What the electric meter tells us about the birth rate

July 4, 2012, Max Planck Society
Fig. 1a: The size of mammals (blue dots) is shown in relationship to their metabolic rate. For humans, the metabolic rate varies considerably (red dots), but is on average (star) much higher than expected. Human weight is set here at 50 kilograms. 1b: While life expectancy among humans rises along with energy consumption, the increase is less than would be expected for a mammal with the same metabolic rate (compare dashed line). 1c: Birth rates decline with rising energy use roughly equally among humans and other mammals. 1d: With rising per capita energy consumption of a country, the gap between the birth of the first child and the onset of menopause is decreasing. © MPI for Demographic Research

(Medical Xpress) -- If a woman were to consume in the form of food the amount of energy she uses, and were to follow the fertility patterns seen in other species, she would weigh as much as two elephants, and would continue to bear children up to the age of 70. This is because, generally, the more energy an organism uses, the bigger it is, the longer it lives, and the later it reproduces. But is this true of humans? After all, humans satisfy their appetite for energy through electrical outlets and other industrial sources.

Humans need to eat around 2,000 , but the in industrial nations burns more than 32,000 calories a day. In wealthy industrial nations like , the average energy consumption of an individual is 110,000 calories, which is still only half of the U.S. value. Because he only eats a small portion of the energy he uses, this level of energy consumption has little effect on the individual’s weight. It does, however, influence his demographic traits, such as lifespan and birth rate, Oskar Burger of the Max Planck Institute for Demographic Research has found in a new study. And this enormous energy usage could even explain the low birth rates seen in highly developed countries, according to the author and his collaborators. 

For almost all animals, body size determines energy use because it determines their metabolic rates, and both body size and energy use are highly correlated with most demographic traits: for example, a giraffe that weighs about 13 times as much as a human burns over 24,000 calories a day, has around 0.6 calves a year, and has a maximum life expectancy of 36 years. A tiny spine mouse, by contrast, needs to consume only around 19 calories a day to maintain its body weight of 45 grams. The mouse has a maximum lifespan of only five years, but gives birth to more than eight offspring a year in that time. These values for the giraffe and the mouse obey so-called “allometric” relationships. The giraffe is 18,000 times heavier than the mouse; the life expectancies and birth rates of these animals change predictably as a function of this change in mass, but they change in smaller steps. The same basic relationship holds across all mammal species, from the smallest to the largest (see Fig. 1b and 1c).

Humans are the exception that proves this rule. If a man were to actually consume all of the energy he uses in the form of food, he would weigh 6,435 kilograms (see Fig. 1a). But the ability of humans to use non-food energy sources has altered this biologically fundamental relationship between body weight and energy use. There is, however, a precise correlation between other demographic traits and energy consumption: a hypothetical female primate that ate 110,000 calories a day would not only weigh as much as two ; she would also have a maximal life expectancy of 112 years, and would bear her first child at the age of 27.

These values fit surprisingly well with the data for average Germans. A comparison between countries with different energy levels also shows that the more watts a country uses, the higher the is (see Fig. 1b), the later children are born, and the lower the birth rates (see Fig. 1c) and the mortality rates of children under age five are.

To prevent a population from shrinking as a result of declining birth rates, the reproductive period should grow in line with energy consumption. But whereas the elephantine creature with the of a mean German woman could still have children at the age of 70, German women typically stop having babies by the age of 42. Oskar Burger sees a possible explanation for the low in many industrial nations in this curtailment of the reproductive lifespan (see Fig. 1d), as almost all the populations of countries that use a lot of tend to have below-replacement fertility.

Explore further: Low fertility in Europe -- is there still reason to worry?

More information: Burger, O. J. DeLong, M. Hamilton, Industrial energy use and the human life history, Scientific Reports 1 (2011) 56

Related Stories

Low fertility in Europe -- is there still reason to worry?

June 17, 2011
The post-war trend of falling birth rates has been reversed across Europe, according to a new study. However, despite an increasing emphasis on family and fertility policies in Europe, this recent development involves social, ...

Why underweight babies become obese: Study says disrupted hypothalamus is to blame

May 2, 2012
It seems improbable that a baby born underweight would be prone to obesity, but it is well documented that these children tend to put on weight in youth if they're allowed free access to calories. Now, researchers believe ...

Eating a lighter lunch can prompt weight loss

August 23, 2011
Losing weight without dieting, going hungry or using an expensive high-protein liquid diet can be as simple as eating a smaller lunch, reports a new Cornell study that is online and will be published in the journal Appetite ...

Recommended for you

Researchers devise decoy molecule to block pain where it starts

January 16, 2018
For anyone who has accidentally injured themselves, Dr. Zachary Campbell not only sympathizes, he's developing new ways to blunt pain.

Scientists unleash power of genetic data to identify disease risk

January 16, 2018
Massive banks of genetic information are being harnessed to shed new light on modifiable health risks that underlie common diseases.

Blood-vessel-on-a-chip provides insight into new anti-inflammatory drug candidate

January 15, 2018
One of the most important and fraught processes in the human body is inflammation. Inflammatory responses to injury or disease are crucial for recruiting the immune system to help the body heal, but inflammation can also ...

Molecule produced by fat cells reduces obesity and diabetes in mice

January 15, 2018
UC San Francisco researchers have discovered a new biological pathway in fat cells that could explain why some people with obesity are at high risk for metabolic diseases such as type 2 diabetes. The new findings—demonstrated ...

Obese fat becomes inflamed and scarred, which may make weight loss harder

January 12, 2018
The fat of obese people becomes distressed, scarred and inflamed, which can make weight loss more difficult, research at the University of Exeter has found.

Optimized human peptide found to be an effective antibacterial agent

January 11, 2018
A team of researchers in the Netherlands has developed an effective antibacterial ointment based on an optimized human peptide. In their paper published in the journal Science Translational Medicine, the group describes developing ...


Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jul 04, 2012
"If a woman were to consume in the form of food the amount of energy she uses"

If I watch a TV show or read a book on a kindle or read information on the Internet I am acquiring knowledge which is not equal to food.

What tortured logic!

Pathetic attempt to get into AR5.
not rated yet Jul 04, 2012
On some level, consumption of energy is proportional to intelligence, as for a very intelligent being in the universe under the laws of physics, there will be at a bare minimum a certain amount of energy per calculation or sensory action of any form which is consumed, wither it's an artificial life form or intelligence such as a robot, computer, or nano-machine collective, or whether it's an animal, a human, or an alien.

Some things we do out of oversight or whatever, which are not very intelligent, such as leaving the television on whilst out of the room and not paying attention to it anyway.

Nevertheless, if we were 10 times more intelligent, but our brains being the same efficiency, the brain would consume 10 times more energy. You might expect us to consume quite a bit more energy as the brain and heart are the two most energy consuming organs in our body, and much of what the heart does is to feed itself and the brain.
not rated yet Jul 05, 2012
The food energy that plants and animals burn is only a small fraction compared to the energy they get from the sun in 1 day. They clearly don't eat that energy, not even plants (photosynthesis is highly inefficient). Energy simply does not mean food.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.