Evolution's Rainbow. Joan Roughgarden
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relationships that involve honesty and cooperation as much as or more than deceit and competition.
Scientists are open about their predilection for seeing deceit everywhere. They write, “Natural selection favors those individuals that are able to increase their own fitness by manipulating the behavior of others,” and “Cooperation might be seen as the opposite of competition . . . it is instead another form of selfish behavior.”35 These attitudes spin how animal behavior is interpreted and predetermine what data are taken.
The expression “female mimicry” prevents the study of gender variation. The words suggest a male deceptively impersonating a female. In biology, mimicry usually refers to such cases as an edible fly that looks like an inedible bee. “Looks like” here means “exactly like,” not “approximately like.” A fly that mimics a bee almost totally resembles a bee. A good magnifying glass and technical knowledge are needed to tell them apart. A bird flying quickly over the ground can’t spot the difference.36 So-called female mimics don’t exactly resemble females, and all the players have a long time to examine each other. I doubt that female mimicry exists anywhere outside the imagination of biologists.
Thus biologists project scripts of their own prejudices and experiences with male-male competition onto animal bodies and use insulting language about animals. Far from being a sexual parasite, why not see the silent male bullfrog as nature’s antidote to excess macho, preventing the controller from grabbing unlimited power? Far from being a cuckolder, why not picture the feminine male sunfish as nature’s peacemaker? Biologists need to develop positive narratives about the diversity they’re seeing. Then a new suite of hypotheses will emerge for testing, taking the place of the shallow, pejorative, and far-fetched ideas that deceit theory requires.
TRANSGENDER SPECIES
Some species have an appearance or behavior that invites the term “transgender.” These species contain polymorphisms of feminine males, masculine females, masculine males, and feminine females all together, and/or gender-crossing behavior. One study offers comparative data on transgender morphology from museum specimens.
Hummingbirds, the world’s tiniest birds, feed on nectar from flowers and on insects. Their name comes from the buzzing sound their wings make as they hover while feeding at a flower. About 340 species exist worldwide. Hummingbirds typically live three to five years. The smallest is the bee hummingbird of Cuba—a bird only 2.25 inches long.
Hummingbirds also have the smallest eggs of all birds—half the size of a jellybean. A female builds the nest and broods the young alone. Males only fertilize. A typical nest is tiny—about the size of a bubblegum ball. A female incubates two eggs for two to three weeks and feeds them for three more weeks. Because females do all the work, males would seem to be in great excess, allowing females to choose among them. Indeed, the males of many hummingbird species are spectacularly colored, which Darwin would argue is what female hummingbirds find handsome. Recently, though, hummingbirds have begun to emerge as the best documented example of transgender expression in birds.
Male sunangel hummingbirds (Heliangelus) of the Andes, from Venezuela through Colombia, Ecuador, Peru, to Bolivia, have colorful feathers on their throats called a gorget. The name comes from a biblike collar of metal armor used in fencing to avoid being pierced in the throat by a sword. In birds, a gorget is a broad band of distinctive color on the throat and upper chest.
Museum specimens revealed that eight of nine sunangel hummingbird species have some percentage of masculine females with gorgets just like those of males. A few instances of males with female coloration were also detected.37 The investigation of masculine females and feminine males has now been extended to forty-two species of hummingbirds from five genera, yielding the first statistical information about transgender expression in birds.38 Of the forty-two species, seven had both masculine females and feminine males, nine had masculine females and no feminine males, two had feminine males and no masculine females, and twenty-four had neither masculine females nor feminine males. Pooling the species with either masculine females, feminine males, or both, yields data on the total variation of gender expression in both sexes. The appearance of a bird’s gorget was divided into four classes, from most femalelike to most malelike. Fifty-two percent of the adult females (288 of 548) were masculine, including 34 percent who were very masculine. In contrast, only 2 percent of the adult males (18 of 745) were feminine.
Not only the total gender variation but also its distribution varied between the sexes. The distribution of masculinity among females was gradual. That is, most of the females were feminine, and the percentage dropped off gradually from most feminine to most masculine. In contrast, the distribution of femininity among males was bimodal. The great majority were masculine, with no intermediates and a small second peak at the most feminine category. Thus the sexes are not symmetric in either the total amount or the distribution of transgender expression.
What does this variation in gender expression mean? Hard to say. The strength of this study on hummingbird museum specimens is its breadth of coverage. The study’s weakness is the absence of field data on how the genders behave. Still, clues are provided by other traits besides the gorget.
Male hummingbirds tend to have shorter bills than females.39 In hummingbirds, bill length is important in indicating the type of flower that is visited—short bills for short, squat flowers, long bills for long, tubular flowers. Overall, short flowers and short-flower users are more abundant than long flowers and long-flower users. Hence, short-billed birds wind up having to defend their flowers and are aggressively territorial compared to long-billed birds, who don’t defend flowers. With its relatively short bill, then, a male typically also has a showy gorget and territorial behavior.
Masculine females have a shorter bill than feminine females, and presumably defend territories containing flowers. It is possible that a male might prefer to mate with a female whose offspring are guaranteed access to resources in the territory she defends. In fact, studies have recently shown that males prefer ornamented (masculine) females in seven bird species, including an auklet, pigeon, swallow, bluethroat, tit, and two finches.40 Thus one guess is that masculine female hummingbirds represent a gender of female controllers maintained in part by male choice.
The feminine males have longer bills than masculine males, even longer than feminine females. Hence feminine males must be using different flowers than the masculine males. The feminine males also have smaller testes than the masculine males, indicating a lower allocation of energy to sperm production. Perhaps the feminine males are pair-bonded to masculine females, as another case of gender meshing. Alternatively, they may have a role in facilitating courtship. Also, mating in some of the species takes place in leks, suggesting a comparison to the feminine male sunfish and ruffs found in leks. Perhaps the feminine males have a role in facilitating courtship at the leks. In any case, the data on museum specimens show that transgender expression is widespread in hummingbirds, inviting follow-up fieldwork.
For a case of transgendered behavior, let us turn to the opposite extreme in data collection, a single individual in the field. Hooded warblers (Wilsonia cirtina) live in woods of the mid-Atlantic United States. They are named for the black plumage that adult males have on their heads—a hood. Some females also have these black hoods, and can’t be distinguished from males by birdwatchers.41 Early on, variation in female plumage color was thought to represent age, but later work showed that the color is permanent, suggesting a genetic polymorphism for color. About 5 percent of the females very closely resemble males.
Of particular interest is one transgendered black-hooded warbler that was discovered in Maryland.42 The bird was originally assumed to be a masculine female, but was later discovered to be gonadally male. Black-hooded warblers are monogamous, and the transgendered bird behaved as the female member of a monogamous pair, consistently showing female-typical behavior throughout two years, including nest building, incubating and brooding young, and not singing or engaging in territorial defense. The bodily appearance of the transgendered bird was typical of males, the behavior typical of females. This bird pair-bonded to a male who was also typically male in both appearance and behavior.43 In this case, a male-bodied bird behaved in all respects like a female, except for laying eggs. Gender identity in this individual hooded