Jean-Louis:
..
>whose flesh is very useful to the body, should have very efficient
>defences: either have a very hard hull, diffucult to open without
>tools, or have a very toxic skin, otherwise too many fruits would be
>eaten and the species wouldn't survive...

My view of the use of the skin is totally different. I consider the skin just
to protect the fruit from rain, mechanical damage, sun exposure and
sometimes from opening the fruit too early (e.g. durian).
It prevents the pulpa from being oxidized. That's, why the skin often
contains lots of antioxidants. It isn't to protect against eating because this
would be a contradiction especially for your example of avocados and
mangosteens: why should an avocado protect itself against eating?
The core still remainsafter eating and the core will indeed defend against
this (have you ever tried it? it contains so much hydrocyanic acid that
you will soon have enough of it).
So in my opinion all fruits have an interest to be eaten. But they generally
are not interested in their cores to be eaten. Some exceptions are durian,
jackfruit and cempedak where the cores are eatable too. But try to eat
 l l cores of a durian. I guess you will fail. Its too much protein. Your instinct
will give you a stop. The strategy of these plants may be, that they have
so much cores that even if several are eaten there remain enough to allow
reproduction and spread.
Another strategy would be that of strawberries and bananas: they contain
lots of very small cores you never will be able to destroy mechanically in
your mouth. Therefore enough of them will remain, pass your digestion
system and will leave you unchanged.

A discussion about the strategies of plants with the toxicity of their cores,
their size (avocado cores mostly are too big to be swallowed), their place
in relation to the pulpa etc. would be interesting. I haven't scanned all of
the archives yet, so if this already has taken place, give me a hint. The
vegetables would be an interesting special case. (How does a kohlrabi defend
against eating it all, making it impossible to reproduce itself?)

Kind instinctive greetings,

Stefan