The incident happened in one of the Zoos. According to one source, female lions attack and kill each other in the zoo, but as the animals are hungry, they attacked and killed one of the female lions. According to the other source, following their instinct of territorial defense the three female lions kept in this enclosure killed the intruding animal. Can you give me scientific explanation which source is true? Thank you

A scientific explanation for animals killing animals?

It's their nature. The strong attack and kill the weak.

Please don't make vague references. What are your sources?

Ya sort of answered your own question. I'm not sure who really knows what those lions were thinking, scientists do try. Find out yourself, whether lions are ever cannibalistic. Cannibalism seems to be rare. Territorialism is very plausible. Maybe, they were stressed, psychotic, or didn't like the color of her knickers. Fitness is part of the equation. Learn what that means.