What large mammal has tough, thick, light-grey skin, and has two large horns coming out of its nose? After reading that description, ideally one’s answer should be a rhinoceros. In most cases, we always picture a rhinoceros to have two horns and light-grey skin, but often forget about the rhinoceros who have darker colored skin and only one horn. There were once hundreds of rhinoceros species however, only five species exist today. The African Black and White rhinos, and the Sumatran rhinos have two horns, but the Indian and Javan rhinos from Asia only have one.
Why is it that some rhinoceros have two horns while others only have one? I believe that the second horn certain rhinoceros have first came along as a beneficial mutation, and then eventually due to natural selection and allopatric speciation, certain rhinoceros now have two horns instead of one. The rhinos that existed about 60 million years ago looked more like today’s horses than the typical rhinoceros we are familiar with. This would explain their unusual diet and rather fast speeds. The weight of a rhino ranges from 750 pounds to 8000 pounds.
The expectation should be that for such a massive animal, they would be rather slow; however, that is not the case at all. Similar to a horse, Rhinos can be rather fast as well. When charging at an animal, the Rhino could reach speeds of 56 km/h. With such violent and defensive ways, it is assumed that rhinos are meat eaters; however, that is not the case either. Similar to horses, rhinos are herbivores. They like to eat grass, foliage of trees, and bushes. Eating habits and speed are behavioral characteristics from their ancestors that have remained the same.
There are even some physical characteristics that have remained the same like being an odd-toed ungulate. While so many characteristics were carried over from the rhinoceros’ ancestors, there are also many new changes that have evolved; the most significant one is the extra horn. I believe that the extra horn came along as a beneficial mutation. When two ancestors of the rhinoceros with a single horn mated, there could have been a mistake in the replication of their DNA and resulted in the extra horn growing above the offspring’s nose.
According to Lamarck’s second principle: The Inheritance of Acquired Characteristics, individuals could pass on to their offspring characteristics they had acquired during their lives. His theory may be incorrect but it helps explain that a species evolves in response to its environment, and becomes better adapted to that environment. The rhinoceros may have started with only one horn on its nose but because of the environment it lived in and the strong need for an extra horn for protection, over time the horn it once had evolved into two horns that we see now.
Since rhinoceros are herbivores, they would never need to hunt for prey. Lions who are carnivores hunt regularly for their food, have many experiences fighting and know how to defend themselves. In a situation where a lion and a rhino battles, the rhino would have no way of defending itself for it has less experience fighting, less power than the lion, and less flexibility. This is why there is such a strong need for an extra horn. The horn would act as protection and by being able to charge at an animal with both horns, there would be an increase in the rhino’s chance of survival.
According to Charles Darwin’s theory of evolution by natural selection, nature favors the reproductive success of some individuals within a population over others. In this case, nature would select against those with only one horn in this geographic area for those are the rhinoceros who are less strong defensively, and will not be able to survive in that particular area. So eventually, sexual selection will factor into the evolution of the rhino and females would only want to mate with rhinoceros who have two horns, resulting in offspring that will have a better chance of surviving and battling against more powerful animals in their habitat.
It is known that the Earth didn’t always look this way. The Earth was once a super continent and overtime, it has broken up into separate pieces of land which are the continents and islands that we know now. It is very likely that the separation of land also separated the population of rhinoceros. This allopatric speciation separated the rhinoceros into two geographically isolated populations. Those who were separated to where we now call “Africa” had the beneficial mutation of an extra horn.
This mutation however, was only shared among themselves because once populations are physically separated, they can no longer exchange genetic information. Rhinoceros who went off to Asia did not develop this mutation, and even if they did, it wouldn’t have been very beneficial to them in any way. While African rhinos tend to feed low to the ground, Asian rhinos usually feed off leaves higher above the ground. If the Asian rhinos did have this mutation, it could act as a barrier from getting food.
Allopatric speciation not only affected the rhinoceros’ physical characteristics but behavioral characteristics as well. Studies show that in general, African rhinos are more aggressive than Asian rhinos. Therefore the different geographic areas these rhinos live in affect both physical and behavioral characteristics of each species. In conclusion, the reason some rhinoceros have two horns while others have one is because of their geographic location. While rhinos in Africa developed this mutation, rhinos in Asia did not.
Allopatric speciation also did not allow the exchange of this new gene between the population of the two geographic areas. Hence, why most Asian rhinos have one horn and African rhinos have two. African rhinos require the extra horn while rhinos in Asia can make do with just their bottom teeth for defensive purposes. Rhinos in Asia won’t ever come across the same animals the Rhinos in Africa do, so the different species don’t share the same need of the horn. Could it be that one day all species of rhinoceros will require that extra horn?
Courtney from Study Moose
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