Variation in developmental arrest among male orangutans: a comparison between a Sumatran and a Bornean population

Abstract

IntroductionThe presence of two sexually active male morphs with different reproductive tactics in a single species is rare among mammals. The most striking case of bimaturism among primates is exhibited by the orangutan (Pongo spp), in which one adult morph, the unflanged male, irreversibly develops into another one, the flanged form, but may remain arrested in the unflanged state for many years. However, it has been suggested that such arrest is less common among Bornean orangutans (Pongo pygmaeus) compared to Sumatrans (Pongo abelii). To investigate this possible inter-specific difference we compared both the number of developing males and the ratios of the two male morphs at two long-term study sites, Suaq Balimbing on Sumatra and Tuanan on Borneo.ResultsFirst, we observed a significantly greater number of flanged than unflanged males per month in the Tuanan study area, whereas the opposite pattern held at Suaq. Second, the same contrast held for the total number of identified individuals over the study, with more flanged than unflanged males at Tuanan and the opposite at Suaq. These differences were mainly due to transient males. For Tuanan, the identification results were confirmed by detailed genetic analyses. Finally, we recorded a higher proportion of unflanged males that became flanged during any given year at Tuanan than at Suaq.ConclusionThese results show that developmental arrest is far more common at Suaq than at Tuanan. Preliminary comparisons suggest that this is a general contrast between the island taxa of orangutans and should help efforts to identify the function and proximate control of developmental arrest in orangutan males.

Male orang-utan bimaturism and reproductive success at Camp Leakey in Tanjung Puting National Park, Indonesia

“…Given that speciation is thought to have occurred at least 400,000 years ago (Locke et al 2011)-and that Bornean and Sumatran orang-utans have since diverged into geographically and reproductively isolated island populations-it is also possible that the findings from either study may not be applicable to the other species (Goossens et al 2006). In particular, there are thought to be more flanged males in Bornean than Sumatran populations and reduced incidence of developmental arrest among Bornean male orang-utans (Delgado and van Schaik 2000;Dunkel et al 2013). As sexual dimorphism is often correlated with polygyny in multiple taxa (Alexander et al 1979;Clutton-Brock 1985;Weckerly 1998), it is possible that orang-utans were once gregarious with 'harems' of females controlled exclusively by a single male (van Hooff 1995).…”

Endocrinological correlates of male bimaturism in wild Bornean orangutans

“…However, note that wild orangutans would be barely weaned at 6 years, and most males are known or estimated to be >20 years old before developing flanges in the wild (cf. Dunkel et al, ; Utami et al, ; unpublished data Tuanan Orangutan Project).…”

“…Polymorphism is uncommon in mammals [Irschick & Lailvaux, 2006; Taborsky et al, ], but two morphs with distinct mating strategies linked to dominance rank can also be found to some degree in some primate species [e.g., Mandrill: Mandrillus sphinx : Setchell & Dixson, 2001; Verreaux sifaka: Propithecus verreauxi verreauxi : Lewis & van Schaik, 2007]. The most pronounced form of this dimorphism is the bimaturism found in orangutan males [ Pongo spp : Dunkel et al, ].…”

“…Orangutan males reach sexual maturity while they are unflanged, whereas they only develop full secondary characteristics (becoming fully flanged) after a highly variable period of time [Dunkel et al, ]. This change is irreversible.…”

“…Inter‐species differences in the unflanged/flanged male ratio indicate a longer period of developmental arrest for the Sumatran species than for the Bornean ( Pongo pygmaeus ) one [Dunkel et al, ]. Differences in mating monopolization potential of males between the two species may have resulted in the more flexible duration of developmental arrest in Sumatra, where male monopolization potential is higher [Pradhan et al, ].…”