Creating a simple swinging vine/rope/line

I am trying to recreate this Mario Party 3 minigame. The first thing I want to do is create those swinging vines. I have created a sprite in GIMP and am now trying to make it swing.

My first thought was to simply throw the asset there, put a Hinge Joint 2D at the top of it, rotate it, reduce its angular drag to 0, and let gravity do the rest. Since there is no drag, gravity should be the only force acting on it and the bottom of the vine should oscillate between back and forth, always achieving the height it started at.

This is not what is happening, though. As time goes on, the vine reaches lower and lower heights. What is causing this dampening?

Here is a video I made, which shows a vine at 90 degrees to begin with, but is approaching 45 degrees.

For reference, here is a screenshot of the object (although not at 90 degrees):

What force could possibly be causing the vine to slow down? Please help