Charles Darwin is famous for his contributions to evolution and natural selection. He developed the theory of natural selection and further realized that there are many forms of natural selection. One of these forms is called sexual selection, which he describes in one of his famous books, The Descent of Man, and Selection in Relation to Sex. The idea rose from the observation that some organisms possess traits or exhibit behaviors that do not appear to help them survive. Examples of such cases include conspicuous plumage in birds, antlers of deer, and lekking behavior in manakins. Understanding how a physical characteristic, such as bright coloration or antlers, has evolved is relatively simple, because they each offer one main advantage: attracting females. Bright colors help males stand out and attract female attention and antlers help males during intra-sexual selection when they compete with other males for limited resources, such as mates. However, lekking behavior is more difficult to understand, because there is no one obvious advantage for exhibiting this behavior.

A lek is an area in which a group of males gather to participate in competitive displays in the hopes of enticing a female who is looking for a mate. Lekking behavior is most common in avian species, including the long-tailed manakin (Chiroxiphia linearis). These birds live in subtropical or tropical forests. Males form a group of two or more and perform elaborate displays together, in order to attract females. There is one “master” who is the older alpha male. The other males are all below him and are the “apprentices”, or beta males. Usually, the master and one apprentice will form a duo and perform a courtship display for a female. If the female is impressed by their display, she will mate. However, only the master male ever mates with the female. The apprentice male does not get to mate at all. His only chance of mating is in the future when his master dies and he gets the opportunity to replace him, which may take up to ten years (McDonald, 2007). It is clear that the apprentice male does not gain any obvious direct benefits from forming this duo. Thus, lekking behavior is a Darwinian puzzle, which means that it is a behavior that appears to reduce the fitness of the individual. This goes against the common theory of natural selection, which states that individuals should possess traits that increase their fitness so they will survive better and pass their advantageous traits on to future generations.

Therefore, this behavior is not fully understood because scientists wonder, why do long-tailed manakin males form duos to perform mating displays and how did this behavior evolve if there are no direct benefits for the apprentice males? Many scientists have researched this and it is evident that although the apprentice male does not gain any direct advantages immediately after performing the courtship display with his master, the indirect, future benefits of being in a duo far outweigh the costs. This indicates that duos could have evolved because they increase the males’ fitness over time.

Numerous theories have been proposed to explain why males engage in reproductive cooperation, where instead of normally being competitors, they cooperate with each other to gain an advantage in reproduction. A study done by Díaz-Muñoz, DuVal, Krakauer, & Lacey (2014) reasoned that male-male cooperation must have some fitness benefits; otherwise, there is no way that this behavior could have evolved. It is possible that males could enjoy direct, indirect, or delayed fitness benefits. They then suggested various theories that could explain male-male cooperation. One explanation is ecological factors; they believe that male cooperation could have an ecological basis. Ecological factors may have significant impacts on social structure and opportunities for male reproductive cooperation. Cooperative male displays seem to be associated with environmental instances where there are limits to social interactions, such as the amount and quality of territory (Díaz-Muñoz et al., 2014). Another explanation is phylogenetic history. They analyzed many other studies and found that male-male cooperation is quite widely distributed among vertebrates and some branches have multiple species that exhibit this behavior (Díaz-Muñoz et al., 2014). This suggests that this behavior has evolved multiple times within animals and within vertebrates (Díaz-Muñoz et al., 2014). Finally, they noted that the most common theory to explain this behavior is kin selection. However, kin selection cannot account for all of the occurrences of male-male cooperation in all species.

Kin selection is a common and well-understood explanation for Darwinian puzzles, such as altruism and cooperation. In kin selection, an individual (a donor) may behave in a way that has no apparent benefit or may even appear to harm them. However, their actions help a relative (a recipient) that shares many of the same genes as the donor. Therefore, although the donor is not gaining any advantages, a relative who shares many of the same genes as the donor has an increased fitness and is able to survive better. Kin selection has been studied in long-tailed manakins as a possible explanation for male-male cooperation and their master-apprentice relationship. Loiselle et al. (2007) analyzed four manakin species that exhibit lek behavior for evidence of kin selection. They first captured the birds and drew blood samples. Then they analyzed their DNA to measure the relatedness between males within leks and males within the entire population. They found that the average relatedness of males in leks was no greater than the relatedness of random males from the population (Loiselle et al., 2007). Their data, therefore, suggests that males join leks at random and relatedness is not a factor that influences their cooperative relationships. They concluded that there must be other mechanisms at work to explain male-male cooperation and lekking behavior in long-tailed manakins (Loiselle et al., 2007).

