Sex Ratios (and E-mail Test)

["David A. Youngker" <youngker@bellsouth.net>]

Hey All -

Well, I've suffered my last problem with MindSpring, and I hate having to
leave an address that's been maintained for so many years, but I'm moving to
BellSouth as an ISP. Changing addys means testing some of my subscriptions
for continuity, but in this case it also allows the opportunity to ask a
general question concerning Apisto biology and biotopes.

Many of the Apistos we keep seem to have a skewed sex ratio within brood
batches that varies with temperature. Mayland and Bork reported on Uwe
Romer's studies, and we've bandied them about at times, and the pattern
seems to follow that the cooler the water the greater the percentage of
females. I've also noted it within my own breeding efforts, and it's an
easily observed phenomenon in my tanks as I tend to keep them on the cooler
side to extend the Apistos' longevity somewhat. And I've been pondering the
situation a little but can't come to any definite conclusions because I'm
lacking some fairly basic information that would help lead to an end point.

The evolutionary reason behind the ratios could depend on one of two logical
factors. It could be an effort to ensure the availability of ripe females
for the shorter-lived males (whose colors, etc. tend to cause them to become
snacks more often than females) or it could be a way to extend the genetic
pool by allowing females more time to expand their territories before
encountering a male.

I know that some plants with the capability of self-fertilization often have
their male and female structures mature at different times to prevent
inbreeding as a rule of habit. This usually means the males first so that
the pollen can travel some distance from the originating stand and ensure
genetic diversity. Something along these lines could be at work within the
genus Apistogramma, leading to the thought of extended territories. In our
own efforts, when a breeding doesn't "work" we often assume the male isn't
"ready", but perhaps instead the _females_ (which most often come from the
same broods and have the same time for development with us) are the true
culprits.

Is there a source of field collection data that samples the environments at
times _other_ than prime collection periods? I know that Apistos, like other
Amazonian fish, are far easier to collect during the dry season when water
levels are down and populations are concentrated, and conversely the wet
season makes collection nearly impossible in a lot of areas. But perhaps
some of the collectors are out there getting "early starts" on the season
and have some of this data within their field notes.

It would be an interesting exercise (at least to me) to try to see how much
time the females would normally have to populate and spread through an area
before the warming waters start to increase the population of the males...


David A. Youngker
youngker@bellsouth.net


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