Evolution's Rainbow. Joan Roughgarden
Читать онлайн книгу.
males. In studies in both 1988 and 1989, more tan-striped males found mates, and found them sooner, than did the white-striped males. So why is the macho white-striped male doing so poorly at attracting mates?
A tan-striped male and a white-striped female work as a team to defend the territory. When the performance of a pair is considered, the team of the tan-striped male and white-striped female is just as effective at repelling intrusions as that of the white-striped male and tan-striped female. The aggressive potential of both teams is the same. The tan-striped males don’t acquire territories until the females arrive to help them out.
Still, why doesn’t a white-striped female pair with a white-striped male? That way she’d form a team with the most aggression of all, and together they would get the best territory of all. Well, tan-striped males provide more parental care than do white-striped males, so when nesting survival is factored in, the white-striped female is better off with a more domestic partner who leaves her to do more of the fighting. Conversely, the tan-striped females provide more parental care than their white-striped counterparts, so a team composed of a white-striped male and a tan-striped female provides the same total parental care as the other type of team.
White-throated sparrows are a neat case of gender meshing. Two kinds of teams provide the same total amount of protection and parental care, but divide the labor differently. These genders represent a genetic polymorphism, in which the body differences are not limited to colored stripes. The brain architecture of the morphs differs. Just as with the morphs of the singing fish, the plainfin midshipman, the differences among the genders extend deep into the body (see also p. 224).
THREE MALE, TWO FEMALE
So far, the most genders that have been described in one species is five: three male and two female genders. The present medal-holder—the side-blotched lizard (Uta stansburiana), from the American Southwest and West—has both males and females of multiple colors, signifying different genders in both sexes. Mortality is high, and the population turns over annually. Three male and two female color morphs occur at a grassland site in Los Baños Grandes in central California:
1 Orange-throated males are controllers. These “very aggressive, ultradominant, high-testosterone” males defend territories large enough to overlap the home ranges of several females.
2 Blue-throated males are less aggressive and juiced with less testosterone. They defend territories small enough to contain only one female, whom they “guard.”
3 Yellow-throated males don’t defend territories. Instead, they cluster around the territories of the orange males, “sneak” copulations, and masquerade as “female mimics.”
4 Orange-throated females lay many small eggs, 5.9 eggs per batch. Orange-throated females, like their male counterparts, are very territorial and, as a result, must distance themselves from one another, achieving a maximum density of only one female per 1.54 square meters.
5 Yellow-throated females lay fewer but bigger eggs, 5.6 eggs per batch. Yellow-throated females, like their male counterparts, are more tolerant of one another and can achieve a maximum density of one female per 0.8 square meter.18
Females lay up to five batches of eggs at monthly intervals during a season. The ratio of males in each gender cycles over time. In one four-year period, blue males predominated in 1991, orange in 1992, yellow in 1993–94, and blue again in 1995. The ratio of females in each morph also cycles, but over a two-year period. The total abundance also fluctuates in a two-year cycle. The female cycle synchronizes with the two-year cycle of total abundance.
Concerning the male genders, an explanation for the male cycling was proposed based on the child’s game of rock-paper-scissors: rock beats scissors, paper beats rock, and scissors beats paper, leading to a never-ending cycle of who’s winning: “Trespassing yellows can fool oranges with their female mimicry. However, trespassing yellows are hunted down by blue males and attacked. Although oranges with their high testosterone and high stamina can handily defeat blues, they are susceptible to the charms of yellows.”19 And so on, in an ecological perpetual motion machine.
Does this theory seem too cute to be true? The problem, as usual, lies with how the nonaggressive male gender is interpreted. First, all the males “sneak,” not only the yellow ones. Thus, the yellow-throated male isn’t distinguished correctly as a sneaker. More important, the yellow male isn’t a female mimic after all. What is supposedly feminine about the yellow-throated male? Early studies indicated that all females had yellow throats. Therefore, the yellow male was thought to resemble a female in throat color. Later study revealed that the orange females had been at a low point of their population cycle during the original study. Once the orange females peaked, the yellow-throated and orange-throated males both resembled corresponding morphs in the females. The loss of throat color as a criterion of femininity left only one other trait: “The most intriguing display that males make, which is restricted to yellow-throated males, is an imitation of the female rejection display. This rejection display is characteristic of post-receptive females and consists of a series of rapid head vibrations [called buzzing]. The male extends his yellow throat, assumes a humped back, and comes in and nips the dominant male on the tail. The parallel between the yellow male . . . and an actual rejection by a bona fide female is extraordinary.”20
Why would performing this one behavior be sufficient to fool an orange male into thinking the yellow male is a female? The blue males aren’t fooled, why only the orange males? This question seems to bother the investigators a bit too. A revealing passage on their website entitled “Are You Blind?” states, “The orange male is somewhat blind and can’t recognize the yellow male in front of him as a male.” A blind lizard that makes its living as a visual predator catching insects? Impossible—the orange males would starve if they were blind. The possibility that the orange male knows what he’s doing and actually wants the yellow male around is never remotely considered.
As for the females, the synchrony between population size and morph ratio in the females suggests that alternating low and high crowding drives the alternating ratios of orange to yellow females. The orange female is more valuable when crowding is low and growth is at a premium. The yellow female is more useful when crowding is high and the ability to pack into a small space is at a premium.21 Thus the polymorphism between orange and yellow females is theorized as a polymorphism between a genotype adapted to low-density conditions and a genotype adapted to crowded conditions.
Still, the alternation of high and low crowding can’t be the whole story. Helping takes place too, not just the negative effects of crowding: “When orange females had more orange neighbors their fitness was reduced, but fitness increased with more yellow neighbors. Yellow female fitness was not affected by the density of either morph. “22 Also, one wonders how the two-year female orange/yellow cycle connects with the four-year male orange/yellow/blue cycle. All in all, this social system with multiple genders would benefit from rethinking and further study.
FEMININE MALES-THE DECEIT MYTH
From what we’ve seen, the notion of a universal male or female template is clearly false. Let’s focus specifically on the males who would seem to most clearly violate the universal male template: the feminine males. The third male gender in bluegill sunfish consists of males that look like females. Are such cross-dressing animals rare? Members of one sex often dress in the clothes of the other. Feminine males especially provoke biologists to froth at the mouth. Why would any self-respecting male want to appear feminine? Well, maybe it’s okay if the purpose is deception. Hey, it’s war out there—a guy does what he has to, even wearing a dress, in order to win.
Let’s look into cases of male-to-female cross-dressing to see if biologists have really demonstrated that the function is deception. If not—banish the thought—we might have to consider that being a feminine male might be adaptive in itself.
The pied flycatcher (Ficedula hypoleuca) is a common insectivorous European bird. Males vary in plumage from a striking black and white to brown, and the colors are inherited. Females are also brown. Some biologists have suggested that the brown males are female mimics, even though brown males have a darker tail and more white on their wings than females. So a human observer can tell