Rhino Horns Have Gradually Decreased in Size over Past Century, Image-Based Study Shows

by johnsmith

Rhinos are among the most charismatic and well-known mammals on Earth, but they face extinction because of human activities. To learn how to save rhinos, scientists need as much information as possible about how their relationship with humans has changed over time. By analyzing thousands of artistic portrayals and photographs, a team of researchers from the University of Helsinki, the University of Cambridge and the Rhino Resource Center found that the relationship between humans and rhinos has been changing from the 16th century onwards. The authors found that African rhino species have become more commonly depicted in the images, compared to Asian rhino species. During the age of European imperialism (between the 16th and 20th centuries), rhinos were commonly portrayed as hunting trophies, but since the mid-20th century, they have been increasingly portrayed in a conservation context, reflecting a change in emphasis from a more to less consumptive relationship between humans and rhinos. Finally, the scientists found evidence for declining horn length over time across species, perhaps related to selective pressure of hunting.

This colorized photo shows American President Theodore Roosevelt standing behind the carcass of a rhinocerous. Image credit: U.S. Library of Congress.

Worldwide, there are five recognized living rhino species across four genera (family Rhinocerotidae).

These are the white rhino (Ceratotherium simum), the black rhino (Diceros bicornis), the Indian rhino (Rhinoceros unicornis), the Javan rhino (Rhinoceros sondaicus) and the Sumatran rhino (Dicerorhinus sumatrensis).

Three of the five species (the black rhino, Javan rhino and Sumatran rhino) lie within the top twelve Evolutionarily Distinct and Globally Endangered (EDGE) species, demonstrating their evolutionary distinctiveness, and all face conservation challenges as a result of human hunting for their horns, as well as habitat loss.

The white rhino (Near Threatened) is the only species not currently threatened according to the IUCN Red List, with the Indian rhino listed as Vulnerable, and the black, Javan and Sumatran rhinos all listed as Critically Endangered.

Modern poaching of rhinos is driven by high demands for horns, particularly in China and Vietnam, where the horn is used in traditional medicines, as a medium for carvings and as a financial investment in a valuable material.

This has resulted in rhino populations suffering precipitous declines. For example, one estimate suggests that 12,750 black rhinos were killed to provide the 36 tons of horn sold in Yemen between 1970 and 1986 alone. In Kenya, there were an estimated 20,000 black rhinos in 1991, but only 631 in 2014.

Hunting, combined with habitat loss, has already led to the extirpation of the Sumatran rhino in mainland Southeast Asia, and the species was declared extinct in Malaysia in 2019. Similarly, the northern white rhino, a subspecies of the white rhino, is now considered functionally extinct, with only two surviving females remaining.

In their new study, University of Helsinki researcher Oscar Wilson and colleagues used data from the Rhino Resource Center (RRC), an online repository curated by experts and holding a collection of over 4,000 rhino images and publications.

They assessed the changing representations and human interactions with rhinos using 3,158 images (1,531 pieces of artwork and 1,627 photographs).

They also measured the horns of 80 rhinos, photographed in profile view between 1886 and 2018.

Horn length was found to have decreased significantly in all species over the last century.

The researchers think rhino horns have become smaller over time due to intensive hunting.

“We were really excited that we could find evidence from photographs that rhino horns have become shorter over time,” said Dr. Wilson, formerly a researcher at the University of Cambridge.

“They’re probably one of the hardest things to work on in natural history because of the security concerns.”

“Rhinos evolved their horns for a reason — different species use them in different ways such as helping to grasp food or to defend against predators — so we think that having smaller horns will be detrimental to their survival.”

The scientists also saw a dramatic shift in human perceptions of rhinos around 1950, when the animals became the focus of conservation efforts rather than hunting.

“We found that we can use images from the last few centuries to visualise how human attitudes towards wildlife have changed, and how artists have influenced these views,” said Dr. Ed Turner, a researcher at the University of Cambridge.

“Many hundreds of photographs showing rhinos shot dead by hunters, taken in the 19th and early 20th century, are included in the collection.”

“These include a photograph of American President Theodore Roosevelt, taken in 1911, standing triumphantly over a black rhino he had just killed.”

Other early images show rhinos as huge, frightening animals chasing humans.

The authors think these images helped justify the hunting of these animals.

“The images suggest that there was very little effort to promote rhino conservation to the public before the 1950s,” they said.

“But after this the focus suddenly changed from hunting the animals to trying to keep them alive.”

“This shift coincides with the collapse of European empires, when African countries became independent and European hunters no longer had easy access to Africa for hunting.”

More recent images appear to reflect a growing awareness of the threats facing the natural world.

“For at least a few decades now there’s been much more of a focus on the conservation of rhinos — and this is reflected in the more recent images, which relate to their conservation in sanctuaries or their plight in the wild,” Dr. Wilson said.

A paper on the findings was published this week in the journal People and Nature.


Oscar E. Wilson et al. Image-based analyses from an online repository provide rich information on long-term changes in morphology and human perceptions of rhinos. People and Nature, published online October 31, 2022; doi: 10.1002/pan3.10406

Source link: https://www.sci.news/biology/shrinking-rhino-horns-11356.html

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