Behavioural monitoring of sterilised long-tailed macaques (M. fascicularis) at Monkey Forest, Ubud (Bali) : evaluating changes in infant caring, sexual and stress-related behaviours

Worldwide, primates and humans are increasingly forced to share space, and often enter in conflict when primates proliferate in urban environments. Birth control is increasingly used to limit population growth as an ethical alternative to culling. Yet, the effects and implications of such programs remain poorly understood, even not documented at all in the wild. In Bali, the Ubud Monkey Forest is a well-known tourist spot characterized by long-term relationships between local communities and long-tailed macaques ( ​Macaca fascicularis ​) which are food provisioned by people. This constant feeding and lack of predation have strongly impacted macaque reproduction by decreasing infant and juvenile mortality and increasing the average lifespan and fertility. Therefore, these macaques locally thrive in overcrowded conditions that lead to intense social tensions amongst them and growing conflicts with humans. In an effort to manage these issues and decrease the over-abundant population of macaques at Ubud Monkey Forest, a sterilisation program (recovering to female tubectomy) has been implemented as a population control strategies. In an effort to fill the gap of information concerning the short and long-term effects of primate reproductive control, the present project participates in the long-term demographic and behavioural monitoring of this managed population. Objectives ​​: Our research objectives were three fold. First, pursuing the demographic surveys of this population to analyse variation in vital rates and assess the efficiency of the birth control program. Second, monitoring the potential impact of sterilisations specifically on the sexual and infant caring behaviors of the female macaques. Third, monitoring their stress-levels using behavioural index. We assumed that, as tubectomy does not trigger immediate hormonal changes, no behavioural change should be observed in sterilised females. Methodology ​​: To reach these objectives, we conducted a four-month ethological study with a comparative approach between sterilised (N=19) and non-sterilised adult females (N=21). Moreover, we also made a distinction between two sub-groups depending on the maternity status of the females (having lactating infant or not). In total, we collected 123 hrs of observational data using a combined method of focal and scan sampling. Demographic census were done using the procession head counting method. Results : ​​Contrary to our predictions, we found some differences between sterilised and control females. Sterilised females devoted less time to resting and caring behaviour, they in turn invested this gained time in social and sexual activities involving adult males. These results show that sterilised females maintained their sexual motivation which was prolonged probably due to the repeated non-conceptive cycles they experience. Sterilised females also seemed more stressed than control females. This effect could be directly connected with the increased sexual activity and proximity with adult males who are more stressful social partners for females. This study brought interesting preliminary results on the short-term changes that can occur in wild macaque females shortly after their sterilization by tubectomy. However, this study was limited and requires a longer period of observations with a greater sample size to test properly the global effect of sterilisation.