Hermaphrodism (zoology)
Hermaphroditism in zoology refers to the condition in which an individual possesses both male and female reproductive organs. This phenomenon can occur in various animal species, where individuals may produce both eggs (typical of females) and sperm (typical of males). Hermaphroditism can be categorized into two main forms: simultaneous and sequential. Simultaneous hermaphrodites can produce both gametes at the same time, allowing for flexible mating strategies, such as in earthworms. In contrast, sequential hermaphrodites start life as one sex and can change to the other later, often influenced by factors like body size or social dynamics.
For example, clownfish are protandrous sequential hermaphrodites, beginning as males and potentially changing to females as they grow larger. Conversely, some species are protogynous, starting as females and able to transition to males under certain conditions. The triggers for these sex changes often relate to environmental factors and social structures rather than just absolute body size. Overall, hermaphroditism showcases the diversity of reproductive strategies in the animal kingdom, highlighting how organisms adapt to their ecological niches.
Hermaphrodism (zoology)
In most species, reproduction involves sex—that is, the joining of genetic material from two individuals to create new, genetically unique offspring. In sexually reproducing species, females are those individuals who produce relatively large sex cells that are full of nutrients (eggs), and males are those individuals who produce relatively small sex cells that have little or no nutrients but which can be produced in much greater numbers (sperm). Generally, there are other differences between the sexes as well—differences in hormones, anatomy, body shape, size, color, and behavior. Such distinguishing features are referred to as sex-limited traits because they appear in only one sex; collectively, they result in sexual dimorphism—literally, the two sexual forms of a single species.
Whether an individual animal develops into a male or a female depends on which genes get turned on early in development. In mammals, genes for maleness get turned on in individuals with an X and a Y chromosome, while genes for femaleness get turned on in individuals with two X chromosomes. In other animals, sex determination may depend on other factors. For example, in many reptiles, development into a male or a female depends upon the embryo's temperature as it develops in its egg.
Hermaphrodites, referred to as intersex individuals in the human species, are individuals that have both male and female reproductive organs—that is, they have both ovaries to produce eggs and testes to produce sperm. In most animals, hermaphroditism results from abnormal development and is extremely rare. In some species, however, hermaphroditism is normal.
Simultaneous Versus Sequential Hermaphroditism
Depending on the species, hermaphroditism can be found in either simultaneous or sequential form. In species with simultaneous hermaphroditism, individuals are simultaneously both male and female; each adult has the ability to produce both sperm and eggs. Depending on the species, a single reproductive encounter between simultaneous hermaphrodites may involve both partners exchanging sperm and eggs (for example, earthworms) or may involve the partners taking turns as male and female (for example, some coral reef fishes).
On the other hand, individuals of species with sequential hermaphroditism start life as either male or female but have the ability to change sex at some later point. In protandrous species, individuals start out as male and have the potential to later change to female. These are typically species that require large body bulk before they can produce eggs, so individuals start out as male and change to female only if they get old enough and large enough to make eggs. An example of protandrous fish are the clown fish. Clown fish travel and live in groups containing one large male and one large female who breed together and a school of smaller fish. The fish in the school are all male. If the large female is removed, her partner fish changes to a female and the next largest fish in the school joins her to make a pair. In this case, the role of the female is revolving. In protogynous species, individuals start out as female and have the potential to later change to male. These are typically species in which males must defend a harem or a territory to mate, so individuals start out as female and change to male-only if they get large enough to fight and win. Several fish species are protogynous.
Triggers for Sex Change
In both protandrous and protogynous species, the trigger for sex change may relate not only to body size but also to the social structure of the individual’s community. For example, a large female of a protogynous species may not change to male if there is an even bigger male present who would clearly win every fight. On the other hand, a relatively small female might change to male if she is the biggest female around and the larger local males suddenly died or disappeared. Thus, body size is a relative, not absolute, trigger for a sex change, and in a very few species individuals can revert back to their original sex if social circumstances change again.
When a sequential hermaphrodite changes sex, it changes not only its reproductive organ or gonad but also all the other sexually dimorphic aspects of its anatomy, physiology, and behavior. Hormones and behavior are the first things to change; then, over a period of time, the hormonal changes induce changes in the gonad and other tissues, including the brain. As a hermaphroditic fish, for example, switches from one sex to another, it may change the size and shape of its fins, its color, its aggression level, and its sexual preferences and rituals. These miraculous changes provide visible proof of the (generally silent) presence in both sexes of sex-limited genes for both male and female attributes.
Principal Terms
Gonad: the organ that produces reproductive cells (sperm or eggs)
Protandry: the condition of starting out male with the potential to become female
Protogyny: the condition of starting out female with the potential to become male
Sequential Hermaphrodite: species or individual with the potential to change from one sex to the other
Sex-Limited Traits: features that are only expressed in one sex
Sexual Dimorphism: the existence of anatomical, physiological, and behavioral differences between the two sexes of a species
Simultaneous Hermaphroditism: the condition of being simultaneously male and female
Bibliography
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