A lek is a gathering of males, of certain animal species, for the purposes of competitive mating display. Leks assemble before and during the breeding season, on a daily basis. The same group of males meet at a traditional place and take up the same individual positions on an arena, each occupying and defending a small territory or court. Intermittently or continuously, they spar individually with their neighbors or put on extravagant visual or aural displays (mating "dances" or gymnastics, plumage displays, vocal challenges, etc.).
The term derives from the Swedish lek, a noun which typically denotes pleasurable and less rule-bound games and activities ("play", as by children). Specifically, the etymology of the word "lek" is from 1871 and means to engage in courtship displays (of certain animals); probably from the Swedish att leka "to play".[3] A strict hierarchy accords the most desirable top-ranking males the most prestigious central territory, with ungraded and lesser aspirants ranged outside. Females come to these arenas to choose mates when the males' hierarchy has become established, and preferentially mate with the dominants in the centre.
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Two main types of lek are distinguished, classical leks and exploded leks. In classical leks, individuals gather within sight of each other to court and compete. Physical contest in these situations is frequent, and plays a major role in the mating rituals of certain shorebird and gamebird species. In Copadichromis eucinostomus, a type of fish, the males build sand castles. The lek member with the tallest mound of sand – almost a meter wide at the base – wins the females. These sandcastles take this ten centimeter (four inch) long animal two weeks to build. Peacocks also form leks to display their tails.[4]
In a classical lek there is no male parental care, with males aggregating at the leks site where the only resource for females are the males from which they select their mate. In exploded leks, some of these may not hold and males may not aggregate in a small area but hold larger territories within which the females may also be able to forage for food.[5]
Exploded leks rely on vocal signals. Male hammer-headed bats have a voice box that occupies more than half their body cavities; during breeding season, they gather for several hours at dawn and dusk to honk for females.[4] A famous example of exploded leks is the "booming" call of the Kakapo, the males of which position themselves many kilometers apart from one another to signal to potential mates.[6]
In some species, the males at the leks show a high-degree of relatedness, but this does not appear to be a trend across species.[7][8][9]
A lek when females come together to compete for males is called a mung (as in the yellow-spotted millipede).
In a lekking reproductive system, what male sexual characteristics can signal to females is limited, as the males provide no resources to females or parental care to their offspring.[10] This implies that females gain indirect benefits from her choice in the form of “good genes” for her offspring.[11] Hypothetically, in choosing a male who excels at courtship displays, females will gain genes for her offspring that will increase their survival or reproductive fitness.
Zahavi declared that male sexual characteristics only convey useful information to the females if these traits confer a handicap on the male.[12] Otherwise, males could simply cheat: if the courtship displays have a neutral effect on survival, males could all perform equally and it would signify nothing to the females. But if the courtship display is somehow deleterious to the male’s survival—such as increased predator risk or time and energy expenditure—it becomes a test by which females can assess male quality. Under the “handicap principle,” males who excel at the courtship displays prove that they are of better quality and genotype, as they have already withstood the costs to having these traits.[12]
Persistent female choice for particular male trait values should erode genetic variance in male traits and thereby remove the benefits of choice, yet choice persists.[13] The enigma of how additive genetic variation is maintained in the face of consistent female preference is named the “lek paradox.” This paradox can be somewhat alleviated by the occurrence of mutations introducing potential differences, as well as the possibility that traits of interest have more or less favorable recessive alleles.
One potential resolution to the lek paradox is Rowe and Houle’s theory of condition-dependent expression of male sexually selected traits. Similar to the handicap principle, Rowe and Houle argue that sexually selected traits depend on physical condition. Condition, in turn, summarizes a large number of genetic loci, including those involved in metabolism, muscular mass, nutrition, etc.[11] Rowe and Houle claim that condition dependence maintains genetic variation in the face of persistent female choice, as the male trait is correlated with abundant genetic variation in condition .[11] This is also called the "Genic Capture" hypothesis.[10]
Genetic variation in condition-dependent traits may be further maintained through mutations and environmental effects.[10] Genotypes may be more effective in developing condition dependent sexual characteristics in different environments, while mutations may be deleterious in one environment and advantageous in another.[10] Thus genetic variance remains in populations through gene flow across environments or generation overlap.
In an alternate but non-exclusionary hypothesis, Hamilton and Zuk proposed that successful development of sexually selected traits signal resistance to parasites.[14] Parasites can significantly stress their hosts so that they are unable to develop sexually selected traits as well as healthy males. According to this theory, a male who vigorously displays demonstrates that he has parasite resistant genes to the females. In support of this theory, Hamilton and Zuk found that male sexual ornaments were significantly correlated with levels of incidence of six blood diseases in North American passerine bird species. The Hamilton and Zuk model addresses the lek paradox, arguing that the cycles of co-adaptation between host and parasite resist a stable equilibrium point.[14] Hosts continue to evolve resistance to parasites and parasites continue to bypass resistant mechanisms, continuously generating genetic variation.[14] The genic capture and parasite resistance hypotheses could logically co-occur in the same population.
The term was originally used most commonly for Black Grouse (orrlek) and for Capercaillie (tjäderlek), and lekking behaviour is quite common in birds of this type, such as Sage Grouse, Prairie Chicken, and Sharp-tailed Grouse. However it is also shown by birds of other families, such as the Ruff, Great Snipe, Musk Ducks, Hermit hummingbirds, Manakins, birds of paradise, Screaming Pihas and the Kakapo, by some mammals such as the Ugandan Kob (a waterbuck), several species of fruit bat and the topi, and by some species of fish and even insects like the midge and the Ghost Moth. The rut of deer is also very similar. There is some dispute among ethologists as to whether the lekking behaviour shown by animals of widely different groups should really be treated as the same, and in particular whether similar selective pressures have led to their emergence.
In a few species (peacocks and the black grouse), leks are composed of brothers and half-brothers. The lower-ranking males gain some fitness benefit by passing their genes on through attracting mates for their brothers (larger leks attract more females). Peacocks recognize and will lek with their brothers, even if they have never met before.[4]