We had that happen too, was out catching grasshoppers to feed to our mantids when my brother and I were about 11 and 13 and the neighbors thought we were looking for snakes and gave us this big ol' garter snake they had caught. We named it a boy name and kept it a nearly two years, until one day I walked in and there were 11 little garter strings all over the cage, had to have been retained sperm from a breeding prior to being caught. That was enough for our long suffering mom, who doesn't really like snakes, and she made us release them at a nearby marshy area. We didn't attempt trying to get them to eat though.