We all know that having kids is expensive, but that truth is more universal than you might think. Across animal species, bearing offspring may be as much as 10 times as energetically costly as previously assumed, according to a new study published May 16 in Science. Specifically, the indirect metabolic costs for mothers carrying young have been seriously underestimated in past biological theories, according to the authors. 

The first-of-its kind analysis excludes the costs of parental care (like lactation or food provisioning) after eggs are laid or offspring are born, and focuses only on the energy inputs required to create those eggs or offspring. Still, it finds that there’s a hefty biological price being paid to bring new life into the world. The findings carry implications for our fundamental understanding of biology and ecology, and also for wildlife conservation and forecasting the future of biodiversity under climate change. 

“This is kind of like re-writing textbooks,” Robbie Burger, an assistant professor of biology at the University of Kentucky who was uninvolved in the new research, tells PopSci. “The cost of biosynthesis has often been neglected and assumed to be pretty insignificant. But this shows that there’s this enormous cost of growing and developing offspring from fertilization to birth that doesn’t go into biomass–it’s all just burned off.” 

“A lot of us have been trying to figure out the direct energetic costs of reproduction, and have sort of subscribed to the assumption that we can ignore these indirect costs,” says Lauren Buckley, an evolutionary ecologist at the University of Washington who wasn’t part of the study team. “These authors are saying that the indirect costs outweigh the direct costs. That has major ramifications for how we’re thinking about energetics of organisms in their environment.”

In reproduction, there’s the energy embodied by offspring–the hatchlings that emerge from eggs or the newborn young that result from live birth. This direct energy comes from the mother and ends up as biological tissue in a new individual. This energetic cost is measured by assessing the makeup of offspring, and estimating the amount of energy contained within. 

But there’s also the indirect costs to reproduction: the energy expended in synthesizing and carrying those offspring, which has previously been largely unquantified. The latter vastly outweighs the former for many species, particularly mammals, according to the new study. The findings demonstrate that manufacturing animals is an inefficient process, similar to running a car engine, says Michael Kearney, an ecologist and biologist at the University of Melbourne who was not part of the new research. The bulk of the energy put into reproduction is wasted as heat and other metabolic byproducts resulting from the chemical and biological reactions that enable new life to form. It takes more energy to synthesize tissue than the resulting tissue itself contains.

The study assessed animals across taxa, splitting them into three groups: egg-laying ectotherms (commonly known as “cold-blooded” animals), live-bearing ectotherms (like many reptiles and arthropods), and mammals, which carry life young and internally manage their body temperatures (i.e. are “warm-blooded”). Costs varied widely between species, but on average the researchers found that for egg-laying ectotherms, an average of about 40% of the total energetic cost of reproduction is indirect. For live-bearing ectotherms, that proportion goes up to about half. And, for mammals, the amount of indirect metabolic energy expended during reproduction composes about 90% of the total energy used in the process from fertilization to birth.

To come to these numbers, the scientists scanned thousands of research articles and formally assessed 171 previously published, relevant papers of animal metabolism and reproduction, encompassing data on 81 total species. (Birds were excluded from the quantitative analysis because of a lack of sufficient past research.) The studies included in the meta-analysis were ones containing data on how much energy individual animals were burning at baseline, energy used while carrying eggs or young, and/or offspring energy content. From this past research, the authors pooled the disparate data, and created a simple mathematical model for understanding both the direct and indirect costs of reproduction. They estimate that the indirect costs of reproduction are a linear function, related to the time spent carrying young or eggs and the metabolic increase associated with that lifestage.

The researchers also modeled the indirect reproductive costs for several species within their three analysis groups. Among mammals, they found brown bats had the lowest indirect costs, at about 75% of the reproductive energy total, while humans had one of the highest. From fertilization to birth, a human pregnancy from fertilization consumes 24 times more metabolic energy than the direct energy stored in a newborn. In other terms, the indirect costs of bearing a child comprise 96% of the total energy burden of reproduction. 

It’s unclear exactly where all of that excess energy goes, but the study authors hypothesize that creating the placenta or the metabolism of the offspring itself burns through the mother’s calories. Further research is needed to know for sure. 

What is clear is if the study findings hold true, then much of animal biology could be due for a re-assessment. “Our work…directly challenges most biological models of animal growth and their life histories,” Samuel Ginther, lead study author and a PhD student at Monash University in Australia tells PopSci.

The new model of reproduction could significantly alter estimates of ecosystem carrying capacity and our understanding of the resources needed to support species’ survival, says Caroline Williams, an integrative biologist at the University of California-Berkeley who wasn’t involved in the research. It could also shift how we understand the tradeoffs that organisms make in growth and survival–things like body size and lifespan may be more impacted by reproduction than we thought, she notes. Aging, too, could be closely related to reproductive metabolism, adds Burger–perhaps the cost of bearing young speeds up an animal’s senescence. 

On the larger scale, the findings amplify concerns for how climate change is affecting life on Earth. Ginther and his colleagues note that ectotherm metabolism is intricately linked to ambient temperature, increasing with rising heat. They hypothesize that global warming could increase the already high metabolic costs of reproduction, and lead to smaller, less vigorous offspring, particularly among cold-blooded animals.

“A lot of organisms are living on the energetic edge, especially in the context of climate change and extreme events,” says Buckley. “If there’s costs that we haven’t previously accounted for, those are additional challenges to the wellbeing of organisms and ecosystems that we weren’t aware of.”

The study “is a really good initial step” towards better understanding gestation and reproduction, says Williams. But there is lots of work left to do. She notes that we need much more data on the real–not modeled–energy costs across different species of animals. As a result of this limited data the study sample sizes were small. Buckley adds that the authors had to make some big assumptions to produce their mathematical model–for instance, that indirect metabolic costs would increase linearly throughout the gestation period. “The assumptions seem reasonable overall, but it will be interesting to see people try to better quantify reproductive costs going forward,” she explains. And lots of questions remain unanswered, such as the comparative costs of lactation vs. gestion for mammals, whether male reproductive energy costs may also be underestimated, and how birds fit into all of this. 

This “puts a spotlight on how little we know,” says Kearney. But now at least we have a critical first glimpse into an underappreciated aspect of motherhood.