Scientists finally think they know why giraffe necks are so long

• Giraffes have the longest necks of any living animal but scientists can't agree on why. 
• Scientists largely agree that males drove the evolution of long necks to compete for mates.
• But a new study offers clues to the contrary, challenging the leading "necks for sex" theory.

Giraffes didn't always look like the elegant giants we recognize — ancient giraffes looked more like deer. But something happened over the past millennia that drove giraffes to evolve the longest necks of any living animal.

What that driver was, however, has been the subject of a 150-year-long debate among evolutionary biologists.

In the 19th century, Charles Darwin and Jean Baptiste Lamarck suggested that giraffes evolved long necks to help them snatch leaves on trees. A later theory usurped Darwin and Lamarck's, suggesting that male giraffes evolved long necks to fight and compete for female mates. This "necks for sex" idea has been the leading theory since the late '90s. But that may soon change.

A recent study published in the peer-reviewed journal Mammalian Biology offers new clues to the debate that could prove Darwin and Lamarck may have been at least partially right all along.

Size doesn't always matter

Male giraffes have longer necks than females. For that reason, biologists have speculated that males drove the evolution of this physical feature.

Lead author of the new study, Douglas Cavener, wasn't entirely convinced. Yes, males have longer necks, but everything else on them is larger too, Cavener, who is a professor of biology at Penn State, told BI.

"I realized that the important question was, 'Do males have proportionally longer necks compared to the rest of their body?'" Cavener said.

So he and his colleagues — including his wife and daughter, who are co-authors on the paper — started to investigate. They estimated neck length from photos of adult Masai giraffes, a species of giraffe native to Tanzania and southern Kenya in East Africa.

They counted pixels in each photo of both captive and wild giraffes to measure various body parts including the neck, legs, and body trunk. When they crunched the numbers, Cavener and his colleagues discovered that the males fell short.

"What we found was pretty surprising, which was that females have proportionately longer necks than males, just the opposite of the prediction," Cavener said, adding that "it turns out that females also have longer trunks proportionally."

"So that kind of turned things upside down," Cavener said.

Why giraffes have long necks

Female giraffes give birth about every two years. Gestation takes about 15 months. So, that means they're pregnant or lactating most of their reproductive lives, Cavener said.

That requires a lot of energy. But female giraffes are picky eaters, so "they'll telescope their neck into a bush to really get" the best leaves, Cavener said.

Cavener said she thinks that because females are consistently in need of more energy and nutrition, this is what drove ancient giraffes to develop such long necks over millennia.

"It sort of goes back to Darwin and Lamarck's theory that this was likely driven by competition for food rather than for mating success. But the important twist is it puts the emphasis on females rather than males," Cavener added.

Cavener said this may be the first study to suggest that females, not males, are the reason for giraffes' long necks. That's important not only for understanding giraffe evolution but how male and female giraffes behave differently, which could help with conservation efforts.

"This study, for me, highlights the importance of understanding the different behavioral strategies used by males and females in their skills for survival," and how those strategies can drive evolution in a species long-term, Zoe Raw — a behavioral biologist and giraffe expert who wasn't involved with the research — told Business Insider over e-mail.

Cavener's study is part of a larger effort to help preserve the species. In 2015, the International Union for Conservation of Nature listed Masai giraffes as endangered. Illegal poaching and other human interference have decimated the population. Hunting Masai giraffes is illegal in East Africa, but poachers still track them down for bushmeat and the purported health benefits in their bone marrow and brains.

While there's typically pushback anytime a new idea enters the conversation, Raw said the new study is convincing enough to challenge the leading "necks for sex" theory.

"Nothing can ever 'prove' what causes evolution, but as far as developing a robust and realistic, evidence-based theory, I think this paper has nailed it," Raw said.