Lek Males Research Articles

Mitochondrial superoxide dismutase overexpression and low oxygen conditioning hormesis improve the performance of irradiated sterile males

The Sterile Insect Technique (SIT) is a successful autocidal control method that uses ionizing radiation to sterilize insects. However, irradiation in normal atmospheric conditions can be damaging for males, because irradiation generates substantial biological oxidative stress that, combined with domestication and mass-rearing conditions, may reduce sterile male sexual competitiveness and quality. In this study, biological oxidative stress and antioxidant capacity were experimentally manipulated in Anastrepha suspensa using a combination of low-oxygen conditions and transgenic overexpression of mitochondrial superoxide dismutase (SOD2) to evaluate their role in the sexual behavior and quality of irradiated males. Our results showed that SOD2 overexpression enhances irradiated insect quality and improves male competitiveness in leks. However, the improvements in mating performance were modest, as normoxia-irradiated SOD2 males exhibited only a 22% improvement in mating success compared to normoxia-irradiated wild type males. Additionally, SOD2 overexpression did not synergistically improve the mating success of males irradiated in either hypoxia or severe hypoxia. Short-term hypoxic and severe-hypoxic conditioning hormesis, per se, increased antioxidant capacity and enhanced sexual competitiveness of irradiated males relative to non-irradiated males in leks. Our study provides valuable new information that antioxidant enzymes, particularly SOD2, have potential to improve the quality and lekking performance of sterile males used in SIT programs.

Comparing methods for estimating the abundance of Western Capercaillie Tetrao urogallus males in Pyrenean leks: singing counts versus genetic analysis of non-invasive samples

ABSTRACTPopulation estimates of male Western Capercaillies Tetrao urogallus were carried out during the mating season using two methods: counts of singing males and non-invasive genetic analysis. Estimates of male numbers were 50% lower using the singing counts compared to the estimates obtained through genetic analysis, and underestimates were greatest when the number of Capercaillies was lowest.

Display activity and foraging costs of a frugivorous lekking bird

Sexual success of males in lek breeding systems is often determined by display activities that require high energetic costs and may represent honest signals of male quality to females. In this context, manakins (Aves, Pipridae) evolved behavioral, morphological, and physiological adaptations that enable complex display maneuvers, and a high rate of fruit ingestion is needed to cope with the energetic costs of lekking activities. We examined the relationship between different displays (rolled-snaps, flights-down, and snap-jumps) and fruit consumption (a proxy for energy intake) by White-bearded Manakin males (Manacus manacus). We conducted a fruit supplementation experiment and recorded male displays in 23 arenas on 5 leks in southeastern Brazil. The flight-down was the display most related to fruit consumption (and energy refueling) by males of the White-bearded Manakin. This display seems to be decisive to the mating success of Manacus males; once inside a lek area, females are driven to display arenas by flights-down of males as an obligatory stage before copulation. This study opens a promising avenue for further field experiments designed to test how local food resources differentially impact display behavior among males in leks.

The impact of lekking on the spatial variation in payoffs to resource-defending topi bulls, Damaliscus lunatus

In mating systems research it is often ignored that the spatial relationship between individuals adopting alternative mating tactics may influence the intensity of their interactions and therefore also their tactical payoffs. I here take a novel approach to investigating the coexistence of lek territoriality and resource territoriality in male topi antelopes by including territorial distance from a lek centre as a continuous variable. Using the body size of territory holders as a proxy for territorial payoffs, I show that the attractive force of lek centres impact negatively on payoffs from surrounding territories only within an 1800 m radius. Thus, lek males are significantly larger than resource defenders inside, but not outside, this impact zone. The impact zones encompass the ranges during oestrus of 91% of the females mating on leks, and because female topi are highly promiscuous, the impact zone corresponds roughly to the area within which resource territorial males experience sperm competition with lek males. These results can be explained by a preference of oestrous females for mating with central males inside, but not outside, the 1800 m zone. The study illustrates how taking spatiality into account can reveal significant differences in tactical payoffs within behavioural categories that are traditionally regarded as uniform.

