@Serdan @Antipodesman

Here is a lovely (as in fascinating) example of ants sacrificing themselves to help their kin.

http://www.newscientist.com/article/dn14821-heroic-ants-pay-the-ultimate-price-to-safeguard-nest.html

Every evening at sunset the ants seal the door to their nest. But a few ants always stay outside to kick sand over the entrance to disguise it better. A large proportion of the ants left outside die of the cold, are blown away or eaten by predators. Hence, every night a number of ants must sacrifice themselves to ensure the protection of the rest of the colony.

This is very interesting as there is no caste that does this, it seems to be a behaviour wired into all the ants which is activated by certain circumstances, i.e. being of the right age to be last in the door. (so they don't get in the door at all!)

Now if we consider Hamilton's rule, this all makes perfect sense. Think of all those genes for "sacrificing yourself to lock the door" swarming inside the colony (or rather inside the queen in the colony, and in the potential queens in the colony). The cost to the individual may be huge (i.e. death) but rB >>>>>>> C because of, first, the fact there are individuals carrying that gene in the nest who will go onto reproduce a helluva lot and second, the fact this behaviour is very important in letting the queen go onto reproduce as it prevents predators sneaking in and attacking the nest (and her) at night.

Obviously RD's book explain this far better than me, but I thought this was a good example.