Two critically endangered Sumatran Orangutans were recently born at Perth Zoo. One is a male named Sungai and the other a female called Lestari.

PERTH Zoo's alpha male orangutan has a fetish for redheads - and he doesn't draw the line at his own species.

Hsing Hsing, who celebrated his 39th birthday on Saturday, has a harem of in-house girlfriends but his roving eye means he is always on the lookout for attractive flame-haired females among the humans who come to see him.

Not only have keepers noticed he stares at such visitors, he's also ripped out pictures of Nicole Kidman from magazines put in his enclosure.

Usually, the orangutans toss the paper around but Hsing Hsing pulls out photos of the Hollywood superstar to keep.

Primate supervisor Holly Thompson says it's a case of "close enough" when the 95kg diabetic ape sees redheads like Kidman, who share 97 per cent of his DNA.

"He's definitely interested - it's amazing," Ms Thompson told AAP.

"He understands that he's attracted to a lovely redhead female orangutan and then he sees a lovely redhead in a magazine and goes, 'oh yeah, close enough - Nicole Kidman, I'm going to rip that page out and put it to one side'."

Hsing Hsing demonstrates other behaviours rarely seen in orangutans. He'll share his food with his best friend Utama, a 35-year-old female orangutan with whom he has a purely platonic relationship.

Ms Thompson said the sharing was touching as orangutans are highly food-motivated.

"Because he's a diabetic, we need to know what food he's having. If he misses some, we supplement it with some more, and so Utama knows how to work him."

Ms Thompson, who has travelled to Indonesia to witness the destruction of orangutan habitat for palm oil, rubber and acacia plantations, says the plight of the orangutans is probably worse than imagined.

While surveys of their night nests suggest as few as 6000 remain in the wild, orangutans often build several nests each day so these numbers may be inflated, she said.