Growing a baby requires a lot of energy, just ask any woman who has been pregnant. Scientists are now uncovering just how much energy it takes. In a study published in the journal Science, Australian researchers estimated that a human pregnancy demands nearly 50,000 dietary calories over nine months.
This is equivalent to about 50 pints of ice cream. Far more than previously thought. Earlier estimates were lower because scientists assumed most of the energy used in reproduction was stored in the fetus, which is relatively small. However, Dustin Marshall, an evolutionary biologist at Monash University, and his students found that the energy stored in a 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 the mother’s body.
This discovery stems from Dr. Marshall’s long-running research on metabolism. Different species face different energy demands. Warm-blooded mammals, for example, maintain a steady body temperature and stay active even when it’s cold, but this high metabolic rate requires constant feeding. Cold-blooded snakes, in contrast, can go weeks between meals.
Dr. Marshall aimed to inventory the energy consumption of various species over their lifetimes. He realized that most females must fuel their bodies and invest additional energy into their offspring. Initially, he found no solid numbers on reproduction costs. Some researchers speculated that indirect costs, the energy females use to sustain their bodies while pregnant, might only be 20% of the direct energy in the baby’s tissues.
Dr. Marshall was skeptical and decided to estimate the costs himself. He and his students reviewed scientific literature for data on energy stored in offspring tissues and the overall metabolic rate of reproducing females, estimated by measuring their oxygen consumption. “Folks were just poodling along, collecting their data on their species, but no one was putting it together,” They found that animal size significantly influences reproductive energy needs. Microscopic rotifers require less than a millionth of a calorie to make one offspring, while a white-tailed deer needs over 112,000 calories to produce a fawn. The species’ metabolism also matters; warm-blooded mammals use three times the energy of similarly sized cold-blooded animals.
The biggest surprise was that in many species, indirect pregnancy costs exceeded direct ones, especially in 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.”
The study offers clues about why some species have higher indirect costs. Egg-laying snakes use less indirect energy than those giving birth to live young, as the latter support embryos growing inside them. Mammals may have high indirect costs due to the placenta transferring nutrients to embryos. Dr. Marshall suspects that humans pay a particularly high cost because women stay pregnant longer than most mammals. The findings might also explain why female mammals invest heavily in post-birth care—they have already invested significantly during pregnancy.