When you think of a snake (any kind of snake, big or small), you probably picture some type of elongated animal without any legs, right? Let’s be honest, “no legs” is one of the main requirements for a snake to really be considered a snake, right? No feet, no legs, and most definitely no claws—just a long, serpentine creature that slithers side to side. But what if that’s not entirely true…
As strange as it is, there are snakes out there slithering around with their own sharp little claws! WHAT? Yes, two different types of snake families actually grow “claws.” But why on earth do these snakes have claws? Let’s take a closer look at the two snakes that have “claws,” and learn why they have them and what they are used for.
Lizards vs. Snakes: Legs or No Legs
Lizards and snakes share much of their ancestral DNA, and are both part of the same scientific order, Squamata. Millions of years ago, however, these two animals took two very different paths on their evolutionary journey. Today’s lizards scurry about on four, fully developed legs while snakes do not have legs at all. Scientists do not know exactly why these two animals evolved so differently. Part of the problem is that snake skeleton fossils are extremely fragile and hard to come by.
However, most of the snake-like fossils of the Cretaceous period (around 70 million years ago) had elongated bodies with very short legs. As they evolved, these snakes first lost their front legs, and then their hind legs. Many researchers believe that today’s snakes are the descendants of these burrowing reptiles, and through evolution they loss their legs to better move through the earth.
Which Snakes Have “Claws”?
Even though snakes today no longer have legs, many still have remnants of hind legs and feet left behind from their ancient ancestors! Where snakes once had back legs, some now have “claws”, called vestigial remnants or spurs. If you look closely at the belly of certain snakes, you can see these tiny spurs on either side of their cloaca.
However, not all snake species have “claws” or spurs. In fact, snakes that have evolved more recently, like corn snakes, do not have any spurs. Older snake species that evolved millions of years ago, however, kept their spurs. These two snakes have their own strange set of “claws” where they once had legs:
Boas are one of the oldest snake species on earth today and have not changed for millions of years. These snakes began to develop nearly 70 million years ago, evolving from the “Boids” of the Cretaceous period. Boids had snake-like bodies with short legs. Over time these creatures lost their short legs, evolving into the boas we see today.
There are at least 45 different types of boas, from the famous boa constrictors of South and Central America to the gigantic Green Anaconda, each with these claw-like spurs. Even the adorably small Rubber Boa in North America (only 15-25 inches long) has its own set of spurs!
The spurs on boas are generally thick and long, and male boas have longer and more visible spurs than females. However, the size and shape of snake spurs varies in each individual. That is why this is not the best way to determine the sex of a snake.
Pythons are another ancient snake species. There are at least 10 different species of pythons that live in Africa, Asia, and Australia. Newly discovered python fossils suggest that these snakes evolved at least 47 million years ago. Like boas, pythons also have vestigial remnants or spurs where their ancestors had back legs. However, the spurs on a python are generally much thinner than those on a boa. They also tend to be more curved like hooks. In addition, python spurs are smaller and usually more inconspicuous.
How and Why Legs or Spurs Develop
The “claws” or spurs on a boa or a python belly are leftover parts from one of their ancient genes. This gene, called the “Sonic the Hedgehog Gene” (SHH) because of its spiky appearance, is crucial for the development and placement of feet and legs. But if these snakes still have the SHH gene in their DNA, why don’t they have legs?
Boa and python X-rays show that these claw-like spurs are connected to a small femur (leg) bone inside the snake’s body, as well as a pelvic girdle. Lizards also have this same SHH gene. However, in lizards the SHH gene creates a fully formed femur, tibia, fibula, and toe digits. So why does the SHH stop short at the femur and just one claw in a snake?
It turns out that pythons actually do develop leg bones when they are still embryos! Python embryos often have femurs, tibias, fibulas, and footplates. However, while the SHH gene guides the development of legs, it is a DNA regulatory enhancer called the “ZP Regulatory Sequence” (ZRS) that actually drives this process.
Recent genetic research shows that boas and pythons have both SHH and its ZRS enhancer, but some of the binding sites have been deleted. Since the proteins cannot bind together in these deleted areas, legs and feet cannot fully form in these snakes. That is why even though python embryos initially develop leg and feet bones, they quickly disintegrate. By the time the pythons are born, only a part of a femur and their pelvic spurs remain.
Why Do Boas and Pythons Still Have Spurs?
Boas and pythons both have the SHH gene and portions of its enhancers in their DNA. This means these snakes have never truly lost their hindlegs, or at least, their DNA has preserved the building model for legs. But if they do not need legs anymore, why did evolution keep this gene? And why develop it into little claw-like spurs?
Many boas and pythons still use their spurs when fighting and for mating. For example, male Madagascan boas, as well as Indian pythons, fight for the right to mate with a female. Snakes often use their spurs during these fights by making them stick out further to scratch or injure their opponent.
Snakes also use their spurs when mating, as observed in emerald tree boas, ball pythons, black pythons, and Burmese pythons. The male snake approaches the female and begins a very complex mating ritual. The male eventually ends up on top of the female and uses his pelvic spurs to move up and down as well as side to side.
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