No peace for estrous topi cows on leks

Male coercion, such as harassment, may be considered the third main component in sexual selection alongside male competition and female choice. In this study on lek-breeding topi antelopes (Damaliscus lunatus), I investigate whether female mating preferences have consequences for male investment in harassment and whether harassing males are more likely to succeed in mating. I then address the question of whether lek evolution in topi can be explained by harassment avoidance. Judging from mating rate, I found that female topi antelopes in estrus preferred lek males to resource defenders. In contrast to lek males, resource defenders demonstrated significantly higher harassment rates before they succeeded in mating than when they did not, and the precopulatory harassment rate was significantly higher on resource territories than on lek territories. After mating on resource territories, harassment dropped to low levels. Thus, resource defenders, but not lek males, seem to employ harassment as a strategy to coerce females to mate against their preference. However, by using various measures of harassment intensity, overall estrous females were found to experience higher harassment levels on lek, and chases by intruders were relatively rare on all territory types. These findings suggest that harassment avoidance is unlikely as an explanation for lek evolution. Copyright 2003.

Mating strategies of topi bulls: getting in the centre of attention

In lek-breeding ungulates, only some males defend clustered lek territories, and others defend dispersed territories or are nonterritorial. In this study of lekking topi antelopes, Damaliscus lunatus, we measured male mating benefits directly by observing matings and investigated why the alternative mating strategies coexist. A multivariate analysis showed that proximity to the lek centre had an overriding, positive, effect on male mating rate. With increasing distance to the lek centre, proximity of a territory to a drainage line became increasingly important in enhancing male mating success. On the other hand, costs of lekking were suggested by higher hyaena density on leks, relatively poor body condition of lek males, and more frequent agonistic encounters, with central lek males more likely to sustain bleeding wounds than others. Probably as a consequence of the intense competition for central lek territories, males defending such territories were larger than others and, judging from horn wear, they were also older than resource defenders. Not only did males defending central lek territories achieve the highest instantaneous mating rate and resource defenders the lowest, but the same order was also likely for overall lifetime reproductive success when we took territorial tenure into account. These results suggest that male mating strategies are phenotype limited, as demands on male quality increase with proximity to the lek centre. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Neighbours, strangers and male-male aggression as a determinant of lek size

Interactions between males on leks may play an influential role in lek formation and the regulation of lek size. In this paper I present the results of a playback experiment that simulated de novo settlement at sites adjacent to currently existing display territories of the ochre-bellied flycatcher, Mionectes oleagineus. In the study population, males displayed both solitarily and at small leks. A large proportion of males held no display territory at all. A stranger's song was played to both solitary and lekking males from 10 m outside their territorial boundaries. In separate playbacks, lekking males were also played neighbour's song. Both lekking and solitary territorial males reacted to the playback by decreasing their song rate, approaching the playback speaker and, on occasion, attacking the model. Solitarily displaying males responded more aggressively to playback of stranger's song than did lek males. Lek males were able to distinguish between their neighbour's and a stranger's song and did so irrespective of whether it was played from the neighbour's territory or from outside the lek. In addition to distinguishing between neighbours and strangers, lek males modified their responses to these different playbacks depending on where the playback originated. These results suggest that male-male interactions can be influential in structuring leks. In M. oleagineus, interactions between males are aggressive and act to limit rather than augment lek size.

Mate choice or harassment avoidance? A question of female control at the lek

Recent studies on the reproductive behavior of fallow deer, Dama dama, propose that harassment from nonterritorial males has a major influence on female movements and mate selection, leading ultimately to the evolution of lek mating in this species. In order to support this statement, one must demonstrate that female movements between lek and isolated territories, and among lek males, lead to a reduction in levels of harassment. We argue that current evidence in favor of this view is inconclusive. A quantification of the total harassment costs experienced by females in lek and isolated territories has never been made. In addition, female movements within the lek may actually lead to higher levels of disruption and harassment: The rate at which females join male territories increases with harem size (the number of females present in the territory), even though harems are disrupted increasingly with size due to a higher frequency of intrusions by nonterritorial males. Females also join male territories at a higher rate while these males are engaged in copulatory sequences, but copulatory sequences are again associated with high levels of disruption and harem instability. In the above studies it is argued that females are nonselective in their mating preferences. This assessment is based on the finding that males that adopt different reproductive strategies do not differ in their mating rates. Here mating rate is measured as the number of copulations received per female-hour. There are a number of reasons, however, why females exhibiting mating preferences might remain longer with preferred males, and so the above preference measure cannot be used to exclude the possibility that females are selective. More research is required to identify the major factors influencing patterns of mate selection and the evolution of leks in this species. We suggest a number of field tests that may help to identify these factors.

