Humans Are Driving Domestic Animals Larger While Wild Animals Grow Smaller

Humans are reshaping animal body size

Over the past 1,000 years, domestic animals have generally grown larger, whereas wild animals have become smaller. A major new study from Mediterranean France identifies a single clear factor behind these contrasting shifts: humans.

People are a powerful evolutionary influence on Earth, and this extensive research, led by University of Montpellier archaeologist Cyprien Mureau, offers a striking illustration of that role.

Mureau's team examined more than 81,000 physical measurements from animal remains recovered at 311 archaeological sites. They found that domesticated species, including chickens and cattle, increased in size during the past millennium, while wild species such as foxes and deer became smaller.

Agriculture and urbanisation intensified the divide

The researchers also modelled environmental influences over 8,000 years, covering climate, vegetation and human land use. The two opposing patterns accelerated abruptly within the last 1,000 years, alongside rapid growth in agriculture and urbanisation.

As growing human populations reduced and fragmented wild landscapes, the team explains, resources became scarcer. Greater hunting pressure, combined with these losses, subjected wild mammals and birds - both herbivores and carnivores - to stronger selection pressures, reducing not only their abundance but also their body size.

Why domestic animals became larger

At the same time, people selected and bred bigger domestic animals for the goods they supplied. Larger sheep yield more wool, heavier cattle provide more meat, and bigger chickens lay more eggs, for example. Domesticated chickens now account for more than three times the biomass of all wild birds combined.

"These findings … [highlight] … in the last millennium, the increasing impact of human activities," Mureau and team write in their paper.

Recent research has also revealed unintended ways in which human activity is altering the bodies of wild animals. Puffins are becoming smaller, as are the wingspans of cliff swallows. Overfishing has left many fish 20 percent smaller, while their life cycles are also 25 percent shorter on average.

These changes are a direct result of humans using natural resources without considering their future environmental effects.

A 2021 study concluded that species offering no immediate benefit to people are more likely to be vital for ecosystem stability. That same stability is necessary for the continued survival of species on which humans do directly depend. Even parasites may have an unusually important part in keeping ecosystems balanced.

But that can happen only if they are allowed to survive.

The research was published in PNAS.