Page 26 - Demo

Chapter 124Orang-utan socio-ecologyOrang-utans (Pongo spp.) are arboreal great apes that are native to Borneo and Sumatra. They share a last common ancestor with humans, that lived approximately 14 million years ago (Goodman et al., 1998). Currently, three species of orang-utans are distinguished (Figure 2): Bornean orang-utans (P. pygmaeus), Sumatran orang-utans (P. abelii) and Tapanuli orang-utans (P. tapanuliensis). Even though differences in behaviour can be found between different orangutan species (Delgado & Schaik, 2000; van Schaik et al., 2008), recent work has emphasized their similarities (van Noordwijk et al., 2018) and suggests that some of the behavioural differences might result from plastic responses to local fruit availability (Roth et al., 2020). In general, orang-utans are characterized by a semi-solitary lifestyle, which is unique among the great apes (Mitra Setia et al., 2008; Roth et al., 2020; van Schaik, 1999). However, occasional social interactions do occur, and provide opportunities for socializing, playing, and mating (van Schaik, 1999). In some populations, the frequency of socialization seems linked to the availability of fruit, with more social interactions occurring during times of abundance (Roth et al., 2020). Besides their peculiar social system, orang-utans are characterized by another uncommon trait, namely male bimaturism. This means that adult male orangutans can be categorized in two distinct morphs: unflanged males and flanged males (Utami et al., 2002). Unflanged males experience a period of arrested development. They are sexually mature and can produce offspring, but are nonpreferred mates for adult females (Knott et al., 2009; Kunz et al., 2022). In contrast, flanged males have fully developed secondary sexual characteristics, such as an extremely large body size, long hair, a large throat sac, and conspicuous flanges on the sides of their faces (Kunz et al., 2022). Furthermore, flanged males can produce long calls that seem to have a dual function: coordinating distance between males and attracting females (Mitra Setia & van Schaik, 2007). Importantly, orang-utan females seem to apply an attraction filter during mate choice: they prefer to mate with flanged males during their ovulation, potentially reflecting selection for good genes (Knott et al., 2009). Thus, the two male morphs are easily distinguishable based on visual and auditory cues, and females seem to discriminate between the morphs during their fertile phase. Because orang-utans are characterized by two distinct male morphs, they provide an excellent species to study the interplay between secondary sexual Tom Roth.indd 24 08-01-2024 10:41