Two juvenile male simakobu alarm-calling while feeding on the flowers of the aren palm
Endemic to the Mentawai Islands of Indonesia, simakobu (Simias concolor) are one of the world’s 25 most endangered primates [1]. They are medium-sized arboreal colobines that feed mainly on young leaves, unripe fruits, and flowers [2-3]. Simakobu typically live in small mixed-sex groups with one adult male and 2-5 adult females, and in all-male bands with 1-3 adult males [4]; however, in some areas, particularly where hunting occurs, they live in small socially monogamous groups [4]. Home ranges are small and exhibit some overlap between adjacent groups, indicating the opportunity for direct competition over resources [4]. Both male and females disperse, and while juvenile males may disperse from natal groups at an earlier age, both adult and juvenile females visit and transfer to other groups [4].
Male-Male Competition
Two juvenile male simakobu play-fighting during a rare venture to the ground.
Intergroup encounters typically involve males only, who often exhibit high levels of aggression, including displays, chases, and fights [5]. Outside of these periods of contact between groups, adult males produce loud calls several times daily during choruses, spontaneously throughout the day, and in response to environmental disturbances [6-7]. Adult and juvenile females exhibit large sexual swellings, which are conspicuously pink and possibly advertise female receptivity [8]. The presence of these swellings in this species is unusual, as no other Asian colobines and no species characterized by one-male groups possess this trait. Sexual dimorphism is pronounced in simakobu, as males are 29% heavier than females and their canines 95% longer [9]. Together, these observations indicate the presence of strong male-male competition.
References 1 Schwitzer C, RA Mittermeier, AB Rylands, LA Taylor, F Chiozza, EA Williamson, J Wallis, FE Clark. 2014. Primates in peril: the world's 25 most endangered primates 2012-2014. IUCN SSC Primate Specialist Group, International Primatological Society, Conservation International, and Bristol Zoological Society, Arlington, VA. 2 Erb WM. 2008. Conservation small grant: Behavioral ecology of simakobu monkeys (Simias concolor) in northern Siberut, Indonesia. ASP Bulletin, 32, 8-10. 3 Erb WM, C Borries, NS Lestari, JK Hodges. 2012. Annual variation in ecology and reproduction of wild simakobu (Simias concolor). International Journal of Primatology. 33: 1406-1419. 4 Erb WM, C Borries, NS Lestari, T Ziegler. 2012. Demography of simakobu (Simias concolor) and the impact of human disturbance. American Journal of Primatology. 74: 580-590. 5 Erb WM. 2012. Male-male competition and loud calls in one-male groups of simakobu (Simias concolor). Ph.D. thesis, Stony Brook University. 6 Erb WM, T Ziegler, NS Lestari, K Hammerschmidt. 2016. Are simakobu (Simias concolor) loud calls energetically costly signals? American Journal of Physical Anthropology. Early View. 7 Tenaza RR. 1989. Intergroup calls of male pig-tailed langurs (Simias concolor). Primates, 30: 199-206. 8 Tenaza RR. 1989. Female sexual swellings in the Asian colobine Simias concolor. American Journal of Primatology 17:81-86. 9 Tenaza RR, A Fuentes. 1995. Monandrous social organization of pigtailed langurs (Simias concolor) in the Pagai Islands, Indonesia. International Journal of Primatology, 16: 295-310.