'World's Smallest Monkey' Baby Born At Sussex Zoo. Drusillas Park in Sussex has welcomed an utterly adorable new arrival – as the world's smallest monkey has given birth to a tiny tot. The baby pygmy marmoset – a species listed as vulnerable in the wild – was discovered by keepers on 5 August who could just about make out a micro ball of fur clinging to mum Rosie's back. Pygmy marmoset newborns measure around 3cm, or roughly the size of a human thumb. They will reach around 12-15cm in length and 100g in weight when fully grown. Since its arrival, the minuscule monkey has been passing the early stages of development with flying colours and can already be seen leaping around independently, returning to mum regularly for cuddles and food. Mum Rosie arrived at Drusillas Park on 30 January and keepers were hopeful that her pairing with resident male, Ben, would result in successful breeding efforts. Keepers observed that Ben appeared to be 'smitten' with Rosie from the moment she arrived, following her everywhere she went and it seems she reciprocated his feelings as the baby must have been conceived about six weeks after their union. Drusillas are now asking followers on their social channels to suggest a name for the mini monkey – the first born there in a decade.