Odds of Pairing out of a group in Gonochoristic Species

Odds of Pairing out of a group in Gonochoristic Species

Postby mpedersen » Wed Jan 23, 2008 2:50 pm

I had to go digging for this chart. Assuming a random sampling of a species which has even sex distribution (1/1 ratio) and sexes are randomly determined and are gonochorist (fixed sexes), here's the odds of having at least ONE male/female pair:

1 fish - 0% chance
2 fish - 50% chance
3 fish - 75% chance
4 fish - 88% chance
5 fish - 94% chance
6 fish - 97% chance
7 fish - 98% chance
8 fish - 99% chance

FWIW,

Matt
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Postby William » Wed Jan 23, 2008 10:20 pm

or the formula

1 - (0.5)^(n-1) where n = number of fish
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Postby David M » Thu Jan 24, 2008 2:07 am

I had to go digging for this chart.


What, you lost your copy of Joyce's book? :wink:

Anyway it's pure crap IMHO, not like we can assume an even distribution of males and females in a sexually mature population of fish. Why would we, it's not true for any other species on the planet. It makes no sense at all from an ecological point of view.

Assume female and seek the mate. 8)
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Re:

Postby mpedersen » Thu Jan 24, 2008 2:23 am

David M wrote:
I had to go digging for this chart.


What, you lost your copy of Joyce's book? :wink:


Doesn't mean I didn't have to dig for it? Actually, I didn't remember this being in her book!

David M wrote:Anyway it's pure crap IMHO, not like we can assume an even distribution of males and females in a sexually mature population of fish. Why would we, it's not true for any other species on the planet. It makes no sense at all from an ecological point of view.

Assume female and seek the mate. 8)


I'm not sure I follow. Why, in a gonochoristic species, especially one that typically forms monogamous reproductive pairs, would there be any benefit to a skewed sex ratio? Of course, the same could not be true for polygamous gonochorists, as it's typically assumed that one male can fertilize multiple females, but then we most often find ourselves in situations favoring sequential hermaphrodism, specifically protogyny.

While I surely haven't done a study of Bangaii Cardinalfish sex distribution, I wouldn't be surprised to find that it's roughly 50/50. Same for any other fish that forms semi-monogamous pairs and is a known gonochorist (i.e. Jawfish).

FWIW,

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