After a 15-month pregnancy, giraffes typically give birth to a single, 6-foot-tall calf. (Photo: Stephanie Pilick/AFP/Getty Images)

Earth's tallest animal is in deep trouble. Wild giraffe populations are plummeting due to poaching and habitat loss, scientists say, with new survey data showing the mammals' numbers have fallen more than 40 percent in just 15 years. And unlike the well-known plight of gorillas, elephants, rhinos and other disappearing African icons, the decline of these serene giants has gone largely unnoticed.

About 140,000 wild giraffes existed as recently as 1999, but now there are fewer than 80,000, according to the Namibia-based Giraffe Conservation Foundation (GCF). That pales in comparison to African elephants, for example, which number around 450,000 but whose decline has drawn closer scientific study and wider publicity. That contrast isn't to diminish the real danger faced by elephants, but it does highlight what GCF director Julian Fennessey calls a "silent extinction" of giraffes.

"Giraffes are the forgotten megafauna," Fennessy recently told Scientific American, noting a scarcity of research as well as public awareness. "They're really not getting the attention they deserve."

Despite their extreme height — adult males can stand nearly 20 feet tall — giraffes have been overlooked by many scientists and conservationists. This is likely due to a longstanding belief giraffes are abundant, experts say, as well as a lack of data proving otherwise. "When I first became interested in giraffes in 2008 and started looking through the scientific literature, I was really surprised to see how little had been done," University of Minnesota Ph.D. student Megan Strauss told the New York Times in October. "It was amazing that something as well known as the giraffe could be so little studied."

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Giraffes eat up to 75 pounds of leaves daily, often traveling miles to find enough food. (Photo: Valentina Storti/Flickr)

The species is not listed as endangered by the International Union for Conservation of Nature (IUCN), which last updated its Red List status in 2010. But while the IUCN still classifies giraffes as "least concern" overall, it has recently listed two of nine subspecies as endangered. The group estimates fewer than 200 West African giraffes remain in Niger, while the population of Rothschild's giraffes in Uganda and Kenya is below 700 and falling, according to a report by the nonprofit Elephants Without Borders.

The IUCN noted in 2010 that Rothschild's giraffes may be near the threshold for a "critically endangered" listing, and also acknowledged emerging hints of trouble for the broader species: "[A] recent preliminary population estimate suggests a decline in the total population has taken place which, if substantiated, could mean that the species will warrant listing in a higher category of threat."

The GCF has been working for the past five years to substantiate such concerns, producing an unprecedented population assessment of giraffes that's expected to be published in 2015. It suggests the species has declined by 42.9 percent in the past 15 years, devastated by an all-too-familiar double whammy for African wildlife: poaching and habitat loss caused by human population growth.

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Giraffes can run up to 35 mph, but they have few natural predators aside from lions, crocodiles and humans. (Photo: Daniel Flower/Flickr)

Giraffes still inhabit 21 countries in Africa, but swaths of their habitat are being repurposed for human use, especially agriculture. Even in places where their native grasslands remain intact, fragmentation caused by development elsewhere can restrict their range and hinder genetic diversity, while climate change can encourage lengthy droughts that may compound other pressures. And beyond their rapidly changing environment — which lead desperate giraffes to feed on farmers' crops, making them seem like pests to local communities — the animals are also increasingly threatened by poaching.

Humans have a long history of hunting giraffes, seeking food as well as thick, durable skin to make clothing and other items. But a belief that giraffe brains and bone marrow can cure HIV has recently gained traction in Tanzania, reportedly pushing prices for a head or bones as high as $140 per piece. And since giraffes are relatively easy for humans to kill, often with a single gunshot, they've also become a popular source of food and extra income among Africa's growing hordes of elephant poachers.

When humans do stick out their necks for giraffes, however, there's evidence it can quickly improve the animals' fortunes. The West African giraffe, for example, was pushed to the brink of extinction in the 1990s by human population growth and a series of droughts. Down to just 50 individuals in 1996, the subspecies won legal protection from the government of Niger, helping it rebound to 250 individuals in 2010. Conservationists have also worked with villages in Niger to plant 5,300 acacia trees since 2012, reducing the need for giraffes to raid crops. Although their recovery remains fragile, Niger is now home to about 400 wild West African giraffes, an increase of 60 percent in just four years.

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.