I saw this on Animal Planet, so I came to see if something had been posted here. Finding nothing, I decided to post it myself.
link: Lioness Adopts 6 Oryx Calves
A little overview for those who don't feel like reading the article:
Oryx is usually a prey animal for lions. However, one lioness named Kamunyak (The Blessed One), decided to adopt an Oryx cub. She wouldn't let the cub out of her sight, and even refused to let it leave when it tried to escape (according to Animal Planet). She would let the calf's mother come and nurse the baby, but would then chase the mother off. Kamunyak's little calf was killed by a hungry male lion 16 days later.
She then went on to adopt and care for 5 more baby calves, always taking care of them until they either escape or died/were eaten.
And copy+paste from the site for some theories:
Kamunyak had been seen hunting warthogs and other small prey. During the first adoption when she remained with the Oryx for 16 days, she kept a 24 hour vigil over the Oryx. Despite being very thin and hungry, when she caught sight of prey she refused to let the Oryx out of her sight.
In February 2003 she was seen in a big fight with two females who are thought to come from the same territory. She was skulking around the edge of a giraffe carcass that the pride of 7 lions had been feeding on. There are several theories that have been proposed to explain this extra ordinary behavior of the female African lion.
1) The question has been raised whether this could have begun on a hunt with an unusually long game of cat and mouse, where after 24 hours she bonded with the calf. Sub-adults have been known to play with mongooses and other small species over a short period of time. However three weeks suggests that the cat and mouse game turned into something else. However, now on her 6th adoption, it seems that the lioness actively goes in search of Oryx calves to kidnap.
2) The Samburu people suggested Kamunyak is barren. However this seems unlikely considering that her body is responding to an overactive maternal drive. Plus she was so young and it is very difficult to tell whether a female is barren.
3) She could have a serious hormonal imbalance, which is triggering this abnormal behavior with another species. There have been records of lioness with huge cysts on their ovaries that affect their behavior, but perhaps not to this degree.
4) According to a scientist who has studied elephant reproduction, phantom pregnancies are quite common in feline species. It could be compared to domestic dogs that have phantom pregnancies and start lactating. If a lioness’ rank affects their endocrinology perhaps a phantom pregnancy is a possible explanation.
5) Kamunyak only adopted Oryx calves. Like all cats, lions have acute vision primed especially to pick up on movement. But they do not seem to be very good at individual recognition from a distance, and rely primarily on their sense of smell at close quarters to identify one another. Oryx calves are remarkably similar in colour to the tawny coat of an African lion, and it is possible that once the lioness had locked onto the smell of "cub" in the calf then it’s lack of a feline physique ceased to matter.
6) The park rangers suggested that she found the calf shortly after it was born and the smell of the amniotic sack on the calf’s body triggered some kind of maternal response.
I was pretty amazed that she took care of one little oryx, but I was astonished to find out that she adopted five more.