Re: Sex Ratio Forecast

[swaldron@slip.net (Steven J. Waldron)]

>I do not have Romer's paper (is there anyone available to fax or mail it
>to me, if it is in English? E-mail me, thanks), but unless the study was
>extremely large, it is unlikely that the estimates calculated allow a
>very precise forecast. They may however provide a useful guidance of the
>general rules and trends of the gender ratio determination, which is
>still an extremely important discovery.
 Hi guys,
Good discussion. I have to admit to ignorance of Romer's findings, I'd
appreciate a copy as well.
I'd like to expand the discussion with another consideration, a bit
speculative but grounded in natural selection: We are assuming a 1:1 ratio
is the norm or ideal in Apistos. For most vertebrates, this seems to be the
case when the return from parental investment in offspring either sex is
equal. But there are exceptions, and some of these have shown the adaptive
nature of a flexible sex ratio. Polygynous apistos at least (e.g. A.
cactuoides) might provide one of these exceptions. A parent apisto's
interest in reproducing is to continue its genetic line, grandchildren in
the least. Male offspring have an enhanced ability for continuing the genes
by virtue of their reproductive strategy (multiple mates). Colorful,
territorial males also might suffer increased mortality  (more risk of
predation, increased competition for territory, longer distance dispersal
etc.)and decreased reproductive success in non-territorial males than
females. Therefore, a sex ratio weighted towards males at conception seems
highly adaptive. As offspring mature and differential mortality between the
genders set in, the ratio is more equalized and may eventually reach 1:1 or
even female biased.
 If we can deduce anything from Romer's data (or at least what I have seen
of it here), it is that different water conditions have variable effects of
offspring sex ratio. This seems a possible adaptive response to changing
ecological conditions and the social structure of the local population
throughout the wet/dry seasons.  Imagine this scenario:  At the end of the
dry season when water bodies have shrunken, fish are more exposed to
predation, flashy, territorial males have been heavily predated upon, it
may be more adaptive to produce more males to fill this vacancy when the
first rains of the wet arrive. The water chemistry changes at this time may
dictate this sex ratio. You can imagine other scenarios for differing water
conditions during the year. I'm rambling, but in essence if we want to
produce a specific sex ratio in captivity, we need to go into the field and
understand the conditions (water chemistry, demography or otherwise) that
are conducive to that ratio naturally. Captive observations of water
chemistry/sex ratio experiments with too few replicates can only (IMHO)
produce too many speculations.
- - Steve Waldron