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. [1] It can also refer to a space used 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, mammals, and arthropods including crustaceans and insects.
A classical lek consists of male territories in visual and auditory range of each other. An exploded lek, as seen in the kākāpō (the owl parrot), has more widely separated territories, but still in auditory range. Lekking is associated with an apparent paradox: strong sexual selection by females for specific male traits ought to erode genetic diversity by Fisherian runaway, but diversity is maintained and runaway does not occur. Many attempts have been made to explain it away, [2] [3] [4] [5] but the paradox remains.
The term derives from the Swedish lek ( [leːk] ), a noun which typically denotes pleasurable and less rule-bound games and activities ("play", as by children), or from the Swedish verb "leka", to play. Written English use of lek dates from the late 1860s and early 1870s. Llewelyn Lloyd's The Game birds and wild fowl of Sweden and Norway (1867) introduces it (capitalised and in single quotes, as 'Lek') explicitly as a Swedish term. [6]
Lekking was originally described in the Tetraonidae (grouse, boldface in cladogram), in particular the black grouse (Swedish: "orrlek") and capercaillie (Swedish: "tjäderlek"), but it is widely distributed phylogenetically among other birds, and in many other animal groups within the vertebrates and the arthropods, as shown in the cladogram. [1] [7] [8]
The presence of a group name means that some species in that group lek; groups with no lekking members are not shown.
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There are two types of lekking arrangement: classical and exploded. In the classic lekking system, male territories are in visual and auditory range of their neighbours. In an exploded lek, males are further away from one another than they would be in a classical lek. Males in an exploded lek are outside visual range of one another, but they stay within earshot. [35] Exploded lek territories are much larger than classic systems and more variable in size. [36] A well-known example of exploded leks is the "booming" call of the kākāpō, the males of which position themselves many kilometres apart from one another to signal to potential mates. [37]
Lek territories of different taxa are stable and do not vary in terms of size and location. [38] Males often return to the same mating sites because of female fidelity. [39] Avian females such as the black grouse and great snipe are faithful to males and not to mating sites. [40] Successful males congregate in the same area as the previous breeding season because it is familiar to them, while females return to reunite with their males. Females do not return to a mating site if their male partner is not present. [40] Another possible explanation for lek stability is from male hierarchies within a lek. In some species of manakin, subordinate betas may inherit an alpha's display site, increasing the chances of female visitation. [40] Rank may also contribute to the stability of lek size, as lower ranking males may congregate to achieve a perceived optimal size to attract females. [41]
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. [1] A positive correlation was also found between attendance, magnitude of exaggerated traits, age, frequency of fights, and mating success. [1] 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% of matings being attributed to only 10–20% of the males present. [42]
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 snipes. [43] Other costs can derive from male combat. For example, male great snipes regularly fight to display dominance or defend their territory, with females preferring victorious males. [43] Aggressive male black grouse are preferred over non-aggressive males and when the males fight they tear feathers from each other's tails. [44] Lekking is associated with sexual dimorphism across a range of bird taxa. [7]
At first glance, it may seem that females receive no direct benefits from lekking, since the males are only contributing genes to the offspring in the absence of parental care or other benefits. [45] However, lekking reduces the cost of female searching because the congregating of males makes mate selection easier. [46] Females do not have to travel as far, since they are able to evaluate and compare multiple males within the same vicinity. Further, having the males in one place may reduce the amount of time a female is vulnerable to predators. When under predatory pressure, female marbled reed frogs consistently choose leks near their release sites; high male calling rates were observed to reduce female search time. [47]
Since sexual selection by females for specific male trait values should erode genetic diversity, the maintenance of genetic variation in lekking species constitutes a paradox in evolutionary biology. Many attempts have been made to explain it away, but the paradox remains. [48] 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. [49] Female choice should lead to directional runaway selection, resulting in a greater prevalence for the selected traits. Stronger selection should lead to impaired survival, as it decreases genetic variance and ensures that more offspring have similar traits. [50] However, 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. [51] This implies that a female gains indirect benefits from her choice in the form of "good genes" for her offspring. [52]
Amotz Zahavi argued that male sexual characteristics only convey useful information to the females if these traits confer a handicap on the male. [53] [54] Zahavi's handicap principle may offer a resolution to the lek paradox, for if females select for the condition of male ornaments, then their offspring have better fitness. Another potential resolution to the lek paradox is Rowe and Houle's theory that sexually selected traits depend on physical condition, which might in turn, summarize many genetic loci. [52] This is the genic capture hypothesis, which describes how a significant amount of the genome is involved in shaping the traits that are sexually selected. [51] There are two assumptions 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. [52] In addition, W. D. Hamilton and Marlene Zuk proposed that sexually selected traits might signal resistance to parasites. [55] One resolution to the lek paradox involves female preferences and how preference alone does not cause a drastic enough directional selection to diminish the genetic variance in fitness. [56] Another conclusion is that the preferred trait is not naturally selected for or against and the trait is maintained because it implies increased attractiveness to the male. [49]
Several possible mechanisms have been proposed as to why males cluster into leks, including the hotshot, hotspot, black hole, kin selection, and predation protection hypotheses, as described below.