Unusual Calling Behavior of Anastrepha robusta Flies (Diptera: Tephritidae) in Nature

Male calling behavior in Anastrepha (Schiner) fruit flies has been described in the following species: bistrigata (Bezzi), fraterculus (Wiedemann), ludens (Loew), obliqua (Macquart), pseudoparallela (Loew), serpentina (Wiedemann), sororcula (Zucchi), striata (Schiner) and suspensa (Loew) (Aluja et al. 1983, 1989, Dodson 1982, Burk 1983, Morgante et al. 1983, 1993, Nation 1972, Polloni & Silva 1986, Robacker & Hart 1985, Teles da S. et al. 1985). In the cases of fraterculus, ludens, obliqua, serpentina, sororcula, striata and suspensa male mating aggregations (leks) have been reported (Aluja et al. 1983, 1989, Dodson 1982, Morgante et al. 1983, 1992, Sivinski 1989). Within a lek (or outside of it) males typically emit a series of courtship sounds (calling songs) through wing fanning (Webb et al. 1984; Sivinski 1988), release a sexual pheromone (Nation 1972), and usually defend a territory (individual leaf or portion thereof) (Aluja et al. 1983). While calling, individuals preferably position themselves on the underside of a leaf (Dodson 1982, Aluja et al. 1989) and only rarely take off and return to the same spot repeatedly within a bout of calling. Here, I report the unusual calling behavior of Anastrepha robusta (Greene) males. The discovery was made during a study of the natural history of Anastrepha spp. in a mixed chico zapote (Manilkara achras L.) citrus (Citrus sinensis Osb., C. reticulata Blanco) guanabana (Annona muricata L.) coconut (Cocos nucifera L.) orchard in Apazapan, Veracruz, Mexico during 1990. On June 21, 1990 a single, very large fruit fly male was sighted in a Citrus reticulata tree performing a series of elaborate loops with repeated landings within looping bouts (Fig. 1). The specimen was captured and identified as Anastrepha robusta. Anastrepha robusta is a rare species belonging to the robusta group (Norrbom 1988). It has only been reported in Brazil, Panama (type material), Guatemala (type locality) and Mexico (Stone 1942). To date nothing has been reported about its biology, ecology and behavior. Eight calling males were observed after the first sighting (1990) and 2 females were captured in McPhail traps placed in a C. reticulata tree. In 1991, no individuals were sighted or trapped at this site. In 1992, 5 individuals (3 Y & 2 d) were trapped in the same location (C. reticulata tree) but no individuals were sighted. All captured individuals have been placed as voucher specimens in the permanent insect collection of the Instituto de Ecologia, A.C. in Xalapa, Veracruz, Mexico. A summary of the 1990 sightings is provided in Table 1 [note that A. robusta males only call during early morning hours (0700-0930 hours)]. While calling, males performed a mean of 9.5 ? 0.5 (SE) loops min-,. The distance a loop extended from the point of departure varied from loop to loop but ranged between ca. 15 and 40 cm. All looping males were observed in the outer canopy of trees (both C. aurantium and A. muricata) and loops were always performed in wide open spaces between trees or branches. While looping, flies flew on both horizontal and vertical planes. Typically, an individual would take off, fly out into the open on an horizontal plane, loop downward and then fly back to the departing point (leaf). On only two occasions were two males seen calling simultaneously at the same site (males were ca. 15 cm apart on two separate leaves ca. 3 m above the ground in an Annona muricata tree). Never were females seen in the vicinity of looping males.

Multiple mating in a lekking bird: why do peahens mate with more than one male and with the same male more than once?

Approximately 50% of marked peahens (Pavo cristatus) mate more than once with lek males. Some females mate with more than one male, others copulate repeatedly with the same male. The frequency of courtship also shows marked variation. Some females repeatedly engage males in courtship interactions after they have succesfully copulated with them. The likelihood of mating with more than one male increases if a female first mates with a non-preferred (unsuccessful male). There is a non-significant tendency for females to copulate with a more successful male when remating. Peahens may mate with a non-preferred male first if they do not encounter a successful male during their initial period of choice, perhaps because the most successful male on a lek was courting another female and/or was defended by another female. There are more aggressive interactions between females in front of preferred males. Preferred males receive more repetitive courtship behaviour and repeated matings. Dominant females are more likely to engage in repetitive courtship and matings. The number of times a female initiates courtship on any one day increases with the number of other females actively courting males at a lek site on that day. We suggest that there is competition amongst females for access to preferred males and that dominant females try to monopolise these males by repeatedly engaging them in courtship interactions. We discuss the implications of these observations for the idea that female may gain directly from mate choice in a species where males contribute nothing but gametes to their offspring.

