Hermaphrodites are organisms having both male and female reproductive organs during their life. The mating system of hermaphrodites vary from simple to complex and often involve unique behavior, sex change, facultative allocation of resources to sexes and conditional reciprocity.

Simultaneous hermaphrodites have both reproductive organs at the same time in their adult life. Interactions between simultaneous hermaphrodites can be complicated and vary with the number and sex of interacting individuals.



In the nematode Caenorhabditis elegans most individuals of this species are hermaphroditic, having both testes and ovaries. As larvae, these hermaphrodites make sperm, which is stored in the nematode’s genital tract. The adult ovary produces eggs, and these eggs become fertilized as they migrate into the uterus. Thus, sperm is already present in the hermaphroditic adult.


Self-fertilization almost always produces more hermaphrodites. Only 0.2 percent of the progeny are males. Males are produce by sperm where the X has been lost.

These males, however, can mate with hermaphrodites; and because their sperm has a competitive advantage over hermaphroditic sperm, the sex ratio resulting from such matings is about 50 percent hermaphrodites to 50 percent males.

Rarely, females are found that are identical morphologically to hermaphrodites and some biologists feels the hermaphrodites have evolved from females. In C. elegans, hermaphrodites can self-fertilize but can switch and reproduce sexual when males are present

In one study mated hermaphrodites (with males) ha a shorter life span than unmated hermaphrodites. Males "fight" with each other and other hermaphrodites for mating privileges. Hermaphrodites also died from opportunistic infections produced by the bacteria they were feeding upon. Cultures then containing males never do as well as those containing only hermaphrodites under stable conditions. Stressed cultures however do produce more males.

Allocation of resources to males or hermaphrodites fits basic ideas regarding the functions and so maintenance of sexual reproduction.

In most hermaphrodites, given the occasional exception, a reduction in female allocation results in more sperm being reproduced and so in most hermaphroditic species there is competition to act as the male partner.

Example 1: Flatworms


Example 2: pond snails

The great pond snail, Lymnaea stagnalis, seems to use a seminal product to manipulate its partner and mates in the male role when enough seminal fluid is available in the prostate gland. Receipt of semen not only initiates egg laying in virgin animals, but also feminizes the mating partner later in life. This increases in the female function have been shown to develop at the expense of growth and production of the sperm, and seminal fluid.

Example 4: banana slugs.

Banana slugs are another example of a hermaphroditic gastropod. The male sexual organ of an adult banana slug is quite large in proportion to its size, as well as compared to the female organ. One partner may bite off the male organ of another (apophallation).


However, reciprocity between partners can occur and be selected for.

Example: Sea bass

Individuals form pairs in late afternoon. Before spawning, two mates alternate courtship displays. The last fish to display releases some eggs. Then the partner releases some sperm (the fertilization is external and eggs are planktonic).

It is thought to be a highly monogamous system because of the egg "trading" that goes on that decreases any chance for "cheating" by individuals acting as males.

If a fairly linear relationship is assumed to exist between ovary size and fecundity, an egg-trading chalk bass will have about 1.5 times the mating success of a pure female in a related species because hermaphrodites appear to devote more energy to growing ovaries.

A pure male would have to fertilize all the eggs of at least two hermaphrodites in order to do better than an egg trader.

In a population of egg traders, an individual will do as well reproductively by exchanging eggs with only one partner as it can having several mates.

This behavior pattern enhances the evolutionary stability of reciprocal spawning and as such, the simultaneously hermaphroditic sex allocation pattern exhibited by the egg traders.

It is still not know what environmental factors "pushed" for hermaphroditism in this species.

Mate choice may also occur in hermaphrodites.

There is also evidence that simultaneous hermaphrodites may discriminate among sperm on the basis of whether spermatophores were exchanged reciprocally, in an nudibranch or seaslug.

In a planarian, individuals gave more sperm to previously isolated individuals, suggesting a preference for lower sperm competition.

Mate choice by hermaphrodites on the basis of size and/or age has been documented. For example, In some simultaneously hermaphroditic fish, individuals with eggs to spawn mate preferentially with larger hermaphrodites or pure males.

Data on the number of offspring sired by first versus second mates shows considerable individual variation in Arianta arbustorum, a land snail, suggesting that these simultaneous hermaphrodites may be able to exercise cryptic female choice.

Studies on artificially selected B. glabrata snail lines have demonstrated that a snail's resistance genotype can influence its reproductive strategy.

Free-moving unselected control snails were provided with a choice of 2 potential tethered mate genotypes, 1 from a resistant-selected line and the other from a susceptible-selected line (Webster and others 2003). The free unselected snails, regardless of infection status, visited, interacted, and mated with tethered resistant and susceptible snails in equal measure. Likewise, there was no genotype or gender bias (snails mated in equal proportions in the male and female gender) if the free snail was uninfected, or if infected and mating with a susceptible-selected partner. In contrast, if the free snail was infected, resistant-selected snails only copulated with it in their less costly male gender, and actively refused to copulate with it in their female gender.

Sexual dimorphism

What is more common in simultaneously hermaphroditic taxa than morphological sexual dimorphism is behavioral sexual dimorphism. That is, it is sometimes the case that individuals will behave quite differently when mating in one role rather than the other.

For example, the sea slug Navanax inermis copulates unilaterally with one individual acting as male and the other, female, and this is associated with distinct male and female courtship behaviors. Simultaneously hermaphroditic fish (sea basses, groupers, etc.) also show sexually dimorphic courtship behavior.