Opened hazelnut with what appears to be an insect egg inside. There may be some teeth marks on the left-hand side of the hole.
No interactions present.
I have this great Collins' Guide to Animal tracks and signs that has a whole page of nuts eaten by various animals. Using that book, this looks like a hole left by a wood mouse. Squirrels apparently split the nut, or where they gnaw a hole it is more ragged in outline.
As for the egg, it looks like it might belong to a snail, but that is a pure guess.
University of Edinburgh and Biodiversity Observatory (OU)
A snail definitely makes sense - there were lots of snail shells in the area where I found the hazel nut.
It did look like a very neat hole, I could barely see any teeth marks so a wood mouse is a definite possibility.
Lat/Lng: 51.4702, -1.0724
OS grid ref: SU645749