22 February 2020 | New Scientist | 45>“ Evidence shows
warming is leading
to increasingly
unequal sex ratios”
By 2100, up to
93 per cent of
green sea turtle
hatchlings could
be femalereduce the impact of higher temperatures
on these creatures.
But there is an even stronger reason to be
optimistic that female-biased populations
won’t lead to extinction. “An excess of
females is actually beneficial,” says Nadav
Pezaro at Stellenbosch University in South
Africa, “as long as there are enough males to
service the entire female population.” There
are only so many offspring that each female
can produce, so they – not males – represent
a limiting factor to population size. As a
result, female-biased populations tend to
have higher reproductive output than groups
with equal numbers of males and females.
Some researchers even think that this
explains why temperature-dependent sex
determination is so widespread. While other
species might struggle to adapt to changing
environmental conditions, those that have
evolved this mechanism could respond with
a population boom.
The pro-female bias has another benefit too:
it is inherently self-correcting. Pezaro and his
colleagues have shown how this may work
in reptiles such as crocodilians, a group that
evolved perhaps more than 200 million years
ago. For these animals, both low and high
incubation temperatures produce females,
while males are produced at intermediate
temperatures. Crucially, individuals differ
genetically in the temperature range that
triggers male development. So, while the
genome of one crocodile might make it male if
incubated between 25°C and 29°C, for another
the range could be 27°C to 32°C. In a particularly
warm year, only embryos carrying genes to
produce males in a high temperature range
will hatch as male. Once mature, these few