A lek is an aggregation of males that gather to engage in competitive displays that may entice visiting females who are surveying prospective partners for copulation. Leks are commonly formed before or during the breeding season. A lekking species is defined by the following characteristics: male displays, strong female mate choice, and the conferring of male indirect benefits. Although lekking is most prevalent among avian species, lekking behavior is found in a variety of animals such as insects, amphibians, and mammals.
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 1861 and refers to the area where "matrimonially affairs" were carried out (of certain animals); probably elliptically from the Swedish lekställe "mating ground".
Lekking species 
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, Guianan Cock-of-the-rock, Musk Ducks, Hermit hummingbirds, Manakins, birds of paradise, Screaming Pihas and the Kakapo. Some mammals such as the Ugandan Kob (a waterbuck), several species of fruit bat and the topi participate in lekking along with and some species of fish and even insects like the midge and the Ghost Moth.
Lekking behavior 
There are two types of lekking arrangements: classical and exploded. In the classic lekking system, male territories are in visual and auditory range of their neighbors. In an exploded lek, males are further away from one another than they would be in a classic lek. Males in an exploded lek are outside of visual range of one another, but they stay within earshot. Exploded lek territories are much more expansive than classic systems and are known to exhibit more variation. 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.
Lek territories of different taxa are stable and do not vary in terms of size and location. Males often return a to the same mating sites because of female fidelity. It has been shown that avian females such as the Black Grouse, and Great Snipe are faithful to males and not mating sites. Successful males congregate in the same area as the previous breeding season because it is familiar to them, while females return to reunite with said males. It has been observed that they will not return to a mating site if their male partner is not present. Another possible explanation for lek stability may result from male hierarchies within a lek. In manakins, subordinate betas may inherit an alpha’s display site, increasing the chances of female visitation. Rank may also contribute to the stability of lek size, as lower ranking males may congregate to achieve a perceived optimal size as a way to attract females.
Costs and Benefits of Lekking 
The main benefit for both sexes is mating success. For males the costs stem from females’ preferences. The traits that are selected for, may be energetically costly to maintain and may cause increased predation. For example, increased vocalization rate caused a decrease in the mass of male Great Snipe. Another cost would be male competition, as females prefer victorious males. Great Snipes regularly fight to display dominance or defend their territory. Aggressive male black grouse are preferred over non-aggressive males and when the males fight they tear feathers from each other tails. At first glance, it would seem that females receive no direct benefits because these males are only contributing genes to the offspring. However, lekking actually reduces the cost of female searching because the congregating of males makes mate selection easier. Females do not have to travel as far, since they are able to evaluate and compare multiple males within the same vicinity. This may also help reduce the amount of time a female may be vulnerable to predators. Female Hyperolius marmoratus under predatory pressure consistently chose leks near their release sites and high male calling rates reduced female search time.
Female Mating Preferences 
A meta analysis of 27 species found that qualities such as, lekking size, male display rate, and the rate of male aggression exhibit positive correlation with male success rates. There was also a positive correlation found between attendance, magnitude of exaggerated traits, age, frequencies of fights, and mating success. This female preference leads to mating skew, with some males being more successful at copulating with females. The variation in mating success is quite large in lek mating systems with 70-80 percent of matings being attributed to only 10%-20% of the males present.
The lek paradox 
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. 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.
The basis of the lek paradox is continuous genetic variation in spite of strong female preference for certain traits. There are two conditions in which the lek paradox arises, the first is that males contribute only genes and the second is that female preference does not affect fecundity. Female selection should lead to directional selection, which would result in a greater prevalence for that trait. Stronger selection should lead to impaired survival because there will be a decrease in genetic variance since more offspring will have similar traits, which is also known as “runaway selection”. Lekking species do not exhibit runaway selection.
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. This implies that females gain indirect benefits from her choice in the form of “good genes” for her offspring. 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. 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. Resolutions have been formed to explain why strong female mate choice does not lead to runaway selection. The handicap principle describes how costly male ornaments provide females with information about the male’s inheritable fitness. The handicap principle may be a resolution to the lek paradox, for if females select for the condition of male ornaments, than their offspring will have better fitness.
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. 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 . This is also called the "genic capture" hypothesis, which describes how a significant amount of the genome is involved in shaping the traits that are sexually selected for. There are two criteria in the genic capture hypothesis: the first is that sexually selected traits are dependent upon condition and the second is that general condition is attributable to high genetic variance.
Genetic variation in condition-dependent traits may be further maintained through mutations and environmental effects. 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. Thus genetic variance remains in populations through gene flow across environments or generation overlap. According to the genic capture hypothesis, female selection does not deplete the genetic variance, as sexual selection operates on condition dependence traits, thereby accumulating genetic variance within the selected for trait. Therefore, females are actually selecting for high genetic variance.
In an alternate but non-exclusionary hypothesis, Hamilton and Zuk proposed that successful development of sexually selected traits signal resistance to parasites. 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. Hosts continue to evolve resistance to parasites and parasites continue to bypass resistant mechanisms, continuously generating genetic variation. The genic capture and parasite resistance hypotheses could logically co-occur in the same population.
One resolution to the lek paradox involves female preferences and how preference alone will not cause a drastic enough directional selection to diminish the genetic variance in fitness. Another conclusion is that the preferred trait is not naturally selected for or against and the trait is maintained because it infers increased attractiveness to the male. Thus, it may be possible that there is no paradox.
Lek Evolution 
Hotshot Hypothesis 
There have been several hypotheses proposed as to why males cluster into leks. The hotshot hypothesis is the only model that attributes males as the driving force behind aggregation. The hotshot model hypothesizes that attractive males, known as the “hotshots,” garner both female and male attention. Females go to the hotshots because they are attracted to these males. Other males form leks around these hotshots as a way to lure females away from the hotshot. A manipulative experiment using the little bustard, Tetrax tetrax, was done to test the various lek evolution models. The experiment involved varying the size and sex ratio of leks using decoys. To test whether or not the presence of a hotshot determined lek formation, a hotshot little bustard decoy was placed within a lek. After the fake hotshot was added to the lek, both male and female visitation to the lek increased.
Hotspot Model 
The hotspot model considers the female density to be the catalyst for the clustering of males. This model predicts that leks will form where females tend to reside as a way to increase female interaction. Female manakin traffic has been observed to be concentrated around leks, bathing sites and fruiting areas, with males aggregated near the most visited fruiting resources. The hotspot model also predicts that lek size is dependent upon the amount of females inhabiting a patch of land. To test if the number of females affects lek formation, a group of female little bustard decoys were added to a lek. The presence of these female decoys did not have an effect on lek size.
Blackhole Model 
The blackhole model proposes that females have neither a preference for size or type of male, but rather that females tend to be mobile and will mate wherever leks may be located. This model predicts that female mobility is a response to male harassment. This prediction is difficult to test, but there was a negative correlation found between male aggressiveness and female visitation in the little bustard population. Evidence supporting the black hole model is mainly found in ungulates.
Kin Selection 
An alternative hypothesis for lekking is kin selection. The kin selection model assumes that males within a lek are related to one another. As females rarely mate outside of leks, it is advantageous for males to form leks. Although not all males within a lek will mate with a female, the unmated males will still receive fitness benefits. Kin selection explains that related males congregate to form leks, as a way to attract females and increase inclusive fitness. In some species, the males at the leks show a high-degree of relatedness, but this does not apply as a rule to lek-forming species in general. 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.
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Further reading 
- Snow, B.K. & Snow, D.W. (1979). "The Ochre-bellied Flycatcher and the Evolution of Lek Behavior." Condor 81(3)