What's happening Wednesday

How vegetation characteristics and fruiting patterns can influence primate ecology – a case study with red langurs

David Ehlers Smith summarises a new paper that we’ve just had published about red langur monkeys.
This week, we’ve had another article published on the ecology of the red langurs (Presbytis rubicunda) here in Sabangau, detailing their home range use and activity patterns. This paper further highlights how the fruiting patterns, and the overall composition of our forest has profound effects on our primates’ ecology, and ties in nicely with what we’ve already discovered about these fascinating monkeys. Today, I want to talk about how all these factors are interlinked.
Previously, we’ve shown that in our mixed-swamp forest, the low fluctuation in fruit availability relative to other Bornean forests that experience “mast fruiting” (periods of mass flowering and fruiting, triggered by certain environmental and climate conditions, subsequently followed by long periods of food shortages) results in our langurs eating more fruit than ever recorded before in any Colobine monkey (the sub-family of monkeys in Africa and Asia that have evolved specialised adaptations in their stomachs to digest high volumes of leaves). Furthermore, we’ve shown how the presence of their favourite food trees, and in particular, those favoured trees that are large enough to produce an abundance of fruit, permits a high population density relative within the Presbytis genus. This is exemplified by their absence in another habitat type within Sabangau, the notoriously low-productivity “low-pole forest”, where their preferred fruit trees don’t grow large enough to provide enough fruit to support them.
In this new paper, we show how this abundance of fruit in the mixed-swamp forest goes even further in influencing the red langurs’ behaviour and ecology. It is widely accepted that frugivores (fruit-eating animals) travel further in their pursuit of their food than folivores (leaf-eating animals), as fruits are less abundant and more unevenly spread out than leaves throughout forested environments. The increased expenditure in energy from travelling further distances is off-set by the nutrition obtained from the higher quality diet. Similarly, frugivorous primates are known to have larger home ranges (the overall area that primate groups or individuals generally inhabit) than folivores, as fruit is rarer in the environment, and therefore a larger area must be defended if all primates living in a home range are to meet their nutritional requirements. In Sabangau, we’ve discovered that the red langurs have the largest home range size recorded in genus Presbytis, and that they travel on average further each day than ever recorded in any folivorous primate on any continent! In this way, and despite their adaptations for leaf-eating, the langurs of Sabangau have actually changed their behavioural ecology to mimic frugivorous primates due to the environmental conditions of our forest. Now, who says tropical peat swamps are boring?!