Generally, present perfect indicates actions that started in the past and are still going on.
An alternative use of present perfect is for things that happened in the past but are still true or for things that have ongoing effects.
Some examples:(1) We have visited Torino in November and found it lovely, though cold.
This doesn't mean we are still visiting, nor that it is still November. It just means that at least once, we did visit in November, and that history is still (and will forever be) true.(2) I have attached the documents to this cover sheet.
This doesn't mean I am still stapling the papers as I write or as the sentence is read...it just means the documents are still attached.
In your OG example, "have been trimmed" should be interpreted in this way--the zoo employees may not still be trimming the horns, but the horns are still short.
Emily Sledge | Manhattan GMAT Instructor | St. Louis
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