Happy World Giraffe Day! The longest day in the year is marked for the world’s tallest animal.

Across Africa, giraffes are undergoing what is being called a silent extinction. Populations have dropped nearly 40 percent in three decades, and the reticulated giraffe, one of the four species, numbers fewer than 16,000. The decline is thought to be caused by habitat loss, fragmentation, and poaching, but lack of long-term conservation efforts in the past makes it hard to know exactly why. Scientists are now pursuing major studies to understand why giraffes are disappearing and what can be done about it.