A lek is an aggregation of male animals gathered to engage in competitive displays and courtship rituals, known as lekking, to entice visiting females which are surveying prospective partners with which to mate. A lek can also indicate an available plot of space able to be utilized by displaying males to defend their own share of territory for the breeding season. A lekking species is characterised by male displays, strong female mate choice, and the conferring of indirect benefits to males and reduced costs to females. Although most prevalent among birds such as black grouse, lekking is also found in a wide range of vertebrates including some bony fish, amphibians, reptiles, and mammals, and arthropods including crustaceans and insects.
Greater sage-grouse at a lek, with multiple males displaying for the less conspicuous females
A posing western capercaillie
Sage grouse lek mating arena, in which each male, alpha-male (highest ranking), beta-male, gamma-male, etc., guards a territory of a few meters in size. The dominant males may each attract eight or more females. Higher-ranking individuals have larger personal space bubbles. Bird leks may have 10-200 individuals. A strict hierarchy accords the top-ranking males the most prestigious central territories. Females come to choose mates when the males' hierarchy is established, and preferentially mate with the dominants in the centre.
A group of three male great-tailed grackles trying to attract the attention of a receptive female
Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior. In other words, before an animal engages with a potential mate, they first evaluate various aspects of that mate which are indicative of quality—such as the resources or phenotypes they have—and evaluate whether or not those particular trait(s) are somehow beneficial to them. The evaluation will then incur a response of some sort.
Mate choice is highly visible in lek mating. Here, black grouse males gather in a quagmire and the females then arrive and observe the male before choosing one.
Ronald Fisher in 1913
The peacock tail in flight, the classic example of a Fisherian runaway
A male satin bowerbird guards its bower from rival males in the hopes of attracting females with its decorations.