Is that necessary to have an assistant principal in horn section?

Sometimes when we go to an orchestral concert, we can see a five-horn section on the stage. Why do we need an assistant principal? Is it because of the lacking ability of the principal horn player? Of course not. The assistant principal sits next to the principal to assist the principal horn player by taking the first part periodically (it depends on the principal’s decision). So, the principal can take a short break to refresh himself/herself and play the solo passages. The assistant needs to fill in the missing parts when the principal is resting. If there are no assistants, the first horn player would bear much more workload when he/she performing is performing a symphony. So I think the assistant means a lot to the horn section.

In April, our school orchestra performed Mahler’s symphony no.9 and my role was the assistant third horn player (or called doubling the third horn part). I was asked to play in so many loud, tutti passages, and even the stopped notes to let the principal rest. So the third horn player can always feel comfortable to play his passages which even include a long solo note. Sometimes I needed to help counting the rest bars in order to let the principal to clear his slides and get ready for the next passage.

This video is the beginning of the Bruckner's symphony no.4, played by the Philadelphia Orchestra. We can see the cooperation of the principal horn, Jennifer Montone and the assistant.