Abstract
The impact of global warming on everything on Earth has been observed and reported by scientists
from every corner of the world for an extended time. Among those phenomena have been the correlation
of sex ratios at birth to increasing global temperatures. From known biological facts, sex determination
can be genotypic (GSD) or temperature-dependent (TSD). A number of previous studies have suggested
that TSD may be very common in many species of lower vertebrates, especially fish and reptiles, with
increasing temperatures generally affecting the sex ratio of a species (leading to many males and few
females). The most frequent citation is noted in a study directed by two Spanish researchers, Natalie
Ospina-Alvarez and Francese Piferrer, reported under the title “temperature-dependent sex determination
in fish revisited: prevalence, a single sex ratio response pattern, and possible effects in climate change,”
published in the July 30, 2008 edition of the scientific journal Public Library of Science (PLoS ONE). They
show that even small changes of just 1-2°C can significantly alter the sex ratio from 1:1 (males : females) to 3:1 in both freshwater and marine species. It is already known that high temperatures inhibit the synthesis
of estrogens, which are essential for female sex differentiation in fish and reptiles.
In mammals, under circumstances of ordinary temperature, male live births exceed those of females.
In humans, the ratio of male births to total births also favor the same trend. It has been hypothesized that
levels of steroid hormones (testosterone and estrogen) in both parents around the time of conception are
positively associated with the offspring sex ratio (proportion of males at birth) of mammals, including
humans. Some people, particularly women, in suboptimal health produce an excess of sons; thus, gonadal
hormones are responsible for adaptive variation.
While our findings of the comparative analysis on the secondary data obtained from the Thai Meteorological
Department and from the National Statistical Office in the 16-year period 1993-2009 show that
increasing temperatures result in an increased prevalence of newborn females over males. Details of
complex analytic patterns are elaborated in the text.
