2020-02-22_New_Scientist

46 | New Scientist | 22 February 2020

males will probably mate with multiple

females, making future hatchlings more likely

to carry genes that favour male development

at warmer temperatures. In this way, mostly

female populations can continue to produce

a modest reserve of males, helping the

population stay afloat and even thrive

as temperatures increase.

This tendency to self-correct, together

with the ability to produce population booms,

could explain how species that produce more

females at higher temperatures have survived

warming in the distant past. But there is a

limit to the adaptability of this mechanism.

Eventually, there won’t be enough males to

mate with all the females and populations

will start declining. Unfortunately, there are

signs that this is happening now as a result

of the rapid pace of modern climate change

compared with past events. Reptiles are at

particular risk because their long generation

spans mean they can’t adapt fast enough.

Loggerhead turtles, for example, take up to

30 years to reach sexual maturity. And a recent

study of freshwater turtles in China predicted

that many populations will be almost

exclusively female within just a few decades.

Global warming poses an even greater threat

for another group of species – those in which

higher temperatures produce fewer females

and more males. Animals with this rarer type

As Earth warms, it is affecting

the balance of males and females

among animals whose sex is

determined by temperature

(see main story). Surprisingly,

temperature can also affect

human sex ratios. A host of

studies show that fewer boys

are born in unusually cold years.

What’s more, when researchers

tracked Japanese populations

in the aftermath of two abnormal

seasonal events – the extremely

hot summer of 2010 and

unusually cold winter weather

of early 2011 – they discovered

that fewer boys were born nine

months later in both cases.

At first glance, these findings

seem to defy logic. In humans,

sex is genetically determined.

Embryos that inherit two X sex

chromosomes become female

and those inheriting an X and a Y

become male. Temperature can’t

override the genetic instructions

we pass down to our children. It

can’t switch a genetically male

embryo into a female. However,

it can influence sex ratios in

another way.

Only about 30 to 50 per cent

of pregnancies result in live

births, with most of the rest

being miscarried within six

weeks of conception. This has

been dubbed “natural selection

in utero”. And when it comes

to survival, male fetuses seem

to be at a disadvantage. For a

start, they fall victim to a host of

developmental issues that don’t

affect females, because they

can’t compensate for any genetic

mutation on their X chromosome

by relying on a normal second

copy. They are also more

sensitive to environmental

conditions during pregnancy –

not just extremes of temperature

but a variety of factors that

induce stress in mothers-to-be.

Women give birth to relatively

fewer boys shortly after dealing

with catastrophes such as

earthquakes, floods and bouts

of smog pollution, as well as

periods of unemployment.

Why male fetuses are more

likely to be victims of these

events remains a matter of

debate. Ralph Catalano at the

University of California, Berkeley,

thinks it might reflect an

evolutionary drive for a pregnant

woman’s body to reject a child

that is less likely to survive under

difficult conditions. And he notes

that males tend to have weaker

immune systems than females,

making them more likely to

succumb to infectious diseases

at any age. Indeed, many species

seem to have evolved a system

for aborting male embryos in

times of stress and scarcity.

Rodents, for example, give birth

to fewer males when placed on

low-fat diets or diets lacking in

calories or essential amino acids.

It is estimated that 15 fewer

boys are born for every 1000

babies in the aftermath of

flood, earthquake or smog.

Climate change is increasing

the likelihood of some of these

stressful events, so in that sense

it is having an impact on the sex

ratios of human populations.

These changes might marginally

increase the difficulty of finding

a sexual partner in affected

regions, but they don’t present

an existential threat to humanity

as rising temperatures may do

to other species.

Girls like it hot

“ Women give

birth to fewer

boys after

dealing with a

stressful event”

Caimans and other
crocodilians
have a way to
defy global
warming

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