Why do Male Long-tailed Manakins Cooperate during Courtship?
The long-tailed manakin is a colorful Central American bird with an unusual pattern of courtship. Like the Greater prairie chicken, groups of male long-tailed manakins display in leks to attract females. Unlike the Greater prairie chicken, however, the long-tailed manakins also court in groups. McDonald (1989) has described display and courtship behaviors of the long-tailed manakin. Each group consists of at least two individuals, an alpha male and a beta male, and sometimes as many as 11 younger individuals. Male long-tailed manakins attract females to the arena with synchronous toledo calls. Courtship consists of leapfrogging and butterfly flights by the alpha and beta males, and small popcorn jumps made by any other males in the group. Courtship is energetically expensive, and typically, only the alpha male will mate, not just with any given display, but over the entire breeding season!
Cooperative courtship thus requires that some individuals forego their own reproduction to further the reproductive efforts of others. Clearly, cooperative courtship does not involve simple reciprocity, as the same individual continues to mate throughout the breeding season. Two other explanations are possible, kinship, and delayed benefit. McDonald and Potts (1993) observed large numbers of leks in the long-tailed manakin over a period of more than ten years to try to understand the unusual mating habits of this species. The exercises below will allow you to review McDonald and Potts' data to draw your own conclusion about the merits of the kinship and delayed benefits explanations.
Investigating kinship:
McDonald used microsatellite markers for four loci to examine the relatedness of 33 groups of cooperating males. They calculated a multilocus coefficient of relatedness (R) using the method of Queller and Goodnight (1989).
Values of the coefficient range from -1 to +1, with 0.5 representing the relatedness of siblings. A value of zero represents the background level of relatedness in the population. A positive value of relatedness for the actor and recipient would therefore indicate greater than expected relatedness, and a negative value would suggest that actor and recipient are more divergent than expected.
For the comparisons of the 33 groups of cooperating males, the average value of R obtained was -0.14, and the standard error (obtained through a jackknifing procedure) was 0.1.
Table 1. Correlation of mating success-new alpha vs. predecessor Distribution of relatedness coefficients For all groups:
For groups where alpha male copulated:
|
<0
|
7
|
|
>0
|
4
|
|
Total Observed
|
11
|
For pairs that danced for females more than 10 times:
|
<0
|
6
|
|
>0
|
3
|
|
Total observed
|
9
|
1. Based on the results described above and in Table 1, are alpha and beta males related? Does increasing relatedness result in increasing success?
Investigating delayed benefits:
McDonald also investigated whether there are delayed benefits for other males, particularly the beta male. From observing leks over a ten-year period, McDonald obtained the results in Tables 2-5
Table 2. Copulations by alpha and beta males, 1983-1992.
Copulations by alpha males 259
Copulations by beta males 4
Total observed 263
Table 3. Obseerved successions to alpha role.
Alpha succeeeded by beta 11
Alpha succeeded by other 0
Table 4. Site fidelity across years.
Females returned to same lek 16
Females went to new lek 11
Total 27
Table 5. Delayed benefits to subordinate males.
Time to beta status 4-8 years
Time as a beta male 1-5 years
Average age of alpha males 10.1
2. How often do beta males copulate in comparison to alpha males, and in comparison to more junior males?
3. Why might younger males delay reproduction? What benefits are there for young males to delay reproduction, albeit for a long time?
Table 6. Mating success of new alpha birds in comparison with old.
|
Visits (per 2 hours) Beta Predecessor
|
Copulations (per 2 hours) Beta Predecessor
|
|
7.34
|
6.02
|
0
|
0
|
|
83.29
|
120.24
|
0.086
|
0.087
|
|
89.34
|
98.84
|
0.014
|
0.045
|
|
98.84
|
173.28
|
0.045
|
0.111
|
|
157.69
|
290.45
|
0.143
|
0.136
|
|
304.22
|
157 69
|
0.217
|
0.143
|
|
290.45
|
420.64
|
0.136
|
0.327
|
|
324
|
260.3
|
0.35
|
0.29
|
4. Recently, Trainer and Parsons (2001) found that long-tailed manakin songs exhibit very little geographic variation. Propose a hypothesis based on the experiments described here to explain the low levels of variability in song observed in this species in comparison with close relatives.
5. Male long-tailed manakins exhibit delayed plumage maturation. Alpha and beta males possess mature plumage, whereas the other individual males in a group often still possess juvenile plumage. McDonald (1993; not available) used stuffed birds with mature and immature plumage to investigate responses of cooperating groups to newcomers. What responses would you expect to see?