Blog 3: Social Systems
Group of male Proboscis monkeys from Labuk Bay Proboscis Monkey Sanctuary, Sandakan
Social Organizations:
Proboscis Monkey's tend to live in group sizes or 10 to 30 while the bachelors groups can be 10 to 20 males. They tend to live in large harem groups with one male with the large bulbous nose and multiple female and their offspring's. (Balolia, 2024). All-male groups include bachelor males who have yet to establish a harem. This harem is referred to as polygynous where as stated above is one male and many females. Many females never leave the group they are born into and help around while the males once they reach sexual maturity they will leave the group to form their own or hang out with other males till their bigger.
A very fascinating thing about Proboscis monkeys organization is their vocalization in conflicts or in communication. Research done by Scheumann et al. (2022) found that the dominate male used his deep honks as an intervention when a conflict arrives and this instantly de-escalates the situation.
An image of how groups are made
Social Structure:
The dominate male will maintain control over the group and mating rights over the females. Who's the dominate male you ask that's the easy part the male with the largest nose. Their dominance is held by their vocalization, posture, and how they fend off rival males. The graph seen above is from Matsuda et al. (2024) and this showed that males will leave the group they are born in and challenge other males from groups or join bachelor groups until they can establish their own harems. According to the same article "frequently maintain close proximity on their sleeping trees along the river." This means that although they are aggressive towards others they all tend to be in the same area for protection and this also limits aggression.
Mating Systems:
Proboscis monkeys from this groups to build a bond with everyone around according to Sjahfirdi (2021) They do this for protection and when its time to mate they can trust their dominant male to provide adequate babies and strength for the group. The males are much larger then the females about 10kg more then the females. Males also have the large bulbous noses making very easy to see whose who in the groups.
When males engage in mating they use their nose and loud vocals to assert who they are. The males will rarely get in fights and let their honks do all the talking for them. The males also don't have much competition within their groups because they are usually the only males around. Females will look to be mounted by the males as Sjahfirdi states "Mounting between male and females had the male climbing the female and as always followed by copulation." They also prefer to mate with the males that have the largest noses and deepest vocals.
A male Proboscis sitting next to a female to show size differences
Care System:
Infant care is usually left to the mother to handle. The mother will provide the nourishment, the protection, and the social aspects of their lives. While other females in the group may help out. Males do not directly help the child in its growth but they play a very important role. The presence of the male provides protection from threats and keeps the child and everyone safe. Not unique but a pretty common behavior with most primates is that the young will hold onto the mother for months before they start to interact with others in the group. They are also extremely shy at first and slowly make their way to interaction.
References
Matsuda, I., Murai, T., Grueter, C. C., Tuuga, A., Goossens, B., Bernard, H., … Salgado-Lynn, M. (2024). The Multilevel Society of proboscis monkeys with a possible patrilineal basis - behavioral ecology and sociobiology. Retrieved from https://link.springer.com/article/10.1007/s00265-023-03419-2
Scheumann, M., Röper, K. M., Nathan, S. K., & Goossens, B. (2022). Third-party vocal intervention in the proboscis monkey (nasalis larvatus). International Journal of Primatology, 43(4), 698–711. doi:10.1007/s10764-021-00273-9
Sjahfirdi, L., & Noviandini, D. (2021). Social and reproductive behaviour of proboscis monkey nasalis larvatus (Van Wurmb, 1787) at Taman Safari Bogor, West Java. Journal of Physics: Conference Series, 1725(1), 012051. doi:10.1088/1742-6596/1725/1/012051
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