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 hotshots, garner both female and male attention. [2] 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. [4] 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, giving weight to the hypothesis. [4]
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. [3] 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. [3] The hotspot model also predicts that lek size is dependent upon the number of females inhabiting a patch of land. [2] 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, tending to refute the hypothesis. [4]
The blackhole model proposes that females have a preference for neither size nor type of male, but rather that females tend to be mobile and mate wherever leks may be located. [4] This model predicts that female mobility is a response to male harassment. [57] This prediction is difficult to test, but there was a negative correlation found between male aggressiveness and female visitation in the little bustard population, suggesting that the model might be correct. [4] Evidence supporting the black hole model is mainly found in ungulates. [39]
An alternative hypothesis for lekking is kin selection, which 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. [5] Although not all males within a lek mate with a female, the unmated males still receive fitness benefits. Kin selection explains that related males congregate to form leks, as a way to attract females and increase inclusive fitness. [40] 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. [58] [59] [60] In a few species such as peacocks and 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, since larger leks attract more females. Peacocks recognize and lek with their brothers, even if they have never met before. [61]
Another hypothesis is predation protection, or the idea that there is a reduction in individual predation risk in a larger group. [4] This could work both for the males within the group and any female who visits the lek. [62] Protection also explains the presence of mixed leks, when a male of one species joins a lek of another species for protection from a common set of predators. This occurs with manakins, [63] as well as other birds such as grouse species. [64]
Sexual selection is a mechanism of evolution in which members of one biological sex choose mates of the other sex to mate with, and compete with members of the same sex for access to members of the opposite sex. These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.
Grouse are a group of birds from the order Galliformes, in the family Phasianidae. Grouse are presently assigned to the tribe Tetraonini, a classification supported by mitochondrial DNA sequence studies, and applied by the American Ornithologists' Union, ITIS, International Ornithological Congress, and others.
Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consist of most animals and some plants. Differences may include secondary sex characteristics, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection. These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.
Behavioral ecology, also spelled behavioural ecology, is the study of the evolutionary basis for animal behavior due to ecological pressures. Behavioral ecology emerged from ethology after Niko Tinbergen outlined four questions to address when studying animal behaviors: What are the proximate causes, ontogeny, survival value, and phylogeny of a behavior?
Stalk-eyed flies are insects of the fly family Diopsidae. The family is distinguished from most other flies by most members of the family possessing "eyestalks": projections from the sides of the head with the eyes at the end. Some fly species from other families such as Drosophilidae, Platystomatidae, Richardiidae, and Tephritidae have similar heads, but the unique character of the Diopsidae is that their antennae are located on the stalk, rather than in the middle of the head as in all other flies. Stalked eyes are present in all members of the subfamily Diopsinae, but are absent in the Centrioncinae, which retain unstalked eyes similar to those of other flies. The stalked eyes are usually sexually dimorphic, with eyestalks present but shorter in females.
Display behaviour is a set of ritualized behaviours that enable an animal to communicate to other animals about specific stimuli. Such ritualized behaviours can be visual, but many animals depend on a mixture of visual, audio, tactical and chemical signals. Evolution has tailored these stereotyped behaviours to allow animals to communicate both conspecifically and interspecifically which allows for a broader connection in different niches in an ecosystem. It is connected to sexual selection and survival of the species in various ways. Typically, display behaviour is used for courtship between two animals and to signal to the female that a viable male is ready to mate. In other instances, species may make territorial displays, in order to preserve a foraging or hunting territory for its family or group. A third form is exhibited by tournament species in which males will fight in order to gain the 'right' to breed. Animals from a broad range of evolutionary hierarchies avail of display behaviours - from invertebrates such as the simple jumping spider to the more complex vertebrates like the harbour seal.
Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between males and females. In one example, males may benefit from multiple matings, while multiple matings may harm or endanger females due to the anatomical differences of that species. Sexual conflict underlies the evolutionary distinction between male and 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.
