Guwahati: A new study has revealed significant differences in the appearance and behavior of the two one-horned Asiatic rhinoceros species, challenging long-standing classifications and supporting a re-evaluation of their taxonomic status.
The study, led by zoologist Francesco Nardelli and paleontologist Kurt Heißig, highlights how millions of years of evolutionary pressures have shaped the distinct adaptations of the Indian (Rhinoceros unicornis) and Sundaic (Rhinoceros sondaicus) rhinoceroses. The critically endangered Sundaic rhinoceros has a slender skull, a broader and lower back of the head, and a shorter nose and teeth suited for browsing leaves. In contrast, the Indian rhinoceros has a more robust skull and taller teeth adapted for grazing on grasses.
The Sundaic rhinoceros, also known as Javan or Lesser one-horned rhinoceros, is presently classified as ‘Critically Endangered’ by the International Union for Conservation of Nature (IUCN). The presence of Rhinoceros sondaicus has been confirmed in northeastern India and Bangladesh, Myanmar, Thailand, Cambodia, Laos, Vietnam, Malaysia, and Sumatra and Java.
The Javan rhino faces a dire situation with an estimated 50 individuals remaining, significantly impacted by poaching and habitat loss, particularly in Ujung Kulon National Park. Since the 2011 death of the last Javan rhino (Rhinoceros sondaicus) in Vietnam, the species now only exists in one country, in one national park – Indonesia’s Ujung Kulon National Park (UKNP).
“The one-horned Asiatic rhinoceroses are examples of evolutionary histories driven by ecological pressures. Adaptations of large terrestrial mammals to various environments are linked to the diversity of food items they can consume, which is reflected in the variation of their dental and cranial morphologies. In rhinoceroses, these adaptations are identified in their teeth and head posture. Evidence on feeding habits aligns with the positioning of skulls within the morphospace, allowing us to infer the existence of distinct feeding types or ecomorphotypes,” the researchers write in their paper, published in the journal ZooKeys. The study has been published in ZooKeys and is available under an open-access license for further examination by scientists and policymakers.
The study provides new insights into the deep evolutionary divergence between the two species. The Indian rhinoceros is larger, with males weighing over 2,000 kg, while the Sundaic rhinoceros is smaller.
The Indian rhinoceros has deep skin folds, and both sexes have a single prominent horn, whereas the Sundaic rhinoceros has polygonal-patterned skin, and females lack a visible horn. The Indian rhinoceros has a broader skull and high-crowned teeth adapted for grazing, while the Sundaic rhinoceros has a slender head and mesodont teeth suited for browsing.
Behaviorally, the Indian rhino forms temporary social groups (crashes), whereas the Sundaic rhinoceros is solitary. In terms of habitat, the Indian rhino inhabits riverine grasslands in northern India and Nepal, while the Sundaic rhinoceros is now confined to Java’s Ujung Kulon peninsula.
Genetic assessments were not feasible in this study due to the lack of comparative genetic research. Studies on population dynamics and demographic history reveal patterns of genetic diversity, gene flow, and population structure within R. unicornis and E. sondaicus populations.
Fossil evidence confirms that these differences evolved independently over time, with each species adapting to its respective niche. The study emphasizes that understanding these distinctions is crucial for conservation.
Based on these findings, the scientists propose a more precise scientific name for the Sundaic rhino: Eurhinoceros sondaicus. “Recognizing Eurhinoceros sondaicus as a distinct genus provides a more accurate reflection of its evolutionary history and ecological specialization,” they assert. “This refined classification not only enhances our understanding of rhinoceros evolution but also provides a clearer framework for conservation planning, helping to tailor strategies for the protection of these critically endangered animals,” the scientists say.
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