Variations in vulture skull shape have been found to coincide with the preferred method each species uses to feed on a carcass.
Learning more about the form and function of living species helps scientists better understand the behavior of extinct birds, like Haast’s giant eagle.
Vultures are unique because they are the only vertebrates that require carrion to survive.
These birds have therefore adapted to a very specialized diet, which is reflected in their bodies and behavior.
Although vultures are often lumped into the singular category of scavengers, a new study sought to see if skull shape differed depending on how a vulture feeds on a carcass.
Researchers compared the skulls of 22 living vulture species and found that skull shape accurately predicts the preferred feeding strategy adopted by a particular species. The results were published in the Journal of Zoology.
Dr Andrew Knapp, a Museum scientist who co-authored the study, said: “Different parts of an animal’s body can tell us important things about how it lives. »
“The skull is really important because there is so much concentrated there. It houses the brain, the oral feeding apparatus, the teeth and the sensory organs. This region contains a lot of information that can tell us a lot on an animal.”
“We can take living species and determine what in their lifestyle might shape the evolution of their skulls, and then we can apply that to extinct species.”
How do different species of vultures feed?
There are 23 living species of vultures divided into two different families. Afro-Eurasian vultures belong to a family called Accipitridae, which also includes birds like the golden eagle and the common buzzard. The second group is the North American vultures of the family Cathartidae, which includes the California condor.
Although the two groups evolve completely independently, different species of vultures within these families have been observed to prefer particular parts of a carcass. These can be divided into three categories: rippers, gulpers and scrappers.
Rippers generally opt for very strong materials like skin and tendons. Gulpers go after the soft insides, like intestines and other internal organs, while scrappers go after the bits that are left behind.
Specialization on different parts of a carcass has allowed multiple species of vultures to coexist and evolve side by side rather than competing for the same type of food. As a result, their shape has adapted to these different behaviors.
Bird skulls have long been an example of adaptive evolution, with different bird beaks being well adapted to various diets. The classic example is Darwin’s finches, whose beaks have all evolved different shapes and sizes depending on where they live and what food is available.
However, a bird’s diet does not yet sufficiently explain the majority of variations in beak shape.
“We always think of diet as something that shapes the evolution of birds’ beaks,” Andrew says.
“But when you look at all birds, diet is not a very good explainer of fitness at all. The reason is that there are many different ways to approach the same diet.”
“If you’re a carnivorous bird, for example, you might swallow prey whole or just tear off pieces of it. When you look holistically at different species, there are many ways to approach the same problem.”
How are vulture skulls different depending on feeding strategy?
The researchers found that the vultures’ different skull shapes fell neatly into three groups that coincided with the three different feeding strategies.
Those classified as “rippers” tended to have larger skulls and sturdier beaks for ripping the tougher tissues from the carcass. The “scrapers” had the thinnest beaks, reflecting the precision needed to pick up small remnants of material around the carcass.
But among the three groups, gulpers had the narrowest skull with the relatively longest beak, ideal for inserting and maneuvering inside a carcass to consume the internal soft tissues.
The researchers also examined the skulls of eight non-vulture raptors of similar size and range. The skulls of these birds tended to be much more similar in shape and much less diverse than those of vultures.
They then performed an analysis to see if they could predict which dietary category a species might fall into based on skull shape alone, excluding factors such as how related the species are to each other. These results were then compared to the observed feeding behaviors of the birds and showed that the analysis had a 100% success rate in predicting the dietary categories of these different vultures or whether the skull belonged to a non-vulture raptor.
The analysis has also been used to determine the feeding method of extinct birds, such as the ancient vulture called Breagyps clarki discovered in the La Brea Tar Pits in California. The skull shape of this species was found to fit perfectly into the “gulper” category, suggesting that the extinct bird would likely have fed on the soft guts of animals like mammoths and ground sloths.
The researchers also studied Haast’s eagle, a giant eagle that disappeared from New Zealand around 600 years ago. Although the species was previously thought to be vulture-like, the model predicts it to be a predatory bird rather than a scavenger, consistent with the theory that they fed on moa unable to fly.
“To estimate the behavior of extinct species, I think you really need to have a good understanding of living animals,” says Andrew.
“Vultures are normally grouped into one category: scavengers. But we get much better results by dividing them into groups, which could be used to help predict the behavior of extinct animals.”
“It’s difficult to attribute anything behaviorally to extinct animals, but I think by understanding it well and demonstrating that it works with living things, we can be confident that it will work when applied to the species of the past.”
Convergent carrion: skull shape predicts vulture feeding ecology. Journal of Zoology. doi.org/10.1111/jzo.13127. zslpublications.onlinelibrary. …ll/10.1111/jzo.13127
Provided by the Natural History Museum
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