Rather than looking at general theories such as kin selection to try to explain leks in long-tailed manakins, other scientists considered looking at specific benefits that they could be getting by forming leks. A study done by McDonald (2007) considered the possible benefits of cooperation and leks. He specifically focused on connectivity and questioned if certain behaviors early in life predict the likelihood of achieving the role of apprentice or master. McDonald created network models to examine whether early social interactions could predict later social networking success and possible mating success of older male long-tailed manakins. His results showed that young males who connected with other males and had a social network were more successful and were able to find mates an average of 4.8 years later (McDonald, 2007). In contrast, network connectivity is less important for older males (10-15 years old) of higher status (McDonald, 2007). This study suggests that it is beneficial for a young male to form a social group with other males because he has a greater chance of finding a mate compared to a solo male that is not part of a social group. As a result, a young male who prefers to join a lek has a greater fitness and is able to pass his genes for preferring social groups onto his offspring. Future generations would be filled with manakins that prefer to form social groups. In contrast, solo males who do not form social networks are less successful at finding mates, so they do not mate and pass their genes onto future generations. Therefore, these benefits of social networking provide some explanation for how lek behavior could have evolved.

For all social species, it is important that there is some sort of order in their social networks to avoid intra-sexual aggression, fighting, and injuries. Dominance hierarchies are common and crucial in maintaining organization within these groups. These hierarches are often based on size, aggressive behavior, strength, and age. In lekking species, there are usually few high ranked individuals and many subordinates. This is the case for long-tailed manakins. Lukianchuk and Doucet (2014a) studied hierarchies in long-tailed manakins and specifically looked at whether older males exhibited more aggressive behavior compared to younger males. Furthermore, if age indicates status, they reasoned that older males should be aggressive towards younger males and young males should rarely, or never, be aggressive toward older males. They also predicted that there should be less aggression between males from males of different ages compared to males who are the same age. They measured the amount of aggressive behavior, such as chases and displacements. Chases were defined as when a male actively chased and followed another male away from the display perch. Displacement was defined as when a male flew towards another male and took his place on the display perch, forcing the subordinate male to move. Their results indicate that older males are more aggressive toward younger males much more frequently than younger males are to older males (Lukianchuk & Doucet, 2014a). They also found that aggressive interactions occurred between males within the same age class more frequently than between males from different age classes (Lukianchuk & Doucet, 2014a). Overall, this study provides support for an age-graded dominance hierarchy among male long-tailed manakins, where older males are more aggressive towards younger males. This is one explanation for how a successful lek of long-tailed manakins can exist.

In addition to there being benefits relating to social networking and connectivity, being in a male-male cooperative relationship has many 

benefits pertaining to their specific displays. Their displays consist of both visual and acoustic elements. There are some common major components of long-tailed manakin displays, such as the visual “hopping display” and the “butterfly display” (Lukianchuk & Doucet, 2014b). A master-apprentice pair will also sing to attract females to their display area. Once they attract females, they will perform a coordinated display of the hopping and butterfly display. At first, the displays are performed together by both the master and apprentice, but it eventually leads to a solo performance by the master male. Lukianchuk and Doucet (2014b) studied and characterized the courtship display of long-tailed manakins to see if they could predict copulation success. From the two main displays, they were able to identify and characterize sixteen more individual display elements. They then analyzed the data to see if the elements of a display followed a stereotypical, predictable pattern. Finally, they examined whether the performance of individual display elements could predict copulation success. They observed long-tailed manakins in Costa Rica and ran various statistical analyses to determine if displays were predictable and if they could predict copulation success. They found that some elements were performed more than others, specifically the leapfrogs, back-and-forths, and tucked wing flicks (Lukianchuk & Doucet, 2014b). They also found evidence that some moves can predict copulation success. Specifically, males who display many upright postures, bounces, angel flights, and bows have greater copulation success (Lukianchuk & Doucet, 2014b). This suggests that these specific moves are particularly important to the display, because a female will find these moves highly impressive so she will be more likely to mate with the master male. Therefore, this study indicates that another advantage of forming leks and being in a master-apprentice relationship could be that being an apprentice enables a male to learn and practice these specific moves that females prefer. This would help him attract mates in the future when he is the master. This would put him at an advantage compared to a solo male who does not form a master-apprentice relationship, because he would not get the opportunity to learn and practice these moves that most impress females.

