Humans are one of the species that spend the most amount of energy on having a baby

The first global calculation shows that the metabolic cost of reproduction is three times higher for mammals than it is for cold-blooded animals

Descendencia primates
Primates are among the animals that spend the most energy on having offspring. In this photo, a family of macaques groom one another while caring for a newborn in a mangrove swamp in southern Thailand.Matt Hunt (SOPA Images/LightRocket via Gett)
Miguel Ángel Criado

A group of scientists has calculated the metabolic cost of reproduction, and humans are one of the species that spends the highest amount of joules (the basic unit for measuring energy) on having children.

A group of biologists collected data on the metabolic cost of reproduction in 81 animal species, with differing reproductive strategies. Among large placental mammals, the norm is one or a few offspring per reproductive event after months of development and a long period of postnatal care. Meanwhile, in most species of ectotherms, the so-called cold-blooded animals, the norm is the mass laying of eggs. But all strategies have a double energy cost: the energy in the offspring and the energy expended to make them.

According to the researchers’ findings, published in the journal Science, the total cost of reproduction is much higher than previously believed, sometimes up to 10 times more. Mammals spend three times as much energy as oviparous ectotherms (species that lay eggs), such as most fish, reptiles and amphibians, and more than double that of viviparous ectotherms, such as some snakes and lizards, whose embryos develop completely in the womb.

Although this is one of the first attempts to quantify the real, physical cost of reproduction in a representative sample of the animal kingdom, the work only calculates the cost up to the moment of the birth or the laying of the eggs. This leaves out energy-intensive processes and behaviors, such as the metabolic cost of producing breast milk or the prolonged care of offspring, which is seen in most mammals. “We did not include the energy costs of postnatal care because there are many ways to quantify it, such as activity patterns, defending the young, cuddling for warmth,” explains Sam Ginther, a scientist at Monash University (Victoria, Australia) and lead author of the investigation.

Of the 81 species studied, the animal (specifically the female) that dedicates the most energy to reproducing is the white-tailed deer or Virginia deer (Odocoileus virginianus), which invests an average of 470,037 kJ (one kJ is equal to 1,000 joules). At the opposite end of the scale is the rotifer (Brachionus plicatilis), which is used to feed fish larvae. For rotifers, the reproductive effort until laying eggs is barely 0.000003 kJ, 11 times lower.

“The total energy cost of human reproduction is 208,303 kJ,” says Ginther. The figure places humans among the top four species in the sample that spend the most on reproduction. “The direct costs are 8,669 kJ and the indirect costs are 199,634 kJ. For comparison, a slice of bread weighing about 28 grams includes 287 kJ. This means that the indirect costs for humans represent approximately 96% of the total cost of reproduction. In this, the human race has the second-largest metabolic load during pregnancy.”

The distinction between direct and indirect cost is key for the authors of this research. Until now, the energy in the offspring had been studied and was well known. It was the result of multiplying the mass of the clutch or offspring of each reproductive event (measured in grams) by the energy density of all the tissues of the offspring (measured in joules per gram). It was a formula that allowed estimation of reproductive cost and comparison between species.

But this left out a much more complex calculation. Intuitively, it is evident that pregnancy, the metabolic demands of a placenta, female mosquitos’ need for proteins to develop eggs and salmons’ effort to return to river headwaters to die of exhaustion just a few minutes after procreating require an enormous expenditure of energy. The problem is calculating the cost. The formula to calculate this indirect cost is somewhat more complicated: the metabolic load (also expressed in joules) is obtained by multiplying the increase in the mother’s metabolism attributable to reproduction (joules per hour) by the duration of pregnancy (in hours).

With this formula, the researchers have confirmed that 48 of the 81 species studied incur more indirect than direct costs. On average, for every joule invested in the offspring, the mother needs another 10 to produce it. And this is more pronounced among mammals, particularly humans. This has great implications for the study of living beings, as highlighted by the senior author of the research, Dustin Marshall, a biologist at the same Australian university: “Our work implies that the way animals grow, when their growth is stunted and indeed their entire existence, is geared to meet the enormous energy demands of reproduction. We have long known that successful reproduction is the essential measure of evolutionary fitness in multicellular animals, but this work shows how much energy consumption this reproduction requires.”

The case of mammals is very striking, as they do not seem to be very efficient when it comes to energy expenditure. The authors give the example of breast milk: in many mammalian species, mothers spend more energy producing it, than the energy it contains. But Marshall disagrees that this is inefficiency: “Yes, mammals expend much more energy to produce offspring of a given mass, which seems less efficient. But those hatchlings, with their advanced development (compared to, say, a fish egg) and rapid growth, suffer much less mortality than less developed hatchlings as they reach adulthood. In the end, the expenditure of mammals represent a high initial investment but with a higher survival rate” And that is rewarded by evolution.

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