There was a small but very unusual science news story out this week that you might have missed. It concerns the discovery of eight new species of nematode roundworms at Mono Lake in the eastern Sierra Mountains of California. That sure sounds exciting doesn’t it?
Well you see Mono Lake is one of the most extreme environments for living things on the surface of the entire planet. Not only is the water in Mono Lake three times as salty as in the ocean but the water is also laced with the poisonous element arsenic while its pH of 10 makes it very alkaline. Few living things can survive the toxic conditions present at Mono Lake so the discovery of eight new species there is quite important.
The most surprising news however concerns one of those eight species, a yet to be formally named member of the genus Auanema that possesses three sexes!!! You heard that right; the newly discovered Auanema sp. has boys, girls and hermaphrodites.
Hermaphrodites you may recall are individuals who possess both male and female sexual organs and who are capable of both bearing offspring and impregnating other members of their species. There are many species that are hermaphroditic in nature just as there are many species with sexed individuals but a species that possesses both sexed and hermaphroditic individuals is very rare. Another oddity about the sex life of Auanema sp. is the fact that it carries its young inside itself in a manner similar to a kangaroo.
The study of life at Mono Lake was conducted by lead author Paul Sternberg of the California Institute of Technology along with researchers Pei-Yin Shih and James Siho Lee and was published in the journal Current Biology. I’m certain that we’ll be hearing more from Drs. Sternberg, Shih and Lee about the strange and extreme life forms at Mono Lake.
The strange sex life of Auanema sp. however begs the question; just how many different ways are there for having sex? This question has become more important recently not only in scientific circles but in the domain of politics. In our society today the question of how we should treat those individuals whose sex lives do not fit into preconceived norms is very contentious. Is it possible that we could learn something about our own sexual behavior by studying the diversity of sex in nature?
To begin my dictionary defines sex as: The property or quality by which organisms are classified according to their reproductive functions. So in our quest to study the diversity of sex we are going to have to look at some of the many ways living creatures reproduce.
Let’s begin with the simplest of all living things, the single celled organisms that we learned about back in high school. The structure of such creatures is so simple that they are not capable of possessing differences between individuals that could be identified as making them either male or female. Instead these creatures reproduce asexually, that is without any sexual contact from another member of their species. Whenever a single celled creature has absorbed enough nutrients to reproduce it will begin the process of cell division that produces, not an adult and child but two identical daughter cells. In asexual reproduction the daughter cells both receive only the genetic information of their single parent.
It is that mixing of the genetic information from two parents that is the advantage of engaging in sex after all. Sex allows new, beneficial mutations to quickly spread throughout a population. Sex increases the variation of characteristics within a species; variations that natural selection can work with in order to better adapt a species to its environment.
As we know, it is in the multi-celled organisms, creatures with groups of cells dedicated to perform certain tasks, in other words organs, that we find differences between individuals that are related to that individual’s function in reproduction. And due to their greater complexity it is among the multi-celled organisms that we find the greater range of sexual behaviors.
We’ve already mentioned hermaphrodites, creatures with both male and female sex organs and who are capable of both being fertilized by and fertilizing another member of their species. Many species of worms, molluscs and flowering plants are hermaphrodites.
Even among species with individuals that can be clearly differentiated into female and male sexual behavior can vary greatly. In the insect order hymenoptera, ants, bees and wasps, an unfertilized female can lay an egg that will develop into a fully functioning male, an actual example of a virgin birth. That male can then have sex with his own mother, fertilizing her so that all of her offspring from then on are female!
Now if you’re thinking that such unusual sexual behaviors are confined to the lower, i.e. invertebrate animals well think again. There are many species of vertebrates, especially fish who exhibit a different type of hermaphroditism known as sequential hermaphroditism. As you might guess from the name a sequential hermaphrodite is an individual who is born as one sex but who may during the course of their life change their sex.
Examples of sex changes in both directions are well recognized. One is the popular clown fish (genus Amphiprion) that exists in groups with a single large breeding female and a smaller breeding male along with some non-breeding members. If the breeding female should die the breeding male will become female and one of the non-breeding members will become the new breeding male.
Another familiar reef fish, the cleaner wrasse (family Labridae) behaves in almost the exact opposite fashion. The single male in a group will dominate all of the females while the largest female dominates all the others and so on down the list until the smallest wrasse gets picked on by everybody. Again if the dominant male should die the top female will change sex and become a male.
Have you noticed how the sex of an individual in these species is determined by their social status? To confirm this interpretation there are reef fish of the species Lythrypnus dalli where the sex of an individual can go back and forth depending on whether it is dominant, male or submissive, female.
Even mammals have a variety of sexual behaviors; the difference between marsupials who carry their young in a pouch and mammals with a womb is well known. There are also many species where a dominant male gathers a harem to himself while smaller males are out of luck. Then there are some mammals who only mate at certain times of the year while others engage in sex year round.
By now I hope you’ve realized that the stereotypical male-female, boy meets girl style of reproduction is not carved in stone, it is not a universal archetype of how things must be. Sex is all about mixing genes so that new, better adapted traits can be spread through a population more quickly giving natural selection a better chance to improve a species. Evolution doesn’t really care how that mixing takes place so there is a great diversity of sexual behaviors across the natural world.