It's difficult to describe a rhino without mention of its horn. This iconic bit of facial artillery is one of the animal's most distinctive features, but new research from the University of Cambridge suggests that, in the last century, rhino horns have been gradually shrinking possibly in response to the pressures of decades of hunting by humans.

Researchers from Cambridge believe that rhino horns are gradually getting shorter due to intensive hunting.

To arrive at their discovery, researchers examined a century's worth of photos held by the Rhino Resource Centre – an online image repository that includes representations of all five species of rhino: white, black, Indian, Javan and Sumatran. Given the strict security restrictions placed on actual rhino horns, the team turned to historical records instead. They scrutinised thousands of images taken between 1886 and 2018 and selected those that depicted rhinos in profile. A collection of 80 photos were used to carefully calculate horn size relative to the animals' bodies.

The results of the study reveal that horn length has decreased significantly in all rhino species over the last century. "We were really excited that we could find evidence from photographs that rhino horns have become shorter over time," Oscar Wilson, a former researcher at the University of Cambridge’s Department of Zoology and first author of the report, said in a press release.

Theodore Roosevelt photographed in 1911 standing above a black rhino he shot and killed.

The researchers believe that horns have been shrinking due to intensive hunting. "In the case of rhinos, people basically have always wanted the largest horn," Wilson explains. As more and more larger-horned animals get removed from the landscape, individuals with smaller features are left to reproduce, passing on their traits to future generations. A similar trend has already been documented in other species like elephants and wild sheep. 

Wilson does point out that the majority of the rhinos used in the study were photographed in captivity, which may also have an impact on horn size. "While we acknowledge that the captivity status of these rhinos may impact our results, the high proportion of wild-born or first-generation captive-born rhinos until the end of the 20th century, and the fact that both Sumatran rhinos and Javan rhinos are challenging to maintain and breed in captivity (meaning that almost all photographs of these species are wild-born), mean that our conclusions on relative horn length are likely to be valid," the researchers write in the journal People and Nature.

But does size really matter? According to Wilson: "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." Just how much of an impact horn shrinkage might have on daily rhino life remains unclear. 

Many rhinos in southern Africa have been dehorned in order to minimise the risk of poaching,. Although horns do grow back, this practice raises questions around the impacts of horn size for these animals.

Across much of southern Africa, governments and conservation authorities are actively dehorning rhinos in an effort to reduce the payload for poachers and ultimately deter illegal hunters from targeting these animals. Although it's a logistically complicated and financially costly undertaking, dehorning has proven effective in reducing rhino poaching without posing any major risk to the animals involved. A recent study conducted in Namibia found no evidence to suggest that dehorning has any effect on the age at which rhinos reproduce, the time period between births, the sex ratio of calves, calf survival, cause of death and lifespan.

The researchers admit that their sample size was fairly small, and more collaborative research needs to be conducted to fully measure the possible impacts of dehorning, but if rhinos are able to live quite happily without horns, then smaller ones may not be that big of a deal. However, if we are only just beginning to explore long-term changes in rhino physiology as a result of human influence, then it is feasible that these animals have already adapted their behaviour to survive in altered landscapes in ways we may not yet fully understand.

Top header image: Matthew Rogers, Flickr