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For species in which females have no associations with males except for
brief periods preceding copulation, mating at leks probably enhances a
female's discrimination of optimal mates, ones advantageous either for
increasing the viability of her offspring or perhaps for reducing the
immediate risks of copulation.
The behavioral mechanisms by which this discrimination occurs are not yet
well established for any species and might well differ among species or
even among populations of the same species.
These mechanisms might involve interactions among females, including
learned traditions, as well as interactions among males and between
females and males. Sexual selection could result from indirect, as well
as direct, consequences of female preferences.
The costs of display and ornamentation for males at leks are indicated by
higher mortality of adult than younger males and greater energy
expenditure by successful males. Advantages to females of choosing mates
at leks remain mostly undocumented.
A general feature of mating at leks is a later onset of successful
reproduction by males than by females. Age-related mating success of
males on leks continues, at least in some cases, beyond the age at which
fully developed morphology is attained and is, again at least in some
cases, related to territorial succession or succession in dominance.
Female preferences for mating at particular locations could provide the
basis for either of these forms of mating succession and result indirectly
in mating with older or more vigorous males.
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Strong directional selection as a result of female preferences, either
direct or indirect, tends to deplete genetic variation for the preferred
male traits. This situation in turn reduces selection for the female
preferences based on discriminations among conspecific males, the
so-called paradox of the lek.
Transient selection for resistence to parasites could maintain genetic
variance for male traits, but evidence necessary to establish this
possibility for lekking species is incomplete. The paradox of the lek
does not arise when there are fixed costs of indiscriminate mating by
females, such as immediate risks during copulation or dysgenic
hybridization.
These generalities apply equally well to any species with brief
interactions between the sexes and no associations of males with resources
used by females. In particular, they apply to species with dispersed,
rather than aggregated, display sites.
In some taxa, the evolution of dispersed versus aggregated display appears
related to risks of predation in different habitats; in others, this
variation has no apparent explanation at present.
All of these species in which copulation occurs at display sites lacking
resources, whether aggregated as leks or dispersed, isolate clearly the
issues of evolution by sexual selection and thus merit our continuing
attention.
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