Evolution's Rainbow. Joan Roughgarden
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controlling a harem of females. The advantage for males of remaining small and for females of becoming large may account for the developmental progression from male to female.7
MALE AND FEMALE SIMULTANEOUSLY
Hamlets, which are small coral reef basses, don’t have to worry about choosing their sex: they are both sexes at the same time. However, they cross-fertilize and must mate with a partner to reproduce. These simultaneous hermaphrodites change between male and female roles several times as they mate. One individual releases a few eggs and the other fertilizes them with sperm. Then the other releases some eggs, which the first fertilizes with sperm, and so on, back and forth.8
No one has offered any suggestions about why hamlets are simultaneous hermaphrodites. Deep-sea fish also tend toward simultaneous hermaphrodism, which for these species is viewed as an adaptation to extremely low population density.9 Hamlets don’t have a strange appearance, nor do any other hermaphroditic fish. Hermaphroditic fish look like, well, just fish. Hamlets are not particularly rare, nor are they derived from ancestors who were rare or lived in the deep sea. So just why hamlets are simultaneously hermaphroditic remains mysterious.
MALE AND FEMALE CRISSCROSSING
Changing sex once may seem a big deal, but some fish do it several times during their life span. An individual may change from an unsexed juvenile to a female, then to a male, and then back to a female. Or it may change from a juvenile to a male, then to a female, and then back to a male. In certain species, sexual identity can be changed as easily as a new coat.
Sex crisscrossing was first discovered in a species of goby, which is the largest family of fish. Gobies are tiny and often live on coral reefs—in this case, on the Pacific island of Okinawa.10 These gobies live as monogamous pairs on branching coral, and the males care for the eggs. About 80 percent of the juveniles mature female, and the rest mature male. Some of the females later switch to male, and some of the males later switch to female. Of those that have switched once, a small fraction later switch back again—the crisscrossers.
Why go to the expense of changing one’s sexual wardrobe? One theory envisages pair formation in gobies as resulting when two larvae drop out of the plankton together onto a piece of coral.11 They awake after metamorphosis to discover that they are both the same sex. What to do? Well, one of them changes sex. Changing sex has been suggested as a better way of obtaining a heterosexual pairing than moving somewhere else to find a partner of the opposite sex when traveling around is risky. Thus this theory comes down to a choice: switch or move. This theory is rather heterosexist, though. As the hamlets show, a heterosexual pair is not necessary for reproduction, because both could be simultaneously hermaphroditic and not have to bother with crisscrossing.
A species of goby from Lizard Island on Australia’s Great Barrier Reef has recently been discovered to crisscross, but in a way that is interestingly different from the Okinawan goby.12 In the Australian goby, all the juveniles mature into females, with some later becoming males. The males, however, can change back into females. In fact, the meaning of male is ambiguous here. The investigators defined a male to be any fish with at least some sperm production. All males, however, contain early-stage oocytes—cells that develop into eggs—in their gonads. So all the males remain part female. The species therefore consists of two genders at any one time: all-female fish and part-male-part-female fish.
Among flowering plants, populations with hermaphrodites and females are common,13 more so than populations with males and females. These mixed hermaphrodite/single-sex species contrast with most plant species, which are entirely hermaphroditic. (Perhaps as more gobies are investigated, a species will be found consisting of females and hermaphrodites, just as in plants.)
Plants also offer the most amusing examples of crisscrossing sex changes. In a tropical ginger from China, some individuals are male in the morning, making pollen, while others are female in the morning, receiving pollen. Then they switch sexes in the afternoon. This phenomenon, called flexistyly, is known in eleven families of flowering plants.14 The ginger’s diurnal sex change is not too different from how hamlets mate, where members of a mating pair switch back and forth between male and female once a minute.
These examples of sequential, simultaneous, and crisscrossing hermaphrodism show that male and female functions don’t need to be packaged into lifelong distinct bodies. Hermaphroditic vertebrate species are successful and common.
INTERSEXES IN MAMMALS
Can mammals be hermaphroditic too, or have we been left out? Not entirely. Mammals described as hermaphrodites are often reported, although the word “hermaphrodite” is misleading.
Let’s work out some definitions. The reproductive system in mammals consists of gonads—the place where eggs and sperm develop—and plumbing, which transports gametes from the gonads to their destination. The plumbing consists of internal pipes and external valves. The internal pipes are fallopian tubes, muellerian ducts, and so forth. External valves include the penis, clitoris, scrotum, labia, and so on. An “intersexed” individual has gonads to make both eggs and sperm and/or combinations of sperm-related and egg-related plumbing parts. With so many parts in the overall system, many combinations are possible.
To be more specific, we can distinguish intersexed gonads, with some combination of ovarian and testicular tissue, from intersexed genitals, with some combination of egg- and sperm-related plumbing.15 We could even distinguish internal genitally intersexed and external genitally intersexed to pinpoint where the combined plumbing is located. Although the gamete-size binary implies that only two sexed functions exist, many body types occur, ranging from all-sperm parts, through various combinations of both sperm- and egg-related parts, to all-egg parts.
To manufacture a hermaphrodite using mammalian components on a vertebrate chassis, two entire sets of gonadal and plumbing parts are needed, one for eggs and one for sperm. Mammals show many partial combinations of sperm- and egg-related parts. All the partial combinations could be stirred together into a putty from which evolution might someday mold a full mammalian hermaphrodite if selection pressure for that arose, a pressure such as those to which coral reef fish have already responded. In some mammalian species, intesexed bodies are a minority; in others, the majority.
Antlers offer easy-to-see clues for possible intersexed individuals. White-tailed deer (Odocoileus virginianus) possess a male body type, called a velvet-horn because these deer retain the special velvet skin over the antlers that is usually shed after the antlers have aged. Velvet-horn males have small antlers, doelike body proportions and facial features, and small testes; they are said to be infertile. Females typically don’t have antlers, but there is a type of female deer with hard, bony antlers and extensively combined plumbing parts, which is believed to be infertile. A distinct fertile antlerless male morph and a distinct fertile antlered female morph occur as well.
The mention of infertility plays to the prejudice that something is “wrong” with intersexes. But the story is more complicated. The frequency of velvet-horns in white-tailed deer is around 10 percent in some areas and can reach as high as 40 to 80 percent.16 Numbers this big contradict the idea that velvet-horns represent a deleterious mutation.
Similarly, a male morph in black-tailed deer (Odocoileus hemionus) called a cactus buck may be a form of intersex as well. Elk (Cervus elaphus, also called red-tailed deer), swamp deer (Cervus duvauceli), Sika deer (Cervus nippon), roe deer (Capreolus capreolus), and fallow deer (Dama dama) all have a male morph with velvet-covered antlers, called a peruke, that is described as nonreproductive. Moose (Alces alces) have males with velvet-covered antlers, called velericorn antlers, as well as perukes and a small number of velvet-antlered females.17
Because female kangaroos incubate their embryos in a pouch rather than a uterus, an intersexed individual might have both a penis and a pouch, mammary glands and testes. Intersexed kangaroos are known among eastern gray kangaroos (Macropus giganteus), red kangaroos (Macropus rufus), euros (Macropus robustus), tammar wallabies (Macropus eugenii), and quokkas (Setonix brachyurus).18
Kangaroo rats are small mammals that are not marsupials at all, but rather rodents native to the American Southwest.