A pair of squawking, thrashing, bald and violently pink twins arrived in the world in Madrid on Sunday, much to the relief of their mother and all those working to ensure the giant panda population continues to claw its way back from the brink.

Madrid’s Zoo Aquarium announced the birth of the as-yet-unnamed cubs on Monday, describing their arrival as “a great contribution to the field of conservation of a threatened and high-profile species that is an icon among those who love and protect nature”.

The siblings are the fifth and sixth cubs born in the zoo to Hua Zui Ba, a female panda, and her partner, Bing Xing, who are on loan from China.

“The first contractions started at 4am and the first happy arrival followed at 8.30am when the mother began to vocalise and instinctively prepare for the moment of birth,” the zoo said in a statement. “After birthing the first cub, she quickly dropped it into her lap so that she could lick it while the cub wriggled around and made loud noises.” Four hours later, its twin arrived.

In an update on Tuesday, the zoo said the cubs had been weighed, clocking in at 0.171kg and 0.137kg respectively, and that their umbilical cords had been tied and disinfected.

“Their sexes remain unknown for the time being as their sexual characteristics are still not obvious to see,” said the zoo. “In the coming days, experts from the Chengdu Research Base of Giant Panda Breeding will be able to confirm it with greater certainty.”

The International Union for Conservation of Nature’s Red List places giant pandas in the “vulnerable” category. Although numbers are increasing, with an estimated population of 1,864 animals – of whom 500 to 1,000 are mature – the species is still threatened by agriculture, mining, development, pollution and extreme weather.