Path: utzoo!attcan!uunet!fernwood!apple!julius.cs.uiuc.edu!zaphod.mps.ohio-state.edu!sdd.hp.com!ucsd!pacbell.com!lll-winken!tristan!loren
From: loren@tristan.llnl.gov (Loren Petrich)
Newsgroups: sci.bio
Subject: Re: FISH:  sex ratio of 1 male : 100 females, why?
Message-ID: <68874@lll-winken.LLNL.GOV>
Date: 27 Sep 90 20:34:01 GMT
References: <1990Sep27.142437.21755@newcastle.ac.uk>
Sender: usenet@lll-winken.LLNL.GOV
Organization: Lawrence Livermore National Laboratory
Lines: 77

In article <1990Sep27.142437.21755@newcastle.ac.uk> J.M.Spencer@newcastle.ac.uk (J.M. Spencer) writes:
>A recent TV program showed a species of fish which was yellow
>and lives in the Red Sea, but I've forgotten the name.  The odd
>thing about this species is that they maintain a sex ratio of 1
>male to every 100 females.  When the male dies, the *largest*
>female turns into a male.
>
>Questions:
>
>1.	How do the fish know when they have the correct ratio?
>	i.e. how do they discriminate between 1:90 and 1:100?
>
>2.	What survival advantage does the species gain from this?

	I think I've seen sex-changing fish mentioned before -- in a
Stephen Jay Gould essay in _The Flamingo's Smile_, "Sex and Size"

	How do the fish know? That is certainly an interesting
question. It may be some pheromone signal.

	What adaptive value? None directly, I'm sure. But it does have
a direct impact on the ability to reproduce. As Gould pointed out in
his article, what sex is bigger depends on what is most convenient for
reproducing. And a good test of that hypothesis is to be found among
"sequential hermaphrodites" -- sex changers.

	For animals that lead essentially isolated lives, it is
generally the female that is bigger, because that is the sex that has
the burden of producing eggs, while the male can get away with
producing only tiny sperm. That is why female praying mantids and
spiders are so dangerous to their mates -- the males are smaller. The
same is true of the jack-in-the pulpit, which starts out male, but
grows into a female. If one is shaded or partially devoured, it
reverts back to maleness again.

	But for animals which interact, other factors come into play.
Whichever sex has to do less work in reproducing becomes the
competititive one, and the sex what has to do more work becomes
choosy. Most of the time, it is the male sex that is the competitive
one, since males usually don't have to do as much as females.
Interestingly, in seahorses and certain crickets, it is the females
who compete for the privilege of mating with the males; male seahorses
become "pregnant" and these male crickets contribute a big sperm
capsule for the female to eat -- in these cases, it is the male that
has the biggest burden.

	So that explains why the biggest fish of groups of this
species is male -- because if one is male, one can spend one's time
defending one's chance to reproduce by watching over a group of
females. Since if more than one fish turns male, they would fight over
the privilege of getting to watch over the group. So that is probably
why only the biggest female turns male -- the others would have an
easier time just staying female. That behavior is common among other
social animals. Bull elephant seals fight fiercely over the beaches
where the cows come to have their calves; whoever controls the most
beach gets to mate with the most cows, thus perpetuating whatever
genetic tendences he would have to act like this.

	Analogies with our species are irresistible; something like
this probably explains why men are usually bigger and stronger than
women. Interestingly, the difference between the sexes is less in our
species than it is in gorillas, or maybe even in chimpanzees (I'm not
sure here). Fossil hominids may provide a clue as to the evolution of
sex-dependent size difference in our species, but there have not been
many fossils to work from. This sex difference may well be partly
vestigial. I do not wish to subscribe to the more simplistic forms of
sociobiology; homan societies have ranged from approximate
egalitarianism to men having essentially all the privileges of a
society.


$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
Loren Petrich, the Master Blaster: loren@sunlight.llnl.gov

Since this nodename is not widely known, you may have to try:

loren%sunlight.llnl.gov@star.stanford.edu