Giraffes gained their long necks for food, not sex

Why have giraffes evolved such long necks? A new study led by Pennsylvania State University biologists explores this question, offering new insights into the evolution of this iconic trait. 

Foraging deep into trees 

The prevailing theory suggests male competition drove neck length, but the research reveals that female giraffes have proportionally longer necks, implying that nutritional needs may have influenced this evolution.

By investigating body proportions of wild and captive giraffes, the experts discovered that neck length may result from females foraging deep into trees for hard-to-reach leaves.

Giraffe necks-for-sex theory 

Jean Baptiste Lamarck and Charles Darwin originally theorized that long necks evolved to help giraffes reach high leaves, avoiding competition with other herbivores. 

More recently, the “necks-for-sex” hypothesis suggested that necks evolved due to male competition, where males swing their necks into each other to assert dominance, called neck sparring. Males with longer necks would be more successful in these competitions, leading to reproduction and passing their genes to offspring.

“The necks-for-sex hypothesis predicted that males would have longer necks than females,” said lead author Doug Cavener, a biologist at Penn State. “And technically they do have longer necks, but everything about males is longer; they are 30% to 40% bigger than females.”

Measuring the necks of giraffes 

The researchers gathered thousands of photos of captive Masai giraffes from public photo repositories such as Flickr and SmugMug, and photos of wild giraffes taken over the past decade. 

The experts focused on measurements relative to one another, such as the length of the neck compared to the overall height of the animal. By analyzing images that met strict criteria, they could consistently take measurements.

Size differences among the genders

“We can identify individual giraffes by their unique spot pattern,” Cavener explained. “Thanks to the Association of Zoos and Aquariums, we also have the full pedigree of all Masai giraffes in North America in zoos and wildlife parks, as well as their birthdates and transfer history.” 

“By considering this information, when the photo was taken, and the approximate age of the animal, we could identify nearly every photo of a captive giraffe. This information was crucial to understanding when male and female giraffes start to exhibit size differences and whether they grow differently.”

At birth, male and female giraffes have the same body proportions. Although males generally grow faster in the first year, body proportions remain similar until sexual maturity around three years of age. Therefore, the study of wild animals was limited to fully grown adults.

Female giraffes have longer necks

In adults, females were found to have proportionally longer necks and trunks, while males had longer forelegs and wider necks. This pattern was consistent in both captive and wild giraffes. 

“Rather than stretching out to eat leaves on the tallest branches, you often see giraffes – especially females – reaching deep into the trees,” Cavener said. 

“Giraffes are picky eaters – they eat the leaves of only a few tree species, and longer necks allow them to reach deeper into the trees to get the leaves no one else can. Once females reach four or five years of age, they are almost always pregnant and lactating, so we think the increased nutritional demands of females drove the evolution of giraffes’ long necks.”

The role of sexual selection 

Sexual selection, either through male competition or female preference for larger mates, likely explains the overall size difference between males and females. 

After the evolution of long necks, sexual selection may have contributed to males’ wider necks. The longer forelegs of males might assist in mating, which is rarely observed.

“Interestingly, giraffes are one of few animals whose height we measure to the top of the head – like humans – rather than to their withers, the highest part of the back, like in horses and other livestock,” Cavener explained. 

“The female has a proportionally longer axial skeleton – a longer neck and trunk – and is more sloped in appearance, while the males are more vertical.”

Conservation efforts to save giraffes 

The research team is also using genetics to identify relationships in wild giraffe groups to understand which males are successful at breeding. This knowledge aims to shed light on mate choice and sexual selection and guide conservation efforts for this endangered species.

“If female foraging is driving this iconic trait as we suspect, it highlights the importance of conserving their dwindling habitat,” said Cavener.

“Populations of Masai giraffes have declined rapidly in the last 30 years due to habitat loss and poaching. Understanding key aspects of their ecology and genetics is critical to devising the most effective conservation strategies to save these majestic animals.”

The study is published in the journal Mammalian Biology.

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