Lek Mating - The Lek Paradox

The Lek Paradox

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.

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.

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.

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.

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.