Peacocks with low mating success are more likely to suffer predation

There is considerable controversy about whatfemales may gain from mate choice in a lekkingspecies where males provide no obvious directbenefits to their offspring (Kirkpatrick & Ryan1991). Some workers have argued that females gaindirect benefits such as safe copulations by matingwith particular lek males (Reynolds & Gross 1990) ;for example, successful lek males may have a lowerchance of having their copulations disrupted andthis is why they are preferred by females. Othersmaintain that females gain indirect benefits fortheir offspring: either their offspring have higherchances of surviving because they have gained`good genes', or their offspring have higher matingsuccess because they have inherited their father's`attractiveness'. If females mate with attractivemales because their offspring have a higher chanceof survival then it might be expected that preferredmales would also show a higher chance of survival,since these males would possess the `good genes'.In a study of lekking black grouse,

Effects of a Severe Drought on the Population Biology of a Tropical Hummingbird

At Finca La Selva, in the Caribbean lowlands of Costa Rica, the Long—tailed Hermit (Phaethornis superciliosus) has a long breeding season (December or January through August or September) during which flowers are abundant, followed by a severe but predictable season of flower scarcity in late October—November, during which most of the annual mortality normally occurs. In 1973, the third of a 4—yr population study of marked lek males, an unusual drought caused a severe flower shortage at the height of the breeding season. During the latter part of the breeding season lek activity of males was reduced, successful breeding by females was drastically curtailed, and masses of both sexes dropped to levels lower than those of the lean season. Interrupted molt, an indicator of energy stress normally seen only in a minority of breeding females, was observed in 25% of adult males, and 50% of females, 1973. Survivorship of lek males through the breeding season dropped from 90% in normal years to °60%, and the total lek population at the start of the 1974 breeding season was one—third lower than before the drought in 1973; the cohort of yearling males was most strongly affected. Recovery of numbers and age structure of the lek male populations to pre—1973 levels required 3—4 yr. Jolly—Seber analyses showed close agreement with survivorship as determined by direct observation (possible because of the lek social system), and permitted estimation of other parameters of interest: populations of marked and unmarked males, probability of recapture of marked males, and recruitment. The average annual survivorship of male P. superciliosus (in normal years) is near the mean value reported for a series of tropical species. A simple model of the reproductive output of female P. superciliosus, assuming no sexual differences in survivorship, suggests that a female breeding only in the prime breeding season (roughly February—July, when over 80% of the fledglings are normally produced) will no more than replace herself in the population; breeding throughout 8—9 mo will permit her to increase her reproductive output by up to 10%, which agrees well with the 3+ yr required by the (male) population to recover from the drought. Possible implications of these findings for tropical bird demography and conservation are discussed.

To lek or not to lek: mating strategies of male fallow deer

We studied the mating system of fallow deer (Dama dama) for 6 years in central Italy. Males in this population could defend territories that were either single, clumped in leks, or satellite to leks. The most highly successful males in our study were in leks. When we considered all males, there were no significant differences in average copulatory success according to territory type because many lek males did not achieve any copulations, which were seen in only a few lek territories. The variance in copulatory success, however, was much greater for leks than elsewhere. Single territories were occupied for shorter times during the rut than lek territories. Fighting among males was more frequent in the lek, even when we excluded highly successful lek males from the analysis. Chases of nonterritorial males and harem size were correlated with the number of copulations achieved by individual males, but did not vary according to territory type. Copulatory success of some individuals increased with age, but there were no age differences among males holding different types of territories. Satellite males switched to lek territoriality in the course of one rut, but switches from single territory to lek territory were rare. We suggest that males in single territories are inferior competitors that select a low-risk, lowbenefit strategy, whereas those in lek territories where no copulations were seen may be attempting to establish themselves on the lek to increase their copulatory success in future years.

