It takes a lot of energy to grow a baby – just ask any pregnant person. But scientists are only now discovering just how much.
In a study published Thursday in the journal Science, Australian researchers estimate that a human pregnancy requires nearly 50,000 dietary calories over a nine-month period. That’s the equivalent of about 50 pints of Ben and Jerry’s Cherry Garcia ice cream, and way more than the researchers expected.
Previous estimates were lower because scientists generally assumed that most of the energy involved in reproduction was stored in the fetus, which is relatively weak.
But Dustin Marshall, an evolutionary biologist at Monash University, and his students found that the energy stored in a human baby’s tissues represents only about 4% of the total energy costs of pregnancy. The remaining 96 percent constitutes the additional fuel required by the woman’s body.
“The baby itself becomes a rounding error,” Dr. Marshall said. “It took us a while to figure that out. »
This discovery came from Dr. Marshall’s long-term research into metabolism. Different species must meet different energy demands. Warm-blooded mammals, for example, can maintain a stable body temperature and remain active even when the temperature drops.
But being warm-blooded also has disadvantages. Maintaining a high metabolic rate requires mammals to constantly fuel the furnace. In contrast, a cold-blooded snake can go weeks between meals.
Dr. Marshall set out to compile a comprehensive 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 provide additional energy to their offspring.
When Dr. Marshall began studying the costs of reproduction, he was unable to find precise figures. Some researchers had guessed that indirect costs – that is, the energy women use to fuel their own bodies during pregnancy – might represent only 20% of the direct energy present in the baby’s tissues. But Dr. Marshall didn’t trust their speculation.
He and his students set out to estimate the costs themselves. They scoured the scientific literature for information such as the energy stored in each offspring’s tissues. They also researched the females’ overall metabolic rate during reproduction, which scientists can estimate by measuring the amount of oxygen consumed by mothers.
“People were just putting their data together about their species, but no one was putting it together,” Dr Marshall said.
By aggregating this data, the researchers estimated the reproductive costs of 81 species, from insects to snakes to goats.
They discovered that the size of an animal has a big influence on the amount of energy it needs to reproduce. Microscopic animals called rotifers, for example, need less than a millionth of a calorie to produce offspring. In contrast, a white-tailed deer doe requires more than 112,000 calories to produce a fawn.
A species’ metabolism also plays a role. Warm-blooded mammals consume three times more energy than reptiles and other cold-blooded animals of the same size.
The biggest surprise came when Dr. Marshall and his students discovered that in many species, the indirect costs of pregnancy were higher than the direct costs.
The most extreme results came from mammals. On average, only 10% of the energy consumed by a female mammal during pregnancy went to her offspring.
“It shocked me,” Dr. Marshall said. “We went back to the sources several times because it seemed surprisingly high compared to the expectations of the theory.”
David Reznick, an evolutionary biologist at the University of California, Riverside, who was not involved in the study, was also surprised by how large the indirect costs could be. “I wouldn’t have guessed that,” he said.
Yet what surprised him even more was that Dr. Marshall’s team was the first to identify these numbers. “It’s disarming,” he said. “You’d think someone had done that before.”
The study provides clues as to 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. Live snakes must support embryos as they grow inside their bodies, while egg-laying mothers can push their offspring out of their bodies more quickly.
There may be a number of reasons why 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 females stay pregnant longer than most other mammals.
Dr Marshall said the new findings could also explain why female mammals spend so much effort caring for their young after birth: because they put so much effort into pregnancy.
“They have already committed huge sunk costs to the project,” Dr Marshall said.
News Source : www.nytimes.com
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