Wiseman, you are not describing group selection. You are still describing individual selection, because the causally effective advantage is to the individual. The benefit to the group is a side-effect. Here is your description of the advantage, which you call the key: “because it will spend less energy developing fox fetuses that won’t survive anyway.” That is an advantage to the individual foxes.
I also have doubts about the specifics of your scenario, but I won’t get into that.
Wiseman, you are not describing group selection. You are still describing individual selection, because the causally effective advantage is to the individual. The benefit to the group is a side-effect. Here is your description of the advantage, which you call the key: “because it will spend less energy developing fox fetuses that won’t survive anyway.” That is an advantage to the individual foxes.
I also have doubts about the specifics of your scenario, but I won’t get into that.