Years ago I went to Auckland Zoo when they held a free entry day.
I vividly remember arriving at the gibbon enclosure to see a gibbon who was clutching at the cage, screaming at everyone going past. His distress was clear to see and afterwards I felt tremendously guilty that I had contributed to his upset, which must have been compounded by the sheer number of visitors that day.
Now I realise that he was Iwani, a male siamang gibbon who has just been euthanised due to being "severely depressed", as reported in the Herald.
It's well known that animals can suffer from depression - pigs on factory farms trapped for weeks on end in sow and farrowing crates are often in utter despair from the constant confinement. Indeed it was this knowledge that first got former pork board PR man Mike King to come over to the animals' side.