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Sunday, 4 October 2020   /   published in Helen Lunn, Horn, Nature, Rhinos

Why Rhinos Need Their Horns

Charles Darwin speculated that horns on animals were a way of advertising fitness for mating and the larger the horn the more attractive a male would be to a potential female partner. He was not able to explain why within one species there was no consistency in the size of horns nor why horns had evolved in the first place, but as usual with Darwin he was not incorrect in his speculations. He was particularly interested in the presence and absence of horns on members of one species of dung beetle because the absence of a horn posed questions about their real purpose. No creature intent on reproduction of its species is going to deliberately hobble its chances of a mate by not developing a sexy and alluring horn or feature that is the clincher in any mating game, so the obvious question was why was it that amongst members of the same species and sex some had horns while others didn’t?

As it happens horns or the lack thereof in dung beetles are more a function of food supply in their developmental phases than anything else, but there are compensatory factors which means that even if a small non horned and a large horned beetle are in play over a female, possession of a large horn does not by any means guarantee that the horned beetle’s genes will be the winners. Beetles however are not rhinos and horns on rhinos are primarily a defense weapon.  A female rhino does not simply accept a male as her mate based on the size of his horn, however the horn of a male white rhino does indirectly have a sexual consequence, since he will use it to defend and keep his territory. A large and intimidating horn goes a long way to seeing off any potential rivals, so when a female accepts his advances and allows him to court her she is indirectly responding to his capacity to defend his territory as a result of him having used his horn to ward off rivals.

There is some difference between the fighting tactics of black and white male rhinos but it is the latter which are being described here as black rhinos are far more argumentative and less inclined to back off.  They are like Vikings who fight to the death, whereas white rhinos are more like Sumo wrestlers, a lot of posturing and only an occasional total wipe out. Even though male rhinos fight to establish territorial dominance, it is not unusual for a dominant male to allow a subordinate male to share his territory. If however there is a challenge to his dominance either from a neighbouring male who intrudes on his territory or the subordinate male makes a move to become the dominant one then its battle on and things can get serious. Rhinos however are huge, averagely 2,300 kg so their fighting technique is rather like that of sumo wrestlers where a great deal of bulk and posturing is key to the brief moments of serious horn to horn encounters.

 Much of the time the dominant male can make his point just with the horn to horn stare but it all depends on the response of the challenger.  If the latter is intent on a fight then the dominant male will adopt a defensive snarl threat while roaring and shrieking. If such a battle proceeds to a fight the males fence with their horns against each other. They only proceed to a cut and thrust parrying  when the fight is really serious at which point they are silent but when they do engage after pushing horns against each other they deliver blows with upward jabbing movements. A fight of that nature can take a very long time and one of them will come off the worse for wear and be obliged to leave. Occasionally these fights can be to the death but it is rare amongst white rhinos for things to get that bad, usually the loser is bloodied and gored but able to live and tell the story. If a subordinate male has won such a battle then he will take over the territory and frequently the loser will stay on in the role of a subordinate male where he will be tolerated as long as he does not make any attempt to use his charms on a female of interest to the dominant male. The basic deal is the subordinate male gets board and lodging and a chance to live his life as one of the world’s largest lawnmowers in return for knowing his place.

 The horn itself is primarily made up of keratin which is found in all human hair and finger nails, but there is also a dense deposit of calcium and melanin at the core of the horn with the calcium serving to strengthen and the melanin protecting it from UV rays. The curve in the horn which is generally back towards the head of the rhino occurs because the keratin in the front grows faster than that at the back of the horn. The horn starts to appear on rhinos at about one year of age and looks like a little knob, but it is enough of a horn for poachers to kill them for. While female rhinos do not battle over territory they need their horn to protect their babies through the two to three years that they spend with them after birth. This is the main period when a rhino is vulnerable to predation. Predators do kill baby rhinos but a horned and angry mother is a formidable creature to take on which is why rhino for dinner is not the most obvious choice of a small pack of lions.

 Knowing how critical a horn is to a rhino in its social life merely highlights the insanity of depriving them of their horns to ensure their survival. The horns do grow back, but in the period of the growing a lot can happen and the delicate social balance which rhinos have evolved over millennia is affected. Male horns grow back twice as fast as female horns underscoring the need on the part of males to have a horn to work out dominance and territory. This never ending territorial battle is a product of the fact that on average a third of all males do not own territory, so the battle for dominance and the right to mate is ongoing. It will only become more frantic as animals have less territory in which to survive.

While it might seem the loss of a horn is a small price to pay for the maintenance of a species, the fact is that removing horns changes social dynamics. There do not appear to be any studies of how males establish dominance in the absence of horns, but losing a critical weapon which seems to lie at the heart of so much social interaction between rhinos, is a questionable price to pay for the survival of a species which established itself on this earth in the Pleistocene era which started 2.58 million years ago. Harvesting horns for survival and protection is like a parent  binding their child’s feet in order to prevent them from going out and getting into danger. It makes no sense. But allowing the extinction of a species that has existed on the planet for over a million years doesn’t make any sense either.