From the book The Coconut Odyssey:
Rats can make fruits fall.
Where common rats are expected
to attack immature fruits, causing
their untimely fall, the manager will
be forced to program more visits to
remove any potentially dangerous
fruit. Rat access can be reduced by
fitting a slippery metal ‘skirt’ around
the trunk, comprising for example
a light gauge (1.2-mm) rectangular
aluminium sheet 700 mm wide and
long enough to wrap fully around
the trunk to be secured with nails
at the overlap. If rats can get access
to the crown from neighbouring
structures or large fruit trees, such as
mango, control will be more difficult.
http://aciar.gov.au/files/node/453/mono101.pdf