He’s big, hairy and has an arresting gaze. She’s an attractive redhead who adores him.

But rather than being romantically entwined, Perth Zoo’s chummiest orang-utans just prefer to hang out together.

Primate supervisor Holly Thompson said the bond between alpha male Hsing Hsing and his biggest fan Utama – who celebrated her 35th birthday on Thursday – was remarkable considering orangutans were typically solitary.

Utama and Hsing Hsing, sitting in a tree… Photo: AAP

What makes their friendship more unusual is its platonic nature. Utama was given a hysterectomy following long-term complications associated with a difficult labour in 2012.

Perth Zoo usually doesn’t keep the male and female orangutans together unless it is for breeding, but made an exception for Hsing Hsing and Utama because of their special bond.

“Unlike the other pairings within the colony, Hsing Hsing and Utama quite often sit together on the same platform sharing food,” Ms Thompson said.

“They simply like to be in each other’s company.

“While orangutans don’t usually share their night nests, Utama frequently makes her nest next to Hsing Hsing’s nest. She loves to be next to him.”

Utama often initiates play with Hsing Hsing by playing hide and seek. But her best friend’s response depends on whether he can keep looking macho.

“Hsing Hsing loves to play with Utama. However, he won’t do this in front of his keepers as he wants to maintain his dominant male image,” Ms Thompson said.

“She also follows him around playfully pulling his dreadlocks.”

The pair even share a star sign: Hsing Hsing turns 39 on Saturday, making them both Geminis.

Perth Zoo has bred 29 orangutans since 1970 and has released two into the wild.