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Some birds have a penis and it becomes erect, but it ues a lymphatic system of enlargement, while mammals use blood:

http://www.scientificamerican.com/article.cfm?id=ostrich-penis-clears-evolutionary-myst...

Complete thread:

Evolution Early animal fossil David Turell 2011-12-12, 02:50 AM
- Evolution: Lungfish and walking David Turell 2011-12-13, 03:26 PM
  - Evolution: Ostrich penis; odd convergence David Turell 2011-12-13, 08:56 PM
    - Evolution: Minor speciation occurs David Turell 2011-12-20, 01:11 AM
      - Evolution: Minor speciation occurs dhw 2011-12-20, 12:56 PM
      - Evolution: Stasis or not? David Turell 2011-12-21, 03:33 PM
        
        Evolution:Pre-Cambrian gene explosion David Turell 2011-12-25, 11:00 PM