That only really works with Lemmings.
And they only commit mass martyrism when the population exceeds what their resources will sustain.
food > lemmings = cool beans
food < lemmings = oh shit * jumps off cliff *
A singular sacrifice in the heat of the moment doesn't really fall in the same category as what the lemmings do.
Actually, the whole lemming thing is a myth.
They don't commit mass suicide. They "commit" mass ACCIDENT. They "stampede" around and sometimes... the ones in the front falls down a cliff and the ones behind doesn't see the fall before its too late. Even if they manage to stop, the ones behind them again won't be able to react immediately.
Also, have you heard of "screecher squirrels?" When they see danger(usually an eagle) they screech. This alert the rest and often saves them. However, the screech usually attracts the eagle to the screeching one. The screecher usually is an older female which have already had at least one set of offspring.
A parent will fight til death for their child. This is self-sacrifice, even though the parent can win the battle.
The species is naturally selected because it defends its children and younger family members.
Shit, this thread isn't for discussion of natural selection... <.<
PP: Got an appointment with the dentist... I bet my "newest" molars have betrayed me again with their shape.