Guwahati: An analysis of artwork and photographs has revealed that rhino horns have been getting smaller and human attitudes towards rhinos have shifted from predation to conservation.
The findings have been published in People and Nature.
An international team of scientists, led by the University of Helsinki, has demonstrated that image databases can be used as an alternative to museum collections when studying long-term changes in human-nature interaction and as material in ecological and evolutionary research.
A joint analysis of artwork and photographs reveals how human attitudes towards rhinos have changed over time (from predation to conservation), and photo-analysis shows a reduction in the size of rhinoceros horns in all rhinoceros species studied, possibly driven by human hunting.
“While the rate of decline in horn length was found to be highest in Sumatran rhinos and lowest in white rhinos, the differences in slopes between the rhino species were negligible,” the study revealed.
Online image repositories can offer a freely accessible, information-rich and cost-effective alternative to museum collections for studying long-term changes in human interactions with nature and ecological and evolutionary change.
The Rhino Resource Center (RRC) is one example, curated by experts and holding a collection of more than 4,000 rhino images, including both artistic portrayals (1481–2021) and photographs (taken between 1862–2021), and representing a potentially valuable case study to investigate the utility of online image repositories for research into large vertebrates and, potentially, other well-recorded smaller taxa.
The scientists identified 1,273 images of Indian rhinos, 851 images of black rhinos, 603 images of white rhinos, 308 of Sumatran rhinos and 123 of Javan rhinos in the RRC dataset.
To assess whether differences in rhino morphology could be identified between species and whether rhino horn length changed over time, the scientists measured several features of morphology on photographs of adult rhinos.
“We only included photographs where the animal was side-on to the camera to facilitate more accurate and repeatable measurements. We excluded photos of any individuals where the horn had been cut, as horn length varies substantially between rhino species and therefore is an important species-specific morphological trait,” the scientists said in the study.
In total, 80 images were identified that were appropriate to use (22 black, 22 white, 18 Indian, 13 Sumatran and five Javan rhino images). Of the 80 images, three were rhinos in a sanctuary, 12 were wild rhinos and 65 were captive rhinos.
Early artwork had a higher representation of Indian rhinos, but the number of images of other species increased over time, particularly white rhinos since the mid-19th century.
Four of the five rhino species that survive today are threatened with extinction, despite their status as one of the most popular and recognisable groups of mammals. Their declines have been driven by hunting for their horns and loss of their habitats.
To best interpret the plight of rhinos, it is important to understand the history of their relationship with humans. Rhinos have been featured in European art for over 500 years, and this represents a valuable source of information for researchers. The RRC has gathered over 4,000 images of rhinos and is the largest database of its kind in the world.
Most images produced in the first 300-400 years since the discovery of rhinos featured Indian rhinos, likely due to the fame of individual rhinos, like the one featured in a woodcut by Albrecht Dürer in 1515 or Clara, who went on a European tour in the 18th century. Recent images have featured more white rhinos. This is the most common species in the wild and captivity today.
During the expansion of European empires, there was an increase in the number of total rhino images and the number showing hunting. The collapse of these empires and the increasing awareness of the threat-facing rhinos have been accompanied by a higher proportion of images painting rhinos in a more positive light and promoting conservation.
Rhinoceros horns are tough to study in museums; natural history collections containing rhinos are largely contained to former European colonies, and in the few European museums where rhino skeletons are found, their horns have been moved to protected facilities or destroyed. The reason for this is the high monetary value of horns among wildlife smugglers, which makes keeping horns a security risk.
The new method based on image analysis, therefore, offers an interesting alternative solution for studying the change in horn size from photographs alone. The researchers obtained evidence of the decrease in the size of the horns of rhinoceros species from the end of the 19th century to the present day by comparing the ratio of horn size to other body dimensions of individuals from photographs from different eras within each rhinoceros species.
According to doctoral researcher Oscar Wilson, the lead author of the study, this follows a pattern seen in other animals. Wilson says: “In other animals which are hunted for trophies, like elephants and wild sheep, the size of these trophies has got smaller over time as a result of natural selection. This suggests that the same thing might be happening with rhino horns.”
However, Wilson stresses that image-based analysis is not limited to rhinos. “Because they are so prominent in European art and the RRC was already well-curated, rhinos were a great place to start this investigation, but there’s no reason that image-based analyses couldn’t be applied to other animals. The same techniques would work very well for elephants or tapirs for example. The potential for the same types of resources to be developed for these animals is really exciting.”
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