Re: Sex Ratios

["Mike Jacobs" <mjacobs2@tampabay.rr.com>]

Mike....Dave........great stuff!!!!!!!!!!!!!!!this is fun listening in!!!

Dave & Mike .............a question that I have had running around in my
head for literally years.  Why do we assume that in nature there is a 50/50
sex ratio?  I could see an advantage in there being an excess number of
males........the males being the "brightest of the sexes" then does that not
lead to them being the first caught in an ambush by a predator therefore an
abundance of males would seem to do the trick?..............or is it as Dave
says that an abundance of females insures the propagation of the
species.........orrrrrr!............does that bring us back to 50/50 and we
are where we started?.....or does mother nature really just swing with the
wind in these cases and whatever happens happens...........is that
randomness part of natural selection.......or is natural selection so
precise that 26C should be maintained or we have
problems?????............fascinating!!!!!!!

I can't put a handle on this question and I dearly love this discussion with
people!

Thanks for the superb e-mail notes..............may I use the past thread of
e-mails in our Tampa Aquarium bulletin?

Thanks.............

Mike

...................by the way.....how do you guys do the degree symbol in
e-mail?????  I always have to write 76degrees F.

Mike Jacobs
Inca 50's In Stock Now!
SOUTHERN APISTOS
Wild Peruvian Imports...Plus
http://www.southernapistos.com
----- Original Message -----
From: "David A. Youngker" <youngker@bellsouth.net>
To: "Apistogramma Mailing List" <apisto@listbox.com>
Sent: Tuesday, January 21, 2003 7:44 PM
Subject: Re: Sex Ratios


> From: Mike & Diane Wise
> Sent: Tuesday, January 21, 2003 1:08
>
> I don't know if you can equate plants & animals...

A valid point, and I'm not equating plants with animals. Plants have such
concerns due to lack of mobility, making it a necessity.

> Animals are much more mobile & will not need the delay
> of 1 sex over the other to disseminate their genetic material...

In this regard, I would think that such a delay would be an _advantage_ to
the "harem" breeders. Again - not a necessity, just an "edge". But even
where the species is monogamous and population densities are greater, it
would help ensure the males' choices of diverse genetic stockpiles.

> Römer has not only studied the physical effect, but also the
> ecological effect of temperatures on sex determination in apistos -
> both in the laboratory & the wild...

Ah, but isn't an animal species' success fairly dependent upon how it
_copes_ with an environmental impact? For instance,

> He found that 26ºC/79ºF was the optimum temperature for
> most apistos. Higher & lower temperatures retarded growth
> & maturity. He also found that primary predators on apistos
> tended to prefer higher & lower temperatures, so for some
> reason 26ºC has few predators...

if 26º is optimum for the Apisto and offers an advantage over natural
predators, then one would expect ratios to be fairly even in distribution at
that range. One would also expect that the normal drive to procreation would
not be overly affected. As you get outside of this range, to either side,
then what of other considerations?

Sperm production and viability may be one thing to think on. Most mammals,
for instance, carry their testicles in a sac that allows temperature
maintenance at a level that conflicts with the creature's own internal body
temperature - which actually runs too high for the sperm's sake. Perhaps
having a ratio heavy in males at higher temperatures ensures that there are
viable males around to affect procreation, while cooler temperatures require
a higher number of females to ensure the acceptance of overly ripe males?

My thoughts tend toward Nature's tendency to choose the path of least
resistance and a penchant for energy efficiency - wasted efforts and energy
don't ensure success. There must be _some_ reason all of this seems geared
toward the male of the population.

> Römer has a good review of his study in his book "Cichlid Atlas" (sections
> 4.2.2.1 through 6). It can explain his ideas better than I.

OK - you got me there. I *still* haven't broken down and gotten hold of a
copy. Certainly a situation I need to rectify...


-Y-


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