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.
References:
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
