In all, 85% of spawning males are either small or medium, with the remaining 15% large males. Although in the minority, large males take part in most of the matings.
Among the large males, the reproductive skew is high and only some of the large males apparently survive the mutual aggression that is necessary to acquire a successful territory. The small and medium males obtain about 14% of the spawnings. Overall, 85% of the territories in which spawning occurs consist of 1 male within 1 female, 11% of 2 or more males and 1 female—usually a large male accompanied by a medium male—and 4% of 1 male and 2 females.
Developmentally, the small and medium males are one genotype, and the large males another. Individuals of the small male genotype transition from the small male gender into the medium male gender as they age, whereas individuals of the large male genotype are not reproductively active until they have attained the size and age fo the large male gender.
Third male as mating facilitator?
Instead of deceit theory or ungendered signaling, Roughgarden proposes a third interpretation:
Once the medium male is sandwiched between the large male and the female during their combined courtship turns, the medium male may protect the female from spawning harassment [from the aggressive large males] through his position between her and the large male.
Also, the medium male may have developed a relationship with the females while schooling with them, and thus able to vouch that the large male is safe.
I suggest that the feminine male is a “marriage broker” who helps initiate mating, and perhaps a “relationship counselor” who facilitates the mating process once the female has entered the larger male’s territory. This service is purchased by the large male from the small male with the currency of access to reproductive opportunity.
Sharing fertilization represents an incentive to stay, not theft...Nothing prevents animals from cooperating in bringing about a mating, as well as in caring for young after a mating...In view of the roles played by the three male genders, let’s agree to call the large male a ‘controller,’ the small male an ‘end-runner,’ and the medium male a ‘cooperator.’
References
For Lake Opinicon studies, see:
M.R. Gross, 1982, Sneakers, satellites and parentals: Polymorphic mating strategies in North American sunfishes, Z. Tierpsychol. 60:1-26.
M. R. Gross, 1991, Evolution of alternative reproductive strategies: Frequency-dependent sexual selection in male bluegill sunfish, Phil. Trans. R. Soc. Lond., ser. B, 332:59-66.
For Lake Cazenovia studies, see:
W.J. Dominey, 1980, Female mimicry in bluegill sunfish—a genetic polymorphism? Nature 284:546–48.
W.J. Dominey, 1981, Maintenance of female mimicry as a reproductive strategy in bluegill sunfish (Lepomis macrochirus), Environ. Biol. Fishes 6:59-64.
Gross, 1991, Evolution of alternative reproductive strategies.
Dominey, 1981, Maintenance of female mimicry as a reproductive strategy in bluegill sunfish.
Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 78-81.