People relax in St James' Park in London, Britain Sept 13, 2016. [Photo/Agencies]

"They come from the Kalahari desert in southern Africa so it can get very warm during the day but they weren't born in the Kalahari desert so they are quite acclimatised to the UK weather," zookeeper Grant Kother said.

"So although they are quite hardy when it comes to warm weather, it is always nice to give them an option to cool down."

Worldwide, this year is set to be the hottest since records began in the late 19th century, due to a build-up of man-made greenhouse gases and an El Nino event that has warmed the Pacific Ocean, the UN weather agency says.

And NASA said on Monday that last month was the warmest August on record. Last year, world leaders meeting in Paris agreed a sweeping plan to shift from fossil fuels this century to limit climate change.