What is the plural of "penis"?

Today was the first Engineering Management tutorial. 4 groups of students, each group from a different country, presented the key information about the local corporate culture they would like to tell foreigners going to their country, and the information they would like to tell their compatriots coming to Germany. We had presentations about Malaysia, Nigeria, Sweden and China. And I witnessed 4 very unique presentation styles that made the session all the more exciting.

Apart from learning about the corporate culture of 4 countries, I stumbled upon the fact that I am not sure about the plural of the word "penis". The story goes like this...

At the end of the last presentation from the Chinese girl, I asked her during the Q&A session: "Is it important for me to be able to eat a tiger penis if I want to strengthen business relationships in China?" And everyone was incredulous, including the Chinese! I repeated the question, and then again but to no avail. As the clock showed well past 12 and students started peeking in through the doors, the professor suggested that I have a one-on-one with the girl to sort things out.

So I approached the girl. And I tried to explain. And surprise surprise...her English was not very good. After she asked me a couple of times: "tiger PEANUTS?", I began to wonder whether she knows what a penis is. And that was big trouble, because I don't know how to say "penis" in German or Chinese. The only options I had was to say "dick" or point towards mine!!! (That reminded me of a guy who once questioned the habit of pointing towards one's bare wrist when asking about the time from somebody else. He retorted: "Do you point towards your dick when you ask where the toilet is?". OK let's cut the crap...) But both seemed rather inappropriate as I was in a class with almost 30 students. Then one of the Chinese guys helped me and explained my question to her. Finally...she understood the question. But she was completely unaware of the fact that tiger PENISES are eaten in China. The guy also told me that they are more of a side-dish than a main course. No big deal, I can imagine that myself. If over 1 billion Chinese had been eating tiger PENISES as main course, tigers would have become extinct ages ago...

Anyway, to sum everything up, here is the webpage that contains the plural of some interesting words, including "penis".
<http://www.straightdope.com/mailbag/mplurals.html>