Fossils of Rhinoceros unicornis appear in the Middle Pleistocene. In the Pleistocene, the Rhinoceros genus ranged throughout Southeast Asia and South Asia, with specimens located on Sri Lanka. Into the Holocene, some rhinoceros lived as far west as Gujarat and Pakistan until as recently as 3,200 years ago.
The Indian and Javan Rhinoceros, the only members of the genus Rhinoceros, first appear in the fossil record in Asia around 1.6 million–3.3 million years ago. Molecular estimates, however, suggest the species may have diverged much earlier, around 11.7 million years ago. Although belonging to the type genus, the Indian and Javan Rhinoceros are not believed to be closely related to other rhino species. Different studies have hypothesized that they may be closely related to the extinct Gaindetherium or Punjabitherium. A detailed cladistic analysis of the Rhinocerotidae placed Rhinoceros and the extinct Punjabitherium in a clade with Dicerorhinus, the Sumatran Rhino. Other studies have suggested the Sumatran Rhinoceros is more closely related to the two African species. The Sumatran Rhino may have diverged from the other Asian rhinos as far back as 15 million years ago.
In size it is equal to that of the white rhino in Africa. Not including the white rhino, it is the largest of all rhinos. Fully grown males are larger than females in the wild, weighing from 2200- 3000 kg (4,800 - 6,600 lb). Female Indian rhinos weigh about 1600 kg. The Indian Rhino is from 1.7 to 2 m tall (5.7 to 6.7 feet) and can be up to 4m (13 ft) long. The record-sized specimen of this rhino was approximately 3500 kg.
The Great One-Horned Rhinoceros has a single horn; this is present in both males and females, but not on newborn young. The horn, like human fingernails, is pure keratin and starts to show after about 6 years. In most adults the horn reaches a length of about 25 centimeters, but have been recorded up to 57.2 centimeters in length. The nasal horn curves backwards from the nose. Its horn is naturally black. In captive animals, the horn is frequently worn down to a thick knob.
This prehistoric-looking rhinoceros has thick, silver-brown skin which becomes pinkish near the large skin folds that cover its body. Males develop thick neck-folds. Its upper legs and shoulders are covered in wart-like bumps. It has very little body hair, aside from eyelashes, ear-fringes and tail-brush.
In captivity, four are known to have lived over 40 years, the oldest living to be 47.
These rhinos live in tall grasslands and riverine forests, but due to habitat loss they have been forced into more cultivated land. They are mostly solitary creatures, with the exception of mothers and calves and breeding pairs, although they sometimes congregate at bathing areas. They have home ranges, the home ranges of males being usually 2-8 square kilometers in size, and overlapping each other. Dominant males tolerate males passing through their territory except when they are in mating season, when dangerous fights break out. They are active at night and early morning. They spend the middle of the day wallowing in lakes, rivers, ponds, and puddles to cool down. They are extremely good swimmers. Over 10 distinct vocalizations have been recorded.
Indian rhinos have few natural enemies, except for tigers. Tigers sometimes kill unguarded calves, but adult rhinos are less vulnerable due to their size. Humans are the only other animal threat, hunting the rhinoceros primarily for sport or for the use of its horn. Mynahs and egrets both eat invertebrates from the rhino's skin and around its feet. Tabanus flies, a type of horse-fly are known to bite rhinos. The rhinos are also vulnerable to diseases spread by parasites such as leeches, ticks, and nematodes. Anthrax and the blood-disease septicemia are known to occur.
Feeding occurs during the morning and evening. The rhino uses its prehensile lip to grasp grass stems, bend the stem down, bite off the top, and then eat the grass. With very tall grasses or saplings, the rhino will often walk over the plant, with its legs on both sides, using the weight of its body to bush the end of the plant to the level of the mouth. Mothers also use this technique to make food edible for their calves. They drink for a minute or two at a time, often imbibing water filled with rhinoceros urine.
The Indian Rhinoceros makes a wide variety of vocalizations. At least ten distinct vocalizations have been identified: snorting, honking, bleating, roaring, squeak-panting, moo-grunting, shrieking, groaning, rumbling and humphing. In addition to noises, the rhino uses olfactory communication. Adult males urinate backwards, as far as 3–4 meters behind them, often in response to being disturbed by observers. Like all rhinos, the Indian Rhinoceros often defecates near other large dung piles. The Indian Rhino has pedal scent glands which are used to mark their presence at these rhino latrines. Males have been observed walking with their heads to the ground as if sniffing, presumably following the scent of females.
In aggregations, Indian Rhinos are often friendly. They will often greet each other by waving or bobbing their heads, mounting flanks, nuzzling noses, or licking. Rhinos will playfully spar, run around, and play with twigs in their mouth. Adult males are the primary instigators in fights. Fights between dominant males are the most common cause of rhino mortality, and males are also very aggressive toward females during courtship. Males will chase females over long distances and even attack them face-to-face. Unlike African Rhinos, the Indian Rhino fights with its incisors, rather than its horns.
In zoos, females may breed as young as four, but in the wild females are usually six before breeding begins. The higher age in the wild may reflect that females need to be large enough to avoid being killed by the aggressive males. The Indian Rhinoceros has a very lengthy gestation period of around 15.7 months. The interval between births ranges from 34–51 months. In captivity, males may breed at five years. But in the wild, dominant males do the breeding, and rhinos do not not attain dominance until they are older and larger. In one five-year field study, only one rhino who achieved mating success was estimated to be younger than 15.
On the former abundance of the species, Thomas C. Jerdon wrote in 1874 in the Mammals of India:
This rhino is a major success of conservation. Only 100 remained in the early 1900s and now their population has increased but even so they are still endangered.
The Indian rhino is illegally poached for its horn, which some cultures in East Asia believe has healing and potency powers and therefore is used for Traditional Chinese Medicine and other Oriental medicines. Habitat loss is another threat. Fewer than 2500 individuals remain in the wild, and the species is endangered.
The Indian and Nepalese governments have taken major steps toward Indian Rhinoceros conservation with the help of the World Wildlife Fund (WWF). The Kaziranga National Park and Manas National Park in Assam, Pobitora reserve forest in Assam (having the highest Indian rhino density in the world), Orang National park of Assam, Laokhowa reserve forest of Assam having a very small population and Royal Chitwan National Park in Nepal are homes for this endangered animal.
Demographic trends of Rhinoceros unicornis. Sources : here