Evolution's Rainbow. Joan Roughgarden
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to have more females than it does males with territories. During spawning, several females arrive at one nest, and because only one female can spawn at a time while the large male is there, the others wait at the rim of the nest. But females may try to crowd in and interrupt the female who is laying eggs. In this situation, the medium male expels the excess females so the spawning can continue. The medium male then stays with the nest for three days, compared with the full week that the large male sticks around.
The spotted European wrasse has three developmental pathways. One type skips early reproduction and matures into a controller. The second starts as an end-runner and transitions into a cooperator. The third starts as a cooperator and transitions into a controller.11 Thus, comparing wrasses to sunfish, the operational sex ratio may determine whether the medium male is feminine or masculine. If females are scarce, as in sun-fish, the large male will need help attracting them, and a feminine male can assist. If females are common, as in wrasses, the large male will need help keeping order at home. In this case, the controller employs a bouncer instead of a marriage broker.
Another particularly graphic case is found among the cichlids, a family of colorful perchlike fishes found in the tropical freshwaters of Africa, South and Central America, India, Sri Lanka, and Madagascar. About 1,500 species are known, or 5 percent of all vertebrate species! Most species occur in the Great Rift lakes of eastern Africa—Lake Malawi, Lake Tanganyika, and Lake Victoria. Cichlids are most closely related to the saltwater damselfishes, wrasses, and parrotfishes, among others.12 They are the freshwater equivalent of the colorful and diverse coral reef fish.
Oreochromis mossambicus, a kind of cichlid, from the Incomati River in Mozambique were studied in an aquarium in Portugal.13 These fish are rather small, around 6 centimeters, and males come in three genders. The controller recruits the cooperator through a courtship that includes remarkable same-sex sexuality.
The black territory-controlling males form dense aggregations, leks, in the sand or mud during the breeding season. A male digs a pit to attract a female, and after courtship, she lays eggs there. The male then quivers and releases spawn over the eggs. The female inhales the mixture of eggs and spawn into her mouth where the actual fertilizations take place. The female then broods the eggs in her mouth, continuing brooding even after the young fry have hatched, for a total of three weeks. The young are “born” when they swim out of the female’s mouth.
The second male gender is the familiar end-runner, who darts into a controlling male’s territory during spawning and adds some of his own spawn to the mix inhaled by the female. The controlling male aggressively repels the end-runners. The third male gender is once again the most puzzling one. These males have a neutral light color and are actively courted by controller males using the full courtship repertoire used for females, including tilting, signaling the nest, circling, and quivering.
Of six hundred courtships observed, two hundred were directed to these light-colored males and the remaining four hundred to females. In three of the male-male courtships, the light-colored male placed his mouth on the genital papillae of the dark territorial male, then the territorial male quivered and released spawn, whereupon the light-colored male moved his mouth as a female does when she inhales the sperm/egg mix. The end-runners did not intrude into these male-male courtships, although they did dart into male-female courtships, indicating that everyone around knew what was going on.
In most groups of these fish, males courted females more than males. In one group, though, males courted males more than females. The authors concluded that “further experimental work is needed.” In particular, the benefits to the controller presumably provided by the third male gender need to be described.
Male genders may also range from territorial stay-at-homers to non-territorial travelers. Although many of the three-male species are organized according to the template of controller, end-runner, and cooperator, not all are. Among tree lizards (Urosaurus ornatus) living in the American Southwest,14 the males come in various colors—nine are known for the species. Some populations have only one color, others only two colors, and still others as many as five.
At one site upstream of the Verde River in Arizona, two colors each account for 45 percent of the total males. The orange-blue form is a punk rocker’s delight—an orange chin with a big blue spot in the middle, a throat fan with orange near the body and a blue band at the tip, and blue on the stomach. In contrast, the orange form is solid orange on the chin, throat fan, and stomach. The orange-blue males are the most aggressive, and their body proportions are short and stocky. They are the controllers, defending territories large enough to overlap the territories of three to four females. The orange males are end-runners, but they come in two subvarieties, nomadic and sedentary. Unlike most end-runners, these males are the same weight or heavier than the controllers, although longer and leaner. These males are not aggressive and defer to controllers when challenged.
In a typical dry year, orange males are nomadic, spending only a day or two at a site before moving on. In a rainy year, orange males settle down for the season, becoming sedentary and occupying relatively small territories, the size of a female home range. The controllers and end-runners are fixed for life, although an end-runner can transition back and forth between nomadic and sedentary styles in successive years.
In this species, the hormonal dimension of gender expression has been worked out. Progesterone determines whether a male matures into a controller or an end-runner. A single injection of progesterone given to a tiny hatchling on the day he hatches from the egg will ensure that he matures into an orange-blue controller. In contrast, males with low progesterone develop into orange end-runners.15 No intermediates occur. Presumably genes produce high or low progesterone levels on the day of hatching, thereby determining whether a male develops into a controller or an end-runner.
As the season progresses and the lizards wait for rain, the oranges listen to the lizard version of Emmylou Harris’s song “Born to Run.” How does an orange male get to feelin’ like it’s time to hit the road rather than settlin’ down for a spell? Orange males are sensitive. When conditions are dry, orange males show high levels of the hormone corticosterone, an indicator of stress. In orange males, corticosterone causes testosterone to decline. This drop in testosterone in turn causes orange males to hit the road and become nomadic.16 Meanwhile, orange-blue males are indifferent to weather conditions—they tough it out no matter what.
The tree lizards illustrate what are called the “organizing” effects of hormones (irreversible effects that occur early in development) and also the “activating” effects (reversible effects, which usually occur later in life). Progesterone on the day of hatching organizes the male body to mature into an orange-blue controller. Corticosterone from the stress of going thirsty during a drought activates the orange end-runner to turn nomadic rather than remaining sedentary.
TWO MALE, TWO FEMALE
When multiple genders occur in both males and females, we may wonder whether some gender combinations don’t mesh together especially well. Would a feminine male paired with a masculine female be just as successful as macho male with a femme female? What about other pairings too?
White-throated sparrows (Zonotrichia albicollis) of Ontario, Canada, have four genders, two male and two female:
1 A male with a white stripe is the most aggressive, calls often, and is the most territorial.
2 A male with a tan stripe is less aggressive and unable to defend a territory from the white-striped male.
3 A female with a white stripe is aggressive, calls spontaneously, and defends a territory.
4 A female with a tan stripe is the most accommodating of all. When challenged with a territorial intrusion, she continues foraging.17
So, in both males and females, the white-striped individuals are more aggressive than the tan-striped individuals. Ninety percent of the breeding pairs involve either a white-striped male with a tan-striped female or a tan-striped male with a white-striped female—attraction between opposites.
The white-striped male appears to have everything going for him. A female who chooses to pair with