Over 140 years ago, Charles Darwin observed that domestic animals have certain traits not seen in their wild ancestors, such as floppy ears, white fur patches, smaller brains and shorter snouts. These changes later became known as “domestication syndrome.”
However, many studies documenting these morphological variations are more than 40 to 50 years old—and some could use an update. For example, one study published in the 1960s compared house cats to European wildcats, which are not direct relatives of today’s furry feline friends, reports Brandon Specktor for Live Science.
Using modern scientific knowledge, a new study replicated these early experiments to confirm that domestic cats (Felis catus) do in fact have smaller brains than their ancestors, according to a paper published in the journal Royal Society Open Science.
“Brain size comparisons are often based on old, inaccessible literature and in some cases drew comparisons between domestic animals and wild species that are no longer thought to represent the true progenitor species of the domestic species in question,” the authors explain in the study.
The research team measured a total of 103 skulls from the collections of National Museums Scotland. They compared domestic cat skulls to African wildcats (Felis lybica), a genetically closer relative than European wildcats (Felis silvestris). To measure cranial volume, they filled each skull with one-millimeter glass beads and weighed how many each skull could hold, per the study.
Modern house kitties have smaller cranial volumes than African wildcats, but both species have smaller skulls than European wildcats. They also measured the skull volume of domestic cats bred with European wildcats. These specimen had brain sizes somewhere in between the small size of their domesticated parent and large size of their wild parent.
“Reduced brain size is one of the most consistent, yet hard to explain, features seen in domestic animals, compared to their wild ancestors,” says study author Raffaela Lesch, who studies veterinary medicine at the University of Vienna, in a blog post.
One explanation for reduced brain size and other traits associated with domestication syndrome is outlined in the neural crest cell hypothesis, which was published in a 2014 study. Neural crest cells are a group of embryonic stem cells that form near the spinal cord of vertebrate embryos and move to different parts of the body as the embryo matures.
These cells give rise to different traits, such as skin and fur pigment, jaws, ears, and the adrenal glands, which are the center of the flight-or-fight response, per a 2014 statement about the original study. A deficit in neural crest cell development could be responsible for those patches of white fur, floppy ears, and baby-face features like little noses, per the hypothesis.
However, when Lesch and the team measured the felines' palates, a proxy for snout length, in the new study, they did not find that today’s pet cats had smaller snouts than their ancestors, reports Alexandra Mae Jones for CTV. Instead, a more accurate predictor of snout length was individual body size—not domestication.
“There may be several reasons for this lack of snout shortening,” the new study's authors explain in the paper. “Domestication might not have affected snout lengths after all, palate length may not be an appropriate proxy for snout length, or the neural crest cell hypothesis may be incorrect in its proposed effect on snout length during domestication.”
Regardless, Lesch says their results illustrate the importance of replication studies.
“In the context of domestication research, it is crucial to replicate these older studies since they are the foundation to many currently debated hypotheses,” says Lesch in the blog. “This also holds true in the bigger picture of science. To counteract factors leading to the replication crisis, especially in historical studies, we have to put effort into establishing that our basic findings are solid and replicable.”