Jois quotes:

>BJOG. 2003 Apr;110 Suppl 20:34-8.  Links
>
>
>Gender aspects of preterm birth.
>
>Ingemarsson I.
>
>Department of Obstetrics and Gynecology, University Hospital, Lund, Sweden.
>
>It was previously believed that sex differentiation took place when the
>undifferentiated gonads formed either testes or ovaries. Studies in recent
>years indicate that sex differentiation begins at conception. The SRY gene
>on the Y-chromosome is already transcribed at the 2-cell stage and triggers
>growth acceleration in the XY embryos. This accelerated growth is believed
>to be important for the male embryo as it allows complete testicular
>differentiation before the levels of oestrogenic hormones become too high
>as pregnancy progresses. It is well known that the death rate is higher for
>male than for female fetuses and that the increase is about 30% in
>chromosomally normal spontaneous abortions (i.e. significantly higher than
>at birth). National figures from Sweden show that boys are more likely to
>be delivered prematurely, accounting for 55-60% of all newborns between 23
>and 32 gestational weeks. Neonatal deaths in these gestational weeks are
>also more common among boys. In 1993, the overall 1-year mortality rate
>(including all gestational weeks) in Sweden was 5.4% for boys and 4.1% for
>girls. The difference in infant mortality (within 1 year) is most
>pronounced at extremely early birth (23-24 gestational weeks) being 60% for
>boys compared with 38% for girls.

These are some of the most robust statistics in existence and were well-known
in Darwin's time. Indeed, he wrote the following in the "Descent of Man"
(Chapter 8), with the last quoted paragraph being the most important:

=====================================

In England during ten years (from 1857 to 1866) the average number of children
born alive yearly was 707,120, in the proportion of 104.5 males to 100 females.
But in 1857 the male births throughout England were as 105.2, and in 1865 as
104.0 to 100. Looking to separate districts, in Buckinghamshire (where about
5000 children are annually born) the mean proportion of male to female births,
during the whole period of the above ten years, was as 102.8 to 100; whilst in
N. Wales (where the average annual births are 12,873) it was as high as 106.2
to 100. Taking a still smaller district, viz., Rutlandshire (where the annual
births average only 739), in 1864 the male births were as 114.6, and in 1862 as
only 97.0 to 100; but even in this small district the average of the 7385
births during the whole ten years, was as 104.5 to 100: that is in the same
ratio as throughout England.* The proportions are sometimes slightly disturbed
by unknown causes; thus Prof. Faye states "that in some districts of Norway
there has been during a decennial period a steady deficiency of boys, whilst in
others the opposite condition has existed." In France during forty-four years
the male to the female births have been as 106.2 to 100; but during this period
it has occurred five times in one department, and six times in another, that
the female births have exceeded the males. In Russia the average proportion is
as high as 108.9, and in Philadelphia in the United States as 110.5 to 100.*(2)
The average for Europe, deduced by Bickes from about seventy million births, is
106 males to 100 females. On the other hand, with white children born at the
Cape of Good Hope, the proportion of males is so low as to fluctuate during
successive years between 90 and 99 males for every 100 females. It is a
singular fact that with Jews the proportion of male births is decidedly larger
than with Christians: thus in Prussia the proportion is as 113, in Breslau as
114, and in Livonia as 120 to 100; the Christian births in these countries
being the same as usual, for instance, in Livonia as 104 to 100.*(3) 

Prof. Faye remarks that "a still greater preponderance of males would be met
with, if death struck both sexes in equal proportion in the womb and during
birth. But the fact is, that for every 100 still-born females, we have in
several countries from 134.6 to 144.9 stillborn males. During the first four or
five years of life, also, more male children die than females, for example in
England, during the first year, 126 boys die for every 100 girls- a proportion
which in France is still more unfavourable."* Dr. Stockton-Hough accounts for
these facts in part by the more frequent defective development of males than of
females. We have before seen that the male sex is more variable in structure
than the female; and variations in important organs would generally be
injurious. But the size of the body, and especially of the head, being greater
in male than female infants is another cause: for the males are thus more
liable to be injured during parturition. Consequently the still-born males are
more numerous; and, as a highly competent judge, Dr. Crichton Browne,*(2)
believes, male infants often suffer in health for some years after birth. Owing
to this excess in the death-rate of male children, both at birth and for some
time subsequently, and owing to the exposure of grown men to various dangers,
and to their tendency to emigrate, the females in all old-settled countries,
where statistical records have been kept,*(3) are found to preponderate
considerably over the males. 

Dr. Stark also remarks (Tenth Annual Reports of Births, Deaths, &c., in
Scotland, 1867, p. xxviii.) that "These examples may suffice to show that, at
almost every stage of life, the males in Scotland have a greater liability to
death and a higher death-rate than the females. The fact, however, of this
peculiarity being most strongly developed at that infantile period of life when
the dress, food, and general treatment of both sexes are alike, seems to prove
that the higher male death-rate is an impressed, natural, and constitutional
peculiarity due to sex alone." 

=====================================

There are three points at which the sex ratio is commonly measured:
fertilization (called the primary sex ratio), birth (the secondary sex ratio)
and just prior to mating (called the "operational sex ratio", but just as
approriately, the tertiary sex ratio).

Diploid heterogamy in mammals (XY males) is a very mild form of haplodiploidy,
but the phenomenon does put the heterogametic gender at some greater
informational risk than the homogametic gender. Whatever defects lie on the
nonredundant X in males are expressed without hope of correction by a
"backup"
copy residing on a now absent homologous chromosome.

There is also good evidence to support the contention that males have been
evolved to be more universally fragile and more error expositive than their
conspecific females. Males in most metazoan species succumb more readily than
females to disease, trauma, exhaustion and starvation; suffer higher embryonic
mortality rates; die in greater numbers from accident and intrasexual combat;
bear generally higher parasitic worm loads; live shorter lives or are driven
from the population after breeding, occasionally dying synchronously.
Polygynous males rarely care for their young, thus they become especially
dispensible to the population in times of ecological stress and may be actively
discriminated against during such periods. 

Male excess mortality in some mammals appears to be directly and
programmatically derived from androgenic hormonal levels, especially
testosterone. Castrated human males live on average 13.6 years longer than
unmodified men. Similarly, in feral Soay sheep (Ovis sp.), wether lambs
(castrated males) significantly outlive both ewe and ram lambs. In contrast,
mortality in normal males due to (aggressive, combative) masculine behaviours
is quite high. Although the sex ratio at birth is 1:1, adult ewes outnumber
normal rams 5:1. The loss of gonads not only markedly alters the aggressive
nature of the males but also diminishes behaviours that would otherwise promote
high levels of physiological and nutritional stress.

In contrast, in the birds, heterogamety is reversed. It is the female who now
has an unpaired chromosome (WZ) and is the gender who is put at greater
informational risk. The effect of this informational reversal appears to have
direct effects on the populational sex ratios of the species and can be very
easily seen in the data taken from wild populations that appears at:

     http://www.cnr.uidaho.edu/wlf448/sexratio1.htm

As mammalian populations age, the percentage of males drops. The pattern is
precisely reversed in birds. As birds age, the percentage of females decline
and we are left with conclusion (using Darwin's words) "that the higher female
death-rate is an impressed, natural, and constitutional peculiarity due to sex
alone."

Wirt Atmar
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 2/15/04 11:00:18 AM