Fins and Feathers (Planetary Crossings Part IV)

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 Keplerian was much stronger, and had to protect her feathered friend. Never could a feathered Keplerian be left alone with a finned Keplerian of another group. The only time a feathered Keplerian was safe with the finned Keplerian of another group was when the feathered Keplerian 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?

(Source: Fantastic Book of Logic Puzzles by Muriel Mandell, illustrated by Elise Chanowitz)

This one was actually quite easy and can be solved in a very methodical fashion. I don't know why it was made last of the group.

First bring both Uti over to the far side. Then come back with the finned Uti. The finned Uti will then return to the far side with the feathered Grundi. Once there ... they? ... can't antagonize the feathered Grundi because of the presence of the feathered Uti. Then come back with the Uti and bring the stronger finned Grundi over. Here the Grundi can't antagonize the feathered Uti because the feathered Grundi is there, so the finned Uti can leave for the near side once more. Then bring the feathered Yomi over to the far side with the finned Uti and return. Lastly bring the finned Yomi over with the finned Uti. That solves this puzzle.