Over time, the general size of animals changes. That’s a normal part of evolution. Sometimes, environmental factors may cause a species to grow smaller; a century later, the animals’ descendants might get bigger again. Researchers have observed alternating periods of increase and decrease over thousands of years. When humans were introduced into the mix and began rearing certain species, the sizes of wild animals and domesticated ones still developed in roughly the same cycles.
But that all changed in the Middle Ages, as scientists from the University of Montpellier in France found out. Their study was published in the journal Proceedings of the National Academy of Sciences, September 1, 2025.
The French team’s findings show that during the Middle Ages and the Modern Era (from approximately 1000 to 2000 AD), the body size evolution of domestic and wild animal species diverged.
“Wild species decreased in body size, while domestic species increased,” Allowen Evin, one of the study’s authors, told DW.
Wild animal species, such as deer, fox, hare and rabbit, got smaller because their forest habitats shrunk or became fragmented due to growing human settlements. Intensified game hunting, beginning in the late Middle Ages, contributed to the process as well.
“In parallel, human control over domestic populations increased, with greater specialization and more systematic selective breeding under controlled management practices,” Evin said.
Among the domesticated species that grew in body size were sheep, goat, cattle, pig, and chicken.
The research team’s results show “the deep and lasting prevalence of environmental influences on all species and the increasing impact of human activities during the [p]ast millennium,” the authors write.
To learn about animal species’ sizes, the scientists looked at 225,780 bones from 311 sites in southern France, spanning the past 8,000 years. Originally, they were only going to look at domestic species, since their research is part of a European Research Council project focused on “eight millennia of changes in domestic plants and animals.” But since they encountered a wealth of information on wild species as well, they expanded their work, resulting in a large-scale comparison.
Evin and her colleagues believe that decoding animals’ evolution in connection to human development tells us a lot about our own history.
Over the past few millennia, “human influence [on the natural world] has steadily increased,” the bioarcheologist said. “We are convinced that understanding our past — and how humans evolved and co-evolved with other species and their environments — is essential for understanding the origin and development of our modern societies.”