Mate choice in lekking sage grouse revisited: the roles of vocal display, female site fidelity, and copying

In lekking sage grouse (Centrocercus urophasianus), females exhibit relatively unanimous mate choice for particular males, but a satisfactory explanation for this unanimity has been elusive. We present analyses of mating distributions from two leks over 4 years that provide evidence for female choice based on differences in vocal display performance of males, the locations at which hens mated in the previous year, and the choices of other females (copying). The unanimity of female choice varied markedly among leks and years in correlation with changes in the mean numbers of hens that mated at the same time and hence the opportunity to copy. The results confirm that hens assess phenotypic traits of males directly but also indicate that the secondary tactics of site fidelity and copying are often important components of female choice. The occurrence of these secondary tactics has three implications: the variance in mating success among lek males will be a poor predictor of the intensity of sexual selection on specific traits; female preferences may generate more clustered dispersions of displaying males than predicted by hotspot settlement models; and direct assessment of males by females may be difficult or costly, a conclusion that supports adaptive models of sexual selection over a nonadaptive Fisherian process. [Behav Ecol 1991;2:165–180]

Alternative Mating Strategies and Reproductive Success in Fallow Deer

Male fallow deer Dama dama, in a free-ranging population in the New Forest, southern England, either defend territories on a lek, defend single territories or are non-territorial. Mating success is highest on the lek, but unsuccessful lek males get fewer matings than some single territory males off the lek. The mating success of non-territorial behaviour, which is adopted by most males at some time during the rut, is uncertain. Non-lek matings in this population are higher than those reported in other lekking ungulate populations. Male fallow deer are not specialists; individuals commonly change strategies as mating opportunities dictate.

Lekking in topi: a consequence of satellite behaviour by small males at hotspots

Male topi, Damaliscus lunatus, either defend large territoriescontaining food resources or join leks. In this study there was large variation in mating success on leks with a few males getting the majority of matings. Resource territories attracted more females than the territories of unsuccessful males on leks. All males on leks spent more time in contest behaviour than males in resource territories so that the majority of lek males that obtained few matings incurred high costs and low benefits. The reason that unsuccessful males stay on leks, rather than adopt the less costly alternative of resource defence, may be that they are smaller than males in resource territories and thus unable to win fights to acquire resource territories. Although scucessful territories were scattered through the study lek, two small territories near the centre consistently attracted many more females than any others. This pattern persisted regardless of which male was in residence; females even came to these territories when the owner was absent. The ability to hold a successful lek territory could be a more reliable indicator of competitive ability, and thus fitness, than any single phenotypic characteristic. Leks may develop from groups of successful resource territories. In the case of topi, the desirable feature of these territories is probably a resting area in open habitat where females concentrate for anti-predator advantage. It is proposed that leks appear when small, satellite males of low resource-holding potential cluster around the males in these successful territories.

Cooperation Under Sexual Selection: Age-Graded Changes in a Lekking Bird

Long-tailed manakins, Chiroxiphia linearis, are birds with a lek mating system and male-male cooperation in courtship display. I studied male-male networks in a color-banded population in Monteverde, Costa Rica, from 1981 to 1987. Males displayed in scattered leks (75-300 m apart) comprising 3-15 males. Within each lek, an alpha and beta male, with strict relative dominance ranks, performed most of the courtship display. Of 50-60 active males per season, only 6-8 males were well-established alpha males in leks with consistent levels of dual-male displays (calls and dances). Demographic data suggest that males may be 8 yr of age or more before attaining beta status. Alpha tenure can last from two to at least four years. Alpha males were rarely or never seen in perch zones other than their primary perch zone (their area of dominance). Lower-ranking males maintained simultaneous affiliations with males in as many as six different zones. Each zone, therefore, acted as a hub in which males with different affiliations around the rim came into contact. Each of the six major perch zones shared at least one affiliate with each of the other zones, and roughly half the males in any particular zone were also known affiliates in one or more other zones. Marked changes occurred in male traits with increasing age and status: (1) significant declines in weight throughout the life span, without loss of dominance status; (2) a 4-yr delay in plumage maturation with distinct subdefinitive, transitional stages; (3) reduction in the number of perch zones with which older males maintained affiliations; and (4) increasing probability of copulatory success. The results are consistent with the hypothesis that subdefinitive plumages in this species serve primarily as accurate indicators of age, which in turn largely determines status, and that males queue for positions in an age-based dominance system. The results do not support the hypothesis that delayed plumage maturation involves mimicry or deceptive signaling by subdefinitive males (≤3 yr old) to high-ranking males (≥8 yr old). Of 85 males monitored from 1983 to 1986, copulations (N = 117) were distributed among 8 males. Four of these males accounted for over 90% of the copulations, with 67% accruing to one male. An index of the opportunity for sexual selection, to be used cautiously in interspecific comparisons, was estimated to range from 15.8 to 31.5 in four successive years. Direct benefits to cooperation by the beta male consisted of rare immediate copulations (N = 2) and eventual ascent to alpha status (N = 3). The high variance of mating success means few opportunities for success by younger males and should favor long-term strategies, such as cooperation and delayed plumage maturation, that enhance the prospects of future success.

