Bornean orangutan seed dispersal and seed dispersal distances in Gunung Palung National Park, Indonesia

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Andrea Blackburn and Chery D. Knott, Anthropology, Boston University, Boston, MA, Tri Wahyu Susanto, Biology, Universitas Nasional, Jakarta, Indonesia, Chery D. Knott, Biology, Boston University, Boston, MA

Background/Question/Methods
Seed dispersal is an essential process for forest growth, maintenance, and regeneration. As a large-bodied frugivore with long gut passage times, we predicted orangutans may be frequently dispersing seeds over long distances (>1km). Our research aim was to measure orangutan seed dispersal distances over varying durations of gut retention time (45, 60, 80, and 125-hours). We followed focal orangutans (Pongo pygmaeus wurmbii) from morning nests to night nests for multiples consecutive days in Gunung Palung National Park, Borneo, Indonesia. We collected orangutan fecal samples (Aug. 2018-Mar. 2020, n=733) and sieved the samples for seeds (>2mm). During orangutan focal follows, we recorded all feeding behaviors and collected continuous GPS tracks of orangutan movements (May 2013 – Dec. 2019, n=586 follow days). We investigated how far orangutans disperse seeds and if dispersal distances vary among orangutan age-sex classes (adult females, flanged males, and unflanged males).
Results/Conclusions
We found orangutans disperse seeds in 71.8% of all fecal samples with a mean of 27.9 ±4.5 (SD=0.95) seeds (>2mm) per fecal sample. Through dispersal distance modeling, our results showed orangutans dispersed seeds longer distances as gut retention time increased. Orangutan dispersed seeds mean distances of 465m at 45-hours (n=113), 539m at 60-hours (n=111), 551m at 80-hours (n=89), and 612m at 125-hours (n=31). We found unflanged male orangutans dispersed seeds farther distances than adult females and flanged male orangutans. Unflanged male mean distances ranged from 618m (45-hours) to 764m (80-hours). Our results support the prediction that orangutans are long-distance seed dispersers (>1km). The maximum seed dispersal distance recorded was 2277m. Orangutans moved seeds more than 100m away from the parent tree in more than 85% of dispersal events. Our results provide evidence that orangutans are important seed dispersers in tropical forests as they are frequent seed dispersers and are dispersing seeds medium (mean distances of 450-600m) to long distances (maximum distances >1km).