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HOW MUCH ENERGY DOES IT TAKE TO MAKE A BABY?  
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Carissa Wong
October 22, 2024
Nature [[link removed]]

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_ Researchers are rethinking what they know. Across the animal
kingdom, models have vastly underestimated the energy females invest
in reproduction. A lack of women in the field might have led
researchers to pay less attention to the costs. _

"Trachypithecus auratus-Mother and baby", by Belgianchoclate
([link removed]) (license CC BY 2.0)

 

Elephants are pregnant for up to two years. Females of some fish
species starve for weeks while holding dozens of eggs and hatchlings
inside their mouths. And pregnant humans commonly endure months of
fatigue and nausea. These are just a few ways in which reproduction
demands energy from mothers across the animal kingdom.

But researchers might have vastly underestimated the energy it takes
to reproduce, according to Dustin Marshall, an ecologist and
evolutionary biologist at Monash University in Melbourne. Australia.

The extra energy it takes a human to produce a baby could be as much
as 24 times more than some influential mathematical models had
estimated, suggest Marshall and his colleagues in a study published in
May1 [[link removed]]. For
animals that rely on external heat sources to regulate their body
temperature, called ectotherms, the difference is smaller. For the
pampas snake (_Tomodon dorsatus_), for example, the value is around
four times more. And for a species of seawater fish called the capelin
(_Mallotus villosus_), it is twice more.

How pregnancy transforms the brain to prepare it for parenthood
[[link removed]]

These findings are not merely a curiosity. Researchers use
mathematical models to examine how energy needs change during animal
lifespans — during growth, feeding and reproduction, for instance.
And these, in turn, feed into explanations of why species evolve in
the way that they do, such as the number of offspring that they have,
or the benefits of live births over laying eggs.

The work by Marshall’s team suggests that these models’ estimates
of the energy requirements of reproduction have often overlooked or
underestimated the energy spent by mothers in making and carrying
their young — sometimes referred to as the ‘indirect’ costs of
reproduction, as opposed to the direct cost, the energy invested and
stored in the offspring. It is akin to assuming that the energy needed
to make a doughnut is equal to the calories in the doughnut itself,
and ignoring the fuel required to cook it.

The results, says Marshall, could influence theories about aspects of
animal lifestyles, and improve predictions about how animals will
respond to climate change.

“The magnitudes of these indirect costs are really quite
stunning,” says ecologist and evolutionary biologist Lauren Buckley
at the University of Washington in Seattle.

HOW TO MEASURE ENERGY USE

In 2020, Marshall’s team decided to comb the literature for dozens
of studies from the 1930s to the 2000s, that had measured either the
rate of energy use in pregnant and non-pregnant females or the energy
stored directly in the offspring. Overall, the team collated enough
data to calculate the indirect and direct costs of reproduction for 81
species, ranging from microscopic animals called rotifers to fishes,
snakes and humans.

For the human data, for instance, the researchers drew on a 2004 paper
that measured the metabolic rate of humans before and during
pregnancy2
[[link removed]]. They
used this to estimate the extra energy required during nine months of
pregnancy — the indirect costs — and combined that with estimated
direct costs, such as the total energy content of a newborn baby.

Metabolic-rate data are not easy to gather. To measure the energy used
by pregnant females, researchers track how much oxygen they are using
and how much carbon dioxide they are exhaling.

In a 2007 study, for instance, Ryan Samuel at South Dakota State
University in Brookings and his colleagues put five pregnant pigs in
individual airtight glass chambers with tubes carrying samples of air
in and out, for one day at a time3
[[link removed]]. “As
the graduate student on the project, I had the overnight shifts to
stay and monitor the animals,” says Samuel, who had to check that
the pigs were comfortable and the equipment was working.

Besides the night shifts, other issues make it hard to track energy
use during pregnancy for periods much longer than one day. “If it
was longer than that, you’d have to figure out a way to remove the
poo and waste from the pigs without breaking the seal of the
chamber,” he says. It’s not surprising that such measurements are
rare in the literature, says Marshall.

