Possible Male Infanticide in Wild Orangutans and a Re-evaluation of Infanticide Risk

Abstract

Infanticide as a male reproductive tactic is widespread across mammals, and is particularly prevalent in catarrhine primates. While it has never been observed in wild orangutans, infanticide by non-sire males has been predicted to occur due to their extremely long inter-birth intervals, semi-solitary social structure, and the presence of female counter-tactics to infanticide. Here, we report on the disappearance of a healthy four-month-old infant, along with a serious foot injury suffered by the primiparous mother. No other cases of infant mortality have been observed at this site in 30 years of study. Using photographic measurements of the injury, and information on the behavior and bite size of potential predators, we evaluate the possible causes of this injury. The context, including the behavior of the female and the presence of a new male at the time of the injury, lead us to conclude that the most likely cause of the infant loss and maternal injury was male infanticide. We suggest that in orangutans, and other species where nulliparous females are not preferred mates, these females may be less successful at using paternity confusion as an infanticide avoidance tactic, thus increasing the likelihood of infanticide of their first-born infants.

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

“…In a dispersed mating system with high costs of association, and where males generally drive association patterns as found here for orangutans, the lack of morphological fertility advertisement can be explained by the selection on the total concealment of ovulation. Given a risk of infanticide (Knott et al 2010(Knott et al , 2019, females must achieve an optimum distribution of paternity assessments (van Schaik and Janson 2000;van Schaik et al 2004) by removing as much information on female fertility status as possible. Accordingly, the absence of morphological fertility advertisement combined with the concealed ovulation in orangutans appears to be the result of a trade-off between the costs of association and the necessity for paternity confusion (van Schaik et al 2004;Knott et al 2010Knott et al , 2019.…”

“…Female orangutans seem to vary their mate preferences with their reproductive status accordingly (Knott et al 2010 ). However, evidence for infanticidal attacks by males remains indirect (Beaudrot et al 2009 ; Knott et al 2019 ; Scott et al 2019 ) and infant mortality is generally extremely low (van Noordwijk et al 2018 ), suggesting that male infanticide in orangutans is extremely rare compared to chimpanzees and that females employ efficient counterstrategies.…”

“…Given a risk of infanticide (Knott et al 2010 , 2019 ), females must achieve an optimum distribution of paternity assessments (van Schaik and Janson 2000 ; van Schaik et al 2004 ) by removing as much information on female fertility status as possible. Accordingly, the absence of morphological fertility advertisement combined with the concealed ovulation in orangutans appears to be the result of a trade-off between the costs of association and the necessity for paternity confusion (van Schaik et al 2004 ; Knott et al 2010 , 2019 ). Future work will have to elaborate on the details of this hypothesis.…”

Sociosexual behavioral patterns involving nulliparous female orangutans (Pongo sp.) reflect unique challenges during the adolescent period

“…Despite these risks, adolescent female orangutans must begin mating if they are to achieve their first conception. Additionally, it has been argued that promiscuous mating is an effective anti‐infanticide behavioral strategy employed by female orangutans (Knott et al, 2010; Knott et al, 2019), which would further necessitate adolescents establishing themselves in the sociosexual landscape if they are to effectively confuse paternity over their first offspring.…”

“…Disinterest in mating with nulliparous females is observed widely across primates (Anderson, 1986; Muller, Thompson, & Wrangham, 2006; Scott, 1984; Smuts, 1985), and has been attributed to adolescent subfecundity and male preference for older, more experienced females (Muller et al, 2006; Scott, 1984). Adolescent female orangutans must overcome male ambivalence during limited social encounters while also protecting themselves from sexual coercion (Knott et al, 2019). As such, we expect them to display unique behavioral strategies compared to parous adult females.…”