans and highlighted that an overall eFAI can explain less variation in ranging behavior. Fruit availability in the habitat (hFAI) could not explain any variation in the movement of orangutans, as expected based on the findings of previous studies, most likely because of the high abundance of fruit in Suaq. In contrast, I detected significant effects of fruit eFAI and non-fruit eFAI on DJL and sinuosity of movement but not on TDD and SI. When flanged males increased their non-fruit consumption, they had shorter DJL. Shorter distances may relate to longer feeding times at their feeding locations. Further results suggest that adult females move the shortest distances but the most tortuous. Moving after independently moving but still dependent offspring or carrying their young offspring may lead to higher sinuosity and slow adult females down. Furthermore, adult females traveled longer and straighter distances when consuming more fruit but shorter distances when focusing on non-fruit food items. This behavior suggests that adult females know the locations of the fruit trees and how to get to them when fruit are available. However, on a focal level, I found that older females tend to stay put when more non-fruit food items are available, and younger females tend to travel more, suggesting they actively search for those items. Results for unflanged males and independent immatures suggest that the ranging behavior of both age-sex classes is not influenced by food availability. For unflanged males, the mating strategy may have a greater effect on their ranging behavior than food availability. In contrast, for independent immatures, social factors most likely play a more prominent role as they are more gregarious and social than adult orangutans. In conclusion, comparing and complementing data sets and establishing new ways of representing food availability for Suaq proved to be meaningful, as I discovered different significant effects of experienced food availability on the movement parameters of Sumatran orangutans. However, differentiating between the availability of different food types is essential when looking at the effects of food availability on ranging. I suggest testing the approach in this thesis further at other research sites, which may help to understand the ranging behavior of orangutans better and protect them