Instead, modellers have found it easier to calculate the energy stored
in offspring — by simply estimating the amount of bone, fat and
protein in the bodies of offspring and combining that with the known
energy content of those components. Modellers assumed that this
equalled the energy cost of reproduction. Some models have included
estimates of indirect costs, but these costs were assumed to range
from just 5% to 25% of the total energy spent on reproduction,
depending on the model.

ENERGY IMBALANCE

Marshall’s team, however, found that the indirect costs of
reproduction are often much higher than the direct costs — contrary
to the assumptions of earlier models (see ‘Costs of making a
baby’).

[Costs of making a baby. Chart comparing direct and indirect
reproduction energy cost for different animals.]

Source: Ref. 1

In mammals, which often grow placentas to provide oxygen and nutrients
and to remove waste, and which maintain a stable internal body
temperature, the researchers found that the indirect costs made up
roughly 90% of the total energy costs of reproduction. Just 10% of the
total energy is contained in the offspring. In humans, 96% of the
208,000 kilojoules (or nearly 50,000 kilocalories) required for
reproduction is taken up by indirect costs.

By contrast, ectotherms invest less of their total reproductive energy
budget in indirect costs, the researchers found. In ectotherms that
give birth to live young, the indirect costs made up roughly 55% on
average of their reproductive energy.

“For mammals, the indirect costs of making babies was huge — it
was striking to see how different they are from other organisms,”
says evolutionary biologist David Reznick at the University of
California, Riverside. Including weaning for babies after birth —
such as the extra energy needed to produce milk — would add even
more costs.

Among ectotherms, researchers found that species that lay eggs, such
as the Antarctic krill (_Euphausia superba_), spend less of their
reproductive energy budget on indirect costs (producing and carrying
the eggs) than do those that give birth to live young. “When you
switch to live-bearing, there’s a significant increase in the
indirect costs — that’s the cost of holding the baby inside you
longer and moving around and carrying it,” says Reznick.

Researchers who spoke to _Nature_ said that they were generally
surprised that no one had quantified indirect costs before. “Most
people have said that in hindsight, it seems obvious that these costs
should have existed — though no one realized they’d be so high,”
Marshall adds. Still others, he says, are surprised that mathematical
treatments of life had assumed such costs to be zero or low.

A lack of women in the field might have led researchers to pay less
attention to the indirect costs of reproduction, Marshall suggests.

Buckley says she also doesn’t find it surprising that a field with
mostly male researchers might neglect to model these costs.

THEORY UPDATE

The larger energy estimates should be plugged into models of how
animals live, says Asta Audzijonyte at the University of Tasmania in
Hobart, Australia, who models how fish are faring amid climate change.
It will take time to make these revisions, says Marshall, because
rewriting the models is not straightforward. But, in turn, this could
shift the weight of evidence behind some theories as to why animals
live the way they do, such as whether fish hit a maximum body size to
conserve energy for reproduction, he says.

Fresh estimates for reproductive energy use could also improve
predictions of how species will evolve amid climate change, says
Audzijonyte. This is because predictive models take into account
factors such as the energy demands of reproduction, how heat alters
the speed of reactions inside animals’ bodies and how big animals
grow.

The study by Marshall and his team is just the first step towards
quantifying the costs of reproduction across the animal kingdom, he
says. Owing to scant data, the researchers often had to extrapolate
the costs of pregnancy from measurements taken at just a few points
during gestation, which limits the accuracy of their estimates, says
Marshall. In future, studies that measure the indirect costs of
reproduction throughout the entire gestation period will be needed, he
says.

“Hopefully, a lot of us will put effort into better quantifying that
indirect cost,” says Buckley.

_Nature_ 634, 768-769 (2024)

_doi: [link removed]

References

*
Ginther, S. G., Cameron, H., White, C. & Marshall, D.
J. _Science_ 384, 763–767 (2024).

Article [[link removed]] PubMed
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