The sexy son hypothesis in evolutionary biology and sexual selection, proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University in Kingston, Ontario in 1979, states that a female's ideal mate choice among potential mates is one whose genes will produce males with the best chance of reproductive success. This implies that other benefits the father can offer the mother or offspring are less relevant than they may appear, including his capacity as a parental caregiver, territory and any nuptial gifts. Fisher's principle means that the sex ratio is always near 1:1 between males and females, yet what matters most are her "sexy sons'" future breeding successes, more likely if they have a promiscuous father, in creating large numbers of offspring carrying copies of her genes. This sexual selection hypothesis has been researched in species such as the European pied flycatcher.
The lek paradox is a conundrum in evolutionary biology that addresses the persistence of genetic variation in male traits within lek mating systems, despite strong sexual selection through female choice. This paradox arises from the expectation that consistent female preference for particular male traits should erode genetic diversity, theoretically leading to a loss of the benefits of choice. The lek paradox challenges our understanding of how genetic variation is maintained in populations subject to intense sexual selection, particularly in species where males provide only genes to their offspring. Several hypotheses have been proposed to resolve this paradox, including the handicap principle, condition-dependent trait expression, and parasite resistance models.
A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), vocalizations, mechanical sound production, or displays of beauty, strength, or agonistic ability.
In the evolutionary biology of sexual reproduction, operational sex ratio (OSR) is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate, or alternatively the local ratio of fertilizable females to sexually active males at any given time. This differs from physical sex ratio which simply includes all individuals, including those that are sexually inactive or do not compete for mates.
Many species have multiple sexual ornaments, whereby females select mating partners using several cues instead of only one cue. Whereas this phenomenon is self-evident and hence long recognized, adaptive explanations of why females use several instead of only one signal have been formulated relatively recently. Several hypotheses exist, but mutually exclusive tests are still lacking.
A mating call is the auditory signal used by animals to attract mates. It can occur in males or females, but literature is abundantly favored toward researching mating calls in females. In addition, mating calls are often the subject of mate choice, in which the preferences of one gender for a certain type of mating call can drive sexual selection in a species. This can result in sympatric speciation of some animals, where two species diverge from each other while living in the same environment.
A biological ornament is a characteristic of an animal that appears to serve a decorative function rather than a utilitarian function. Many are secondary sexual characteristics, and others appear on young birds during the period when they are dependent on being fed by their parents. Ornaments are used in displays to attract mates, which may lead to the evolutionary process known as sexual selection. An animal may shake, lengthen, or spread out its ornament in order to get the attention of the opposite sex, which will in turn choose the most attractive one with which to mate. Ornaments are most often observed in males, and choosing an extravagantly ornamented male benefits females as the genes that produce the ornament will be passed on to her offspring, increasing their own reproductive fitness. As Ronald Fisher noted, the male offspring will inherit the ornament while the female offspring will inherit the preference for said ornament, which can lead to a positive feedback loop known as a Fisherian runaway. These structures serve as cues to animal sexual behaviour, that is, they are sensory signals that affect mating responses. Therefore, ornamental traits are often selected by mate choice.
Sexual selection in birds concerns how birds have evolved a variety of mating behaviors, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Commonly occurring sexual dimorphisms such as size and color differences are energetically costly attributes that signal competitive breeding situations. Many types of avian sexual selection have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signaling adaptations ensure that sexually selected ornaments such as plumage coloration and courtship behavior are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviors.
Sexual selection in mammals is a process the study of which started with Charles Darwin's observations concerning sexual selection, including sexual selection in humans, and in other mammals, consisting of male–male competition and mate choice that mold the development of future phenotypes in a population for a given species.
Sexual selection in scaled reptiles studies how sexual selection manifests in snakes and lizards, which constitute the order Squamata of reptiles. Each of the over three thousand snakes use different tactics in acquiring mates. Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward. It is common for neck biting to occur while the snakes are entwined.
Sexual selection in amphibians involves sexual selection processes in amphibians, including frogs, salamanders and newts. Prolonged breeders, the majority of frog species, have breeding seasons at regular intervals where male-male competition occurs with males arriving at the waters edge first in large number and producing a wide range of vocalizations, with variations in depth of calls the speed of calls and other complex behaviours to attract mates. The fittest males will have the deepest croaks and the best territories, with females making their mate choices at least partly based on the males depth of croaking. This has led to sexual dimorphism, with females being larger than males in 90% of species, males in 10% and males fighting for groups of females.
Hirtodrosophila mycetophaga is a fairly large drosophilid fly, with a mean length of 4.0–4.5 mm. It has thus far only been found in Australia. It mates on bracket fungi, preferentially those with a lighter-colored surface in order to enhance mating displays. In addition to these physical displays, flies emit specific sounds in order to attract and ultimately copulate with females.