Saturday, August 11, 2012
Gender part six: Simultaneous Hermaphrodism in Animals
Note: This program first aired August 4, 2012.
The animal kingdom is full of mind boggling diversity, when it comes to making baby animals. Years ago an astute listener sent me a book, called How Animals Have Sex: A Guide to the Reproductive Habits of Creatures Great and Small, which documents in an extremely humorous way, this amazing diversity. The book highlights the reproductive strategies of many creatures from the animal world, including a few of today’s topic, the simultaneous hermaphrodites.
The word hermaphrodite comes from Greek mythology. Hermaphroditos was the son of Aphrodite and Hermes. One day he met a beautiful water nymph Salmacis, who kissed him and prayed that they be united forever. The Greek gods were extremely literal in this case, and Hermaphroditos was forever after recognized as part male and part female.
Animals that practice simultaneous hermaphrodism are like Hermaphroditos, by having gonads with both functional ovarian and testicular tissue, meaning they can make both eggs and sperm at the same time (scrabble players take note, the organs are called ovotestes, singular ovotestis). Animals that can do this include many gastropods, like our common garden slugs and snails, many other groups of molluscs, flat worms, many segmented worms like earth worms and leaches, some bivalves, the ectoprotcs like bryozoans, and even a few fish. There are estimates that as many as one third of all animals, excluding insects, are hermaphrodites.
Some of these species are self fertile, and can use their own sperm to fertilize their own eggs, others have evolved barriers against self fertilization, like many hermaphroditic plants. When hermaphroditic animals mate, a typical scenario is to swap sperm. Some species practice a variation on that theme, with one individual acting as male and the other as female during mating. Then they switch roles and mate again. Sometimes there is a difference between the gonadal gender and the functional or behavioral gender of an individual. They may have fully functional male and female gonads but choose to present as either one or the other for social reasons, often having to do with relative size or population density.
Phylogeny is evolutionary history. The phylogeny of hermaphrodism is unresolved at best. It appears that hermaphrodism is what we call a derived condition, as opposed to an ancestral condition. This means that hermaphrodism is not the original, or ancestral state of animal sexuality, it has been derived at some later time in animal evolution. Because it is seen in many relatively unrelated groups this also means it has evolved independently several times. This is called convergent evolution, and when we see that happen, we know the populations involved are on to something good, something that works.
The thing that works is that hermaphrodism allows organisms to maximize their chances of getting their DNA into viable offspring. It can literally double their chances. Beyond the opportunity to fertilize yourself in a pinch (which we know from previous discussions isn’t actually advisable), having double duty gonads functionally doubles the population size of a group by having each individual act as both a male and a female. So not only do I have a chance at having offspring if I take your sperm and fertilize my eggs with it, I can double my chances by giving you some of my sperm to fertilize your eggs with. If I live in an area with low population density, this is especially helpful, because it means I can mate with anyone I encounter, instead of being one gender and wandering about looking for an individual of the opposite one. For these groups of organisms, these benefits outweigh the metabolic costs of having to make two different kinds of gametes and the elaborate sexual organs needed to manipulate these gametes.
This may not be at all what the Greek gods had in mind when they transformed Hermaphroditos, but it has made life easier for many of our invertebrate friends, and our experience of the natural world just that much richer.
This is the book I wish that I had written: Strorm, David, How Animals Have Sex Gotham Books, NY, NY 2005 ISBN: 1592401910 I include the ISBN because you know you want to buy this book.
Leonard, Janet ed. The Evolution of Primary Sexual Character in Animals Oxford University Press (partial preview available on Google Books)
Editorial in Seminars in Cell and Developmental Biology “Mode and tempo in environmental sex determination in vertebrates” 2009
Avise, J. C., Mank J.E. “Evolutionary Perspectives on Hermaphrodism in Fishes” Sexual Development 2009, 3: 152-163