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Washington State University

In the Blink of an Eye: How Humans Shrink Animals Around the World

Posted by rdsayler | October 16, 2023
Female Northern Cardinal
[Female Northern Cardinal (Cardinalis cardinalis). Source: Wikipedia. Credit: Rhododendrites. License: CC BY-SA 4.0 International.]


If humans are good at one thing, it’s our ability to change the world, whether we intend to do so or not. Human impacts on the world range from immensely large (think global climate change) to those small and barely noticeable – perhaps even including the size of a bird’s eye.

The surprising reality is that humans are directly and indirectly shrinking the size of many species of animals, not just birds. While we never intended to do so, human activities and the unique environments we create are physically and rapidly shrinking the living world around us in multitudes of ways.

Dr. Jennifer Phillips, an avian ecologist in the WSU School of the Environment, along with her colleagues at Texas A&M University-San Antonio, recently reported in the journal Global Change Biology that individuals of several bird species, including Carolina Wrens (Thryothorus ludovicianus) and Northern Cardinals (Cardinalis cardinalis), that were resident in core urban environments with brighter lights and illumination had smaller average eye sizes than birds living in edge habitats farther away from urban environments.

By contrast, several migratory bird species did not show any changes in eye size depending on where they lived. These different responses to urban light sources may be explained by resident urban birds adapting to the much brighter ambient light levels in cities and urban environments where night-time lighting is ubiquitous, while migratory birds that spent a greater proportion of time away from such urban areas did not.

The investigators also found smaller body size among one bird species in urban environments, the Painted Bunting (Passerina ciris), but concluded that this size difference was due to younger and smaller males being more likely to inhabit less desirable urban-core habitats.

This study helps illustrates that many relatively unseen changes are occurring in species as they adapt to novel ecological and evolutionary forces created in human-dominated and urban environments in particular.

“A growing body of research suggests that global warming is messing with the body sizes of all kinds of creatures, from cold-blooded frogs to warm-bodied mammals, and often making animals smaller.” – Benji Jones for 

According to an ecogeographical principle called Bergmann’s Rule, among broadly distributed groups of warm-blooded species, species of larger body size are more likely to be found in colder regions compared to species living in warmer climates. For example, the largest penguins occur in the bitter cold landscape of Antarctica while smaller species of penguins are found in warmer areas in Argentina and elsewhere.

Bergmans Rule Illustrated by Penguin Distributions
[Figure: Bergman’s Rule illustrated by geographic distribution of species of penguins. Source: Wikipedia. Credit: Karel Frydrýšek. License: CC BY-SA 4.0 International.]

A similar ecogeographical principle, Allen’s Rule, predicts that animals in colder climates generally will have a smaller surface area to volume ratio, essentially meaning that individuals in cold climates will minimize the surface area of their extremities (e.g., smaller ears, legs), along with other adaptations, to better conserve heat energy. Conversely, individuals in warmer climates and temperatures may have relatively smaller body sizes or larger extremities (e.g., ears).

While there are many exceptions to these rules for different species in different environments due to competing selection forces, one general prediction is that under warmer temperatures smaller body sizes may be an advantage by allowing heat to dissipate easier and prevent overheating. In addition, a smaller body may affect energy and metabolic demands and subtly improve survival.

Thus, under the environmental pressures created by rapid global warming, individuals of many species may be selected to develop or evolve smaller body sizes to better deal with the increased heat and all of the myriad ecological and physiological changes now confronting most organisms on Earth.

Evidence for Shrinking Birds & Mammals

A recent study with large sample sizes (e.g., >100,000 specimens) have documented a general decline in the size of numerous species of birds over about the past 40 years. Moreover, smaller bird species appear to be changing body size more rapidly than larger species.

The evidence that a warming planet may shrink many living animals is also supported by fossil evidence from species living millions of years ago. Fossile evidence demonstrates that when the Earth warmed dramatically during the early Eocene about 56 M years ago, some mammals and other species became smaller (see: Vox – Animals are shrinking. Blame climate change.)

The Shrinking World

What may surprise many people is that it isn’t only birds and mammals that may be shrinking in size due to increasing heat. The phenomenon has been documented for frogs, fish, snakes, salamanders, insects, rodents, and other groups of organisms. However, ecological and physiological factors other than simple heat dissipation can be involved and may end up shrinking some animals for a variety of different reasons (see: Scientific American).

For example, the developmental metabolism of many organisms is affected by temperature, which means that under warmer temperatures, young may develop and mature more quickly which results in a smaller adult or mature body size, which may then directly influence their ability to reproduce, produce as many offspring, or survive. Such a process may partly explain why some amphibian and fish species may exhibit smaller body size under higher water temperatures.

In addition, increasing heat may influence available food resources and cause shifts in foraging ecology, food webs, and energy and nutrient availability, which may then restrict growth and development of young and immature animals, again resulting in smaller body sizes.

Shrinking Populations & Species

In addition to the declining size of some individual organisms, humans are shrinking the average size of many studied or monitored wildlife populations around the world (see: Living Planet Index; see critique: Our World in Data) as well as reducing the numbers of species on Earth. The decline in size of individual wildlife populations is dramatic while at the same time the human population is projected to grow by additional billions of people.

Graph of Declining Wildlife Populations
[Graph of change in the Living Planet Index (LPI) since 1970. Credit: Our World in Data.]

“Since 1970… the size of animal populations for which data is available have declined by 69% on average.” Living Planet Index


Don’t blink, but climate change is affecting many species on Earth in complex ways. The shrinking size of individuals of various species, along with drastic reductions in biological diversity and abundance of many species of plants and animals, may be an increasingly common and life-threatening phenomenon as temperatures rapidly increase on Earth.

R. Sayler
WSU School of the Environment


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