Aonyx cinereus, commonly known as Asian Small-clawed Otter or Oriental Small-clawed Otter, is a native otter species in Asia that belongs to the family Mustelidae. It is the smallest of 13 otter species found globally (Prakash et al., 2012). Their adult body weight barely exceeds 5kg with a body length measurement usually not exceeding 61 cm long, including the tail (Hussain et al., 2011). According to Larivière (2003), the Asian Small-clawed Otter has well-developed plantar pads with considerably narrower and shallower webs and with short blunt claws that do not extend beyond the ends of the digital pads. These distinct features are used to distinguish the tracks of A. cinereus from the other otters.
The social group structure of Aonyx cinereus consists of up to 15 individuals that coordinate predator defense within their groups (Kruuk & Kruuk, 2006; Ladds et al., 2017). This social and intelligent species is widely distributed from India eastwards to the Philippines, Taiwan and southern China (Hussain et al., 2011; Wilson & Reeder, 2005). They are commonly found in freshwater and peat swamp forests, lakes, streams, reservoirs, canals, mangroves and coastal wetlands (Melisch et al., 1996; Roberton, 2007). According to a study conducted by Abdul-Patah et al., (2014), this otter species is most commonly seen in rice fields and casuarina forest areas in Malaysia.
A. cinereus has a plain brown dorsal body colouration or sometimes with a hint of reddish or yellowish-brown, while the underpart is a paler shade of brown and frequently seen with a grey cast. The head of this otter is marked by greyish, nearly white, coloration starting from the edge of the upper lips down to its throat. This distinct coloration eventually forms a tide mark running from below the eye toward below the ear (Pocock, 1941). It has two layers of fur; the shiny guard hairs that are water repellent and the dense fluffy underfur with up to 450,000 hairs per square inch that traps air and helps the otter to stay dry and insulated while in the water (Mason & Macdonald, 2009). Therefore, it is common to see the otters spending quite a lot of time grooming and reintroducing air into their coat.
The presence of stiff whiskers or vibrissae play important roles in detecting movements of prey underwater. Unlike other species, A. cinereus will use their paws that are sensitive in locating and capturing prey to dig through the murky water or mud (Mason & Macdonald, 2009). In addition to feeding on large quantities of fish, rodents, snakes and amphibians, a study conducted on the scat of this species suggests that its diet also consists mainly of insects, crabs and gastropods (Kanchanasaka & Duplaix, 2011). The broad and robust build of the last two upper teeth of this otter enables them to crush hard-shelled prey (Popowics, 2003).
A. cinereus communicates by secreting a strong musky scent on its faeces which originates from the scent gland near its tail. The scent-marked faeces or spraint are often smeared on tree trunks, trails and rocks to mark and establish territory boundaries (Davies, 2009). Apart from scent markings, A. cinereus can produce a variety of sounds comprising more than 12 distinct vocalizations (Timmis, 1971). The vocalizations are classified into four call types: chirps, squeals, barks, and screams. Different types of calls express different levels of stress faced by different otter individuals. Aside from projecting a high-pitch vocal that serves as a distress call to attract other group members for help, a monitoring study on the vocalization of A. cinereus found that the different vocal intensity is associated with the aggression behaviour within the otter group (Scheifele et al., 2015).
Last assessment made by the International Union for the Conservation of Nature (IUCN) Red List in 2015, the Asian Small-clawed otter is listed as Vulnerable. Throughout its range, the population size of this otter is thought to be declining especially over the past 30 years (Wright et al., 2015). The main factor affecting their survival is habitat loss due to anthropogenic activities such as the development of farmland and aquaculture as well as the expansion of human settlements in peat swamp forests and mangroves. Reduction in food resources is also an important factor. Besides over-exploitation of fish resources due to overfishing, the loss of fish resources due to heavy metal and organochlorine pollution also affects the survival of this otter (Hussain et al., 2011). Conservation measures such as restoration of habitat vegetation and stream pollution treatments are urgently needed to ensure the continued existence of this species.
