Rhinoceros, derived from the Greek ‘rhino’ (nose) and ‘ceros’ (horn), are the second largest land mammals on earth. There are five extant rhino species under the family Rhinocerotidae, two African species - black rhino and white rhino - and three Asian species - Javan rhino, Sumatran rhino, and greater one horned rhino. These mega-herbivores, considered to be amongst the most evolutionarily distinctive animals alive today and commonly described as a ‘keystone species’, once numbered more than 500,000, inhabiting the vast savannas, shrublands and forest landscapes of Africa and Asia.
However, since the emergence of rampant anthropogenic pressures, primarily rhino poaching and habitat loss, rhino populations have suffered extensive declines. Today, rhino population estimates stand at around 27,000, and three species - the black rhino, Javan rhino and Sumatran rhino - are classified as “Critically Endangered” under the IUCN Red List. The white rhino is classified as “Near Threatened”, whilst the greater one horned Rhino is listed as “Vulnerable”.
Due to their sheer size and herbivorous diet, rhinoceroses play a critical role in maintaining the health and vitality of their surrounding ecosystems. With each rhinoceros eating 25 to 50 kilograms of vegetation per day, these gentle giants control the overgrowth of plants, create pathways through shrubland, and help with seed dispersal and fertilisation through the 20 odd kilograms of dung they deposit across their territorial range.
So if a rhinoceros eats up to 50 kilograms of plants and excretes 20 to 30 kilograms of dung on a daily basis, exactly how much does a rhino weigh and how did they grow to be so large on a vegetarian diet?
What the different rhino species weigh...
The biggest of all five species of rhino is the white rhinoceros, of which there are two subspecies: the northern white rhino and the southern white rhino.
Also known as the square-lipped rhino, the white rhinoceros is grazer and feeds primarily on African short and long grasses, using its square mouth and broad, Iips to crop grass as efficiently as possible in the savanna. It has a longer, larger head than the black rhino, as well as an incredibly muscular neck, since the animal spends the majority of the day with its head lowered close to the ground.
At birth, a baby white rhino weighs approximately 40 to 60 kilograms. Adult females then grow to about 1,800 to 2,000 kilograms whereas adult males typically weigh 1,800 to 2,500 kilograms; yet larger individuals can exceed 3,500 kilograms, with the biggest white rhino ever recorded weighing 4,600 kilograms. Both males and females measure between 1.5 and two metres in height. Despite their enormous size, white rhino are agile and can run up to 40 kilometres per hour for short distances.
Although only two northern white rhino remain at Helping Rhinos’ partner project Ol Pejeta Conservancy in Kenya, both of which are female, the southern white rhino is currently the most populous rhinoceros subspecies with approximately 16,000 individuals roaming the savannahs of Kenya, Namibia, Zimbabwe, Zambia and South Africa.
The smaller of the two African rhino species is the black rhino, of which there are three extant subspecies: the South-central black rhino; the Eastern black rhino; the South-western black rhino. The Western black rhino was declared extinct in 2011.
Also known as the hook-lipped rhino, the black rhinoceros is a browser and they acquire the majority of their sustenance from picking leaves, branches, twigs, tree bark, and fruits off trees and shrubs. They have been known to display a certain fondness for certain vegetation, such as acacia shrubs. It is due to this particular feeding behaviour that the black rhino developed its distinctive lip shape, as their pointed, prehensile upper lip allows them to grasp branches, hold foliage and pick fruits with ease. Their bite produces a neat, angular mark on woody vegetation, similar to pruning shears.
At birth, baby black rhinos weigh between 30 and 45 kilograms. Females then grow to approximately 900 kilograms, whereas adult males typically weigh between 1,350 to 1,600 kilograms, with large individuals reaching up to 1,800 kilograms. Both males and females stand at around 1.5 to 1.75 metres at the shoulder. Black rhinos have a smaller hump on the back of their necks, as well as a smaller head, when compared to the white rhino as their feeding habits require less neck strength. Due to their smaller size, however, black rhinos can run faster than both white and Indian rhinos, observed running at speeds of 55 kilometres per hour with an impressive ability to change directions quickly.
With a habitat range that once spanned sub-Saharan Africa, black rhinos suffered the most drastic population decline of all rhinoceros species, losing 96% of their numbers between 1970 and 1993. Nevertheless, concentrated anti-poaching efforts and strategic translocations to protected areas and private game reserves in historic rhino habitats has allowed black rhino populations to gradually recover over time, with a current population estimate 6,487. These are distributed across Namibia, Kenya, Tanzania and Southern Africa.
