J.M.Spencer@newcastle.ac.uk (J.M. Spencer) (09/27/90)
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? ----------------------------------------------------------------------------- Sender : Jonathan M Spencer Mail : Computing Lab, University of Newcastle-upon-Tyne, NE1 7RU, UK Phone : +91 222 8229 ARPA : J.M.Spencer%newcastle.ac.uk@cs.ucl.ac.uk JANET : J.M.Spencer@uk.ac.newcastle UUCP : !ukc!newcastle.ac.uk!J.M.Spencer
loren@tristan.llnl.gov (Loren Petrich) (09/28/90)
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
frazier@oahu.cs.ucla.edu (Greg Frazier) (09/28/90)
In article <1990Sep27.142437.21755@newcastle.ac.uk> J.M.Spencer@newcastle.ac.uk (J.M. Spencer) writes: >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? Loren gave a great answer to this, to which I will just at that an obvious survival advantage is that one male (fish) can cover 90 to 100 females, so there is no point in having more males. -- "They thought to use and shame me but I win out by nature, because a true freak cannot be made. A true freak must be born." K. Dunn, _Geek_Love_ Greg Frazier frazier@CS.UCLA.EDU !{ucbvax,rutgers}!ucla-cs!frazier