Trapline Foraging by Hermit Hummingbirds: Competition for an Undefended, Renewable Resource

Traplining hummingbirds face a conflict between waiting for undefended floral nectar to accumulate to higher levels of profit and losing accumulated nectar to competitors. The goal of this research was to learn the rules and temporal patterns of flower visitation by Long—tailed Hermit Hummingbirds (Phaethornis superciliosus). Monitors of visits by color—marked individuals, particularly lek males, to natural flowers established patterns of feeding in space and time. Monitors of visits to nectar feeders that were refilled at fixed intervals established patterns of adjustment to nectar availability and competitive losses. Competing hummingbirds tended to segregate among undefended feeding sites, with one individual becoming the primary user of each site. Competition caused hummingbirds to visit a feeder frequently, often before a scheduled refill. In this way one individual could harvest most of the provided nectar. Under conditions of nearly exclusive use of a feeder with artificial nectar, the hummingbirds adjusted their visits to operant (fixed—interval) schedules of food presentation. The scheduled harvesting of floral nectar by traplining hermit hummingbirds thus entails the adaptive use of short, elapsed time intervals.

Conditional lekking in ruff (Philomachus pugnax)

In our study of ruff, a lekking shorebird, we found that the “lek ratio” — the proportion of the total population of males occurring on leks was low, averaging 12% over the breeding season (Fig. 1D). Off-lek males spent approximately the same proportion of time as lek males in displaying to females (Fig. 4). We defined three spacing tacties that male ruffs use to position themselves to court females: “Following” — directly pursuing females, and two types of lekking behavior, “Intercepting” — waiting for females at resource-rich sites, and “True Lekking” — waiting for females at places without any resources other than the males themselves. Males switched among these tactics, causing the lek ratio to vary over the season (Fig. 1 D). Lek ratio increased when the number of females present in the study area plummeted at the end of May, and was positively correlated throughout the season with the copulation rate of the preceding day, suggesting that males were tracking the behavior, as well as the number, of females available (Table 1, Fig. 1). Early in the season, males off leks spent most of their time feeding (Fig. 4), and lek ratio was positively correlated with temperature (Table 1), suggesting that some males may have been unable to lek during cold weather. Males on leks mated at significantly higher rates than Followers (Table 4). On average, males at our interception lek were less site faithful and less peristent than males at true leks, and the interception lek itself disappeared after females stopped coming to use its adjacent resource (Table 2, Fig. 5). The most successful individuals in our population were the True Lekking males, rather than the Interceptors. In addition to the conditinal lekking tactics described here, ruff display a dimorphism in behaviorat leks. “Independent” males defend small courts on leks, while “Satellite” males share courts and mutually display with independents Both independents and satellites may use all three conditional tactics. We propose that satellites evolved as specialized. Followers, adept at tracking the movements of females among leks.

Disruption, Dispersion, and Dominance in Lek-Breeding Birds

Disruption is any behavior that interferes with male-female courtship interactions or copulation and leads to a decrease in the number of copulations completed by a male, or an increase in the time and energy required to complete them. Here I examine the origin and maintenance of disruption, its effects on the fitness of the individuals involved, and its influence on dominance relationships and spacing of males at leks. Disruptive behavior is most common among lek-breeding birds, probably because its frequency is proportional to the amount of contact between members of the same sex. Amount of contact, in turn, is inversely proportional to distance between males, so that clustering of males on leks may result in increased disruption. Disruption usually is performed by a rival male who, as a result of his actions, may be able to mate with the female disrupted. Such behavior should be highly advantageous if, on average, the disruptor is able to decrease his age of first reproduction or to obtain more copulations. On the other hand, it is disadvantageous to the disrupted pair. Disruption also may occur as a by-product of high levels of male aggressiveness that are favored by selection because they increase the ability of a lek male to obtain and hold a high quality court or a high rank in a dominance hierarchy and to attract and excite females for copulation. Selection should operate to minimize the amount of disruption at leks through the evolution of strict dominance hierarchies among lek males, or by increasing the separation of lek males which may result in the formation of exploded leks.