Vietnam Primate Conservation Status Review 2002 - Hoang Lien ...

3.4.5 Status
3.4 DELACOUR’S LANGUR - Trachypithecus delacouri
Table 3.4.5-1 gives an overview of the estimated groups and individuals in the 19 isolated subpopulations
of Delacour’s langur. The total number is 49-53 groups with a total of 270-302 individuals
for the whole species.
11 additional animals are kept at the Endangered Primate Rescue Center; six of these were born there.
In average there are 5-6 indivuals/group. Compared to similar species of Indochinese langurs
(T. francoisi, laotum, poliocephalus, leucocephalus) this indicates already a reduction in the
population density. The average number of individuals/group in a normal population density
is around 9 (compilation see Nadler & Ha Thang Long, 2000, see also 3.3.5).
The abundance of the sub-populations indicates a very high number of small isolated groups (table
3.4.5-2). 20% of all existing Delacour’s langurs occur in isolated populations with a maximum of 10
animals. Such small populations are extremly sensitive and vulnerable. The loss of the important
male for reproduction - mostly only one per group - causes the annihilation of the whole sub-population.
The long term existence of these sub-populations is also doubtful for genetic reasons.
Without management and strict regulations the loss of these sub-populations and consequently of
20% of the population of this species is foreseeable.
55 % of the population, sub-populations with 11-30 animals are also particularly vulnerable if it is
not possible to stop the hunting. Unfortunately, it can be assumed that there will be no effective
control of hunting within the immediate future.
Only two solitary sub-populations comprising 30-35 individuals, the necessary to survive, are known.
Luckily one sub-population exists in Van Long Nature Reserve. This area also has an advantageous
geomorphological structure which makes access more difficult and can be easily controlled. However,
the genetic structure of this sub-population is unknown and studies are necessary to assess the
genetic diversity as a basis for long term survival.
The most important, and for some sub-populations, the only reason for the decline is hunting pressure.
During the FZS surveys an attempt was made to record all hunted Delacour’s langurs (Table 3.4.5-
3). Certainly this list is not complete. Some hunters were not found, some did not remember the exact
numbers and some were afraid to give correct information. But these numbers are alarming enough.
The recorded total number of hunted animals over a period of 10 years is 316. This equates with an
annual loss of more than 30 individuals. The numbers hunted in the late 1980’s could not be calculated
exactly but it was estimated at around 50 or more animals per year.
Since 1996, there has been a clear reduction in hunting levels. It has been influenced by the break
down of some sub-populations and the disappearance of langurs in some areas. Possibly it has also
been influenced to a small degree by the improvement of ranger activities and increased law
enforcement.
Based only on the known hunted langurs, the population has declined by 50-55% over the last 10
years. Normally, the decline of a population decreases with the reduction of the absolute numbers of
individuals. The last single individuals are not so easily hunted anymore. The Delacour’s langur has
not had a closed distribution area for a long time. A single group as a reproductive unit is much more
vulnerable. In some cases where a sub-population is only represented by a small number of animals,
the capturing of one group can result in the collapsing of the whole sub-population. If only one group
exists in a sub-population the sub-population is easily extirpated.
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