Trainer, McDonald, and Learne (2002) reasoned the same and looked at how the acoustic abilities of long-tailed manakins are affected by being in a lek. They analyzed the songs and singing performances of long-tailed manakins in Costa Rica. Specifically, they estimated the singing consistency of each male and how in sync the two males were in their performance, which they called frequency matching. They assessed whether their singing ability improved with age and predicted that the development of their singing ability might be a result of the formation of a prolonged partnership that benefits males over time. Their results indicated that males’ singing ability increases as they age (Trainer et al., 2002). They also found that the frequency matching of duos was greater than what they would have expected if the duos were random (Trainer et al., 2002). Additionally, they noted that apprentice males also learned more song characteristics over the years from practicing with the master male. Similar to the study done by Lukianchuk and Doucet (2014b) on visual displays, their overall results suggest that a major benefit for the apprentice male is an increased ability to give a display that will end with copulation. Therefore, when he becomes the master male, he has strong singing capabilities and is able to attract females. Altogether, the development of display competence may be an important long-term strategy for reproductive success of male long-tailed manakins.

Together, these studies show how male long-tailed manakins gain indirect, future benefits by forming leks and engaging in cooperative behavior. Initially, lekking behavior could have evolved many years ago and spread throughout many avian species as a result of their environment. As the master-apprentice relationship evolved, it would have become clear that this relationship was advantageous for males as they underwent sexual selection. It is true that an apprentice has to put up with many years of no reproductive success and no direct benefits. However, during this time, he is able to increase his indirect fitness by learning and practicing songs and dances so by the time he is the master, he is well experienced and has a high probability of enticing and mating with a female. Furthermore, it is advantageous for him to form a group because female manakins specifically look to mate with males in a group display. A solo male would have no chance of mating with a female, because he would not know how to perform a display and females would not consider him in a group. Therefore, this Darwinian puzzle can be solved by considering the benefits of forming a social network, including an increased probability of finding mates later in life, the visual and acoustic skills that they learn from their master, and then finally the chance to become a master once their master dies. These advantages all outweigh the immediate disadvantage of not being able to 

mate with a female as an apprentice. Therefore, because the advantages outweigh the disadvantages, it makes sense how lekking behavior has evolved in male long-tailed manakins.

References

Díaz-Muñoz, S. L., DuVal, E. H., Krakauer, A. H., & Lacey, E. A. (2014). Cooperating to compete: Altruism, sexual selection and causes of male reproductive cooperation. Animal Behaviour, 88, 67-78.

Loiselle, B. A., Ryder, T. B., Durães, R., Tori, W., Blake, J. G., & Parker, P. G. (2007). Kin selection does not explain male aggregation at leks of 4 manakin species. Behavioral Ecology, 18(2), 287-291.

Lukianchuk, K.C., & Doucet, S., M. (2014a). A young Manakin knows his place: evidence for an age-graded dominance hierarchy among long-tailed Manakins. Ethology, .20. 693-701

Lukianchuk, K. C., & Doucet, S. M. (2014b). Cooperative courtship display in Long-tailed Manakins Chiroxiphia linearis: predictors of courtship success revealed through full characterization of display. Journal of Ornithology, 155(3), 729-743.

McDonald, D. B. (2007). Predicting fate from early connectivity in a social network. Proceedings of the National Academy of Sciences, 104(26), 10910-10914.

Trainer, J. A., McDonald, D. B., & Learne, W. A. (2002). The development of coordinated singing in cooperatively displaying long-tailed manakins. Behavioral Ecology, 13 (1) 65-69.

Related Links:

• The dance_edited.pdf (PDF)