Oct. 16, 2012 -- The world's 25 most endangered primates were revealed in a report this week at the UN's Convention on Biological Diversity COP11, held in Hyderabad, India.
This Colombian variegated spider monkey (Ateles hybridus) has such a low population density that the number has proven impossible to count thus far.
"The most significant threats to the 25 most endangered primates, and to many other primates in all of the four habitat regions (Africa, Madagascar, Asia, Neotropics), are habitat destruction and unsustainable hunting," report co-editor Christoph Schwitzer, head of research at the Bristol Conservation and Science Foundation, told Discovery News.
The list features nine primate species from Asia, six from Madagascar, five from Africa and five from the Neotropics. In terms of individual countries, Madagascar tops the list with six of the 25 most endangered species.
Vietnam has five, Indonesia three, Brazil two, and China, Colombia, Côte d'Ivoire, the Democratic Republic of Congo, Ecuador, Equatorial Guinea, Ghana, Kenya, Peru, Sri Lanka, Tanzania and Venezuela each have one.
The San Martin titi monkey from Peru (Callicebus oenanthe) exists now in extremely fragmented, small groups. Its estimated decline is 80 percent over the last 25 years.
Delacour's langur (Trachypithecus delacouri) from Vietnam, in the year 2000, had an estimated population of 281 to 317 individuals. Sixty percent of the total population lives in isolated subgroups with less than 20 langurs per group.
Grauer's gorilla (Gorilla beringei graueri) is one of two subspecies of eastern gorilla in Africa. Many of them have disappeared during the last 30 years.
Russell Mittermeier, chair of the IUCN/SSC Primate Specialist Group and president of Conservation International, said: "Primates are our closest living relatives and probably the best flagship species for tropical rain forests, since more than 90 percent of all known primates occur in this endangered biome."
"It's also important to note that primates are a key element in their tropical forest homes," he continued. "They often serve as seed dispersers and help to maintain forest diversity. It is increasingly being recognized that forests make a major contribution in terms of ecosystem services for people, providing drinking water, food and medicines."
Madame Berthe's mouse lemur (Microcebus berthae) is the world's smallest primate.
It has declined severely over the years due to habitat loss and fragmentation. Illegal logging and slash-and-burn agriculture take place in its southwest Madagascar range.
The blue-eyed black lemur (Eulemur flavirfrons) from Madagascar was thought to be extinct, but was "rediscovered" in 1983. Its population has declined by over 35 percent in just three years due to habitat loss, slash-and-burn agriculture, logging, hunting for bushmeat, and live capture for the pet trade.
This primate "exemplifies a species mainly threatened by habitat destruction through land conversion for subsistence agriculture," according to Schwitzer.
"The situation has been aggravated by the political crisis in Madagascar since early 2009, which has led to an almost total lack of law enforcement in the country when it comes to illegal logging or bushmeat hunting."
This red-ruffed lemur was photographed in North Carolina, but the species (Varecia rubra) is native to Madagascar. Its range there is very restricted, due to human encroachment.
The silky sifaka (Propithecus candidus) of Madagascar has a restricted range in the northeastern part of the country. Although it lives at a very high elevation, it has also been impacted by human activities.
Less than 250 individuals are believed to exist.
The Indri (Indri indri) of Madagascar is the largest extant species of lemur. It is known for its eerie wailing song, which has been fading away due to population loss.
Half of the entire population has disappeared over the past 36 years.
The grey-shanked douc (Pygathrix cinerea) is mostly found in mountain evergreen forests of Vietnam. But hunting and use in the medicine trade have decimated numbers, so only 600-700 individuals are believed to exist now.
The Roloway monkey (Cercopithecus roloway) has declined by over 80 percent, with many local extinctions in Africa.
Its relatively large size and the value of its meat and skin have made it a preferred game species. Forest loss has also contributed to its demise.
In terms of why this, and the following featured species, are represented by illustrations and not photographs, Schwitzer explained, "Some of the species on the list of the world's 25 most endangered primates are indeed so rare that not many photos exist of them, let alone any good ones."
The Bioko red colobus (Piliocolobus pennantii pennantii) is down to less than 5,000 individuals now. Its meat is considered to be a luxury food in Equatorial Guinea.
"Most notably from the early 1980s, when a commercial bushmeat market appeared in the town of Malabo, this species has been hunted close to extinction," explained Schwitzer. "Its very limited range meant that it was susceptible to quick population decline."
The Tana River red colobus (Piliocolobus rufomitratus) was estimated to have a population of 1200-1800 in 1975. Even that low number has gone down to about 1,100 now.
There have been "drastic changes in vegetation" in its habitat, according to the report. Dam construction, irrigation projects and water diversion along with other problems are all contributing to this species' decline.
The Northern sportive lemur (Lepilemur septentrionalis) of Madagascar has a very small fragmented range. High pressure from hunting continues to reduce its already small population size.
The Northern sportive lemur (Lepilemur septentrionalis) has a very small fragmented range in Madagascar.
According to the report, it is the world's most restricted and least protected lemur. Its population size is so small that scientists have been unable to make an exact count.
The pig-tailed, snub-nosed langur (Nasalis concolor) is endemic to Indonesia. It is a preferred game species in some areas and is also sought after for the pet trade.
The golden-headed langur (Trachypithecus poliocephalus poliocephalus) is now confined to the island of Cat Ba in the Gulf of Tonkin, northeastern Vietnam.
Just 60-70 individuals remain. Many live in all female, non-reproducing groups, so its future looks grim at the moment.
The Eastern black-crested gibbon (Nomascus nasutus) had an historical range east of the Red River in China and Vietnam, but now that has been very restricted by human encroachment.
The total estimated population is just 110 individuals living in 18 groups.
The Western purple-faced langur (Semnopithecus vetulus nestor) from Sri Lanka inhabits an area of high human density.
Everything from dogs to power lines has led to its severe decline. Its population has gone down by more than 80 percent over just three generations.
Some researchers even believe this species is now extinct in the wild.
The Ecuadorian brown-headed spider monkey (Ateles fusciceps) from Ecuador has an unknown population density. Scientists believe that its population has declined by around 80 percent in recent years due to habitat loss.
The Ka'apor capuchin monkey (Cebus kaapori) has drastically declined by over 80 percent during the past three generations.
Its forest homes in Brazil have been largely destroyed due to logging and other activities. Hunting and the pet trade are other threats to this species.
The Tonkin snub-nosed monkey (Rhinopithecus avunculus) from Northeastern Vietnam has had its population reduced due to hunting pressures and habitat degradation.
Currently five completely isolated small groups of these monkeys are known to still exist.
The Northern brown howler monkey (Alouatta guariba guariba) is now restricted to a small area north of the Rio Jequitinhonha in Brazil.
Only 250 adult individuals of this species are thought to exist. Disease, hunting, and deforestation are all primary threats.
The Rondo dwarf galago (Galagoides rondoenis) of Tanzania now lives in very small and fragmented patches of forest.
This tiny primate has a distinctive call and uniquely shaped bottle brush-like tail. Its population density is so small that researchers have been unable to successfully estimate it.
The Javan slow loris (Nycticebus javanicus) of Indonesia was only recognized as a species in 2006, but this nocturnal, arboreal primate is already highly endangered. Intensive hunting pressure is reducing its already low numbers.
Jannette Wallis is vice president of Conservation at the International Primatological Society, which also helped to put the report together. She reminds that such a list provides researchers with a focus for future conservation efforts.
Prior versions of the list, she said, have "been used quite effectively by primatologists around the world when seeking policy changes, raising global awareness, and educating local populations about primate protection and conservation."
PHOTOS: Animals at Risk