After careful research I can only find two instance in which it is said that lions shed their mane but the information appears to be unsupported by good evidence and all the other instances of lions having thinner manes are due to environmental conditions such as the landscape in which they live and/or the ambient temperature.
Ousted from prides
Apparently, it was reported in a book published by National Geographic that male lions who were ousted from a pride by incoming lions suffered from a thinning mane in order to feminise them which made them less of a threat and accordingly which in turn allowed them to stick around the pride without causing disharmony. That sounds far-fetched to me and I don’t have any supporting evidence for it. There is nothing in the best books on the lion that I have which supports this information. For the time being I think we can ‘park’ it but nonetheless it is worth reporting for the sake of completeness.
Lost a battle
In the book Ghosts of Tsavo: Stalking the Mystery Lions of East Africa by Philip Caputo he writes, “Also, it [the mane] warns potential rivals that its owner hasn’t lost a battle in a long time, as male lions tend to shed their manes after a loss in combat. In short, a large mane is a sign of a lion’s ability to defend his pride, and a challenger who sees a pride leader adorned with one will think twice before going to war”. Once again there is nothing else that I can find which supports this hypothesis but this does not mean it is incorrect. As it is a fundamental part of lion life you’d have thought that it would be mentioned in other reference books about the lion.
Shedding the mane
As mentioned, the best books I have e.g. Wild Cats of the World by Mel and Fiona Sunquist do not refer to male lions shedding their manes. That doesn’t mean it doesn’t happen but it looks like it doesn’t.
What appears far more certain is that environmental conditions have an influence over the density and fullness of a lion’s mane. The natural order of events for the African or Asiatic lion is that they develop as good a mane as they can and one that is as dark as possible because it is more impressive for females and to other species of animal. The silhouette of a male lion sporting a full dark mane sends a strong signal to others as to who is in charge. Also, the lion’s mane protects the animal from bites in combat.
But if the lion lives in a landscape where there is dense thickets of commiphora bushes, which blanket the Kenyan region of Tsavo, the advantages of a mane are outweighed by the disadvantages as it becomes entangled in the bushes or perhaps it is torn out by the thorns.
Logically, it also seems that ambient temperature has an impact on the impressiveness of the lion mane. But this is not a case of shedding hair in the mane to make it thinner but more about not growing a thicker more dense mane because of heat stress. It is thought that bushy manes evolved to attract females in cooler climates where heat stress is not an issue. This is more a matter of lions deliberately delaying mane growth to cope with the harsh temperatures of the Tsavo region of Kenya, for instance. Zoo lions in America tend to grow thicker manes when the ambient temperature allowed it.
My conclusion is that (1) the experts are still studying the lion mane after all these years, and there are still some unknowns but (2) there is no hard evidence at the momment (Jan 2021) to suggest that lions shed their manes. It’s more a case of modifying the natural process of growing as full a mane as possible because of environmental conditions. Like everything natural it’s a balancing act to enhance survival.
SOME MORE ON THE LION’S MANE: