The current number of living individuals of the Bornean subspecies of the Sumatran rhino (Dicerorhinus sumatrensis harrissoni; also known as the Bornean rhino) is possibly around forty or less. Sabah now offers the only likely prospect for saving this sub-species, and the best prospect for saving the species in Malaysia.
Based on morphological characters, Groves (1965) favoured separation of the Borneo form of the Asian two-horned or Sumatran rhinoceros (Dicerorhinus sumatrensis) as a distinct sub-species (D. r. harrissoni), with D. r. sumatrensis regarded as a single form occurring in Sumatra and Peninsula Malaysia. Based on mitochondrial DNA, however, Amato et al (1995) concluded that all Sumatran rhinos in Indonesia and Malaysia should be regarded as a single conservation unit. These results are of great significance : the Sumatran rhinoceros is now so highly endangered that mixing of Bornean and Sumatran forms in captive situations represents a potentially significant means to increase the number of births.
The fact that the ecologically similar Javan rhino and Malay tapir became extinct in Borneo before the expansion of the human population suggests that natural factors may have played a role in the low population density of rhinos. Pressure from hunting of rhinos for their horns (a mainstay of ancient Chinese medicine) has likely been ongoing over long periods. The Bornean rhino was widely distributed in at least some localities in eastern and central Sabah in the late nineteenth century (Payne 1990), but the population was likely depleted below natural carrying capacity by that time. The species was already regarded as very rare and endangered in Sabah by 1961 (Burgess, 1961). Payne (1990) showed that isolated ones or twos of rhinos occurred in many parts of eastern Sabah as recently as the 1970s, but the species quickly became extinct at most sites during the 1980s. Davies and Payne (1982) noted that all Sabah rhino records compiled in 1981 showed that a natural salt source or mud volcano was present within a maximum of 14 km for all records, suggesting that the species distribution may be limited by access to certain minerals. More recently, it has been noted that all confirmed Sabah rhino records from late nineteenth century to present occur on sedimentary (most) and volcanic (some) derived soils, with none on crystalline basement and ultramafic rocks. Although overall rhino numbers have continued to decline in Sabah since 1980, the general pattern of rhino distribution (very small breeding populations in what are now Tabin Wildlife Reserve and Danum Valley Conservation Area, with a few scattered individuals elsewhere) has remained remarkably constant.
The argument that it is too late to save this rhino because of inbreeding is not valid. The African and Indian rhinos species were in a similar situation about a century ago, as were several other large mammal species such as the European bison, Arabian oryx and Pere David’s deer, all of which were built up to much larger numbers with appropriate passion and action by a small number of people.
Major threats faced by rhinoceros today
Very low numbers
The Sumatran rhino is one of the most endangered animal species anywhere in the world. Less than 40 rhinos are believed to survive in Sabah. Even if half are females, and some are too old or too young to reproduce, perhaps only six to seven remaining rhinos have the potential to give birth. With a birth interval of three years under optimum conditions, no more than two rhinos are being born annually (see below). The Allee effect (Allee, 1931) refers to a phenomenon whereby a positive correlation exists between of individual fitness (e.g. survival probability, fertility, reproductive rate) and population density of the species (Courchamp et al, 2008). As numbers of individuals of a species decline to a low very level, the various factors associated with very low numbers (e.g. narrow genetic base, locally skewed sex ratio, difficulty in finding a fertile mate, reproductive pathology associated with long non-reproductive periods; see below) conspire to drive numbers even lower, to the extent that death rate eventually exceeds birth rate, even with adequate habitat and zero poaching. In the absence of specific actions to bring Sumatran rhinos together and boost production of offspring, therefore, there is a strong possibility that the Sumatran rhino may go extinct even if protection of rhino habitats and rhinos can be maintained and improved.
