Evolution's Rainbow. Joan Roughgarden

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СКАЧАТЬ for granted and hoped to catch a perch or other rarer fish. I would never have guessed that these everyday freshwater fish from the United States and Canada would someday challenge the foundations of gender and sexuality.

      One sunfish species, the bluegill sunfish (Lepomis macrochirus), has been studied in detail at Lake Opinicon, Ontario, Canada, and at Lake Cazenovia, in upstate New York.⁷ Spawning males consist of three distinct size/color classes, and together with females, fall into four morphological categories, corresponding to four distinct genders:

      1 Large males are about 17 centimeters long and eight years old. Their gonads constitute 1 percent of body weight, and they have a light body color with a yellow-orange breast.

      2 Medium males are about 10 centimeters long and four years old. Their gonads are 3 percent of body weight, and they have a dark body color marked with dark vertical bars.

      3 Small males are about 7 centimeters long and three years old. Their gonads are 5 percent of body weight, and they have a uniformly light body color with neither a yellow-orange breast nor dark vertical bars. The testes may occupy most of the body cavity, crowding the stomach and displacing the intestines.

      4 Females are about 12 centimeters long and six years old, making them larger than the medium males by about 2 centimeters and older by about two years. At breeding, females bulge somewhat with eggs. Females have a dark body color with vertical bars, like the medium males. The medium males somewhat resemble small young females because of the similarity of color pattern.

      The yearly spawning episode lasts only one day. In preparation, large males aggressively stake out territories next to one another in aggregations of a hundred or more, called leks, along the bottom of the lake at a depth of 1 meter. Large males are called on to defend their space against neighbors about once every three minutes. Large males make nests for eggs in their territories by scooping out a depression in the mud with their tails. Females aggregate at the locales with many males and do not visit isolated or peripheral nests. Females prefer nests belonging to large aggregations because the presence of many males affords more protection from egg predators.

      The large males are not Mr. Nice Guys. Their acts of aggression include biting, opercular spreading, lateral displays, tail beating, and chasing. Although primarily directed at intruding males, aggression sometimes is directed at a female in the territory—domestic violence, sunfish style. The male apparently tries to control the speed and timing at which a female lays eggs. Females simply leave if harassed too much in this way.

      The females arrive in a school, and one by one they enter the territories of the large males. When a female arrives, a large male begins to swim in tight circles, with the female following. Every few seconds as the pair turns, the female rotates on her side, presses her genital pore against that of the large male, and releases eggs that the large male fertilizes. The egg release is visible as a horizontal dipping motion.

      A female may spawn in many nests. A large male accumulates up to thirty thousand eggs from various females during the one-day spawning episode. A female lays about twelve eggs at a time with her dipping motion, so this total egg accumulation involves some female laying in the nest about once every thirty seconds. The scene is fast. Still, large males somehow find the time to enter the nests of neighbors, and about 9 percent of the fertilizations in a nest are by a neighboring large male.

      Meanwhile, the small males are active. They stay at the borders between territories of large males and in the periphery, often close to rocks or in vegetation. Eggs remain viable in lake water for about an hour and sperm for only a minute. When the female releases eggs, the small males dart in quickly to release sperm over the eggs and carry out their own fertilizations. The large males try to repel the small males from their territories, but the small males are more numerous than the large males—about seven to one in shallow-water colonies. Chasing all these small males, as well as neighboring large males and the occasional predator, takes a large male away from fertilizing the eggs being laid in his territory. In these circumstances, the females spawn readily with small males while the large male is busy with all his chasing.

      There are more small males in shallow-water colonies than deep ones because there is more vegetation for cover. It is important to hide because predators—large-mouth bass, small-mouth bass, and pike—lurk in the lake. Thus the ratio of small to large males depends on the surrounding environmental context. All in all, the small males seem to be the gender counterpart of silent bullfrogs, silent singing fish, jack and parr salmon, and antlerless male deer.

      The medium males—the third male gender—are really surprising. No one knows where the medium males live most of the time, but they may school with the females. A medium male approaches the territory of a large male from above in the water and descends without aggression or hesitation into the large male’s territory. The two males then begin a courtship turning that continues for as long as ten minutes. In the end, the medium male joins the large male, sharing the territory that the large male originally made and defends.

      Although the medium male sometimes joins the large male before a female has arrived, more often the medium male joins after a female is already present. The large male makes little if any attempt to drive away the medium male, in contrast to the way the large male drives away small males that dart into the territory. When a female and two males are present, the three of them jointly carry out the courtship turning and mating. Typically, the medium male, who is smaller than the female, is sandwiched between the large male and the female while the turning takes place. As the female releases eggs, both males fertilize them.

      Occasionally, two females may be within a large male’s territory at the same time. Although the large male mates with both females, the three do not participate in any common ritual similar to the three-way interaction of the female with a large and a medium male.

      After the day’s excitement is over, each large male remains in his territory for eight to ten days to guard the eggs. The large male repels nest predators. During this period he never leaves the nest to forage and loses body weight.

      In all, 85 percent of spawning males are either small or medium, with the remaining 15 percent large males. Though 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 percent of the spawnings. Overall, 85 percent of the territories in which spawning occurs consist of one male with one female, 11 percent of two or more males and one female—usually a large male accompanied by a medium male—and 4 percent of one male and two 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 of the large male gender.

      Explaining the medium male gender has caused big-time confusion among biologists. Three theories have emerged.

      DECEIT

      The most popular theory is that by sharing some female coloration and participating in courtship turning, a medium male deceives a large male into thinking he’s a female.⁸ This female-disguised male then steals some of the fertilizations that rightfully belong to the large male.

      

      I don’t find this theory plausible. Sunfish, which prey on tiny shrimp in the water during the day, have great eyesight. As predators, they estimate size and distance very well, constantly using these abilities to decide which prey to catch and which to ignore. Lots of visual cues distinguish a medium male from a female, including simply size and shape. СКАЧАТЬ