We all know that having a baby is expensive, but that truth is more universal than you might think: Across animal species, producing offspring may be 10 times more energetically costly than previously thought, according to a new study. New Research Released on May 16th ScienceIn particular, the authors say, previous biological theories have significantly underestimated the indirect metabolic costs to mothers who are carrying a child.
“This is the first analysis to focus solely on the energetic input required to produce eggs or offspring, excluding the costs of parental care after eggs are laid or offspring are born (such as nursing and feeding). Nevertheless, it shows that bringing new life into the world comes at a significant biological cost. The findings have implications for our fundamental understanding of biology and ecology, as well as for wildlife conservation and future predictions of biodiversity under climate change.”
“This is like rewriting the textbook.” Robbie Burger“These findings are important to our understanding of the science of how animals are able to function and how they function in a variety of ways,” said Dr. Popular Science“The costs of biogenesis have often been ignored and considered very small, but this shows that the enormous costs of growing and developing an offspring from fertilization to birth don’t end up in biomass, they’re all burned up.”
“Most of us are trying to understand the direct energetic costs of reproduction and subscribe to the assumption that the indirect costs are negligible,” he said. Lauren Buckley“The authors say that indirect costs outweigh the direct costs, which has major implications for how we think about the energetics of organisms in their environments,” said John F. Kennedy, an evolutionary ecologist at the University of Washington who was not part of the study team.
In reproduction, energy is embodied in the offspring, either the young that hatch from the eggs or the newborns that are born at birth. This direct energy comes from the mother and is formed into the biological tissue of the new individual. This energetic cost is measured by assessing the composition of the offspring and estimating the amount of energy contained within it.
But reproduction also has an indirect cost: the energy expended synthesizing and carrying offspring, which until now has been barely quantified. The latter is much greater than the former for many species, especially mammals, according to the new study. The results prove that producing animals is an inefficient process, similar to powering a car engine, the researchers say. Michael Carney“Most of the energy put into reproduction is wasted as heat and other metabolic by-products from the chemical and biological reactions that allow new life to form. It takes more energy to synthesize tissue than is contained in the resulting tissue itself,” says the University of Melbourne ecologist and biologist, who was not involved in the new study.
The study assessed animals across taxa and divided them into three groups: egg-laying ectotherms (commonly known as “cold-blooded” animals), viviparous ectotherms (such as many reptiles and arthropods), and mammals, which raise their young and regulate their body temperature internally (i.e., are “warm-blooded”). While costs vary widely between species, on average, the researchers found that for egg-laying ectotherms, about 40% of the total energy costs of reproduction are indirect. For viviparous ectotherms, that figure rises to about half. And for mammals, the amount of indirect metabolic energy consumed during reproduction accounts for about 90% of the total energy used in the process from fertilization to birth.
To arrive at these figures, the scientists combed through thousands of research papers and formally evaluated 171 previously published relevant papers on animal metabolism and reproduction, covering a total of 81 species (birds were excluded from the quantitative analysis due to a lack of previous research). Studies included in the meta-analysis were those that included data on the amount of energy individual animals were burning at baseline, the energy expended while carrying eggs or young, and/or the energy content of the offspring. From this previous work, the authors synthesized various data and created a simple mathematical model to understand 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 life stage.
The researchers also modeled the indirect reproductive costs for several species in the three analysis groups. They found that among mammals, bats have the lowest indirect costs, accounting for about 75% of their total reproductive energy, while humans have among the highest. From fertilization to birth, human pregnancy consumes 24 times more metabolic energy than the direct energy stored in the newborn. In other words, the indirect costs of having a child account for 96% of the total energy burden of reproduction.
It’s unclear exactly where the excess energy goes, but the researchers hypothesize that the formation of the placenta and the offspring’s metabolism may itself use up calories from the mother. More research is needed to know for sure.
What’s clear is that if the findings hold true, much of animal biology will need to be reevaluated: “Our study directly calls into question most biological models of animal development and life cycles.” Samuel Ginther“The results suggest that the effects of aging are not conclusive,” said lead author and doctoral student at Monash University in Australia. Popular Science.
Caroline Williams, an integrative biologist at the University of California, Berkeley, who was not involved in the study, says the new reproduction model could significantly change estimates of ecosystem carrying capacity and our understanding of the resources needed to support species survival. It could also change our understanding of the trade-offs organisms make to grow and survive. Things like body size and lifespan may be more influenced by reproduction than we thought, Williams notes. Aging may also be closely linked to reproductive metabolism, Berger adds. Perhaps the costs of having children cause animals to age faster.
From a larger perspective, the discovery adds to concerns about how climate change is affecting life on Earth. Ginther and his colleagues note that the metabolism of ectotherms is intricately linked to the temperature of their surroundings and increases with increasing temperatures. They hypothesize that global warming could further increase the already high metabolic costs of reproduction, resulting in smaller, less active offspring, especially in cold-blooded animals.
“Many organisms live on their energy limits, especially under climate change and extreme weather conditions,” Buckley says. “There are costs that we haven’t considered before, and they’re additional challenges to organisms and ecosystem health that we weren’t aware of.”
The study is a “really good first step” toward better understanding pregnancy and reproduction, Williams says. But there’s a lot of work to be done. She points out that more data is needed on the actual (unmodeled) energetic costs of different animal species. As a result of this limited data, the study’s sample size was small. Buckley adds that the authors had to make some big assumptions to create the mathematical model, for example that indirect metabolic costs increase linearly throughout gestation. “The assumptions seem reasonable overall, but it’ll be interesting to see people try to quantify the costs of reproduction more precisely in the future,” she explains. And many questions remain unanswered, including the comparative costs of lactation and pregnancy in mammals, whether the energetic costs of reproduction for males have also been underestimated, and how birds fit into all of this.
This “shines a light on how little we know,” Carney says, but at least we’ve gotten our first glimpse into an underappreciated aspect of motherhood.
