Seasonal variation in the diet and activity budget of the northern yellow-cheeked crested gibbon Nomascus annamensis

Crested gibbons (genus Nomascus) are among the most highly endangered and understudied primates on the planet. One member of the group, the northern yellow-cheeked crested gibbon Nomascus annamensis, is similarly threatened and has been subject to few published studies. Because understanding the behaviour and ecology of a species and how it adjusts to changes in its environment are integral to its conservation, our aim was to determine how N. annamensis adapts its diet and behaviour to seasonal changes in northeastern Cambodia. To this end, we employed instantaneous scan sampling to quantify the diet and activity of a habituated group in Veun Sai-Siem Pang National Park from 2007 to 2014. Our results indicate that activity varied seasonally, and that this variation was likely due to changes in feeding activity. There was a pronounced increase in leaf consumption and a decrease in fruit consumption during the dry (NovemberApril) season. With the increase in folivory, gibbons spent more time eating overall and less time on other activities compared to the wet season (MayOctober). Overall, our data suggests that N. annamensis exhibits behav-ioural patt erns similar to other gibbon species.