Mother-offspring proximity maintenance as an infanticide avoidance strategy in bornean orangutans (Pongo pygmaeus wurmbii).

Sexually-selected infanticide by males is widespread across primates. Maternal protection is one of many infanticide avoidance strategies employed by female primates. Bornean orangutan (Pongo pygmaeus wurmbii) mothers with younger offspring are less social with males than mothers with older offspring. Additionally, the distance between a mother and offspring decreases in the presence of male conspecifics, but not female conspecifics. We hypothesized that mothers are responsible for the change in mother-offspring proximity when males are present. Using a year of behavioral data from orangutans in Gunung Palung National Park, we tested whether the Hinde Index, a ratio of the number of approaches and leaves between two individuals, was indicative of mother or offspring proximity maintenance across different social groupings. The semi-solitary social organization of orangutans allows us to observe different social groupings. We found that the mother-offspring Hinde Index was typically indicative of offspring maintenance of proximity. However, the presence of male conspecifics was associated with an increase in the Hinde Index which indicates that mothers are responsible for the decrease in mother-offspring distance when males are present. The decrease in mother-offspring distances and increase in Hinde Index when males are present indicates that mothers react to the presence of males in a protective manner. We suggest this may be an infanticide avoidance behavior by mother orangutans.

Slow loris (Nycticebus borneanus) consumption by a wild Bornean orangutan (Pongo pygmaeus wurmbii).

Vertebrate predation and consumption by wild Bornean orangutans (Pongo pygmaeus spp.) is rare. In contrast to recorded observations of slow loris consumption by Sumatran orangutans (Pongo abelii), no cases of this have been previously published for Bornean orangutans in the wild. In 2017, we observed the capture and consumption of a slow loris (Nycticebus borneanus) by an adult unflanged male Bornean orangutan at Tuanan Orangutan Research Station, which is located in the Kapuas region of Central Kalimantan. The unflanged male was together with an adult female and her 3.5-year-old offspring throughout the event. However, despite the mother and her offspring watching the male closely and occasionally begging while he consumed the loris, he resisted all food-taking attempts. This study reports, to the best of our knowledge, the first documented case of slow loris predation and consumption by a Bornean orangutan, and thus provides an important data point for understanding primate predation on other primate species.