Perth Zoo's orangutan, Utama, spent her 35th birthday in the company of her best friend, Hsing Hsing.

Hsing Hsing, the zoo's dominant male orangutan, will turn 39 on Saturday.

Orangutans are typically solitary, but Hsing Hsing and Utama live together at the zoo.

Primate supervisor Holly Thompson said the orangutans enjoyed a remarkable relationship.

"Usually, we don't keep the males and females together unless it is for breeding purposes, but these two have a really special bond," she said.

"They've been together a lot over the years. Utama often initiates play with Hsing Hsing by playing hide and seek. She also follows him around playfully pulling his dreadlocks.

"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."

Orangutan Utama at Perth Zoo. ( Supplied: Perth Zoo )

Ms Thompson said the relationship was particularly unusual because it was platonic.

Utama had a hysterectomy following a difficult labour in 2012 and subsequent complications.

"Unlike the other pairings within the colony, Hsing Hsing and Utama quite often sit together on the same platform sharing food," she 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."

There are believed to be less than 6,000 Sumatran orangutans left in the wild.

They are considered critically endangered.