The Effects of Forest Fires on the Behaviour and Health of Bornean Orang-utans in Sebangau Peat-Swamp Forest

Abstract

Forest fires are one of the greatest threats to endangered species in tropical peat-swamp forests. Over the last two decades, it has become evident that forest fires are increasing in frequency and severity at an alarming rate as a result of increasing anthropogenic land modification and climate change. Once considered a depauperate habitat type, peat-swamp forests have now been shown to contain considerable diversity and endemism. Of particular attention is the importance of peat-swamp forests for the critically endangered Bornean orang-utan (Pongo pygmaeus). Peat-swamp forests contain high densities and some of the last remaining strongholds of orang-utans in the world. The way in which fire affects orang-utans, in terms of their individual fitness, ability to survive and reproductive success may provide an early warning system and key insights into future fire-induced population trajectories. This study investigates variation in orang-utan activity budgets, physiological condition and gastrointestinal parasite infection before and after major fire events. I analysed 8056.8 hours of behavioural observations collected between 2011 and 2019 using generalised linear mixed models to assess if a major fire event in 2015 impacted the amount of time orang-utans allocate to daily activities. I revealed that orang-utans decrease their active period and allocate less time to resting over the long-term, four years post-fire, but increase their time allocation to resting in the short-term, up to nine months post-fire. I did not detect any decline in physiological condition after the 2015 fire event from analysis of ketones and specific gravity in 262 urine samples collected between 2009 and 2019. I employed the use of the Mini-FLOTAC faecal analysis technique to investigate changes in gastrointestinal parasite prevalence and load after a major fire event 2019. I collected 77 faecal samples between March and December 2019 from which I identified and counted the presence of gastrointestinal nematode eggs and larvae. Using zero-inflated negative binomial models and Fisher’s exact tests, I revealed that overall nematode intensity was greater post-fire but detected no significant change in parasitic prevalence. From these findings, I suggest that orang-utans may have been employing energy conserving behavioural strategies to cope with fire disturbance, but this was unlikely due to a negative energy balance. The findings of a greater nematode intensity warrants further investigation into alternative physiological markers and the impact of forest fires on wildlife health. Understanding how orang-utans are affected by forest fires will considerably improve the ability to manage and conserve remaining populations in forests prone to fire.