Non-susceptible species lack lysine at this position, which has a large side chain; macaques instead have asparagine, which contributes to HBV resistance.

Importantly, this improvement was unique to those who actually fell asleep -- the macaques that experienced quiet wakefulness without falling asleep did not show the same performance boost.

Worcester Crown Court heard graphic details of the suffering experienced by young macaques in Indonesia - with the footage sold to people around the world.

Earlier this year, a Japanese macaque named Honshu escaped from a zoo in Scotland.

The goal was to create bespoke films in which baby long-tailed macaque monkeys were abused, tortured and sometimes then killed on film.