I found several great macadamia nut websites. One with Macadamia Oil!!
[who knew???]  and rxs for cakes and brittle etc. One site had an email
setup so I asked the question re how macad nuts open naturally, ie in
the absence of human/animal intervention.  This was the reply. The site
is:

http://www.civicnet.org/webmarket/hawaii/waiukahe/index.html

Company: Waiukahe Macadamia Nuts  Hawaii


"When the nut is ready to sprout the shell has a soft spot and splits
open so that the baby tree or sprout can emerge.  The nut is not very
human delicious at this point but the wild pigs go for them!  Carol"

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