Download Evolution of Leks

Evolution of Leks
SHEKIB DASTAGIR, KIM DI MINNI’, JEFF PRITSKY, HAMID SAADATI
Department of Biology, College of Arts and Science, New York University
New York, NY, U.S.A
(Submitted on 2 Deccimber 1997)
The most likely reason for males banding in groups known as “leks”) for courtship behavior is because it provides
an opportunity for females to assess a wide range of males for selection of the best possible traits-and is
advantageous for males because leks increase their chance for encountering females. Three hypotheses have been
proposed for how leks occur: “preference, ” “hotshot,” and the “hotspot” hypotheses. The female “preference”
hypothesis states that, females prefer mating at leks because of the advantages they gain compared to mating at
solitary sites. This idea suggests that the risk of predation is reduced; a reduction in the cost of searching for mates
should be observed, as well as a comparison of male qualities for potential mates. One experiment refuted this
hypothesis. Evidence has shown that rarely does the same predator strike the same nest twice. In fact, the study
showed that in over five hundred hours of observation ofleks, only two predation attempts, both unsuccessful, were
observed. The “hotshot” hypothesis suggests that females prefer to mate with attractive males. In the process
unattractive males simply join these attractive males to increase their mating chances by feeding off their
attractiveness. This hypothesis should give evidence of kleptoparasitism. While there is evidence of
kleptoparasitism on leks, adversaries believe that the “hotshot” mechanism cannot be the sole explanation for
lekking. The “hotspot” hypothesis suggests that patterns of female movements and/or dispersions determine where
males settle: leks form where female are most likely to be encountered or where resources are most abundant.
Evidence strongly supports this hypothesis as opposed to the other two. Studies have proven that males will tend to
cluster on the points of greatest female home range overlap, or greatest female density. In a study done by Wescott,
his experiment showed that if abundant resources occurred in certain places males will set up their leks there. In any
case, his experimental results render support to the idea that males set up leks where the chances of encountering
females are high. Though it is confirmed that leks offer advantages for both males and females, the mechanism by
which leks occur is less understood. But, perhaps because of favorable evidence the “hotspot” hypothesis is a more
plausible explanation for lek occurrence.
The most likely reason for males banding in groups (known as“leks”) for courtship
displays is because it provides an opportunity for females to assess a wide range of males for
selection of the best possible traits-and is advantageous for males because leks increase their
chance for encountering females. A lek is a cluster of males on display territories that appear to
offer ample opportunity for sexual selection. Leks usually offer advantages to both females and
males. For the female it allows comparisons for mate choice, and for the male it offers a good
chance of encountering females. There are three hypotheses for how these advantages arise: the
“preference, ” “hotshot,” and “hotspot.” The “preference” hypothesis suggests that either males
or females gain advantages compared to mating at solitary sites, therefore making leks a
prosperous place to mate. The “hotshot” hypothesis implies that as a result of high variance in
male mating success the leks will form The fiial and third hypothesis, “hotspot,’ suggests that
leks form where females are most likely to be encountered. Thus, dispersion of the female
determines where males settle. Of the three, hotspot is supported by most of the evidence, and is
appealing because it is fundamental to ideas about the evolution of mating systems.
A lek is a traditional Scandinavian name for a site used by males for epigamic (intrasexual
and intersexual) display (11). Males gather in variably clustered arenas for reproductive purposes.
The main attribute of a lek is the gathering of males in one area, where females visit to assess
potential mates for the purpose of copulation. At this time, each male defends a display territory
within a lek resulting in intense competition, ultimately leading to the ranking of males in a
dominance hierarchy, which determines who sires most of the next generation. Experience,
ability, and age are all incorporated into the hierarchy. Females thus obtain for their offspring the
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genes responsible for the dominant male’s “superior” traits. The selection process of a good male
is a process of a dominance hierarchy (12).
An example of lekking can be found in themanakin. Forming a vibrating mass, the males
perch side-by-side, facing the same direction, in the same position on a twig, calling to the females
in a recurrent rhythm. After a few seconds, the male positioned lowest on the twig flies up to two
feet in the air and hangs momentarily facing the female. He screeches “dik, dik, dik,” then lands
at the upper end of the row of males at the side of the motionless female.He joins the other
males in “tripping” by pivoting immediately in the direction of the other males. The lowest bird
performs in a similar manner (11).
The birds’ actions then become more and more frenzied, and
then suddenly stops. The oldest, dominant male does a brief precopulatory solo display and
mounts the female. Subordinate males are not being altruistic and only cooperate in group
displays to excite and attract females. When the dominant males are absent, the less dominant
males occasionally copulate. Some eventually achieve a more dominant status and develop
expertise (11).
In the evolution of leks, one may ask why should promiscuous males gather inleks, in
which a few dominant individuals mate more frequently? What are the conditions that favor lek
clusters over dispersed non-monogamous males trying to attract the females? Several possibilities
stand out, as in the female “preference,” “hotshot,” and “hotspot” hypotheses.
