Hermaphrodites reproduce sexually
Evolution of the sexes
Most of the time, reproduction is directly associated with sexuality. But there are also completely different strategies based on the principle of non-sexual, i.e. asexual, reproduction.
Many plants, for example, form offshoots; Bacteria and other protozoa multiply by simply dividing them into two parts. The freshwater polyp Hydra reproduces by budding, in that new individuals grow on its body, which after some time pinch off.
Sexuality and the Origin of the Sexes
First of all, sexuality means that genetic material is exchanged between two cells and thus recombined. For this purpose, cells specialized in reproduction, the so-called sex cells, emerged at an early stage of evolution.
The sex cells contain the maternal or paternal genetic makeup. The union of both cells produces offspring, each with a unique combination of genes from both parents.
In contrast to asexual reproduction, the result of sexual reproduction is a progeny that is genetically different from one another and from their parents.
As a result, the offspring have new properties that enable them to better adapt to the environment, which make them more viable and thus more reproductive. All of this is the basis for the further development of life.
But sexual reproduction brought with it new problems compared to asexual. Reproduction can no longer be managed alone. A suitable sexual partner has to be found.
For this purpose, two different types of cells developed from the originally uniform sex cells. On the one hand small, mobile ones, the sperm, on the other hand, large immobile ones with a supply of food, the egg cells.
In many further developmental steps, two sexes arose within the species, males and females, which clearly differ from one another in their body characteristics and behavior.
The actual reproductive organs are called primary sexual characteristics. Secondary sexual characteristics are all other differences such as body size, color, body attachments and vocalizations.
These characteristics (examples: deer antlers, feather coloration and song in birds, voice and body hair in men, breasts and hip-waist ratio in women) have the purpose of attracting and impressing the respective opposite-sex sexual partner in order to achieve reproductive success with him.
Parental investment and partner choice
Both sexes have one goal in common: to secure the offspring. However, they achieve this goal with different amounts of effort. Biologists speak of brood care expenses or parental investment.
The brood care expenditure means the parental expenditure that increases the chances of survival of the offspring, but at the same time reduces the possibility of having more offspring.
As a rule, the effort for the female sex is greater than for the male. Even the production of the much larger eggs costs more energy than the production of the sperm. In addition, there are brood or pregnancy and the subsequent care of the offspring.
During this long period of time, the female cannot have any more offspring, which reduces her chances of reproduction. The male, on the other hand, can theoretically produce unlimited further offspring during this time.
Therefore, according to the parental investment theory, females are particularly picky about their sexual partner. They pay particular attention to criteria that signal a male's willingness to invest more than just sperm donation in their offspring.
Males who present themselves as good defenders and nourishers, offer the female valuable resources (for example, good territory occupied by her) are preferred. In addition, a longer "engagement period" is often put forward in the animal kingdom in order to test loyalty.
In mammals, there is a particularly large imbalance between the sexes due to long gestation periods and the sole care of the offspring by the females. Humans also belong to the group of mammals.
Sociobiologists have investigated the consequences of the theory of parental investment for humans. Studies have shown that women around the world tend to prefer older, socially and professionally successful men with good financial resources, who often have a higher status than themselves.
Men of all ages choose predominantly younger and physically attractive women who are still fertile for a long time.
So does the man like to present expensive status symbols or spend most of the time in the office in order to meet the woman's selection criteria? And does the woman do everything for an eternally youthful appearance to signal the man of continued fertility and to bind him to herself?
We cannot go that far, because humans have not only developed biologically but also culturally and sociobiology can only explain small parts of our behavior.
When the partner is missing - hermaphroditism as a reproductive strategy
Sexual reproduction is particularly problematic for firmly attached organisms such as plants, burrowing animals such as earthworms or parasites (e.g. tapeworms). It is impossible or very difficult for everyone to find a sexual partner. One solution is hermaphroditism, in which every living being has both female and male sexual organs.
Twittering requires a lot of energy, as both sex organs are formed in one organism. Therefore, this form of reproduction has only proven to be effective in the course of evolution in organisms whose search for a partner is severely restricted by their way of life.
In many cases, hermaphrodites can actually fertilize themselves, but this is avoided if possible. As a rule, a partner is also sought in the case of hernia. Because all individuals of a species can take on both the female and the male part, every conspecific is a potential sexual partner. This doubles the chance of reproduction compared to species that are sexually separated.
Living beings that are male and female at the same time are called simultaneous twitterers. These include, for example, the earthworm and the Roman snail.
The consecutive hermaphrodites are distinguished from the simultaneous weather. With this reproductive strategy, living beings change sex in the course of their lives. The animal either begins its life as a female and later becomes a male or vice versa.
An example of a consecutive hermaphrodite is the tropical clownfish. A male and a female usually live together with several initially sexless conspecifics in a group. If the female dies or is removed, the male changes sex. It becomes the female and thus the new leader of the group. The most senior sexless fish becomes the new male.
Erratum:It's not that easy with the clownfish. Now males become females or is it the other way around? Not all sources agree. Our article so far has been referring to a standard work in biology (Campbell, 2006) - not a good idea as it turned out. The information was wrong. The error has now been eliminated (source: Buston, P. M. (2004). Territory inheritance in clownfish. Proc. R. Soc. Lond. B 271, 252-254.).
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