The Javan rhino is the second largest Asian species of rhino, and is comprised of three subspecies: the Indonesian Javan rhino, the Indian Javan rhino (declared extinct), and the Vietnamese Javan rhino (declared extinct in 2011). Previously found across northeast India and the Sunderbans, mainland Southeast Asia and on the islands of Sumatra and Java, the species is currently represented by 76 Indonesian Javan rhino found in the Ujung Kulon National Park.
Although the Javan rhino has been observed consuming a wide variety of different plant species, numbering in the hundreds, approximately 40% of the animal’s daily food intake comes from a handful of preferred types of plants: leaves, young shoots and twigs. Javan rhino prefer to feed in unshaded areas, where the quality and quantity of food is generally higher.
Few Javan rhinos have been weighed in the wild, but the species is believed to be comparable in size to the African black rhino. Both males and females stand at approximately 1.4 to 1.7 metres tall, and weigh between 900 and 2,300 kilograms. In fact, data gathered from Ujung Kulon National Park and from skeletons housed in museums indicate that Javan female rhinos might be slightly larger than males. In Vietnam, prior to the species being declared extinct from the Cat Loc part of the Cat Tien National Park in 2011, the remaining resident Javan rhino were unusually small, measuring 1.2 metres and weighing less than 800 kilograms.
The Javan rhino suffered devastating population declines early on, restricted to Ujung Kulon by 1930 with a few small, isolated populations in Vietnam, Laos and Cambodia. Knowledge on the behavioural and ecological background of the species is therefore scarce. Nevertheless, strict protections awarded to the Javan rhino, including translocations to new secure habitats, appear to be promising, with conservationists aiming to recover at least 2,000 to 2,500 individuals to ensure the species’ long-term survival.
The Sumatran rhino is the smallest species of rhino, and is comprised of three subspecies: the Western Sumatran rhino, the Northern Sumatran rhino (declared extinct in the wild), and the Eastern Sumatran rhino (declared extinct in the wild). Previously found across Southeast Asia, from Bhutan and northeast India to southern China, Cambodia, Indonesia and Thailand, the remaining 34 to 47 Sumatran rhino inhabit a few key sites on the islands of Sumatra and Borneo.
Also known as the Asian two-horned rhinoceros, the Sumatran rhino is a browser and consumes a wide variety of tropical forest vegetation. They primarily feed on young tips of plants growing from the ground, leaves from sapling trees, fruits, and secondary vegetation resulting from landslides and fallen trees. Despite their relatively smaller size, Sumatran rhino consume 50 to 60 kilograms of vegetation on a daily basis.
Sumatran rhino weigh approximately 40 to 50 kilograms at birth, and calves subsequently gain one to two kilograms daily. Adult Sumatran rhinos typically reach a height of 1.3 to 1.5 metres and weigh in at around 500 to 960 kilograms. The skin of the Sumatran rhino is thinner than that of other species, roughly 10 to 16 millimetres thick, and is relatively soft. Due to their smaller size, Sumatran rhino are fast and agile, climbing mountains, steep slopes and balancing on riverbanks with relative ease.
Despite the fact that Sumatran rhino populations remain in existence, these remnant rhino live in small, fragmented, non-viable populations with limited breeding opportunities. The species has been a primary target of poachers for decades and has suffered extensive habitat loss throughout its range, two pressures which remain relatively unabated. As a result, they have been classified as a critically endangered species since 1996.
GREATER ONE HORNED Rhino
The greater one horned rhino is the second largest species of rhino, and is native to the grasslands, savannas and shrublands of India and Nepal.
Also known as the Indian rhinoceros, the greater one-horned rhino feeds seasonally on a variety of vegetation, including grasses, leaves, fruits, tree branches, as well as aquatic plants. They eat approximately 1% of their own body weight on a daily basis, choosing between 183 different plant species throughout the year, and despite their enormous size, they are excellent swimmers and can dive and feed on submerged vegetation.
Greater one-horned rhino calves can weigh a whopping 58 to 70 kilograms at birth. Adult females then grow to around 1,900 kilograms, whilst males typically reach a weight of 2,500 to 3,200 kilograms, with the largest Greater one-horned rhino weighing in at 3,800 kilograms. Both males and females stand at approximately 1.75 to two metres tall. Contributing to the weight of the Greater one-horned rhino is 4 centimetre-thick, folded skin, similar to armour plating, a 2.5 centimetre-thick layer of subcutaneous fat, as well as a single front horn which weighs up to 3 kilograms.
The species suffered drastic population declines prior to the 20th century, reaching the brink of extinction with a population of approximately 200 rhino. However, due to concentrated conservation efforts in key range states, the current population stands at around 4,014 rhino with subpopulations across eight protected areas in India and four in Nepal.
Why Are Rhinos So Heavy?
So now we know how much each species of rhino weigh, the next big question is: how did the rhinoceros become so heavy, especially given the strict herbivorous diet they follow?
