Ankarana Special Reserve in northern Madagascar was created in 1956. It is a small, partially vegetated plateau composed of 150-million-year-old middle Jurassic limestone. With an average annual rainfall of about 2,000 millimetres (79 in), the underlying rocks are susceptible to erosion, thereby producing caves and underground rivers—a karst topography. The rugged relief and the dense vegetation have helped protect the region from human intrusion.
The plateau slopes gently to the east, but on the west it ends abrubtly in the "Wall of Ankarana", a sheer cliff that extends 25 kilometres (16 mi) north to south, and rises as high as 280 metres (920 ft). To the south, the limestone mass breaks up into separate spires known as tower karst. In the center of the plateau, seismic activity and eons of rainfall have dissolved the limestone away in deep gorges, and sometimes redeposited it in ribbons of flowstone. In places where the calcific upper layers have been completely eroded, the harder base rock has been etched into channels and ridges known as tsingy.
The largest sinkhole in the Ankarana karst region, Mangily sinkhole, measures up to 700 meters across and 140 meters deep with a volume of 25 million cubic meters. The area is sacred to the Antankarana people, who have historically taken refuge from encroaching enemy armies in its caves and other natural rock shelters.
Beginning in the 1960s, expatriate Frenchman Jean Duflos (who married locally and changed his name to Jean Radofilao) did a huge amount of exploration of the cave systems and subterranean rivers of the Massif, much of it on his own or with visiting speleologists. A total of about 100 kilometres (62 mi) of cave passages within the massif have been mapped. One of the most accessible caves, La Grotte d'Andrafiabe, alone comprises at least 8.035 kilometres (4.993 mi) of horizonatal passages. Indeed the Massif contains the longest cave systems in Madagascar, and probably in the whole of Africa.
Expeditions that first began cataloguing the animals and plants of the Special Reserve created around the Ankarana Massif in the 1980s are described in Dr Jane Wilson-Howarth's travel narrative Lemurs of the Lost World and in the scientific press. Discoveries included unexpected sub-fossil remains of large extinct lemurs and surviving but previously undescribed species of blind fish, shrimps and other invertebrates. Several expedition members contributed photos to an illustrated introductory guide to Madagascar which features the Crocodile Caves.
During the 1986 expedition, Phil Chapman and Jean-Elie Randriamasy collated a bird list for the reserve and recorded 65 species from 32 families representing nearly a third of all bird species that breed in Madagascar. They also noted one interesting aspect of behaviour. They reported that there was an unusual strategy used by many of the small insect-eating songbirds. Species such as the Paradise Flycatcher (Terpsiphone mutata), the Common Jery (Neomixis tenella), the Greenbuls (Phyllastrephus zosterops and P. madagascariensis), the Bulbul (Hypsipetes madagascariensis), the Sunbird (Nectarina souimanga) and the Vagas (Lepopterus madagascarinus and Xenopirostris polleni) foraged together in mixed bands. Within each band different species seemed to specialise in where and how they searched out their insect prey. Some species concentrated on the trunk and branches of trees, some on slender boughs, others searched beneath the leaves. By acting together in this way they probably increased foraging efficiency as each species could catch others’ escaped prey. They were also safer from attack by predators, as the group as a whole was more likely to spot approaching danger.
The Ankarana Reserve is an important refuge for significant populations of the crowned lemur (Eulemur coronatus) Sanford's brown lemur (Eulemur sanfordi) and other mammal species. The following lemurs are also recorded from the area: northern sportive lemur (Lepilemur septentrionalis), brown mouse lemur (Microcebus rufus), fat-tailed dwarf lemur (Cheirogalus medius), fork-marked lemur (Phaner furcifer), eastern woolly lemur (Avahi laniger), Perrier's sifaka (Propithicus diadema perrieri), aye-aye (Daubentonia madagascariensis) and the western lesser bamboo lemur (Hapalemur griseus occidentalis). In addition subfossils of the following lemurs have been found at Ankarana: greater bamboo lemur (Prolemur simus), indri (Indri indri), the sloth lemurs (Babakotia radofilai), Mesopropithicus dolichobrachion and Palaeopropithicus cf ingens plus Pachylemur sp., the huge Megaladapis cf madagascariensis/grandidieri, and the baboon lemur Archaeolemur sp.
The southern entrance of the park is situated in Mahamasina at the Route nationale 6 some 108 km south-west of Antsiranana and 29 km north-east of Ambilombe.