Fascinating subject hey?! I didn't know about this problem until sometime in 1999 and have been trying to keep track of the results since then.
Over a period of three years, 1995 to 1998, the Pilanesberg National Park lost approximately 40 white rhino to abhorrent elephant behaviour. Pilanesberg was one of many Parks that received young elephants for re-location from the Kruger National Park as part of the Kruger National ParkÃ¢â‚¬â„¢s annual culling operations. It was when the young bulls left the herds that problems arose. Young elephant bulls usually leave the breeding herds at about the age of 12 Ã¢â‚¬â€œ 15 years old and join up with older bulls that then guide them through their "adolescent" life. Pilanesberg did not have older bulls and hence the young bulls became delinquents and their hormones caused them to run amok within the white rhino population. The older bulls suppress a phase in the young elephants called musth (heightened aggression associated with testosterone and other hormones). The young Pilanesberg bulls were coming into musth too early, from about the age of 15 years old, and did not know how to properly court the breeding herds and were hence shunned from the breeding herds. With no where else to go the bulls turned to the white rhinos and getting nowhere with them, started to take out their frustration on the rhinos, the result being 40 dead white rhinos.
The same source has (a lot) more here
: this site contains links to photographs of rhinos killed/injured by elephants which may be considered not suited for younger children or people with sensitive stomachs.
Unfortunately Pilanesberg wasn't the only park to encounter the same problem, according to this BBC news topic
dated February 2000.
Aggressive young orphaned elephants are reported to have killed 36 rhinos, including rare black ones, in a game park in eastern South Africa.
According to conservationists, the young elephants have been provoking confrontations with the rhinos since they were introduced to Hluhluwe-Umfolozi Park in KwaZulu-Natal.
The elephants were orphaned when their parents were culled in the early 1990s in an effort to control the elephant population in Kruger National Park. As they have matured, so they have become more aggressive.
Attacks on rhinos have been growing over the past two years, with 13 killed, including two black rhino, in the last five months of 1999, South African newspapers report. A park ranger said he had witnessed an elephant knocking a rhino over, trampling it and driving a tusk through its chest.
Conservation vet Dave Cooper said: "There was a spate of killings, and it was as if they were purposeful. The rhinos were ripped to pieces." He said that elephant and rhino routinely clash in nature "but this sort of behaviour, when elephant actively go out and chase rhino, is totally abnormal".
Fellow conservationist Tony Conway said similarly aggressive behaviour had also been seen in Pilanesberg National Park in Northwest Province - another home for the Kruger Park orphaned elephants.
However, the killings at Pilanesberg stopped when six adult elephant bulls were introduced to the park. The young ones' behaviour patterns returned to normal under their influence.
Officials at Hluhluwe-Umfolozi Park have asked Kruger Park to send it 10 adult bulls in the hope that their presence will have the same effect on the young elephants there.
The park's top attractions are its rhino - both the white or square-lipped rhino and the rarer black or hooked-lipped rhino. There are only about 1,000 black rhino left in South Africa.