Rhinoceros belong to a taxonomic order known as Perissodactyla, or odd-toed ungulates (hoofed mammals), since they have only three weight-bearing toes (or hooves). Interestingly, this is not a feature shared by the other largest land mammals on earth: hippos are even-toed ungulates (Artiodactyla), and elephants lack hooves altogether. Although scientists do not fully understand the evolutionary reasons behind the divergence of even-toed and odd-toed ungulates, the fact that most large hoofed mammals are herbivorous and share certain physiological characteristics has helped shed light on how these species were able to survive over time.
Herbivorous species evolved independently numerous times across the world, as they adapted to what sustenance was available in their habitats at the time. Herbivorous diets make sense in terms of natural selection, as there is typically an abundance of vegetation in the natural environment and this limits the need for competition with carnivorous or omnivorous species for food. Despite the time and effort it takes to consume and digest vegetation, the vast amount that is readily available means that herbivores can consume a large amount of calories per day relatively easily.
Over time, these herbivorous species developed specialised digestive systems to process quantities of vegetation and extract as many nutrients as possible, resulting in large animals with large stomachs and intestines.
One such type of digestion is hind-gut fermentation, meaning that micro-organisms found in the last part of the animal’s intestines break down ingestible parts of their food. Odd-toed ungulates (such as rhino) and elephants are hind-gut fermenters. This digestive process allows animals to feed on small quantities of low-quality vegetation throughout the day and therefore survive in conditions where the nutritional value of sustenance is low (such as many parts of Africa). They are able to extract more nutrition from their feed and process it more rapidly, meaning that they can consume vegetation in bulk. The largest mega-herbivores in history have all been hind-gut fermenters, and studies have shown that the fastest growth in body mass over evolutionary time occurred in hind-gut fermenters.
Despite the large body mass of many herbivorous species, they still found themselves vulnerable to predation from carnivores, who possessed large canines, claws and talons for attacking prey with ease. Since almost all ungulates are herbivores, scientists believed that hooves were developed in order to escape such predators. Hoofed mammals use digitigrade locomotion (walking or running on their toes/hooves), which allows them to run faster and for longer distances since it uses less muscles and expends less energy. Larger species, such as rhinoceros and hippos, also developed shorter, thicker bones to support their weight whilst maintaining fast running speeds.
As mentioned, baby rhino are already incredibly heavy when born, ranging between 30 and 70 kilograms depending on the species. Why is it that baby rhinos weigh so much?
Rhinoceros have one of the longest gestation periods of any land mammal, second to elephants, with pregnancies typically lasting between 15 and 16 months (although white rhinos have been known to carry young for up to 18 months). While larger animals tend to have longer gestation periods due to the size of their offspring, there are also advantages to lengthier pregnancies.
Long gestation periods allow offspring to become well developed before leaving the mother’s body. Newborns that are able to walk and run shortly after birth have greater chances of survival, as they are less vulnerable to predation. This is particularly true for herbivorous species, which tend to produce very mature offspring at birth, as they must outrun predators from an early age. Newborn rhino are able to stand within an hour of birth and start nursing within the first five hours.
With a lifespan of 35 to 40 years in the wild, rhino also develop at a much faster rate than humans, with female rhino becoming sexually mature by the age of three and a half years and ready to mate by the age of five. Infant rhino drink approximately 20 to 30 litres of milk and gain roughly one to two kilograms on a daily basis. Although they begin grazing on foliage after two to five months, mothers will typically nurse their offspring for 12 to 20 months. As calves are highly vulnerable to predation, they will remain with their mothers until the age of three.
Skin & Horns
Apart from the general bodily mass of rhinos, there are two other factors that contribute to the animal’s immense weight - their skin and their horns.
Rhinoceros have the thickest skin of any land mammal, acting as a protective armour with a highly specialised structure and material properties. Reaching a thickness of up to five centimetres, the dorsolateral skin of a rhino is made of dense layers of collagen fibres, making it elastic and tense - properties that make rhino skin somewhat similar in strength to tendons. However, despite their resilience to attacks from predators or fellow rhino, the skin of rhinoceroses is vulnerable to sunburn. They spend the majority of their day wallowing in mud to keep cool and protect their skin from sunburn and insect bites.
Although the size of a rhino’s horn differs between each species, subspecies and individual rhino, these keratin-rich appendages usually measure between 25 to 100 centimetres, with the longest horn ever recorded reaching a length of 150 centimetres, and weigh between one to five kilograms. A rhino’s horn continues to grow throughout their life (growing by up to seven centimetres per year), and is used by the rhino for defence, maternal care, and foraging behaviours. Unfortunately, rhino horn is a primary target for poachers in both Asia and Africa as some believe that it has powerful medicinal properties, despite the lack of scientific evidence supporting these claims.
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