In the “preference” hypothesis males or females prefer mating at leks because of
the advantages they gain compared with mating at solitary sites. Leks might be preferred
because: 1) they reduce the risk of predation; 2) they allow a more efficient comparison of males;
3) they reduce the cost of searching for mates; and 4) they result from an arbitrary Fisherian trait
(12), (which is defined as a hypothetical process of “runaway” sexual selection, which proposes
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that females may increasingly tend to prefer a male secondary sexual trait, that becomes
progressively enhanced through directional selection associated with this female mate choice
behavior (4,11).
Assuming success comes about through female preference for mating inleks, the possible
reasons for a female showing such a preference are predation pressure, clustering, and/or
“runaway” sexual selection (figure 1). Firstly, females may prefer clustered males because such
males entail low costs, since clustered males will deter predators away from nest sites; therefore,
nesting success is predicted to increase with distance from lek site (12). Research by Phillips,
extended this idea by not only invoking males as a decoy, but also as sentinels for females. This
hypothesis thus predicts that nesting success will be highest at some intermediate distance from
lek site (12). However, opponents argue that this explanation is unlikely (3). In fact, opponents
assert that rarely does the same predator strike both adults and nests (3,4). For example, in the
black grouse, goshawks are the principle predators on adults, whereas nests are ravaged by
Corvids and foxes (4). From this we can gather that predator deterence could not work in this
case. Further evidence shows that, in over five hundred hours of observations at leks, only two
predation attempts, both unsuccessful, were observed. In both instances, neither the actual lek
under attack nor leks in adjacent territories appeared to notice the incident, and continued to
display uninterrupted. In addition, predators were not more or less common at leks than
elsewhere on the study site. These observations can’t rule out an influence of predation on
lekking. However, they do suggest that predation is probably not the primary force and
maintenance in the production of leks(4,5,14).
Males in leks also could be preferred because clustered males allow females an easier
comparison of males and reduce search costs then if males are spread (2,3). Also the fact that
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males are aggregated may allow females to compare males in competitive interactions directly.
Although these conclusions seem credible, these issues have not been directly tested(2,3).
Another reason for female preference of males in larger leks is that female preference for
larger leks can evolve and be maintained by “runaway” sexual selection, which results in mating
advantages. This is similar to how any sexually selective male trait could evolve under female
preference (3,12). The final reason for female preference of clustered males could be that males
on large leks provide females a better breeding opportunity.“First, the absolute quality of the
best male could be better in the larger site. Second, the average male quality could be correlated
with lek size” (12).
The second hypothesis called the “hotshot” hypothesis suggests that females prefer to
mate with attractive “hotshot” males and thus search the habitat for them. Unattractive males
simply join these hotshot males to feed off their attractiveness. Thus, the clustering of males
would be the result of the hotshot being attractive not only to females, but also to less attractive
males (7,14). This hypothesis is an alternative explanation for lek evolution. It is thought that the
clustering of males, is an effect of some males being preferred by females, and therefore, the
unpreferred males should gain by becoming parasites on preferred males.IJltimately, males
cluster into leks because of the alluring effect of the mating construct (7). Mating constructs
come about because (1) “Some courtholders are chosen by severalfemales;”(2) “unsuccessful
males abandon their court;” and (3) “hotshots begin attracting abundant males” (7).
This mechanism should thus show evidence of kleptoparasitisim activity (the exploitation
of a dominant male’s “attractiveness” by a less dominant males to increase his chance of mating)
on leks, and of distinct differences in mating success explained by differences in attractiveness
and/or dominant status. It also follows that leks should be unstable especially with respect to low
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quality males, which would most probably change lek sites. Removal of the best male should
result in decreased visitation rates and loss of attendance by subordinates, which contributes to
the departure of the hierarchy. Removal of several hotshots should result in an increase in
disruptions and fighting rates and eventually the break down of the leks.On the other hand, the
removal of subordinates would have no effect on female visitation rates(6,7).
Opponents of the “hotshot” view argue that few field studies have addressed the
predictions of the “hotshot” explanation However, several studies have shown evidence for
kleptoparasitism. In an experimental study, using great snipe, the males attracted females by
repeated display behavior; and the rate of display by any given male was positively correlated with
male success. Attractive males ended up in the center of the lek presumably because less
attractive males compete for positions close to attractive ones. When peripheral males were
removed, such territories were instantly taken over by neighboring or floating males, whereas
removal of central birds resulted in reallocation of the lek. Similarly, peripheral birds attacked
dummies accompanied by playback calls broadcasted within their territories, whereas central birds
were indifferent to dummy males (6).
While there is some evidence of kleptoparisitism on leks, adversaries believe that the
hotshot mechanism cannot be the sole explanation for lekking.Proponents of this belief contend
that if lekking is simply a consequence of kleptoparasitism and females are attracted to a site only
by the presence of one or more hotshots, then it would be in their best interest to move if they
take the females with them (5, 10). But these investigations also state that “to explain” why
hotshots stay, “either site limitations, or tradition has to be invoked” (5). If this were the case,
the hotshot argument breaks down to an argument that is similar to other hypotheses of lek.
