Humans Triggered a Change in Rhinos

Rhinoceros horns have become smaller over the past 135 years, likely as a result of trophy hunting and poaching, according to researchers, who say the change might not be for the better. Rhinos continue to be targeted for their horns, used in traditional medicines and for trophies, resulting in strict protocols governing their use—so strict in fact that researchers have difficulty accessing horns for study, per a release. In this case, researchers used photos of 80 rhinos from all five species in profile view, taken between 1886 and 2018. (The 65 rhinos photographed in captivity were either formerly wild or the offspring of formerly wild animals.) Across all photographs, they measured the horn, head, and body, to get the horn length in proportion to body size.

They found the horns of all species—white, black, Indian, Javan, and Sumatran—have gradually decreased in size relative to body length over time. The change was most pronounced in critically endangered Sumatran rhinos, whose numbers have dwindled to the dozens as a result of poaching and other threats, per New Scientist. "Preferential hunting selection for individuals with larger horns or tusks resulted in individuals with smaller features surviving and reproducing more, passing on these traits to future generations, and resulting in an evolutionary change," reads the study published in People and Nature, per the Guardian. This trend has been observed among other animals, including elephants and wild sheep, but this is a first for rhinoceroses, according to the release.

"This is bad news for hunters obviously, but ... for rhinos as well, because if the hunters want the same amount of horn, they're going to have to shoot more rhinos," lead author Oscar Wilson of the University of Helsinki tells New Scientist. In the release, he adds that "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." On the bright side, researchers observed positive changes in human attitudes toward rhinos across centuries' worth of photographs and drawings, away from fearful perceptions used to justify killings up to 1950. Now, "we're viewing rhinos way more positively than we ever have," Wilson says. (More rhinoceros stories.)