Red langurs live in groups made up of one male and multiple females. The resident adult male will always be at risk of being ousted by an invading male and losing his females. In 2012, a group of red langur monkeys experienced a change in leadership. This got our Red Langur Project Leader, David Ehlers Smith, thinking about the nature of these encounters between different groups, and what this may mean about how the monkey groups interact.
I recently published a paper (available here) on the context of encounters in this group before the takeover, and the behaviour of the group after the invading male had assumed control of the females. The results are very interesting, indeed.
Group-living primates enjoy numerous benefits, including the ability to defend resources within their home range from other groups. But, reÂsource contest often results in aggression between groups.
.JPG) |
| Photo by Dave Ehlers Smith/OuTrop |
There are several theories that help explain why the monkeys are aggressive. One assumes that males defend their females (as male success is limited by the ability to mate), and another that females defend their resources (as female success is limited by the ability to find food).
It is also hypothesised that females allow males to defend them and in turn benefit from having their resources protected (the ‘hired guns effect’), or also that males may defend a territory and the resources within it for females to use, and in return the male is granted breeding access to the females.
What we found in the Sabangau population of red langurs was that the resident male was most often the aggressor against invading groups, but also that most interÂgroup encounters occurred within their ‘core range’ of valuable resources. Therefore, it is likely that the females in the group benefited from indirect resource defence as a result of aggression by the resident male as a ‘hired gun’.
.JPG) |
| Photo by Dave Ehlers Smith/OuTrop |
Further to this, after the group takeover, the home range and core range of the new group (consisting of the new male and the existing females) overlapped pre-takeover ranges and post-takeover the group reused nearly half of the previously known sleeping trees. This suggests that the invading male established himself in the territory of the resident females, who were then able to continue to use the resources.
So, it seems that indirect mate defence via protection of resources may have been used by the invading male to obtain reproductive access to females. This study represents the first preliminary evidence for both indirect-resource-defence-via-hired-guns and indirect-mate-defence-via-resource-defence theories in langurs – another fascinating first for Sabangau!