The only information on inter-birth interval for the Sumatran rhino comes from Cincinati Zoo, where three young were born at intervals of 2 years and ten months between 2001 and 2007. For wild Sumatran rhinos, actual birth interval is likely to have been less in recent decades, because of the paucity of sites with fertile females and males present.
Cranbrook (2009) points to the long inter-birth interval of these taxa, and refers to Johnson’s (2006) modeling of different levels of off-take applied to large mammals, whereby a small increase in juvenile mortality can hold recruitment rates below a level needed to replace breeding adults. If Danum and Tabin are each assumed to contain 15 rhinos, and that about half are females, and that of those females some are too old or too young to reproduce, perhaps only three or four rhinos in each area will be reproductively active. With a birth interval of three years under optimum conditions, only one rhino will be born into each population annually – this would explain the apparent zero rate of population increase in these protected areas. Even this may now represent an optimistic scenario.
Reproductive tract pathology
At least half the female rhinos caught between 1984 and 1995 had reproductive tract pathology (Schaffer, 2001), a phenomenon associated with lack of breeding and carrying of fetuses to successful birth that appears to particularly afflict rhinos (Hermes et. al, 2006). The fact that at least some wild female Sumatran rhinos have exhibited this pathology at time of capture indicates that not all wild female rhinos are breeding, presumably due to insufficient fertile males to meet and mate.
Skewed sex ratio
A period of active capture of rhinos from sites in Sumatra in 1959, and from Sumatra, Peninsula Malaysia and Sabah where forest was being converted to plantations between 1984 and 1995 and Sabah revealed differences in sex ratio. Of nine rhinos caught in the Siak River area, Riau, Sumatra, in 1959, only one was a male (Skafte, 1964). At that time, Riau was largely forest-covered. Of twenty rhinos caught at various locations in Sumatra between 1985 and 1992, a period of accelerating forest loss, eleven were females. Later, in September 2005, two immature female rhinos were caught (named Rosa and Ratu), each having apparently moved into inhabited semi-forest areas from Bukit Barisan Selatan and Way Kambas National Park respectively. Of twelve rhinos caught in Peninsular Malaysia between 1984 and 1994 from several separate regions, nine were females. Since female Sumatran rhinos are believed to have smaller home ranges than males, and siting of rhino traps was based on well-used rhino paths, and the sample size is small, this bias towards females is not unexpected. Yet a severe bias in sex ratio in the opposite direction was observed in Sabah where, between March 1987 and November 1995, a total of ten rhinos were captured. Of those, nine were caught within an area of about 120,000 hectares which would up to around 1980 have been contiguous forest cover. Of the nine, one was a mature female and eight were mature males. Although the sample size is small by normal standards in biology, there are unlikely to have been many, if any, rhinos not located during the conversion of 120,000 ha of forest. Thus, the remnant rhinos in this small population were almost all mature males. The tenth rhino, caught in April 1989, was a young female (named Lun Parai) that had arrived near a major road and which may have come from Tabin Wildlife Reserve, the nearest large block of forest some 25 km away in a straight line. Not much can be gleaned from these records and a similar situation will not happen again, as there is now much less forest and much fewer rhinos. It is clear, however, that a biased sex ration may occur in very small populations of Sumatran rhino. The observations from Sabah also suggest that female rhinos, potentially easier to locate than males because of their presumed use of smaller areas, had already been selectively taken by hunters before the start of government-sponsored trapping for a captive breeding programme. Also, despite the very small sample size, the three cases of young rhinos moving out of forest into areas inhabited by humans suggests that young adult rhinos may tend to move far from their natal area.
Hunting and trapping
At any time, a single rhino poaching or inadvertent trapping event may represent the tipping point that pushes the species to a trajectory of extinction in Sabah.
Cause for optimism
The rhino in Sabah represents one of the few examples of a tropical large mammal where forest loss is not a major issue threatening the species. Conservation efforts can focus, therefore, exclusively on minimizing human-induced mortality and on bringing fertile females and males together.
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