Q: does the mac require antivirus software?

Only if you're using Windows on it or frequently exchange files with computers running Windows.

Viruses aren't the only type of malware. Use common sense and keep Mac OS X up to date.

(138518)

You posted your question in the iMac (PowerPC) forum. Those Macs were discontinued a very long time ago.

Assuming you are using an iMac that was built in the past decade and is running a recent version of OS X, protection against malicious alteration is already incorporated. Install nothing else. Read Effective defenses against malware and other threats.