Giraffes’ disproportionate necks may be the result of females searching for leaves deep in trees that would otherwise be difficult to reach.
It is the conclusion of a study led by biologists at Penn State University that explores how this trait could have evolved and provides new insights into this iconic question.
Contrary to classical theories, a recent hypothesis holds that competition between males influenced neck length, but the research team found that female giraffes have proportionally longer necks than males, suggesting that high nutritional needs of females may have driven the evolution of this trait.
The study, which explored the body proportions of both wild and captive giraffes, is described in a paper in the journal Mammalian Biology. The findings, the team said, indicate that the neck length may be a result of females searching for leaves deep in trees that would otherwise be difficult to reach.
In their classic theories of evolution, both Jean Baptiste Lamarck and Charles Darwin suggested that giraffes’ long necks evolved to help them reach leaves high up in a tree, avoiding competition with other herbivores.
However, a more recent hypothesis, called “sex necks,” suggests that the evolution of long necks was driven by competition between males, who clash their necks against each other to assert dominance, called “fighting necks.” “. That is, males with longer necks might have been more successful in competition, leading them to reproduce and pass their genes on to offspring.
“The sex-neck hypothesis predicted that males would have longer necks than females,” Doug Cavener, holder of the Dorothy Foehr Huck and J. Lloyd Huck Chair in Evolutionary Genetics and professor of biology at Penn State and lead author of the study.
“And technically they have longer necks, but everything about males is longer; they are 30% to 40% larger than females. In this study, we analyzed photographs of hundreds of wild and captive Masai giraffes to investigate the proportions relative body characteristics of each species and how they might change as giraffes grow and mature.
The researchers collected thousands of photographs of captive Masai giraffes from the publicly accessible photo repositories Flickr and SmugMug, as well as photographs of adult wild animals they had taken over the past decade.
Since absolute measurements, such as total height, are difficult to determine from a photograph without a reference point of known length, the researchers focused instead on measurements relative to each other or on body proportions, e.g. the length of the neck in relation to the total height of the animal. They restricted their analysis to images that met strict criteria, such as only using images of giraffes perpendicular to the camera, so they could systematically take a variety of measurements.
“We can identify individual giraffes by their unique spot pattern,” Cavener said. “Thanks to the Association of Zoos and Aquariums, we also have the complete pedigree, or family tree, of all North American Masai giraffes in zoos and wildlife parks, as well as their birth dates and transfer history.
“Therefore, by carefully considering this information, when the photo was taken, and the approximate age of the animal, we were able to identify the specific individual in almost every photo of a captive giraffe. This information was instrumental in understanding when male and female giraffes they start to show differences in size and if they grow differently.
At birth, male and female giraffes have the same body proportions. The researchers found that although males generally grow faster in the first year, body proportions are not significantly different until they begin to reach sexual maturity around three years of age. Because body proportions change early in life, the team limited their study of wild animals, whose ages are largely unknown, to fully grown adults.
In adult giraffes, researchers found that females have proportionally longer necks and trunks, or the main section of their body, which does not include the legs or the neck and head. Adult males, on the other hand, have longer front legs and wider necks. This pattern was the same in both captive and wild giraffes.
“Instead of reaching out to eat leaves on higher branches, giraffes, especially females, are often seen reaching deep into the trees,” Cavener said. “Giraffes are very picky eaters: they eat the leaves of a few tree species and their longer necks allow them to reach deep into the trees to grab leaves that no one else can. Once the females reach four or five years old, they are almost always pregnant and lactating, so we think the higher nutritional demands of females drove the evolution of giraffes’ long necks.”
The researchers noted that sexual selection (either competition between males or preference among females for larger mates) was likely responsible for the overall size difference between males and females, as is the case for many other large hoofed mammals that They are polygamous (where one male mates with many females).
The authors suggest that, following the evolution of the long neck, sexual selection (including body-pushing and neck-fighting behavior) may have contributed to males having wider necks. Additionally, the males’ longer front legs may aid in mating, which researchers say is a brief and challenging process that is rarely observed.
