Did Lemmins really suicide?

[sp1nn3r]

One popular explanation for their behavior is that the travelers are actually
committing suicide to relieve overpopulation. By heading off to die, the suicidal
lemmings leave shelter and food for those who stay behind. These surviving
individuals will perpetuate their species, saving it from extinction.

At what level selection is postulated to happen in this theory?
How would you use Gary Larson’s cartoon (below) to evaluate the hypothesis critically?

[Cat]

No, they are not committing suicide. When population becomes too large, not everyone gets enough food. Some will go off to look for better place.