Why Are Animals Shrinking?

Although it may seem strange, for decades animals have been shrinking little by little. Surprised? Find out why.
Why Are Animals Shrinking?

Last update: 17 August, 2021

For the past 40 years, a phenomenon has occurred that intrigues biologists and ecologists alike: animals are shrinking generation after generation. It has taken quite a few years for this data to come to our attention, as it’s a slow and subtle process. Even so, recent studies have investigated this issue in depth.

How is it possible for animals to get smaller? Although there are still ends to be tied, it seems that global warming, once again, has something to say in the matter. If you want to know what these two phenomena have to do with each other, read on, and you’ll find the answers.

Bergmann’s rule and global warming

It’s nothing new for individuals of the same species to be of different sizes depending on the time interval or the location. In fact, in ecology, there is a principle known as Bergmann’s rule, which postulates that individuals in a population of endothermic (warm-blooded) animals will be larger in colder climates and smaller in warm places.

This is directly related to body surface area: large animals have an easier time conserving heat and smaller animals lose it better in hot climates. Therefore, according to this standard, it would be expected that animal body sizes would change where global warming has affected temperatures.

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Fossil evidence that animals are shrinking

The fact that animals are shrinking doesn’t come as a surprise to us. Throughout Earth’s history, global temperatures have changed due to extreme weather events (such as ice ages). Through the fossils of the early Eocene it has been confirmed that many living beings reduced in size with increasing temperatures of between 5 and 8 degrees over 10,000 years.

These studies don’t only serve to confirm that temperatures influence body size, but they also provide a basis on which to predict the effects of global warming in our future. By their calculations, the planet is expected to be 1.5 degrees warmer by 2040.

Animals are shrinking: the data

There are many studies that have found a relationship between climate change and the fact that animals are gradually shrinking. In 2019, for example, 52 bird species in the Field Museum’s collection were found to have reduced in size by 2.6% between 1978 and 2016.

There are even animals that are getting bigger due to this same effect, as global warming slows down the arrival of winter, helping large mammals to extend their growth period.

The case of the menhaden fish (Brevoortia tyrannus) is also known to have reduced its size by 15% in the last 65 years. The field mice of the Doñana National Park in Spain are others affected – today, they weigh a third of what they did 40 years ago. The Basque whale (Eubalaena glacialis) has reduced its length by almost 1 meter (3.3 feet) since the 1980s.

Explanations for endotherms and ectotherms

The metabolism of each animal and the environmental temperature are also related to this event. In cold climates, the body’s basal metabolism increases, allowing the organism to grow further during its maturation, thus complying with Bergmann’s rule. If temperatures rise due to global warming, this growth is reduced.

And what about ectothermic species, like reptiles and amphibians? A reduction in their average size has also been documented and there’s an explanation for these animals too. Warm temperatures, for example, accelerate the development phases of frogs and their growth rate isn’t maintained, In this way,  they’re smaller when they complete their metamorphosis.

The decrease in the size of an animal by actions unrelated to natural selection can be a devastating event for its survival.

The sixth mass extinction

In the history of the Earth there have already been five mass extinctions of species, after which the planet has managed to recover. However, this restoration took millions of years to develop. Now, with the action of man, the sixth extinction is closer than previously believed: the rate of disappearance of species is 100 to 1000 times higher than expected.

The mere disappearance of an animal or plant implies much more than us not being able to see that species ever again. The interdependence of species is a delicate balance, and taking the “legs” off the “ecosystem table” causes it to wobble more and more.

Predators that run out of food, reduced dispersal of seeds and pollen, increased populations of other species and a long etcetera: the examples of the destabilization of food chains are innumerable.

Why are animals shrinking?

Although efforts to predict and prevent the future effects of human action on the planet are intense, the complexity of ecosystems always means that unexpected factors appear. Experts assure us that, until now, the measures that are being carried out are simply “patches”. If the root problem isn’t tackled, then there’ll be no solution for the effects of climate change.

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  • Turner, R. E. (2017). Smaller size‐at‐age menhaden with coastal warming and fishing intensity. Geo: Geography and Environment4(2), e00044.
  • Abigail, R. D., Clyde, W. C., Fricke, H. C., Gingerich, P. D., & Abels, H. A. (2017). Repetitive mammalian dwarfing during ancient greenhouse warming events. Science Advances3(3), e1601430.
  • Haynes, L. L., & Hönisch, B. (2020). The seawater carbon inventory at the Paleocene–Eocene Thermal Maximum. Proceedings of the National Academy of Sciences117(39), 24088-24095.
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