MadSci Network: Development |
In its simplest form, sex is the ability to exchange genetic material between two individuals usually for the purposes of procreation. When procreation is the goal, one partner tends to contribute a greater amount of nongenetic material to the procreation effort and we call that partner female - by default the one that's left is the male. For most forms of life, fertilization, that actual exchange of genetic material, takes place outside the physical body. This means that quantity of the nongenetic contribution, and therefore the distinction between male and female, can be very blurry. In most cases it doesn't really matter. Sometimes, a better way to distinguish male from female is to examine the reproductive strategy. The female invests a relatively greater amount of energy and effort into a fewer number of potential offspring (she values quality over quantity). The male invests as little as possible in each individual potential offspring, but tries to produce as many potential offspring as possible (he values quantity over quality). Still, there are some creatures that hedge their bets, and play both strategies - these are the hermaphrodites.
The true hermaphrodite is a creature with a complete, and functional, set of both the male and female sexual organs. This is different from a creature that is self-fertilizing, such as a plant, since that represents a re-mixing of genes but not an exchange. There are several examples of true hermaphrodites in nature, such as the earthworm. With all true hermaphrodites, two individuals must still exchange genetic material.
Notice, that so far I have not used the X and Y chromosome definitions of sex. The use of this system is limited only to mammals, and that leads to many misunderstandings. For example. XX (the homozygous condition) is female in mammals, XY (the heterozygote) is male and an individual born with a single X chromosome (XO or Turner's Syndrome) develops in a vaguely female fashion. This gives the impression that "female" is the default setting - but nothing could be further from the truth. In birds, the sex chromosomes are designated as M and N. The individual born MM (homozygote) is male, and the MN individual is female - giving the impression the "male" is the default. But, in some types of reptiles, amphibians and fish there are no "sex" chromosomes. In most cases, male and female is determined by the environment or even the temperature. There is even a type of adult fish that will change sex depending upon population structure, if there are too many males some will become female or vice versa. Clearly sex is much more complicated than a single gene (or locus) on a chromosome.
So, what are we dealing with when we discuss human hermaphrodites? Actually they are more properly called pseudohermaphrodites (which is the term used by biologists) or "Intersexuals" (which is the term that seems to be preferred in the politically correct circles - check out the web page at www.isna.org). If we examine the course of human embryological development we can see that we are all originally equipped with the potential to form both the male and female genitalia (which are actually outgrowths of early types of kidneys), but we have only one set of gonads. During normal male development, the gonads form into testis, the male based genitalia enlarge and the female based genitalia regress. In female development, the opposite happens, gonads become ovaries male genitalia regress and female genitalia enlarge. Sexually ambiguous individuals develop in a way that both sets of genitalia enlarge. Notice, that these individuals still only have one set of gonads. These gonads will either be testis or ovaries and that will truly identify the sex of the individual. In some cases, the gonad will itself be ambiguous and share histological characteristics of both testis and ovaries, but in this case it will be able to function as neither. So the individual that develops is not a true hermaphrodite since they lack any functional reproductive organ.
These types of "sexual" confusions are more confusing to us as humans than they are in the natural world. Cases of clitoral hypertrophy (a penis-like organ on the female) is quite common in nature and is actually the "normal" condition in many types of animal (such as the hyena and the spider monkey). Similarly, undescended testis are common in elephants. It becomes confusing in humans because much of our role in society is determined by sex. If we don't know whether someone is male or female (whether this is due to biological ambiguity or some type of cross-dressing) our culture does not instruct us well as to how we should interact with that person.
Under most circumstances, we have as much control over choosing our culture as we do over choosing our sex. Therefore, I am hesitant to criticize anyone too harshly on this issue. Our culture demands that we categorize individuals, and when male and female do not seem to apply well, we fall back on the term Hermaphrodite, and that term is about as culturally useful, and potentially hurtful, as any other. But: I will state clearly, as a biologist, that a true hermaphrodite cannot happen in humans. The person you are looking at is either male or female, possibly neither -- but never both.
References: Hickman, CP, LS Roberts & FM Hickman (1984) Integrated Principles of Zoology. Mosby College Publishing: St. Louis Gerhart, J & M. Kirschner (1997) Cells, Embryos and Evolution. Blackwell Science: Malden, MA
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