Study challenges “necks-for-sex” hypothesis of giraffe evolution

The exceptionally named “necks-for-sex” hypothesis of giraffe evolution suggests their long necks are the result of competition among males, but new research suggests it could be something much more prosaic.

Male giraffes practice “neck sparring,” violently swing their necks into each other to assert dominance, so the hypothesis formed that males with longer necks may have been more reproductively successful.

But, more recently, biologists have proposed that neck length may instead be driven by females’ foraging behaviour. Being able to forage deeply into trees for otherwise difficult-to-reach leaves would be an advantage for females, who have increased nutritional demands due to gestation and lactation.

Now new research has found that female Masai giraffes (Giraffa tippelskirchi) have proportionally longer necks and trunks than males.

“The necks-for-sex hypothesis predicted that males would have longer necks than females,” says Doug Cavener, a biologist at Pennsylvania State University in the US and lead author of the study in the journal Mammalian Biology.

“And technically they do have longer necks, but everything about males is longer; they are 30% to 40% bigger than females.

The researchers analysed photos of hundreds of wild and captive Masai giraffes to investigate the relative body proportions of each species and how they might change as giraffes grow and mature.

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

A diagram of a male and female giraffe's body. The images show that males have wider necks and longer front legs. Whereas females have longer bodies and longer necks.
Although male and female giraffes have the same body proportions at birth, they are significantly different as they reach sexual maturity. Females have proportionally longer necks and longer bodies than males, which might help with foraging and child rearing, while males have wider necks and longer front legs, which might help win fights against other males and with mating. Credit: Penn State

This supports previous research, which found female South African giraffes (G. giraffa) have proportionally longer necks than males.

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

“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 researchers also found that adult male giraffes have longer forelegs and wider necks than females, probably to assist in mating and for male neck sparring behaviours, respectively.

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

The team is also using genetics to better understand which males are successful at breeding, with the aim to guide conservation efforts for this endangered species.

“Populations of Masai giraffes have declined rapidly in the last 30 years, in part due to habitat loss and poaching, and it is critical that we understand the key aspects of their ecology and genetics in order devise the most efficacious conservation strategies to save these majestic animals.”

Sign up to our weekly newsletter

Please login to favourite this article.