even the leanest mamals have some fat. Boiling the meat and bones of the squirrel / rabbit / chipmunk / mouse will release most of that fat to the water. Boiling will also kill most of what may be bad for you in the animal. The organs and brain are some fattier but also taste bad and the digestive organs may contains high amounts of toxins which are unaffected by cooking. If you are trying to subsist on trapping and fishing then you may attempt to snare a larger mamal such as a deer or elk. This can be done with expedient snaring material if you don't have long or strong enough snare wire. I'm not sure but 100' of 60# spiderwire could probably become 30' of 200# rope with a little braiding - that might be a decent snare for a deer if you are nearby to finish off the kill once the animal is snared. The kill could be accomplished with the "Xacto blade on a stick" spear. or the simple "rock to the head" method - messy and brutal but hey, you're starving here. Spider wire is thin enough to cut even if it isn't strong enough to hold as such it might effectively hobble the deer even if the deer can break free.

In anycase if you are trying to get a higher fat content from your squirrels, just boil them up and throw out the carcass and drink the broth. The fat will be in the broth and the extra protien will be thrown out - of course you need to catch twice as many squirrels to live this way but it solves the fat problem.