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On a tiny island called Cayo Santiago off the coast of Puerto Rico exists a colony of about 1,800 rhesus macaques.

Last year, they captured a Japanese macaque that was on the loose for five days after escaping from the park.

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.