Grouping behavior of Sumatran orangutans (Pongo abelii) and Tapanuli orangutans (Pongo tapanuliensis) living in forest with low fruit abundance

Abstract

In contrast to the African great apes, orangutans (Pongo spp.) are semisolitary: Individuals are often on their own, but form aggregations more often than expected by chance. These temporary aggregations provide social benefits such as mating opportunities. When fruit availability is high, costs of aggregating should be lower, because competition is less pronounced. Therefore, average party size is expected to be higher when fruit availability is high. This hypothesis would also explain why orangutans in highly fruit-productive habitats on Sumatra are more gregarious than in the usually less productive habitats of Borneo. Here, we describe the aggregation behavior of orangutans in less productive Sumatran habitats (Sikundur and Batang Toru), and compare results with those of previously surveyed field sites. Orangutans in Sikundur were more likely to form parties when fruit availability was higher, but the size of daily parties was not significantly affected by fruit availability. With regard to between-site comparisons, average party sizes of females and alone time of parous females in Sikundur and Batang Toru were substantially lower than those for two previously surveyed Sumatran sites, and both fall in the range of values for Bornean sites. Our results indicate that the assessment of orangutans on Sumatra as being more social than those on Borneo needs revision. Instead, between-site differences in sociality seem to reflect differences in average fruit availability.

Experimental evidence for yawn contagion in orangutans (Pongo pygmaeus)

“…CY has thus far been observed in highly social species 6,7,15,17,[19][20][21][22] (but see: 1,[24][25][26][27] ). Orangutans have meaningful social interactions that occur more often than is expected by chance alone 76 , but these interactions occur at a much lower frequency compared to bonobos and chimpanzees 66,67 . Interestingly, our results show that orangutans exhibit CY, suggesting that a high degree of affiliation within a species is not necessary for CY to occur.…”

The importance of orangutans in small fragments for maintaining metapopulation dynamics

“…Our collation of reports from agricultural landscapes demonstrates substantial use of forest fragments by orangutans in farmland. It is increasingly evident that orangutans are a highly flexible and adaptable species that can maintain high population densities in production forests Oram, 2018;Roth et al, 2020), and most wild orangutan populations in Borneo are currently found outside of strictly-protected forests (Santika et al, 2017;Voigt et al, 2018). Orangutans in these landscapes can cope with degradation in habitat structure such as canopy opening (Davies et al, 2017), disperse on the ground when necessary , reproduce, and even successfully raise young to maturity (van Noordwijk et al, 2018).…”

The cost of associating with males for Bornean and Sumatran female orangutans: a hidden form of sexual conflict?

“…Here, we evaluate the costs of association for female orangutans with both females and males at two study sites, Suaq ( P. abelii , Sumatra) and Tuanan ( P. pygmaeus , Borneo) using behavioral and endocrine data. Because of large within-species variation in terms of their socioecology (e.g., Vogel et al 2015 ; Roth et al 2020 ) and little evidence for life history differences between species (van Noordwijk et al 2018 ), we refer to study site rather than species differences, as we evaluate only one study site per species. We included female-female associations as a comparative category with the assumption that females have similar incentives to associate with each other (van Noordwijk et al 2012 ) as opposed to male-female associations and that these therefore are likely cost-sensitive (sensu van Schaik 1999 ).…”

“…In the absence of high predation pressure due to their arboreal life style (van Schaik and van Hooff 1983 ), food abundance is the major constraint to population density and sociality in orangutans (van Schaik 1999 ; Hardus et al 2013 ; Vogel et al 2015 ). Fruit availability is not only thought to be responsible for the higher association frequency in West Sumatran ( Pongo abelii ) (average daily party size ranging from 1.6 to 1.9 individuals) compared to both Eastern Sumatran ( P. abelii ) as well as Bornean orangutans ( Pongo pygmaeus ) (average daily party size ranging from 1.05 to 1.3 individuals) (van Schaik 1999 ; Mitra Setia et al 2009 ; Wich et al 2011 ; Roth et al 2020 ), it also likely constrains associations within populations over time (van Schaik and Fox 1996 ; Fox 1998 ; Wich et al 2006 ; Roth et al 2020 ; J.Meric de Bellefon et al, unpublished data). A high degree of scramble competition has been held responsible for the low female sociability, and direct female-female contest competition has also been reported (Utami Atmoko et al 1997 ; Knott et al 2008 ; van Noordwijk et al 2012 ; Marzec et al 2016 ).…”