A total of 504 observations of nest building behaviour and the re-use of sleeping nests exhibited by 10 individuals of Sulawesi babirusa (Babyrousa celebensis) were made in spacious outdoor enclosures on Bali, Indonesia. We recorded behavioural characteristics associated with the timing of nest building behaviour, nest sites, and selection of nestbuilding materials. Both sexes frequently participated in the construction of sleeping nests during the short period from 17:00 hr to 18:00 hr, which corresponded to the period between the evening feed and the time of sunset. Young babirusa contributed from the age of 15 weeks. The babirusa favoured specific nesting sites in the enclosures determined by available structural supports such as walls, trees or corners. Various kinds of plant materials available in the enclosure were used. In addition to picking up fallen plant matters, they also pulled various parts off trees while standing on the hind limbs. In one instance, parts of a wooden fence were used due to the lack of other nesting materials. When housed as a group, consistent leadership of the nesting behaviour was not recognised; they seemed to prefer cooperative behaviour and built a communal nest. We concluded that nest building behaviour, for resting and sleeping, was a normal and important behaviour of the babirusa and that the provision of nest building materials to captive babirusa was an effective form of enrichment.

The impact of environmental stimuli on the expression of nesting behaviour of Sulawesi babirusa (Babyrousa celebensis) was explored over a period of 1339 days from 01 st August 2006 until 31 st March 2010 in a large animal enclosure on Bali, Indonesia. We found that nest building behaviour and the re-use of sleeping nests occurred more frequently in the dry season than the wet season and was clearly influenced by both a drop in the ambient temperature and an increase in wind speed. Additionally, net effective temperature (NET) computed from a combination of ambient temperature, relative humidity, and wind speed indicated a close relation to the nesting behaviour frequency; a decreasing trend of the expression rate of nesting behaviour with increasing NET in the early evening. Moreover, the trend of the expression rate by 12-hr NET fluctuation from 05:00 hr to 17:00 hr was also found out in all seasons. Meanwhile, 24-hr rainfall had a negative impact on the expression rate. These findings suggested that NET was a useful meteorological index to discuss the motivation to exhibit the nesting behaviour of babirusa, and also babirusa probably found thermal comfort in the sleeping nest.