Creatures Wiki

A Lek is the name for the dance performed by a Norn when it has a high sex drive. Similar behavior is commonly exhibited in a variety of real-life animals; there is more information at Wikipedia.

Steve Grand mentioned this in his book, Creation: Life and How to Make It (pp. 222-3):

One of the other things I made capable of affecting the female sex drive was a pheromone produced by males - a sort of virtual musky scent. I made them produce this substance whenever they chose to perform a specific behaviour in the presence of a females - a little mating dance, or lek, to be precise.
The idea was that males who learn to do the dance should successfully impregnate more females than those who do not, so their genes should therefore become more apparent over many generations. But this effect is rather subtle, and its effects are hard to discern. To this day I don't know whether the mating ritual has ever been witnessed and recorded by any of the people who keep and study Creatures.

At least one person, Markham Carroll, appears to have noticed this, as he mentions in his introduction to his Norn Posters.