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What a fascinating experiment! It reminded of a text we read in biology class concerning the physiology and genetics of animal behavior. Male zebra finches are the only ones to produce courtship songs. Very early in a male songbird's life, for example, certain cells in its brain produces estrogen, a hormone which affects target neurons in an area of the developing brain called the higher vocal center. The presence of this hormone leads to a complex series of biochemical changes which result in the production of more neurons in areas of the brain that control singing. With female zebra finches, their brains do not produce estrogen and, as a result, the number of neurons in the higher vocal center declines over time. Experiments have shown that when these female birds are given estrogen, they produce more neurons in the HVC. However, they do not sing later on in life unless they were also treated with androgens. I found this example very ironic as male zebra finches had higher estrogen levels than the females and the females could only sing with the addition of androgens (a male hormone). As Caitlin stated above, different numbers of neurons (in my case) play a large part in observed behavior differences between sexes.