Newsgroups: sci.bio
Path: utzoo!rising
From: rising@utzoo.uucp (Jim Rising)
Subject: sex ratios
Message-ID: <1988Mar31.124839.9957@utzoo.uucp>
Organization: U of Toronto Zoology
Date: Thu, 31 Mar 88 12:48:39 GMT

I have always assumed that there tended to be a male biased sex
ratio at conception in mammals is because the sperm carrying the 
Y chromosome is somewhat smaller and faster than those carrying
X.  Higher early (pre-natal; juvenile) mortality may be due to 
deleterious recessives on the X--for which the males are "homozygous."
In birds, the females are the heterogametic sex.  I don't know much
about ratios at conception (bet it's known for chickens), but would
bet that there's higher female conception and early mortality.  In
virtually all species there appears to be a biased adult sex ratio,
with more males than females.  That, of course, increases the variance
in male fitness--and the intensity of sexual selection--even in 
monogamous species (because some males have no mates).

--Jim Rising
-- 
Name:   Jim Rising
Mail:   Dept. Zoology, Univ. Toronto
        Toronto, Ontario, Canada    M5S 1A1
UUCP:   {allegra,ihnp4,linus,decvax}!utzoo!rising