They were very willing to go for a romp but they knew nothing about jumping, as yet, and all they could do was to show their eagerness to be out for a run by milling up and down the fence.
If that were the case, there were other ways of opening corrals and Alcatraz knew them all.

He tried the fence with his shoulder, leaning all his weight.

More than once he had smashed time-rotted fences in this manner, but he found that these posts were new and well tamped and the boards were strongly nailed.

He gave up that effort and went about looking for a gate.

Gates were not hard to find.