A gibbon pendulum at Bassenthwaite

We were treated to a fabulous display of gymnastics by this gibbon. I timed the pendulum swings: 25 seconds for 10 swings. The formula for a pendulum is time period T=2pi x sqrt(length/g). This means that the length of the pendulum is 1.5 metres. The big question is to where that is measured. The gibbon is holding on in two places. 1.5 m is further down than the top hand. I suspect that 1.5 m will be down to the centre of mass.