@ThylacineAlive Studies shows that many species of browse, from trees native to Africa or to other places, have very lower iron content. In every way, appropriate browse (non-toxic) would be better than grass/legume (hay). I don't wrote nothing about whether they give or not browse to the animals. In part you can be right, but It should take a lot of longer periods than 50-100 years in captivity, for black rhinos to evolve so they can consume diet rich on iron without any detrimental effects to their body.
I never said that studies didn't show that browse has a lower Iron content...
I also never said that the rhinos evolved differently, just pointed out that animal's change in captivity. Certain needs change, behavior can change, and diet especially can change.
I must correct myself about post that Chester is holder of largest population of black rhinos in Europe. I can't find about current situation, but in 2008, Port Lympne holded 16 black rhinos, Dvur Kralove 15, and Chester on the third place in Europe with 7 black rhinos. (http://eaza.portal.isis.org/activities/cp/yearbook20072008/36_Rhinoceros_TAG.pdf ). The population in Europe did not increased significantly from 79 black rhinos in 2008 (including 2 South-central in Frankfurt), to 84 (now just one left South-central at Frankfurt) in 2015, but that should be close to biological capacity of this population.
