Bawean warty pig
Did you know? The Javan warty pig is characterised by a great elongation of the face and pronounced sexual dimorphism, with the males being much larger than the females (ca. 90 kg as opposed to 45 kg).
Groves and Grubb (2011) upgraded the Bawean warty pig to full species status with the name of Sus blouchi. The Bawean warty pig distinguishes itself from its close relative, the Java warty pig, by morphological (smaller skull) and phenotypical (different coat colour and shape of facial warts) characteristics (Blouch, 1988; Groves & Grubb, 2011). Whole-genome sequencing has recently demonstrated that the Bawean warty pig diverged from its sister species S. verrucosus, at least 166 k years ago following a founder event (Schleimer et al. 2023). The genomic assessment clearly supports the single species status of S. blouchi, as was previously proposed based on morphometric data.
The Bawean warty pig is endemic to the small remnant volcanic island of Bawean (197 km²) in the Java Sea
(S 05°46’0.00”, E 122°40’0.00”) approximately 120 km north of Java and 260 km south of Kalimantan. The island used to be part of the larger Sundaland landmass that existed until the Late Miocene some 5 million years ago (Smit S. 1947).
The Bawean warty pig has also a pronounced sexual dimorphism but is much smaller than its sister species S. verrucosus, weighing less than 70 kg in males and 50 kg in females. It also differs from the Java warty pig by morphological (smaller skull) and phenotypical (different pelage colour and shape of facial warts) characteristics (Blouch, 1988; Groves & Grubb, 2011).
Topographically, the island presently consists of numerous small mountains, with the highest peak being Gunung Besar at 646 m asl (Nijman V 2006). The weather on the island is seasonal with heavy rains from December through March and a dry season from April through November (Hamada JI et al 2002). The current human population is about 90,000, having grown relatively little from an estimated population of 44,000 inhabitants in the early 1900s (Veth PJ 1903). Due to the Islamic culture, there is no Sus scrofa on the island. Natural predators of Warty pig-piglets are stray dogs, monitor lizards and pythons.
Historically, the vegetation of this island likely consisted of deciduous monsoon forest (MacKinnon JR 1997), but people probably used and converted these forests extensively over hundreds of years. The remaining forests were protected in forest reserves in the 1930s (De Indische Courant 1935), which became the Bawean Nature Reserve in 1995 (Semiadi & Meijaard 2013). The Bawean Nature Reserve roughly coincides with the forest boundaries of the island and is divided into five wildlife reserves (38 km2), six strict nature reserves (7 km2) and three communal wildlife reserves (1.6 km2). Despite these designations, small-scale illegal logging and slash-and-burn continues to occur in protected forests due to lack of clear area boundaries and law enforcement (Nijman 2006; Nur Syamsi, pers. comm.). According to Nijman (2006), the island is characterised by four habitat types, all of which are used by warty pigs.
// Community forest: Community owned forested gardens at the assumed borders of the protected areas consisting of a mixture of cultivated trees such as Spondias pinnata, Artocarpus heterophyllus, Tectona grandis, Tamarindus indica, Bambussa spp., Arenga pinnata and undergrowth dominated by either shrubs or grasses.
// Teak plantation: Monoculture T. grandis stands inside the protected areas with undergrowth dominated by grasses and sparse herbaceous plant and shrub cover.
// Shrubland and degraded forest: Patches inside the protected areas characterized by high, young (DBH<30cm) tree density and clear signs of logging and burning. Undergrowth is either dominated by a mix of grassland and herbaceous plants, or dense shrub cover. Tree species primarily represent those found in community forest, such as mixtures of Bambussa spp., A. pinnataand T. grandis.
// High forest: Mature secondary or tertiary forest characterized by Ficus variegata, F.s septica, Podocarpus rumphii, and multiple Eugenia species, interspersed with dense patches of small trees.
Adapted from: Rademaker et al. (2016)
Movements, home range and social organisation
Bawean warty pigs can be active at any time of day or night. However, due to intensive hunting, they are strictly nocturnal outside of protected areas. GPS-based data from an adult female indicate a home-range size of 260 ha during the wet season (Feb. - May), which is almost exclusively in forested areas. During the dry season (Aug-Oct), when portions of the fields are irrigated, the pigs come to dig for invertebrates. When the rice is ripe in March-April, the pigs may come to the rice fields and cause crop damage. Three to four females with piglets can form groups. Interestingly, males can also form bachelor groups and even two old males can roam together. In general, Bawean warty pigs are thought to form a fission-fusion dynamic (ANB unpubl. data).
Bawean warty pigs can give birth throughout the year. Preliminary data from cam trapping stations indicate that about half of the warty pigs recorded were piglets, indicating a high reproduction rate. The estimated mean litter size was 3.49 piglets (n=45 litters). Females build simple nests of grass and reeds and piglets are born without stripes (ANB unpubl. data).
Status and conservation
The Bawean warty pig is not yet listed as a distinct species by the IUCN. Together with its sister taxon, the Java warty pig (S. verrucosus), it is currently still listed as "endangered" under IUCN Red List criteria (Semiadi et al., 2016). The Bawean warty pig is not protected under Indonesian law, is considered a pest, and is regularly hunted. Considering its extremely isolated, endemic distribution and the threat of African swine fever, it should be classified as critically endangered in the next update of the IUCN Red List (Drygala pers. comm.).
ANB Association for Nature and Biodiversity www.anbio.org
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