Human-fed red squirrels in Britain develop weaker jaws and flatter skulls
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A large team of researchers affiliated with several institutions in the U.K. has found evidence that red squirrels fed by humans in a town on England's west coast lost muscle mass and experienced changes to the shape of their skulls. In their study published in the journal Royal Society Open Science, the group compared the remains of squirrels in the area before, during and after human feeding began.
The red squirrel is the only species of squirrel native to England. Prior research has shown that the little mammals are native to parts of Scotland and northern parts of England, with some in Wales and a few on some nearby islands. Unfortunately, gray squirrels from North America were introduced to England in the late 1800s. Since that time, the grays have slowly been taking over. As their population has grown, the numbers of reds have dropped dramatically—they are now a protected species.
As red squirrel population numbers dropped, people around England began feeding them peanuts, a legume with a very soft shell. In one part of England, a town called Formby, both private citizens and conservationists began feeding the local red squirrels peanuts—so many that the handouts became their primary source of food from the 1990s to the 2010s.
Eventually, mammal experts began warning residents about possible downsides to such feeding, such as starvation due to drop-offs during inclement weather. That led them to change to much harder nuts, such as pine or hazelnuts, the type the squirrels would have been eating naturally.
Eager to learn about what sorts of physical changes the squirrels might have undergone during their peanut-eating days, the research team studied the remains of squirrels donated over a number of years from before, during and after the peanut-eating years.
In all, the team studied 113 skulls and 387 jaws, and found some patterns. One was that the skulls of the Formby squirrels that had eaten a lot of peanuts were flatter than those of their Scottish counterparts. The researchers also found jaw muscle attachments were situated closer to the joint that allows for chewing. Both changes, the researchers note, are signs of a weaker, less-efficient bite.
Eating nuts with soft shells had reduced their ability to eat nuts with harder shells—a change that could have proven disastrous if the squirrels were suddenly faced with a reduced peanut supply. The researchers suggest their work shows the adverse impact human feeding can have on wildlife.
More information: Kim E. Chandler et al, Morphological change in an isolated population of red squirrels (Sciurus vulgaris ) in Britain, Royal Society Open Science (2025). DOI: 10.1098/rsos.240555
Journal information: Royal Society Open Science
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