If ever an animal required full use of its ears, it would be the rhino.
Rhino may be large, but as far as visual acuity goes they are analogous with the diminutive bat. Nevertheless, these short sighted behemoths have carved a niche for millions of years on the predator-dense African savannah. It makes sense, therefore, that rhino have survived by being extremely large (i.e., > 1000kg’s), have two rapier like horns and a skin thick enough to repel all but the most determined of predators.
Moreover, being blessed with exceptional hearing and an acute sense of smell has enabled them, for the most part, to ameliorate danger. Despite this, large predators such as lions and hyenas have been known to kill rhino calves and sub-adults. Significantly, modern humans have become extremely determined and adept rhino predators – reducing populations by over 96% within 100 years. This rapid population decline has created fragmented populations scattered across Africa that requires human intervention to move rhino between reserves thus preventing inbreeding and density dependent effects.
I was always astonished, therefore, to see odd looking black rhino, without any visible external ear pinnae, roaming within African game reserves (see photographs). Despite the obvious peculiarity in appearance, ‘earlessness’ in black rhino raises some pertinent questions for one of the world’s most critically endangered large mammals: what causes it, can they hear and survive / breed?
To ear or not to ear ? That is the question.
Missing ears in black rhino have been well documented over the last century. Ill-formed or missing ears can be common in some black rhinoceros populations (e.g. Hluhluwe-iMfolozi Park, South Africa: 3.7–4.0% of individuals; Mkhuze Game Reserve, South Africa: 7.1% of individuals). While the cause remains uncertain, it was initially speculated to be due to a genetic deformity from inbreeding effects. In fact, so strong was the belief that Addo National Park management went to the trouble of sterilising an adult male earless black rhino recently translocated from Hluhluwe-iMfolozi Park (HiP) in the 1970s. The overwhelming fear was that the male would transfer ‘earlessness’ genes to the Addo population – creating similarly odd looking black rhino and perhaps even affect tourism. Subsequent investigation showed that this might have been folly as records revealed that the male had not always been earless.
Since then, speculation has shifted more towards implicating non-human predators such as lion and spotted hyena. The idea is compelling as both lion and hyena have been observed attacking calves and grabbing at other rhino extremities. Opportunistic hyenas, in particular, have been seen grabbing at the ears while dragging the squealing calf from its dangerous but short sighted mother. Missing ears, might also be caused by predators but also caused by accident, disease or parasitic deformities. Ticks, for example, regularly cause deformities to the ears of cattle and some wild ungulates. The veins in ears are sometimes covered in ticks such that the blood flow is blocked and the tissue becomes necrotic and cause the ear to fold over. A phenomenon known as: ‘droopy ear’. Indeed, I once came across a black rhino calf in the field with such a droopy ear (see photograph). Thus, the cause of ear loss and droop might be different i.e., former predators and latter parasites.
While several records exist of earlessness in black rhino, there is very little information of its effect on rhino behavior and subsequent survival. HiP records do show that an adult earless male and female rhino lived to 30 years plus, a ripe old age for a wild black rhino, even though they had no ears for most of their lives.
My experiences in the field suggest that earlessness does appear to make rhino absolutely stone deaf. I once came across the long lived earless male mentioned previously. He was asleep in the long grass on a ridge with only one tree nearby. Not seeing him laying there I almost stood on him before running and climbing up the nearby tree. Not relishing an encounter with a temperamental black rhino in an open field I attempted to alert him acoustically. As much as I tried I could not alert him and after a while trusting that he couldn’t hear me I moved away. Close inspection of several anaesthetized earless black rhino appears to show that in some cases scar tissue from once present ear pinnae closes over the ear canal (see photograph). For other rhino the ear canal is open even though the pinnae are completely gone. It may be that in some cases black rhino are made deaf by scar tissue after a predator removes the ear.
Regardless of the cause, already short sighted and also deaf earless black rhino do appear to be able to survive and breed. I observed the long lived earless male on one occasion successfully mating with a female rhino –deaf he may be, but earlessness fortunately does not appear to be a hindrance with the opposite sex.
Earlessness – a warning?
Earlessness, however, might be a warning bell for other rarely considered problems for conservation managers. Should genetic deformities be the cause then it might indicate that some populations are suffering inbreeding effects and require augmentation. On the other hand, if predation on juveniles was the primary cause then earlessness might be indicative of potentially high calf mortality. Calf predation might be presently under-reported because both births and carcasses are rarely detected.
Thus, there is uncertainty about how important earlessness is in black rhinoceros population dynamics and clearly more research is needed. Earlessness does unfortunately have some negative effects. Although long-lived, the earless male was killed by elephants one night. Unknown at the time, I could hear from my hut the noise and commotion from the elephant herd as they trampled him. Unfortunately, he probably couldn’t hear them until it was too late.
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