Gibbons are the smallest of the extant ape species and are, with the exception of the Genus Homo, the most numerically successful. There are four extant taxa, all of which are found in tropical forests of south-eastern Asia: Symphalangus, Nomascus , Hylobates and Hoolock. Only the siamang (Symphalangus syndactylus) and two hylobatids – the white-handed (H. lar) and agile gibbon (H. agilis) – have overlapping distributions.
Morphologically, gibbons are very different to their ‘great ape’ counterparts. Not only do they share several primitive characteristics with old-world monkeys but they exhibit exaggerated limb proportions and extreme basal separation of the first digit of both hands and feet. Like most primates, gibbons are predominantly arboreal. Unlike other arboreal primates which are largely obligate quadrupeds, gibbons have evolved to travel and forage using three main modes of horizontal locomotion: leaping, bipedalism and, largely and primarily, brachiation.
Brachiating is, put simply, a method of locomotion whereby the animal is suspended and travels hand-over-hand, much like a child on the monkey bars in a playground. Gibbons are the only ape – and one of only two primate taxa – which utilise brachiation as the primary mode of locomotion. The other primates said to brachiate are the spider monkeys (Ateles spp.) though their reliance on their prehensile tails disqualifies them as true brachiators. Using this method, gibbons are able to move through the canopy at great speed, often using their momentum to cross large gaps between trees. They have even been reported to snatch birds from the air. Brachiating also allows the species access to relatively inaccessible food as they are able to pull thinner fruit-laden branches, which would otherwise be inaccessible, down and towards themselves.
To facilitate such a lifestyle, gibbons have evolved several adaptations, primarily in the forelimbs. Their arms are greatly elongated and their hands permanently hooked, allowing the animal to swing like a pendulum, greatly increasing the efficiency of the movement. Uniquely, gibbons also have a ball-and-socket-type joint in the wrist, granting increased agility and allowing sudden changes of direction, even while moving at high speed.
Though gibbons are famed for brachiating, there were few studies conducted which examined their musculoskeletal structure and sought to link that data to the mechanical action of brachiation. Michilsens et al. (2009) compared gibbons to ‘occasional brachiators’ such as chimps and bonobos, the semi-brachiating spider monkeys and non-brachiating species. They found that while the organisational structure of gibbon musculature is broadly similar to that of other primates, there are differences in shoulder flexors, extensors and rotator muscles which result in an increased capacity for power generation. Further, gibbon wrist and elbow flexors are capable of generating a high degree of force and the species exhibit exaggerated muscular proportionality with the greatest density around the shoulders. This adds to the pendulum effect and can be used by a flexing of the arm to change various facets of the swing. Tellingly, muscle dimensions in different gibbons species are directly comparable when body mass is normalised, meaning that the largest gibbon (the siamang) exhibits the same relative muscular dimensions as the smallest (Nomascus sp.).
New Zealand. The species was first described in 1886 but was not collected from Dent Island until 1975. In 1990, the breeding population was estimated at 60-100 birds, which declined further to 25 pairs in 1998. The species has been successfully introduced to Codfish and Campbell Islands. The total population is now estimated to be around 200 birds, though this includes captive specimens. Alien species, severe weather and avian diseases are primary threats.
endemic to Ecuador where it inhabits the north-western slopes of the Volcan Pichincha and the Cordillera de Toisan. The Pinchincha population numbers approximately 160 individuals and is restricted to an area of around 34km2. The total wild population has been estimated at between 250 and 999 individuals. Deforestation and habitat destruction is the greatest cause of past decline and the greatest ongoing threat to the species.
where it likely exists as a tiny and spatially restricted population. Little is known of the species, even amongst local people. It is thought that it exploits a more terrestrial lifestyle than that of other Aegotheles species. Based on a similar species – A. cristatus – it is thought that introduced predators and habitat loss are primary threats.
The Bengal florican is a bustard – the Family Otididae which includes the heaviest flying bird, the great bustard (Otis tarda) – which occurs in two populations; one in India, through Nepal, the other in southeast Asia. These populations fragmented into small sub-populations. The total global population is thought to be between 250 and 999 individuals. There have been no recent surveys of the Indian population. The primary threat is loss of key grassland habitat.
saw its population decimated in three generations. The remaining wild population – estimated at around 250-300 individuals – is found in Llanos de Mojos, Bolivia. Drastic reduction of the trade in wild-caught birds has halted the decline and there are signs of recovery. Other threats are hunting for feathers and meat, inter-specific competition and inbreeding of fragmented sub-populations.
