All Puzzles

Hostile members of the Uti, Grundi and Yomi groups
were travelling to a conference. There were two members
from each group, one finned and one feathered.
The finned Martian was much stronger, and had to
protect her feathered friend. Never could a feathered
Martian be left alone with a finned Martian of another
group. The only time a feathered Martian was safe with
the finned Martian of another group was when the
feathered Martian of that enemy group was also
present.
The trip was quiet until they came to a deep ravine.
The only way to cross it was by swinging across on a
rope. But the rope was only strong enough to hold two
of them. And it wasn’t heavy enough for them to swing
it back over the ravine without someone to weigh it
down. How did they all cross the ravine?