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The third hypothesis or the “hotspot” hypothesis is appealing because of its underlying
mechanism, that “male dispersion is a function of female distributions, which is fundamental to
our ideas about mating systems evolution in general” (15). This hypothesis suggests that patterns
of female movements and/or dispersions determine where males settle: leks form where females
are most likely to be encountered or where female densities are highest. This concept is crucial to
understanding the ideas involving mating systems and to its evolution(9,11,15). Males in leks are
able to pursue other options to maximize their reproductive success, due to the fact that they are
freed from the constraints of parental care._ “These options include moving to display sites where
their exposure to potential mates is maximized” (15). It is reasonable to expect that hotspot
effects should be apparent in lek mating(5,6,14,15).
Non-defendable resources could determinehotspot to which females are attracted such as
food, nest sites, and water holes. But even in the absence of the resources, hotspots can arise
from overlapping female home ranges. Therefore, clustering of males is determined by the
clustering of females which in turn can be determined by the clustering of resources. Those
species in which females have overlapping home ranges also will have more male encounters
In support of this theory, a study has proven that males will tend to cluster on the points
of greatest female home range overlap, or greatest female density, and that female home range
size accounts for 25% to 30 % of male clustering (Figure 2) (15). That leks are sited on the
points of greatest female density is supported in some species of birds. For example, inRuffs,
leks are often situated near small ponds that females probably visit for feeding and drinking (3).
In the Costa Rican rainforest, four species (a tyrant flycatcher, two species of manakins,
and a humming bird) set up leks at roughly similar places from year to year (14). This suggests
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,
..
that there are two environmental explanations for location of leks. First, the topography of the
landscape may funnel females into certain areas (leks are often in valleys separated by ridges).
Second, some of the species have overlapping diets and thus they have similar resource needs. If
resources occur at certain places, then males will set up their leks there (14). In any case, the
results render at least limited support to the idea that males set up leks where the chances of
encountering females are high (5,14).
An interspecific case of the hotspot hypothesis has been extended to several species.
Wescott began by considering a group of sympatric lekking bird species. These species use
similarly distributed resources and are similar in size. These species should exhibit similar ranging
patterns if primary resources determine the ranging pattern.Both the routes they use to travel to
resources and the points at which their movements are concentrated, should roughly be in the
same location if these species have similar environmental constraints on their movements.Where
the female densities are highest, is where the males will settle because this is where the greatest
number of potential mates is maximized (14).
An experiment was conducted at a site in Costa Rica. It included four bird species, the
ochre-bellied flycatcher (Mionectes oleagineus ), the red-capped manakin (Pipra mentalis ), the
blue-crowned manakin (Phaethornis superciliosus) ,and the Long-Tailed Hermit Hummingbird
(Phaethornis supercilious) ,which form leks in the forest understorey (Figure 4) (15). The fiist
three of these species are frugivores that show an overlap in resource use. The changes in fruit
availability are known to change the response in local abundance of these three species.The forth
species is a traplining nectarivore. All four species, probably face the same environmental
constraint on their movements because they all move large distances (as much as one kilometer).
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Wescott conducted searches of the study site to map the site, make trails, and survey
vegetations. During the searches, lek locations, or the number of displaying males and the
location of each lek center were determined. Figure 3 summarizes lek locations for the four
species in each year of the study as well as the number of leks and males. The leks of the four
species were significantly clumped. This clumping was so extreme at some sites that all four
species could be encountered displaying simultaneously on partly overlappingleks (14).
The clustered leks that were documented suggest there is a common factor affecting all
species. Movement is likely caused by environmental features such as vegetation and topography.
According to the hotspot hypothesis, males should settle at these sites, thus producing the
observed clusters of leks. It is suggested that the clustering of leks is due to the shared influence
of the environmental features and female movements, and this is supported by two observations
(14, 15). First, leks are associated with topographical features that would cause movements to a
particular area. Second, environmental features such as boundaries between different vegetation
types, might also have an effect
The clustering of leks may be explained through other mechanisms. One of them (others
have been discussed earlier) is that the four species share preferences for particular display habitat
of limited availability. This was proven unlikely becauseMionectes oleagineus males do not show
preferences for particular display habitats. Although all four species often reside in one area, they
choose different levels of the understorey for their displays resulting in marked differences across
the species in the habitat types ultimately selected (14).
In short, leks give females the advantage to choose from a large number of males for
mating. Similarly, males also have an advantage, for they increase their chances of interacting
with a female. This assertion clearly pervades the theme of the paper and is not refuted by any of
the observations and experiments. However, how the evolution of leks occurs (preference
hypothesis, hotshot hypothesis, and hotspot hypothesis) has not been completely ruled against. It
has been our contention that the evidence for lek evolutions suggests that the “preference” and
“hotshot” hypotheses are less likely to be models for the evolution of leks. Rather, we lean
toward the “hotspot” hypothesis because it has played a larger role in the evolution of lek
behavior as suggested by most of the evidence.
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