(Furnariidae), a family famous for the horneros (Furnarius spp.) which build oven-like clay nests. The Rayadito is more demure, nesting in natural cavities. It is found only on the island of Alejandro Selkirk in the Juan Fernandez Islands, Chile. Recent surveys put the population at c. 248 birds though based on availability of suitable habitat, the total population may number as many as 1,000. Primary threats are habitat loss, climate change and an increase in red-backed hawks (Buteo polysoma exsul).
of the larger olive ibis (Bostrychia olivacea). It is endemic to São Tomé, São Tomé e Príncipe where it inhabits catchments in the south-west and centre of the island. Threatened by habitat loss and intense hunting pressure, the population has dwindled to fewer than 50 birds.
patches of forest to the north of the island of Reunion in the Indian Ocean. One of these sub-populations now consists of fewer than 25 individuals. From 2003 to 2007, numbers of unpaired males increased by over 10% every year, indicative of a heavy male skew in the population. Predation by rats, disturbance, invasive plants and inter-specific competition continue to contribute to the decline of the species. Fewer than 25 breeding pairs remained as of 2007.
woodpecker was thought extinct for several decades. However, a male was caught in a mist net in 2006; several specimens have been recorded since. It is known from several states where it is thought to be associated with bamboo and babassu palm forest. The current population is cautiously estimated at 50-250 individuals. Due to the number of captures and the likely size of its range, however, it is likely that the population is large and the species may soon be downlisted.
name of this species is a misnomer. Rather than being a true hawk (Accipiter spp.), Ridgeway’s is in fact a buzzard (Buteo spp.). Once recorded from Haiti and many associated islands, the species is now likely restricted to Los Haitises National Park in the Dominican Republic. The species is undergoing an annual decline of c. 5-10% of breeding pairs at one site which also experiences 10-15% annual forest loss. There were estimated to be between 80 and 120 breeding pairs as of 2006. A translocation effort by the Peregrine Fund is ongoing.
efforts since 1981 when the population was just 23 individuals. Despite contstant effort, the species remains one of the rarest shorebirds in the world. The wild population has almost certainly been saved from extinction by the annual release of large numbers of captive-bred birds. Once widespread across North and South islands of New Zealand, breeding is now restricted to the Waitaki Valley on South Island. The total population stood at 78 birds as of 2007-08, 20 of which were breeding pairs. Predators, nest-site disturbance and stochastic weather events are the primary threats.
Norfolk Island where the remnant population is confined to Norfolk Island National Park. Despite occasional reported sightings, official surveys have been unable to find a single individual. If the species persists, it does so in low numbers. It is threatened by rat predation and the destruction and alteration of suitable habitat.
along the island chain which includes Papua New Guinea and the Solomon Islands. The extent of its breeding range or area of occupancy whilst at sea are unknown. The global population is thought to be larger than originally estimated due to the sighting of around 160 birds between New Britain and New Ireland in 2008. The species may be threatened at its breeding grounds by invasive predators.
Raso (7km2) in the Cape Verde Islands. The species’ population appears to fluctuate in response to climactic conditions with recorded numbers veering from a low of 20-50 pairs to around 250 individuals. The species is especially vulnerable during periods of low abundance as the sex ratio becomes skewed 3:1 towards males. Primary threats are grought and nest predation.
criteria when assessing species abundance. The primary criteria include population size, geographic range and the projected population size over a given period. In order to full quantify their assessment, the organisation also takes other factors such as habitat quality, exploitation, pollution, competitive species, hybridisation and introduced species into account. This approach is designed to incorporate as many parameters and influencing factors as possible so that accurate assessments can be made. It is so highly regarded that several countries including Canada, Mexico and Australia use IUCN criteria as a basis for their national strategies when assigning conservation priorities to rare or endangered species.
status according to three parameters: population size (somewhere large and everywhere small), geographic range (wide and narrow) and habitat specificity (broad and restricted). There are of course limitations inherent in this system, not least the suggestion that applying these criteria is a subjective and context-dependent endeavour. but it appears to be broadly effective nonetheless. It is worth noting that several studies, each surveying disparate taxa, have been unable to classify any organisms in the category of narrow distribution, broad habitat specificity and small population size.
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