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SCIENTISTS CALCULATED THE ENERGY NEEDED TO CARRY A BABY. SHOCKER:
IT’S A LOT.  
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Carl Zimmer
May 16, 2024
New York Times
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_ The energetic cost of a human pregnancy is about 50,000 dietary
calories — far higher than previously believed, a new study found. _


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It takes a lot of energy to grow a baby — just ask anyone who has
been pregnant. But scientists are only now discovering just how much.

In a study
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Thursday in the journal Science, Australian researchers estimated that
a human pregnancy demands almost 50,000 dietary calories over the
course of nine months. That’s the equivalent of about 50 pints of
Ben and Jerry’s Cherry Garcia ice cream, and significantly more than
the researchers expected.

Previous estimates were lower because scientists generally assumed
that most of the energy involved in reproduction wound up stored in
the fetus, which is relatively small.

But Dustin Marshall, an evolutionary biologist at Monash University,
and his students have discovered
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human baby’s tissues accounts for only about 4 percent of the total
energy costs of pregnancy. The other 96 percent is extra fuel required
by a woman’s own body.

“The baby itself becomes a rounding error,” Dr. Marshall said.
“It took us a while to wrap our heads around that.”

This discovery emerged from Dr. Marshall’s long-running research on
metabolism. Different species have to meet different demands for
energy. Warm-blooded mammals, for example, can maintain a steady body
temperature and stay active even when the temperature drops.

But being warm-blooded also has drawbacks. Maintaining a high
metabolic rate requires mammals to constantly feed the furnace. A
coldblooded snake, in contrast, can go weeks between meals
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Dr. Marshall set out to compile a complete inventory of the energy
consumed by dozens of species over the course of their lives. He
recognized that most females must not only fuel their own bodies, but
must also put additional energy into their offspring.

When Dr. Marshall began looking into the costs of reproduction, he
couldn’t find solid numbers. Some researchers had guessed that
indirect costs — that is, the energy females use to fuel their own
bodies while pregnant — might come to only 20 percent of the direct
energy in the baby’s tissues. But Dr. Marshall didn’t trust their
speculation.

He and his students set out to estimate the costs for themselves. They
scoured the scientific literature for information such as the energy
stored in each offspring’s tissues. They also looked for the overall
metabolic rate of females while they were reproducing, which
scientists can estimate by measuring how much oxygen the mothers
consume.

“Folks were just poodling along, collecting their data on their
species, but no one was putting it together,” Dr. Marshall said.

By aggregating such data, the researchers estimated the costs of
reproduction for 81 species, from insects to snakes to goats.

They found that the size of an animal has a big influence on how much
energy it needs to reproduce. Microscopic animals called rotifers, for
example, require less than a millionth of a calorie to make one
offspring. By contrast, a white-tailed deer doe needs more than
112,000 calories to produce a fawn.

The metabolism of a species also plays a part. Warm-blooded mammals
use three times the energy that reptiles and other coldblooded animals
of the same size do.

The biggest surprise came when Dr. Marshall and his students found
that in many species, the indirect costs of pregnancy were greater
than the direct ones.

The most extreme results came from mammals. On average, only 10
percent of the energy a female mammal used during pregnancy went into
its offspring.

“It shocked me,” Dr. Marshall said. “We went back to the sources
many times because it seemed astonishingly high based on the
expectation from theory.”

David Reznick, an evolutionary biologist at the University of
California, Riverside, who was not involved in the study, was also
startled at how high the indirect cost could get. “I wouldn’t have
guessed that,” he said.

And yet what surprised him even more was that Dr. Marshall’s team
was the first to pin down these numbers. “It is disarming,” he
said. “You think, someone has done this before.”

The study offers clues about why some species have higher indirect
costs than others. Snakes that lay eggs use much less indirect energy
than snakes that give birth to live young. The live-bearing snakes
have to support embryos as they grow inside their bodies, whereas
egg-laying mothers can get their offspring out of their bodies faster.

There may be a number of reasons that mammals pay such high indirect
costs for being pregnant. Many species build a placenta to transfer
nutrients to their embryos, for example. Dr. Marshall suspects that
humans pay a particularly high cost because women stay pregnant longer
than most other mammals do.

Dr. Marshall said that the new results may also explain why female
mammals put so much effort into caring for their young after they’re
born: because they put in so much effort during pregnancy.

“They’ve already got massive sunk costs in the project,” Dr.
Marshall said.

_Carl Zimmer [[link removed]] covers news
about science for The Times and writes the Origins column
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_Get the Times in your inbox with any of these free newsletters
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