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Living creatures who carry fetuses to term experience an energy spike, long thought to be caused by the fetus.

But now, a team of Australian researchers found that it is the bearer’s own body that demands such high amounts of energy, according to a new study.

While energy invested in the offspring – the direct cost – was well documented, the energy expended to make babies – the indirect cost – was “unquantified but often assumed to be small,” researchers wrote in the study.

The scientists combined data from thousands of previous studies covering a wide range of species including goats, snakes, insects, lizards, and humans.

In the end, researchers examined the costs of reproduction for 81 species and found that the size of an animal has a big influence on how much energy it needs to reproduce.

Microscopic animals called rotifers need less than a millionth of a calorie to produce an offspring, while a white-tailed deer doe needs more than 112,000 calories to do so.

The metabolism of a species also plays a part, for example, warm-blooded mammals use three times the energy than coldblooded fauna of the same size do

Their most striking finding was that the direct cost of bringing an offspring to term was typically smaller than the indirect one. Even by excluding lactation, they estimated that the metabolic load of bearing a baby represented around 90 percent of total reproductive costs for mammals.

“We went back to the sources many times because it seemed astonishingly high based on the expectation from theory,” co-author Dustin Marshall, who teaches evolutionary biology at Monash University in Melbourne, told the New York Times.

The researchers understood that for mammals and other warm-blooded animals, energy was also spent on keeping up their temperature – and of course, that of the womb.

They also found that placentas were “metabolically expensive.”

Overall, they established that pregnancy costs a human nearly 50,000 calories not necessarily spread evenly across the pregnancy term.

That equals about 75 steak dinners, or 219 Mars bars.

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