As members of a species with separate sexes, humans tend to forget about the suite of organisms out there that are hermaphrodites, simultaneously carrying male and female genitalia. Hermaphroditism is present in 24 of the 34 animal phyla (taxonomic groups), and is quite common in the invertebrate world.
In fact, for many organisms hermaphroditism is the rule rather than the exception.
When animals are both male and female at the same time, the scene is set for a good deal of biological conflict.
We have already seen many examples of conflict between the sexual needs of males and females (traumatic insemination, for example), so what is an organism to do when such conflict exists within itself?
The Hermaphrodites’ Dilemma
Hermaphrodites from various phyla display behaviours and physiologies consistent with the notion that sexual strife is alive and well at the level of the single organism.
Aptly termed the “hermaphrodites’ dilemma”, each individual has the urge to be sexually gregarious, courtesy of its male self, and sexually choosy, thanks to its female self. This dilemma has led to the evolution of some extremely bizarre and violent sexual rituals.
Let’s envisage two hermaphrodites meeting. There are at least three scenarios to consider.
1. Reciprocal copulation
First, there are reciprocal copulations where both individuals act as both partners during an encounter. Two individuals, two penises, and two vaginas.
2. Unilateral copulation
Second, there is unilateral copulation, where one individual acts as a male and the other acts as a female. Two individuals, one active penis, and one active vagina. Kind of like traditional sex, except that the male partner also has an inactive vagina, and the female partner has an inactive penis.
Imaginably, it is more biologically advantageous to be the male in said scenarios because it is much cheaper to transfer gametes and move on to the next partner than it is to be the one who is left to maintain the fertilized embryos.
Lastly, although not all species are capable of it, many hermaphrodites can self-fertilize. One individual, one penis and one vagina.
Biologists often argue that self-fertilization should maintain a baseline of the sexual activities of any individual as a safeguard for fertilization assurance. Once an individual has transferred enough sperm to ensure total fertilization, said individual should invest energy in finding higher-quality sperm (as a safeguard against inbreeding depression), and transferring sperm of their own.
So within any one individual there is both autosperm (self-sperm) and allosperm (sperm donated by others). Interestingly, only a tiny portion of the sperm received by an individual (as low as 0.025-0.1 percent) is stored for further use in copulations.
The lucky sperm that manage to reach storage can remain in the spermatheca from a few weeks to over a year, depending on the species. Most of the sperm that an individual hermaphrodite receives is digested in an organ called the bursa copulatrix – clearly a function that has evolved on behalf of the female part of the hermaphrodite.
This post is an adapted excerpt from my book “Wild Sex: The Science Behind Mating In The Animal Kingdom“