Evolution's Rainbow. Joan Roughgarden

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СКАЧАТЬ body. A body becomes “sexed” when classified with respect to the size of the gametes produced. Thus, gender is appearance plus action, how an organism uses morphology, including color and shape, plus behavior to carry out a sexual role.

      Now we’re free to explore the zoological (and botanical!) counterpart of human gender studies. So, we may ask: How much variety occurs in gender expression among other species? Let’s take some favorite stereotypes and see. We’ll look mostly at vertebrates; even more variety occurs with invertebrates and plants.

      An organism is solely male or female for life. No, the most common body form among plants and in perhaps half of the animal kingdom is for an individual to be both male and female at the same, or at different times during its life. These individuals make both small and large gametes during their lives.

      Males are bigger than females, on the average. No, in lots of species, especially fish, the female is bigger than the male.

      Females, not males, give birth. No, in many species the female deposits the eggs in the pouch of the male, who incubates them until birth. In many species, males, not females, tend the nest.

      Males have XY chromosomes and females XX chromosomes. No, in birds, including domesticated poultry like chickens, the reverse is true. In many other species, males and females show no difference in chromosomes. In all alligators and crocodiles, some turtles and lizards, and the occasional fish, sex is determined by the temperature at which the eggs are raised. A female can control the sex ratio among her offspring by laying eggs in a shady or a sunny spot.

      Only two genders occur, corresponding to the two sexes. No, many species have three or more genders, with individuals of each sex occurring in two or more forms.

      Males and females look different from one another. No, in some species, males and females are almost indistinguishable. In other species, males occur in two or more forms, one of which resembles a female, while the others are different from the female.

      The male has the penis and the female lactates. No, in the spotted hyena, females have a penislike structure externally identical to that of males, and in the fruit bat of Malaysia and Borneo, the males have milk-producing mammary glands.⁸

      Males control females. No, in some species females control males, and in many, mating is a dynamic interaction between female and male choice. Females may or may not prefer a dominant male.

      Females prefer monogamy and males want to play around. No, depending on the species, either or both sexes may play around. Lifelong monogamy is rare, and even within monogamous species, females may initiate divorce to acquire a higher-ranking male.

      One could tick off even more examples of gender stereotypes that are often thought to be “nature’s way” but that have no generality within biology. Instead, let’s look closer at the lives of these organisms to see whether what they do makes sense to us. Be prepared, though, to shrug your shoulders and wonder about the mystery of life.

      Note that by defining gender as how an organism presents and carries out a sexual role, we can also define masculine and feminine in ways unique to each species. “Masculine” and “feminine” refer to the distinguishing traits possessed by most males and females respectively. Cross-gender appearance and behavior are also possible. For example, if most females have vertical stripes on their bodies and males do not, then a male with vertical stripes is a “feminine male.” If most males have antlers and females do not, then a doe with antlers is a “masculine female.”

      Politically, locating the definition of male and female with gamete size keeps society’s gender categories at arm’s length from biology’s sex binary. We don’t have to deny the universality of the biological male/female distinction in order to challenge whether the gender of whole organisms also sorts into a male/female binary. In humans specifically, a gender binary for whole people is not clear-cut even though the difference between human sperm and egg is obvious—a size ratio of about one million to one.

*A gamete is a cell containing half of its parent’s genes. Fusing two gametes, each with half the needed number of genes, produces a new individual. A gamete is made through a special kind of cell division called meiosis, whereas other cells are made through the regular kind of cell division, called mitosis. When two gametes fuse, the resulting cell is called a zygote. A fertilized egg is a zygote.

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      Sex within Bodies

      Although the binary in gamete size is practically universal, the way male and female functions are packaged into individual bodies does not fit into any consistent polarity. We tend to think that males and females must be in separate bodies because most of us are, as are most of the animals we live with, such as our pets, domesticated stock, and the birds and bees around our parks. However, many species have other ways of organizing sexual functions.

      An individual body who makes both small and large gametes at some point in life is called a hermaphrodite. An individual who makes both sizes at the same time is a simultaneous hermaphrodite, and one who makes them at different times is a sequential hermaphrodite. Most flowering plants are simultaneous hermaphrodites because they make pollen and seeds at the same time. Pollen is the male part of a plant and the ovule is the female part. A pollinated ovule turns into a tiny embryo that detaches, to be blown away by the wind or carried away by an animal.

      Among animals, hermaphrodism is common in the ocean.¹ Most marine invertebrates, such as barnacles, snails, starfish, fan worms, and sea anemones, are hermaphroditic. Many fish are too. If you go snorkeling at a coral reef in Hawaii, the Caribbean, Australia, or the Red Sea, chances are that about a quarter of the fish you see will be hermaphroditic. Or take a look at some of the colorful fish popular in tropical aquaria—they are often sequential hermaphrodites. Most species of wrasses, parrot fish, and larger groupers are hermaphroditic, as are some damselfish, angelfish, gobies, porgies, emperors, soapfishes, dottybacks, and moray eels (all from shallow waters), and many deep-sea fish as well.²

      Hermaphrodism is a successful way of life for many species; my guess is that hermaphrodism is more common in the world than species who maintain separate sexes in separate bodies (called gonochorism). The separate-sex/separate-body state is often viewed as “normal,” suggesting that something unusual favors hermaphrodism in plants, on coral reefs, and in the deep sea. Alternatively, hermaphrodism may be viewed as the original norm, prompting us to ask what there is in mobile organisms in the terrestrial environment that favors separate sexes in separate bodies.

      WHAT FISH CAN TELL US

      FEMALES CHANGING TO MALE

      Sex change is only one of several interesting aspects of coral reef fish society. The bluehead wrasse is named for the blue head of the largest males. When small and just entering sexual maturity, fish of both sexes look similar. Later three genders develop. One gender consists of individuals who begin life as a male and remain so for life. Another gender consists of individuals who begin as females and later change into males. These sex-changed males are larger than those who have been male from the beginning. The third gender consists of females who remain female. We’ll call the two male genders the “small unchanged males” and the “large sex-changed males,” respectively. The large sex-changed males are the biggest individuals of the three genders, and they attempt to control the females. In some species, the large sex-changed males maintain and defend the females, and in others they defend locations that females appear to prefer.

      Fertilization is external—a female releases eggs into the water and a male then releases a cloud of sperm around the eggs to fertilize them. The unfertilized eggs are out in the open and can potentially be fertilized СКАЧАТЬ