John Ball Zoo says a pygmy hippo attacked and killed a sitatunga this week.
A sitatunga is a medium-sized antelope.
Officials say the incident happened during a controlled introduction between the two animals.
The animals had been visually introduced over the last month and were expected to live in the same habitat.
Zoo officials say the hippo attacked the sitatunga very suddenly.
Staff were unable to resuscitate the sitatunga.
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