For the rest of you, read on. This is fascinating stuff!
Out in the real world, meaning the one we humans haven't
artificially insulated ourselves from with technology, where the realities of
life and death are governed by chemical and physical laws, animals must stick
to a strict budget. It isn't a financial budget, but an energy budget that must
be balanced. Every living organism, for its survival and the survival of the
species, does whatever it takes to optimize energy use.
Energy In = Work to Survive
Where:
Energy In = the number of useable calories an organism takes
in, and
Work to Survive = using those calories for growth, finding
food, shelter, defending your shelter, finding a mate, the process of mating,
and repair of tissue damage or injury incurred from any of the above activities.
If you don't take enough energy in, you can't do all of the
activities needed to survive. The whole point of survival in the animal kingdom
is to reproduce. The more offspring you have, the more likely your genes are to
survive through subsequent generations, thereby making you somewhat immortal.
As anyone on a budget can tell you, making sure that what you take in can cover all your expenses, you have to make some trade-offs. The more you spend on having fancy things and defending them, the less you have to wine and dine potential mates. But, without those accoutrements that announce your relative fitness, even your superiority, to prospective mates you won't get that opportunity.
No surprise that the perfect balance is different for males
and females. In most species, females do the majority of the work in the
offspring department. Whether they carry those embryos internally, or scatter
them to the currents in the ocean, they still have the greater energy
investment in egg production. Their eggs provide all the initial nutrition and
organelles, as well as DNA, to the newly fertilized offspring. Sperm are packs
of DNA and nothing more. It doesn't take much energy to make them.
This difference in energy expenditure for reproduction is
one of the driving forces behind sexual dimorphism: differences in physical
appearance between the males and females of a species. In particular, this
accounts for size differences: When males have to protect territory or a harem
from other males, spending energy to get bigger makes sense. When you can swim
in, release lots of sperm, and leave, being small and quick is beneficial.
Likewise, if a female produces many eggs, a far more energy intensive
undertaking than sperm production, being large is beneficial.
Since "survival of the fittest" doesn't really
mean not dying, but actually reproducing, "success" in the natural
world means producing healthy offspring that go on to be successful themselves,
the more offspring you produce, the better. If a female with a larger body
cavity can produce more eggs per reproductive episode, and the more eggs
produced increases her potential reproductive success, it's to her benefit to
be a big female. Conversely, if larger males have greatly increased reproductive
success (i.e. gain control of a harem), it would be beneficial to be a big male.
This is known as the size advantage model, developed by
Ghiselin in 1969, that states "if an individual could significantly
increase its reproductive success after reaching a certain size by being a
different sex, it would be to their advantage to switch to that sex." And
on the coral reef, they do!
Here are some of the brilliant reef fish that can change sex--sometimes
in a matter of a few hours.
 |
| The Blueheaded Wrasse |
The cleaner wrasse, Labroides dimidiatus. Typically, live in harems with a dominant
male around cleaning stations. When the male is removed, the largest female
becomes a functional male. The Blueheaded wrasse, Thalassoma bifasciatum, do not form harems, but the dominant or terminal
males take temporary control of spawning sites. Removal of terminal males
results in the largest females transforming.
 |
| Moonheaded Wrasse |
 |
| Rock Beauty |
 |
| Clownfish in their anemone |
Anemone
fish, like the skunk anemonefish Amphiprion
akallaopisos, and clownfish,
like Nemo, are protandrous. They change from male to female. Anemonefish live
in monogamous pairs composed of a large female and a smaller, functional male.
The pair may share their anemone-home with other, small, stunted (not sexually developed)
males (juveniles; all anemonefish are born male). When the female is removed,
the functional male changes sex and the largest juvenile becomes a functional male.
Snook (Centropomus undecimalis) are one of the few known Caribbean protandrous fish.
Some reef fish, like gobies, can actively change sex in
either direction. This lets them maximize their genetic fitness under any
environmental situation.
There are many amazing things in the natural world. We'd do well to study them, be amazed by them, and learn from them. Our lesson from these fish? Embrace change, especially when it benefits us!
DeLoach, Ned and Paul Humann (1999). Reef Fish
Behavior: Florida , Caribbean, Bahamas Jacksonville
Hendrickson, Robert (1984). The Ocean Almanac.
Doubleday. New York , NY
The National Audobon Society (1997). Field Guide
to Tropical Marine Fishes: Caribbean, Gulf of Mexico ,
Florida , Bahamas ,
Bermuda . Alfred A. Knopf, Inc. New York , NY
Rice, Aaron N. (undated).
Physiology of Sex-Change in Reef Fish. Available at: http://www.bio.davidson.edu/Courses/anphys/1999/Rice/Rice.htm
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